DEVELOPMENT OF A WIND TUNNEL FORCE BALANCE AND RELATED PRACTICAL EXERCISE MANUAL A Thesis Part B report submitted in partial fulfilment of the requirements for the degree of Bachelor of Engineering in Mechanical Engineering By Gethin Barden Student No. s203897 Supervisor: Dr. Daria Surovtseva and Micah Thorbjornsen Thesis coordinator: Kamal Debnath School of Engineering & Information Technology Faculty of Engineering, Health, Science and the Environment Charles Darwin University Darwin May 2014 2 This page intentionally left blank Development of a Wind Tunnel Force Balance Practical Gethin Barden 3 ABSTRACT DEVELOPMENT OF A WIND TUNNEL FORCE BALANCE AND RELATED PRACTICAL EXERCISE MANUAL Keywords: Wind tunnel, two-component force balance, load cell, lift, drag, pressure distribution A two-component force balance has been constructed and installed in the Charles Darwin University (CDU) wind tunnel to increase the capability of experimental investigation into fluid flow around submerged bodies. The balance consists of a series of struts that transfer forces to two load cells orientated perpendicularly in order to separately measure horizontal (drag) and vertical (lift) forces. Various attachments have been manufactured for various model types. The mechanism has been tested over a range of air velocities from 0-40m/s with the various attachments and suitable models. Comparison of the experimental results with the results of theoretical modelling using a CFD software package revealed an excellent agreement between theoretical and experimental interpretation of fluid forces. A detailed practical exercise manual was developed to assist future students and researchers in conducting experiments involving investigation of fluid flow over submerged bodies. One of the major practical benefits resulted from this project is that the created setup allows for the direct measurement of fluid forces which was not available previously. Development of a Wind Tunnel Force Balance Practical Gethin Barden 4 ACKNOWLEGEMENTS The author is grateful to and would like to thank Dr. Daria Surovtseva and Micah Thorbjornsen for their insight and supervision throughout this thesis. He would also like to thank the Stone family and Brendan von Gerhardt for their assistance in construction of the apparatus and their ongoing support. A final thanks is to the author’s family who have supported him throughout this thesis. Development of a Wind Tunnel Force Balance Practical Gethin Barden 5 Table of Contents SYMBOLS AND ABBREVIATIONS ...................................................................................... 7 LIST OF TABLES ..................................................................................................................... 8 LIST OF FIGURES .................................................................................................................... 9 1. PROJECT INTRODUCTION ........................................................................................... 10 2. LITERATURE SURVEY ................................................................................................. 11 2.1. Existing Force Balance System Designs ....................................................................... 11 2.1.1. Model Mounting ........................................................................................................ 11 2.1.2. Force Measurement .................................................................................................... 13 2.2. Balance Effects on Air Flow and Fluid Forces.............................................................. 14 2.3. Force Balances ............................................................................................................... 14 2.3.1. 2.4. External Force Balances ............................................................................................ 14 Testing And Calibration Techniques ............................................................................. 15 2.4.1. Methodology .............................................................................................................. 15 2.4.2. Comparative Results .................................................................................................. 16 2.4.2.1. Pressure Distribution Analysis ............................................................................... 16 2.4.2.2. Computational Fluid Dynamics Software .............................................................. 17 2.4.2.3. Coefficients Method ............................................................................................... 17 2.5. Model Selection ............................................................................................................. 18 2.5.1. Model Construction and Installation Details ............................................................. 18 2.5.2. Model Coefficient Data ............................................................................................. 19 3. PROJECT SCOPE ............................................................................................................ 21 4. DESIGN STUDY .............................................................................................................. 22 4.1. Apparatus Constraints ................................................................................................... 22 4.2. Force Measurement Setup ............................................................................................. 22 4.2.1. Selected Measurement Devices and Setup ................................................................ 22 4.2.2. Force Balance Software ............................................................................................. 22 4.2.3. Initial Design & Testing ............................................................................................ 24 4.2.4. Improved Design & Testing ...................................................................................... 27 4.3. Clark-Y Aerofoil on Flat Plate Mount Analysis ............................................................ 30 4.3.1. Pressure Distribution Method .................................................................................... 31 4.3.2. CFD Simulation Results ............................................................................................ 31 4.3.3. Data Comparison ....................................................................................................... 32 4.4. 5. 5.1. Flat Plate Model on Rear Plate Mount Analysis ........................................................... 34 PEER TESTING AND EVALUATION ........................................................................... 36 Effect of dimples on objects in air flow ........................................................................ 36 Development of a Wind Tunnel Force Balance Practical Gethin Barden 6 5.1.1. Cylinder ..................................................................................................................... 36 5.1.2. Orgive ........................................................................................................................ 37 5.2. ENG480 Projects ........................................................................................................... 39 5.2.1. Aerofoils .................................................................................................................... 39 5.2.2. Magnus Effect Demonstration ................................................................................... 40 5.2.3. Wind Turbine ............................................................................................................. 41 5.3. Peer Testing Summary .................................................................................................. 42 6. PRACTICAL OUTLINE DESIGN ................................................................................... 44 7. CONCLUSION ................................................................................................................. 45 8. RECOMMENDATIONS .................................................................................................. 46 9. APPENDIX ....................................................................................................................... 47 Appendix A: Practical Exercise Manual .................................................................................. 47 Appendix B: Design Calculations ............................................................................................ 59 Appendix C: Force Balance Drawing....................................................................................... 60 10. REFERENCES .............................................................................................................. 61 Development of a Wind Tunnel Force Balance Practical Gethin Barden 7 SYMBOLS AND ABBREVIATIONS D Drag L Lift CD Drag Coefficient CL Lift Coefficient p Pressure p∞ Atmospheric Pressure U Free stream Velocity θ Angle normal to object surface α Angle of attack A Area ρ Fluid density τw Shear stress between fluid and object F Force M Moment I Moment of Inertia σy Yield stress σb Bending stress τavg Average shear stress E Elastic Modulus δ Deflection Development of a Wind Tunnel Force Balance Practical Gethin Barden 8 LIST OF TABLES Table 1 – Forces (N) at various wind tunnel speeds Page 19 Table 2 – Test results for flat plate with preliminary design Page 26 Table 3 – Preliminary test results Page 26 Table 4 – Clark Y aerofoil wind tunnel testing Page 30 Table 5 – Lift force calculated via pressure distribution Page 30 Table 6 – COMSOL computational fluid analysis results Page 31 Table 7 – Flat plate drag force measurements Page 33 Table 8 – Flat plate drag force comparison Page 34 Table 9 – Smooth cylinder results Page 36 Table 10 – Smooth orgive results Page 37 Table 11 – Data from supersonic aerofoil testing Page 38 Table 12 – Wind turbine results Page 41 Development of a Wind Tunnel Force Balance Practical Gethin Barden 9 LIST OF FIGURES Figure 1 – Sting and strut mounting methods Page 11 Figure 2 – Half model mount of Bombardier Business Jet Page 11 Figure 3 – Phidgets Micro Load Cell Page 12 Figure 4 – 6-component platform balance Page 14 Figure 5 – Diagram of small object and forces acting upon it Page 15 Figure 6 – Clark Y profile Page 18 Figure 7 – Force Balance Program Interface Page 22 Figure 8- Support strut design created with 3D modelling software Page 23 Figure 9 – Small pronged mount for smaller models. Page 24 Figure 10 – Large pronged mount Page 25 Figure 11 - Airfoil type model mount Page 25 Figure 12 – Alternative Apparatus diagram Page 27 Figure 13 – Installed apparatus Page 28 Figure 14 – Model mounts Page 29 Figure 15 – COMSOL image of Clark Y Page 31 Figure 16 – Clark Y Lift force comparison Page 32 Figure 17 – Smooth cylinder mounted in wind tunnel Page 35 Figure 18 – Orgive mounted in wind tunnel Page 36 Figure 19 – Supersonic aerofoil fitted with pressure taps Page 38 Figure 20 – Magnus Effect Page 39 Figure 21 – Magnus effect apparatus to demonstrate lift force Page 40 Figure 22 – Wind turbine secured in tunnel Page 41 Development of a Wind Tunnel Force Balance Practical Gethin Barden 10 1. PROJECT INTRODUCTION Fluid flow is involved in nearly any engineering application. It is an important consideration to design for, or against. Fluid mechanics is especially important in mechanical engineering applications. One of the primary tools used by engineers to design objects that are to encounter fluid flows is a wind tunnel which draws air through a chamber and can be used to visualise and measure the effects that the fluid imparts on the model. Charles Darwin University (CDU) currently has a working wind tunnel with test section dimensions of 450mm x 450mm x 1000mm. The wind tunnel is primarily used in teaching the ENG480 Applied Fluid Mechanics unit and for university displays to local senior school students. The fan is rated to provide airflow of up to 40m/s, and the wind speed is measured by finding the stagnation pressure through the use of a Pitot tube. Currently, a very basic setup for lift measurements has been installed. Some features in which the CDU wind tunnel lacks include a method of displaying the boundary layer, a method of taking pressure and temperature distributions along a surface and, importantly for this thesis, a method of directly measuring drag and lift forces. This thesis will primarily focus on providing an apparatus to measure lift and drag with the secondary focus of producing a Practical Exercise Manual (Appendix A) for ENG480 students to utilise in their studies. In order to improve the wind tunnels functionality, a more accurate, stable and versatile method of measuring the two major aerodynamic forces, lift and drag, is required. Previous students of ENG480 created a device to roughly measure the drag force through springs and their extension. Issues with this balance was that it was unstable, could be used at low velocities only and the spring extension proved difficult to measure accurately. A force balance to measure drag, while at the same time measuring lift, was highlighted as a necessary improvement to the wind tunnel needed for the unit. As the device is intended to be used primarily for ENG480 this thesis revolves around the design of a practical or workshop for students to investigate the lift and drag properties of various models. In the previous year, students designed models that would demonstrate lift, drag and boundary layer. It is planned to continue to allow students to devise and create their own models and, though these models will be constrained by the strut attachments, the force balance should allow for various model designs to be built and tested. Development of a Wind Tunnel Force Balance Practical Gethin Barden 11 2. LITERATURE SURVEY 2.1. Existing Force Balance System Designs There are a number of force balance designs available for lift and drag measurement in a wind tunnel and the drawbacks and benefits are well described. Balances are categorized by components, or the number of forces measured. For example, a one-component balance measures only one force whereas a three-component balance measures three (typically lift, drag and pitching moment). Pitching moment is vital for aerofoil testing but not so important for regular wind tunnel use. Force balances comprise of two aspects that need to be considered, the method of mounting the model and the method of measuring the forces. Two requirements that any mount must comply with are being reasonably aerodynamically discrete and supply a sufficient hold on the model (Onera, 2009). 2.1.1. Model Mounting There are several methods of mounting models in wind tunnels which characterise the methods used to calculate forces. One of the more common methods used is the sting, which is a strut that rises typically from below to connect to the rear of the model. For example, the majority (80%) of the wind tunnels at NASA’s Langley Research Centre’s in 1965 used sting supports (Schaefer, 1965). The sting is most commonly fitted with internal strain gauges or load cells to measure up to six component forces. To manufacture a sting is a complex process and available stings are expensive to purchase1. A simpler method is to mount the model on supports, usually called struts, which are then connected to an external balance system. An example of this is the University of Washington Aeronautical Laboratory’s wind tunnel which typically uses a single strut with a two or three pronged fork to better secure the model (University of Washington Aeronautical Laboratory, n.d.). This method can be designed to be less complex to manufacture than a sting though as the number of components increases so does the difficulty of design and manufacture. These two mounting types, seen in Figure 1, can also be designed to be completely removable from the wind tunnel. Stings generally allow for a greater range of angle of attack than external balances (Barlow et al, 1999), it is however believed that the strut design can be made to accommodate easy alteration of angle of attack. Both of these characteristics, removability and the ability to alter the angle of attack, were desirables in the scope for this project. 1 A quote for a sting without software and positioning system (force measuring component only) was in excess of $50,000. With software and positioning system, price increased to over $120,000. Development of a Wind Tunnel Force Balance Practical Gethin Barden 12 Figure 1 – Sting and strut mounting methods (NASA, 2012) Other methods often used in model mounting include making half models (Figure 2) and fixing to an external balance through the sides or bottom of the tunnel. While this mounting and balance system often produces good results due to the absence of the drag-inducing strut or sting, it does not offer the same flexibility as a sting or strut, and interference with the tunnel wall is an issue that needs to be recognised and accounted for. There are also issues with larger than normal rolling moments with external balances and half models as only one side is present (Barlow et al, 1999). Other types of model mounts do exist but these others are typically centred on a certain design and, for example the half model side mount, do not offer the flexibility desired by ENG480. Figure 2 – Half model mount of Bombardier Business Jet (National Research Council of Canada, 2009) Development of a Wind Tunnel Force Balance Practical Gethin Barden 13 2.1.2. Force Measurement Measuring the forces produced by the fluid-model interaction is the purpose of a force balance design. Purely mechanical methods exist such as the use of springs or counterweights (NASA, 2011). These methods are limited by generally having small operating force ranges. Springs are governed by their spring constant and have a limited amount of extension before they stop behaving linearly. They can also have a tendency to creep if the tolerable load is exceeded and if a small load is applied it generally proves difficult to measure the spring’s extension. Though counterweights would potentially have a larger range, it is still limited and, for high accuracy, would require a large number of weights. It would also be difficult to design against a tedious process of weight management. With the development of technology, data acquisition is becoming more electronics based as these devices allow for the data to be sampled at higher rates, be analysed and manipulated more easily and reduce sources of human error. As such, electronic methods will be investigated. Two electronic methods include the use of strain gauges and load cells. Strain gauges are an alloy foil or set of wires with known resistance properties that are applied to a specimen. As the foil or wires are stretched, the resistivity changes. The resistivity relies on the strain-sensitivity of the alloy foil, the material of the carrier and the gage pattern though other characteristics can also affect the performance (Vishay Precision Group, n.d.). From the extension of the foil (the strain), a stress can be found by knowing the properties of the specimen. A load cell takes this technology further and attaches gauges to a specimen of known material and dimensions (see Figure 3). By doing this there is no requirement for calculating the stress-strain relationship of the material. Load cells also come in a huge range with cells capable of measuring forces to a high degree of accuracy. Figure 3 – Phidgets Micro Load Cell (Phidgets Inc, 2012) Development of a Wind Tunnel Force Balance Practical Gethin Barden 14 2.2. Balance Effects on Air Flow and Fluid Forces It is expected that a strut or sting will add some drag to the model. Whether this drag is negligible or not needs to be investigated. There are three parts that make up the drag component added due to the model support. The first is the direct drag of the support. Part of the support is exposed directly to fluid flow which causes drag as fluid impacts and flows over the support. The design of the strut may have a significant effect on the magnitude of this drag and it needs to be taken into account during the design. To reduce the effect, the support should be streamlined and the cross-sectional area that encounters the fluid flow should be small. The second component of the additional drag is the interference the support has on the fluid flow around the model. Where the support meets the model there will be small gaps and vertices at the edges of the support. The air flowing past the model must accelerate to pass through this reduced area. This acceleration and disruption causes added turbulence and, in regards to a sting, will affect the wake region behind the model. The final part is the interference that the model has on the support. This will, in the same way the supports affect the model, also add to the turbulence and effect the vortices in the wake region (Barlow et al, 1999). 2.3. Force Balances The next part of the system is the force balance. The measurement devices can be inside the wind tunnel (internal) or outside the wind tunnel (external). If an internal measurement device is used it needs to be discrete enough so that it will not disrupt the fluid flow and increase drag. Internal devices are generally located inside the supports or the models themselves. As the balance will be for multiple models, a measurement device on the inside of the model is not suitable. External measurement devices are located outside the wind tunnel and so will not directly influence the fluid flow. This allows for external balances to be as large as required. 2.3.1. External Force Balances Two types of external balances exist. A single specimen fitted with strain gauges and more complex linkage and member types which decouple the forces mechanically before being measured by single force measurement devices. The single specimen balance offers the advantages of being significantly smaller compared to the decoupling balances (Foss et al, 2007). There are four methods of decoupling that external balances typically use, the wire, the platform, the yoke and the pyramidal (Barlow et al, 1999). The first of these, the wire balance, is not used as often these days as other more robust and versatile alternatives exist. Issues with Development of a Wind Tunnel Force Balance Practical Gethin Barden 15 wire breakage leading to model loss and calibrating difficulty are two of these reasons (Barlow et al, 1999). The other three systems use members and linkages to decouple forces. The platform balance (see Figure 4) is the most commonly used but not exceptionally accurate in measuring moments (Barlow et al, 1999). As measuring moments are outside the scope of this thesis this is not an issue. The other two types increase in complexity of design and calculation, especially the pyramidal balance. As the measurement of moments is outside the scope of this project and a two component balance is what is required the focus will be on either a single specimen type or an external platform balance. Figure 4 – 6-component platform balance (NASA, 2012) 2.4. Testing And Calibration Techniques Testing and calibration will be a large part of the apparatus implementation part of this thesis. Several methods have been proposed by different groups with differing levels of relevance (Miranda, 2000; Reis et al, 2013; Gonzalez et al, 2011; Arney & Harter, 1964). In addition to this, several numerical techniques are available in order to verify the reliability and accuracy of the measurement device. 2.4.1. Methodology In most wind tunnel balance calibrations, static loading is the first step towards calibration (Gonzalez et al, 2011). Before calibration commences, the mount should be installed in its final position and the direction of the fluid recognised. With this, the strut is loaded with known loads and the reading on the sensor recorded. A known weight or load is applied and the load entered into the software. A second load, preferably with a large range to reduce errors, is then applied Development of a Wind Tunnel Force Balance Practical Gethin Barden 16 and that load entered into the software (Phidgets Inc, 2013). It is a simple matter to then interpolate between or extrapolate the loads to calculate any subsequent load applied. The second step is the dynamic calibration (Gonzalez et al, 2011; Arney & Harter, 1964) which uses a reference model to make final adjustments. An aerofoil model with known lift and drag forces at certain operating conditions is generally used to ensure sufficient decoupling of force has occurred and any additional coupling requirements are resolved. 2.4.2. Comparative Results The following techniques of acquiring forces caused by fluid flow over submerged bodies will be used to ensure the apparatus is giving an accurate representation of the forces. 2.4.2.1. Pressure Distribution Analysis One method that will be used for this investigation is the analysis of the pressure distribution between the front and rear of the model and also between the top and bottom of the model in cases of aerofoils (Smiadak, 2008; University of Iowa, 2002). These pressure distributions will be obtained through the use of a manometer and specially manufactured models. The models will then be tested in the wind tunnel at a range of air velocities. Fluid mechanics laws state that drag is the force that is developed over an area by the difference in pressure between the front and the back of the submerged object plus the friction force caused by friction between the object surface and fluid. The following equation and figure shows this relationship. Figure 5 – Diagram of small object and forces acting upon it in submerged fluid flow (Munson et al, 2009) Drag=D= ∫ 𝑝 𝑐𝑜𝑠𝜃 𝑑𝐴 + ∫ 𝜏𝑤 𝑠𝑖𝑛𝜃 𝑑𝐴 (1) Experimentally, this integral can become a sum. If the intervals of an object with uniform width are set as certain lengths and pressure measurements taken at these intervals, a pressure Development of a Wind Tunnel Force Balance Practical Gethin Barden 17 distribution can be found and the interval areas used to find the equivalent force (University of Iowa, n.d.). The following equation gives the force component due to the pressure difference, Dp. 1 𝐷𝑝 ≈ ∑𝑖 2 [(𝑝∞ − 𝑝𝑖 ) + (𝑝∞ − 𝑝𝑖−1 )]𝑐𝑜𝑠𝜃𝑖−1,𝑖 ∆𝑠𝑖−1,𝑖 (2) Where i represents the interval number, s is the interval element length and ∞ represents the submerged fluid. This method can also be used for lift by manipulating the equation. As this only provides the pressure difference force component, it can be used, if done accurately, as a close approximation for thin objects where friction drag will be negligible (a good example is a flat plate perpendicular to the flow). This value will then give a force value that is occurring experimentally and can be used to verify literature coefficient calculations. 2.4.2.2. Computational Fluid Dynamics Software The next method that will be used to investigate the drag force effects that the balance and support has on the drag force of the model is computational fluid dynamics (CFD) software. This software models, using algorithms, the flow of fluids over objects. It has the capacity to give values for forces such as drag and lift for various model types. The software does this by finding the stress on opposing surfaces (ie. top and bottom or front and back) and calculating the difference (Bychkov, 2013). This is similar to the pressure distribution method except with an entirely theoretical model and more data points. The software, which has been selected is COMSOL Multiphysics; one of the field’s leading programs. Three-dimensional computer-aided design models will be created for each of the test models. These will be examined at varying speeds and the forces produced from this will be compared with literature coefficient calculations. Initial testing of the software show that the results from COMSOL Multiphysics simulations and literature coefficients are similar. 2.4.2.3. Coefficients Method The final method of calculating the drag, D, and lift, L, forces on a submerged object is by using tabulated values for the lift coefficient, CL, and drag coefficient, CD, which have been derived from experimentation for certain profiles and shapes. These coefficients can then be used in the following formulae (Munson et al, 2009):1 𝐿 = 𝜌𝑈 2 𝐴𝐶𝐿 2 𝐷= 1 2 𝜌𝑈 𝐴𝐶𝐷 2 Development of a Wind Tunnel Force Balance Practical (3) (4) Gethin Barden 18 Where U is the free stream air velocity, ρ is the air density and A is the frontal area of the object. This method will give us the theoretical amount of drag force for the object only. This will allow for comparison of how much extra drag the apparatus adds to the system and how the lift force is affected. 2.5. Model Selection The following models are proposed to be used: Flat plate perpendicular to the flow  Basic aerofoil shape with numerous coefficient sources available The reasons for selecting these models is that they are simple in shape and should be able to test the functionality of the apparatus. The plate will be used to test the pronged mounts in the preliminary design and the flat rear plate mount of the secondary design to ensure they are capable of supporting the models they are required to. They will only be used to test whether a correct drag force is being obtained. The aerofoil will be used with the aerofoil mounts and will test both lift and drag. 2.5.1. Model Construction and Installation Details Flat Plate The flat plate will be used to test the drag component. The model will have the dimensions 200mmx200mm and will be 10mm thick. The model will be installed perpendicular to the flow so that a large pressure difference can be measured and used as comparison data. The flat plate will be installed on the small or large model prong mount. Coefficient data for this model will be gained using the data for a rectangular prism of width to thickness ratio of 0.05. Model will be constructed from 3-ply plywood. Aerofoil The aerofoil that will be used for testing will be the Clark Y type aerofoil. This aerofoil has been used and tested extensively since its creation with numerous designs spawning from it. Due to its flat bottom and simple shape, this aerofoil is less complex to build compared to other foils. Data regarding the aerofoil’s performance has been found and tabulated in the next section. The model will be constructed of polystyrene and has a 250mm chord and is 230mm wide. To manufacture, a profile of the aerofoil (downloaded from aerofoil construction site AirfoilTools.com; Figure 6) will be printed and this profile used in the shaping of the polystyrene. This manufacturing technique should allow for an accurate model to be produced. Development of a Wind Tunnel Force Balance Practical Gethin Barden 19 Figure 6 - Clark Y profile (AirfoilTools.com, 2013) 2.5.2. Model Coefficient Data Table 2 below details drag coefficient values found for each model, the respective source of each coefficient and the drag forces experienced under 10m/s, 20m/s and 30m/s air velocities. The air density that will be used will be 1.13kg/m3, a pressure that was calculated from the Bureau of Meteorology website (2013). Drag and lift coefficients will be taken for a Reynolds number of 100,000 though this can vary up to 200,000 for some of the models at 30m/s the coefficients do not change excessively over this time. For the Clark-Y aerofoil, the angle of attack will be set at its natural angle of 3.2o. The area of the aerofoil for lift is equal to the span of the wing by the length of the chord which, for the aerofoil used, is 0.0575m2. The area for drag is equal to the wing span by the height of the foil which is equal to 0.0067m2. The two Clark Y aerofoil coefficients (Zimmerman, 1933; Silverstein, 1935) can be seen in Table 1 to be very different. This is due to the Zimmerman coefficient taking into account the aspect ratio whereas the Silverstein coefficient does not. The Silverstein coefficient has been calculated for an aspect ratio of 6 whereas the aerofoil in question has an aspect ratio of only 0.9. Due to the aspect ratio being larger for the Silverstein coefficient, the vortices that would be produced in this situation will have less of an impact due to the greater width. Vortices occur on the outer edges of the wing and are caused by the high pressure air at the wing tips travelling around the wing tips to the low pressure side on top of the aerofoil. These vortices create a downwash which counteracts some of the lift being produced (Anderson, 2001). Similarly, the aspect ratio also has an effect on the flat plate. A study by Fail et al (1959) indicated that as the aspect ratio increased so did the drag coefficient. The generally accepted drag coefficient for a flat plate perpendicular to the flow is about 1.98 (Munsen et al, 2009; Engineering Toolbox, n.d.). This, however, is a general case and generally for a plate of infinite size. For a plate with aspect ratio of 1 and an area of 25 square inches (5 inch x 5 inch which is approximately equivalent to 200mm x 200mm), the drag coefficient was found to be equal to 1.17, generally due to a slightly higher base pressure behind the plate compared to the higher Development of a Wind Tunnel Force Balance Practical Gethin Barden 20 aspect ratios. Table 1 displays the forces that are expected to be encountered in testing the flat plate. Table 1 - Forces (N) at various wind tunnel speeds Object Coefficient Drag (Lift) Flat Plate 1.97(0) Clark Y Source 10m/s Drag (Lift) 4.45(0) Munson et al, 2009 1.17(0) Fail et al, 1959 2.64(0) 0.049(0.80) Silverstein, 0.02(2.60) 1935 0.045(0.28) Zimmerman, 0.02(0.91) 1933 Development of a Wind Tunnel Force Balance Practical 20m/s Drag (Lift) 17.81(0) 30m/s Drag (Lift) 40.07(0) 10.58(0) 0.08(10.40) 23.80(0) 0.17(23.39) 0.07(3.64) 0.16(8.19) Gethin Barden 21 3. PROJECT SCOPE This thesis encompasses a large practical section. It aims to develop a working practical for drag and lift measurement in the wind tunnel for ENG480 students. It is believed the thesis results will greatly assist students in understanding fluid flow over submerged bodies and allow comparison of theoretical methods. The primary objectives of this thesis are: Construct and install an apparatus to directly measure lift and drag forces  Test apparatus to ensure that measured results are similar to actual forces  Develop a manual for the use of the apparatus, including installation and disassembly  Develop a practical exercise document for use in the ENG480 unit Secondary objectives include: Build additional models for ENG480 unit examples  Develop a method to record data over period of time and display both forces concurrently In order to progress the design, several constraints and requirements were put on the development, these include: Apparatus had to be built due to budget constraints and the unique nature of the CDU wind tunnel.  Design must be able to withstand a 300N drag force and a 50N lift force which have been calculated to be the peak loads.  Design must be capable of withstanding 40m/s air velocities while laden with model.  Able to support and test a number of models of various shapes such as flat pates, spheres and aerofoils.  Lift and drag components need to be achieved through direct measurement.  Be completely removable from the wind tunnel with no negative effect on airflow Other additional design characteristics that are desirable, though may not be achievable in the allotted time, include: Design allowing the alteration of the angle of attack without removing the wind tunnel test chamber from the diffuser.  Angle of attack of the model made to be adjustable.  A small access hole to access the chamber without having to detach the test chamber from the diffuser or nozzle. Development of a Wind Tunnel Force Balance Practical Gethin Barden 22 4. DESIGN STUDY 4.1. Apparatus Constraints Models used in conjunction with the force balance should be constrained to follow good practice and the design limits. The width and height of the models should be kept to less than 0.8 times the width and height of the test chamber (Barlow et al, 1999). In this instance, that limit is 360mm for both height and width. The reason for this constraint is that models that exceed this limit may encounter effects from the walls of the wind tunnel. An aerofoil, for example, may produce more lift than it would realistically due to vortices not being able to move around the ends of the model. Calculations should be completed to ensure that the design force limits are not exceeded. Lift force should not exceed 50N or 5kg as this exceeds the operating range of the load cell. Similarly, drag should not exceed 300N if the 30kg load cell is equipped. If the 5kg or 20kg load cells are equipped then the limit should be 50N or 200N respectively. 4.2. Force Measurement Setup An integral part of the apparatus is the force measurement devices. These devices take the forces and convert them into data that can be easily interpreted. 4.2.1. Selected Measurement Devices and Setup The selected method of measuring forces will be the use of load cells. The load cells that have been selected for this apparatus are 5kg Phidgets load cells (Figure 3) With most models, it is unlikely that 50N of drag force will be exceeded. However, if it is expected that the force will be greater than this, a 20kg and 30kg load cell have also been procured. This type of cell was selected as it is easily used and calibrated, programs for the data acquisition system can be implemented on a variety of platforms including LabVIEW, delivery time is fast and the load cells are inexpensive. In order to use the load cells, an electronic bridge is required and this was also purchased. Initial testing of the load cells have shown them to work well and provide accurate results. The load cells will be connected externally to the strut by threaded rod and bolted to brackets which will support the entire apparatus. 4.2.2. Force Balance Software Once data has been obtained by the load cells, it needs to be manipulated to give a reading that is easily understood by the user. A software program is required for this step. LabVIEW was chosen as the platform that would be used to write the program as it is more user friendly than the other alternatives to people who are not experienced in programming. The program needs Development of a Wind Tunnel Force Balance Practical Gethin Barden 23 to be able to convert the load cell data from mV/V to newtons. As the load cells behave linearly, the data can be converted by applying a linear equation to it. The program allows for two equations, one for lift and one for drag, to be inputted and once done so will automatically convert the data to a force value. Calibration is conducted by applying a static force to the balance in the component directions (ie horizontally or vertically). To produce this force, a precision digital force meter was procured. Once the force is applied, the known force and the subsequent mV/V data value is recorded. This is done for a second point in the same direction and from these two points the linear equation can be derived and inputted into the program. The interface of the program (Figure 7) needs to be suitable for ENG480 students to understand with relative ease. As a result, the design is simplistic and detailed instructions are provided on the interface for ease of use. In addition to this, there is further information regarding procedures, including a calibration guide, in the Force Balance Practical Exercises Manual (Appendix A). Figure 7 – Force Balance Program Interface Development of a Wind Tunnel Force Balance Practical Gethin Barden 24 4.2.3. Initial Design & Testing For the sake of simplicity, the following design was suggested to initially test the load cells. The design involves a vertical strut, with which calculations on deflection could be easily calculated by hand, with load cells orientated perpendicularly to measure each force. Doubts existed whether this design would accurately measure the correct forces as the lift and drag forces may be interfere with each other. This would be because the forces were not being successfully decoupled. If this appeared to be the case, and no proportionality existed between theoretical and measured results, then an alternative design would be implemented. Figure 8- Support strut design with part details created with 3D modelling software As can be seen in figure 8 above, the support design is relatively simple. Part 1 comprises of two halves which are bolted to the main strut by two bolts to ensure rigidity. The initial design had the part countersunk into the main strut but issues with the computer controlled milling machines available at the university meant that this could not be completed and so the design was modified to accommodate for this. Part 2 is clamped between the two halves of part 1 with a nut and bolt (and spring or flat washers as required). The angle of attack is adjustable by untightening the bolt holding part 2, changing the angle and then retightening. The materials being used for the prototype attachment parts (parts 1 and 2) will be ABS plastic for the model support fittings. This plastic, if laid axially, yields under tension at approximately 20MPa and a yield under compression of 38MPa (Ahn et al, 2002). The parts would generally be under compression and the smallest cross-sectional area is 120mm2 and would be capable of Development of a Wind Tunnel Force Balance Practical Gethin Barden 25 supporting 4.56kN before compression failure. As, at this point of minimum cross sectional area, the part is supported by the strut, this will reduce buckling and make destruction due to the maximum force of 300N very improbable. The strut itself was be made from Plexiglas®, which has a yield strength of 72MPa at 23oC and an elastic modulus of 3.3 GPa (Evonik Industries, 2013). To reach the vertical centre of the tunnel from where the strut would be secured, a span of approximately 300mm is required (y-component). The thickness of the material is 10mm, though no significant load should be taken in this direction (z-component). The last dimension is the horizontal component parallel with fluid flow (x-component) which is the dimension that would have the most load applied to it. Calculations (see Appendix B) have shown that, with a safety factor of 4, this x-component length needs to be 55mm. Deflection calculations were then applied to these dimensions and it was found that at max load a deflection of 5.8mm would occur. As a result, the x-component was increased to 70mm which allowed for a maximum deflection of 2.9mm. This deflection calculation was tested by applying a 15kg load to the strut which, under this load, should and did deflect by approximately 1.5mm. Three model mounts were constructed to accommodate for models of various size and type. The three mounts will be interchangeable with part 2 and images of each with a description can be seen below in figures 9, 10 and 11. Figure 9 – Small pronged mount for smaller models. Three prongs inserted into predrilled holes in model. Development of a Wind Tunnel Force Balance Practical Gethin Barden 26 Figure 10 – Large pronged mount Figure 11 - Aerofoil type model mount. Model placed atop mount and bolted into place using a piece of thin wooden board as a washer Testing for drag was undertaken with the initial apparatus in the wind tunnel. The design was tested using the model of a flat plate loaded on the small model mount as the large model mount had manufacturing flaws. To initially calibrate the device, a static load of known force is applied in the drag component direction. This was then used to calibrate the load cells using the software. A run without a model was completed to ensure vibrations or failure did not occur. The wind tunnel was taken to its full speed of 50Hz (40m/s) with no ill effects noted. The model was introduced and recalibrated using a 10 pound (45.3N) static weight. As the wind tunnel increased the fluid velocity to 30m/s the model started to vibrate though not catastrophically. The speed was increased to 40Hz (about 32m/s) without any additional vibration though the speed was not increased after this point. It was expected that with the larger mount these Development of a Wind Tunnel Force Balance Practical Gethin Barden 27 vibrations would have been reduced as the model would be connected over a wider area. The results from this testing can be seen below in Table 2. Table 2 – Test results from Flat Plate with preliminary design Fluid Velocity (m/s) Horizontal Load Cell (N) Vertical Load Cell (N) 10 0.6 1.5 20 9.1 7.6 30 15.0 17.7 As can be seen from Table 2, the apparatus measured significant forces in both the lift and drag load cells. This indicates that the apparatus is not able to successfully decouple the forces. To further illustrate this, a comparison with literature values and computational fluid simulations can be seen in Table 3. The results immediately indicate that the forces, separately, are well below the expected values of drag force for the flat plate. The sum of the two measured forces however is similar, particularly at 20m/s, to the calculated forces. This indicates that the load cells are successfully obtaining nearly all the force but are unable to split the force into its components. Table 3 - Preliminary test results Fluid Velocity Horizontal Load Vertical Load Coefficient COMSOL (m/s) Cell (N) Cell (N) Method (N) Simulation (N) 10 0.6 1.5 4.4 4.2 20 9.1 7.6 17.8 17.0 30 15.0 17.7 40.1 38.5 In addition to the unsuccessful decoupling of the force, the vibrations apparent during testing were also cause for concern. These vibrations, though they may be reduced with a larger mount, will cause difficulty in reading the data and may cause damage to the wind tunnel. As a result, the second design will be investigated and implemented. 4.2.4. Improved Design & Testing In the case that the preliminary design did not decouple the forces, then an alternative design, similar to Figure 12 below would be constructed and installed. This design was not completed originally because of the potential need of extra space required underneath the tunnel, the added complexity of design and a lack of time in the first semester to construct this system. Development of a Wind Tunnel Force Balance Practical Gethin Barden 28 This system would better decouple the lift and drag forces so they can be individually measured. The strut is attached to members that are pinned and transfer forces along their axes. Figure 12 –Improved Apparatus Figure 12 shows the secondary setup which has a support (1) that is located inside the wind tunnel and connects to an external strut system outside. As lift and drag forces are applied to the end of the support, they are transferred through the support where the drag component is removed by the first (3) horizontal strut. This force is transmitted to the drag load cell (4). The remaining lift force is then transmitted along the second (2) horizontal strut and down the vertical lift strut (5) to the lift load cell (6). A counterbalance point (7) has been included in the design though it is not necessary if load cells are being used instead of springs (which were used in a previous design). The support and strut system (Figure 13) have been made completely of lengths of aluminium (25mm wide by 3mm thick). Aluminium was chosen as it will resist corrosion, it has sufficient strength for low weight and is easily accessible. The main issue with using the aluminium was finding a way to connect certain pieces together. Aluminium can be welded to itself but cannot be joined as easily to steel which may cause issues due to the use of steel nuts to fasten the struts to the load cells. Pieces 3 and 5 have nuts set into the ends so that they can be connected Development of a Wind Tunnel Force Balance Practical Gethin Barden 29 to the load cells. Obtaining aluminium nuts is difficult and welding something so small is considerably difficult. Eventually, the nuts were fixed with strong adhesive which has been successful. All connections for the force balance system are made with M5 fasteners. M5 fasteners were used as this is what is required by the load cells purchased. By designing the force balance with M5 fasteners only, assembly is made easier by preventing mix ups and replacement parts are easier to source. Figure 13 – Photo of completed force balance installed in the wind tunnel As with the initial design, three mounts will be used with this device (Figure 14). The first is a flat plate mount for aerofoils or models that need to be held from below. This model is secured in place by two M6 bolts. A second plate is included to be placed on the top in order to distribute the clamping force of the bolts. Similar to the force balance, this mount has been constructed from aluminium. The second mount, also primarily for aerofoils, has four telescopic arms that grip the aerofoil from the sides. The arms are steel rod and they slide within aluminium tubing. Steel was used in this instance as it is more malleable and less brittle than aluminium (Ashby & Jones, 2005). Each arm is secured at the desired position by an M5 bolt. This mount was designed in order to accommodate models with a round bottom surface that would not be able to be mounted on the flat plate. The third mount is the rear plate mount that allows models to Development of a Wind Tunnel Force Balance Practical Gethin Barden 30 be mounted from the rear. Models are mounted with four fasteners. For further information regarding the dimensions of the force balance and model mounts see section 1 in Appendix A (Force Balance Practical Exercise Manual). Figure 14 – Aerofoil mount diagrams The second design was tested initially with a Clark Y aerofoil. The load cells were calibrated statically by a 10N load applied in both the horizontal and vertical directions. At this stage of calibration, the apparatus was not perfectly square and so it was noted that a drag force did influence the lift load cell slightly and vice versa. The design was tweaked so that the apparatus was square which resulted in the successful separation of the lift and drag forces. The results from the initial test (see Section 4.2.5) with the Clark Y aerofoil were acceptable and similar to calculated values. 4.3. Clark-Y Aerofoil on Flat Plate Mount Analysis Testing with the Clark-Y aerofoil on the flat plate mount was conducted. During testing no illeffects were noticed and the model was subjected to the maximum fluid velocity of 40ms-1. The following data regarding the lift and drag results for the Clark-Y aerofoil can be seen below in Table 4. Development of a Wind Tunnel Force Balance Practical Gethin Barden 31 Table 4 – Clark-Y aerofoil wind tunnel testing Fluid Velocity (m s-1) Lift (N) Drag (N) 10 0.7 0.1 20 2.9 0.9 30 8.3 2.0 40 16.8 3.8 4.3.1. Pressure Distribution Method After measuring the forces using the force balance, pressure measurements along the profile of the aerofoil were taken using a manometer. These manometer readings were then converted to pressure differences and the segment lengths calculated. Once this was completed, they were inputted into equation 2 from Section 2.4.2.1 and the lift was calculated. The resultant lift force can be seen in Table 5. Table 5 - Lift force calculated via pressure distribution. Fluid Velocity (m s-1) Lift (N) 10 1.12 20 4.28 30 8.65 40 12.82 4.3.2. CFD Simulation Results In addition to wind tunnel testing, CFD simulations were conducted on the model. COMSOL simulation software (Figure 15) was used for this procedure and the following results in Table 6 achieved. The figure shows how the velocity changes along the aerofoils profile. As can be seen, the areas on top of the aerofoil have a higher velocity flow than those underneath indicating that there is a lower pressure on the top surface. Development of a Wind Tunnel Force Balance Practical Gethin Barden 32 Figure 15 – Image of COMSOL being used on the Clark-Y Aerofoil at 20m/s Table 6 – COMSOL computational fluid analysis results Fluid Velocity (ms-1) Lift (N) Drag (N) 10 0.74 0.12 20 2.97 0.48 30 6.72 1.08 40 12.00 1.91 4.3.3. Data Comparison At first view, the data in Table 6 is comparable to the data obtained for wind tunnel testing (Table 4). Particularly at the slower two velocities. However, the higher velocities differ with both the drag and lift being higher in the wind tunnel than in the simulations. Figure 16 shows a graph that compares lift values for the various methods used to calculate and measure the forces. Development of a Wind Tunnel Force Balance Practical Gethin Barden 33 Figure 16 – Comparison of lift forces Figure 16 indicates that lift forces calculated with Zimmerman’s coefficient and the computational fluid simulations are quite similar to the results achieved in the wind tunnel. The Silverstein coefficient, which was significantly higher than Zimmerman’s, produces significantly more lift. Due to this reasoning and the evidence shown in results from simulations and testing it can safely be assumed that the results achieved with Silverstein’s coefficients are not representative of this model. Some discrepancy does exist between the test results, simulations and Zimmerman coefficient results. Zimmerman coefficient does not take into account the surface roughness of the material and while the simulation software has inputs for the surface roughness these are not likely to be exact. This would have some effect on the discrepancies. Another source of discrepancy could be in the shape of the aerofoil. While close to the desired shape, irregularities will exist in the test model and this could affect the lift results. Interference from the mount could also be affecting the lift of the model. Results for the lower velocities however are very similar, Development of a Wind Tunnel Force Balance Practical Gethin Barden 34 particularly between the test results and simulations, and so the results obtained for lift are acceptable. The results for the drag force of the wind tunnel tests compared to the simulation and coefficient methods showed that the wind tunnel test drag forces are significantly higher than the simulation results which are again higher than the coefficient results. This was expected as additional drag force was expected to be created by the mount. The drag force would be caused predominantly from the mount parts in contact with the model aerofoil. These areas will cause added turbulence which will increase drag. For an aerofoil model with the flat plate mount the additional drag at mid-range fluid velocities (20-30m/s) has been calculated to be 85% more which increases as the fluid velocity increases. 4.4. Flat Plate Model on Rear Plate Mount Analysis To ensure that the drag cell was working correctly and independently to the lift cell a flat plate of know dimensions was tested. This model was also tested in section 4.2.3 with the preliminary design and coefficient data can be seen in Table 2. The model was mounted using four bolts to the rear plate mount and was tested through a range of fluid velocities from 9.5ms-1 to 33.25ms-1. Drag force data can be seen below in Table 7. Table 7 – Flat plate drag force measurements Fluid Velocity (ms-1) Drag Force (N) 9.50 1.40 19.00 6.51 23.75 10.70 28.50 15.46 33.25 20.80 The flat plate was relatively heavy compared to other models tested and the effect this had on vibrations was evident. The vibrations in the horizontal component were not excessive due to the constant force in that direction. The vibration in the vertical component was substantial and this meant that an accurate lift reading could not be obtained. Despite this, it was evident that the lift force was relatively constant as the vibrations stayed centred around the -3.5N mark (-3.5N were the initial conditions due to the weight of the plate). The reason that the vibrations were an issue with this system is because of the large ratio between the contact area of the mount and the area of the model as well as the centre of gravity of the model not being directly Development of a Wind Tunnel Force Balance Practical Gethin Barden 35 over the top of the support. A lot of force acts upon the outer extremities of the plate and so when vibration occurs there is considerable momentum involved. When the data from testing is compared to the calculated and computational fluid analysis values (Table 8), the difference is considerable. The measured drag force has been linearly interpolated due to the data being taken at frequencies that did not match the desired fluid velocities. This will introduce some error as the relationship is polynomial but this error is insignificant compared to the discrepancy between the calculated and measured results. The measured results are 30% less than the Fail et al (1959) coefficient calculated drag forces and nearly 250% less than the Munson et al (2009) coefficient calculated drag force. Meanwhile, the COMSOL drag force is nearly 300% greater (for 20 and 30m/s) than measured. Due to the small aspect ratio of the plate, it is suggested that substantial vortices are forming behind the flat plate which increases the base pressure, reducing the drag. The sharpness of the edges of the plates, and the considerable roughness of the sides, may also have affected the amount of vortices as both of these characteristics would introduce turbulent flow more quickly. The difference in results could be caused by some friction in the force balance causing a loss of force during transmission to the load cells. Table 8 – Flat plate drag force comparison Fluid Velocity Coefficient Drag COMSOL Drag Force Measured Drag (m/s) Force (N) (N) (interpolated) (N) 10 2.65-4.45 5.37 1.67 20 10.58-17.81 21.61 7.39 30 23.80-40.07 49.06 17.15 40 42.30-71.26 88.37 Development of a Wind Tunnel Force Balance Practical Force Gethin Barden 36 5. PEER TESTING AND EVALUATION 5.1. Effect of dimples on objects in air flow In conjunction with this thesis, a fellow thesis student has been completing his own studies in regards to fluid flow over smooth and dimpled geometries. Due to the geometries being relatively simple the results were to be used as a guide as to how the force balance was performing. The objects included a cylinder and an orgive shape. Two models were created for each geometry with the only difference being the surface finish; one model was smooth and the other dimpled. These models would then be tested in the wind tunnel to determine if there would be a reduction in drag by dimpling the surface. Computational fluid dynamics simulations were conducted to provide values for comparison. 5.1.1. Cylinder The cylinder to be tested was hollow inside though had its ends capped and had a diameter of 50mm and a length of 100mm. The model was mounted directly onto the support (Figure 17) so that the additional drag of a model mount would not affect the results. The cylinder was held in place by two M5 threaded rods and 4 nuts. Figure 17 - Smooth cylinder mounted in the wind tunnel Testing was undertaken at 10m/s intervals starting at 5m/s. The cylinder performed well with limited vibrations. The supporting rods were kept in the same orientation for both dimpled and smooth models to ensure that they did not have an unexpected effect on the results. The results for the smooth cylinder from testing and computational fluid simulations can be seen in Table 8. Development of a Wind Tunnel Force Balance Practical Gethin Barden 37 Table 9 - Smooth cylinder results Fluid Velocity (m s-1) Wind Tunnel Drag (N) Simulated Drag (N) 10 0.183 0.193 15 0.467 0.492 20 0.846 0.891 25 1.381 1.454 30 1.976 2.080 35 2.572 2.707 40 3.152 3.318 The results from this testing indicate that less drag is occurring in the wind tunnel than in the simulations. From 15m/s through to 35m/s it is noted that the percentage error between the wind tunnel results and the simulated results is between 7% and 12%. At fluid velocities below 10m/s it is observed that the force being measured for small to medium sized models is often not attained effectively and there is often a large error. It should be noted that for most models simulated there was very little difference in drag for sub-10m/s velocities. It was unexpected that the wind tunnel drag would be less than the simulated drag. This could be caused due to friction losses in the apparatus or some error in the shape of the model. The dimpled model experienced similar forces to that of the smooth model. From the simulation results, a drastic decrease in drag of approximately 57% was expected from the dimpled model. Though the dimpled model did produce less drag, it was not percentage based as expected. The reduction in drag over all speeds was approximately constant and centred around 0.16N. Resultantly, the dimpled simulation results would be quite different percentage-wise to the test result. Issues with surface finish could have affected the performance of the dimples and increased drag. Also, due to the complex surface structure of the dimples, the computational fluid dynamics software had difficulty running the dimpled model at high mesh frequencies. This could mean that the simulation drag data is lower than it should be. 5.1.2. Orgive The orgive to be tested comprises of two sections. A 50mm diameter cylinder that is 80mm tall which connects to a 50mm diameter based cone with a 4.5mm diameter tip and is 90mm tall. This model was mounted from the rear using a flat plate with two bolt holes (Figure 18). It was expected that the mount would increase drag slightly though not significantly. This was due to Development of a Wind Tunnel Force Balance Practical Gethin Barden 38 the mount being thin and completely behind the back of the orgive. The main effect the mount would have would be on the wake of the orgive. Figure 18 - Orgive model mounted in wind tunnel The results from the orgive (Table 9) indicate that the drag force for the wind tunnel results and the simulation results are quite similar. The percentage error between the test and simulation results within the 15 to 30m/s is approximately 11%. This error indicates that the model mount may have slightly increased the drag on the model. Alternatively, the surface finish was not exceptionally smooth and this may have had an effect on the results. Table 10 - Smooth orgive results Fluid Velocity (m s-1) Wind Tunnel Drag Force (N) Simulation Drag Force (N) 10 0.025 0.038 15 0.076 0.086 20 0.160 0.153 25 0.236 0.239 30 0.382 0.344 35 0.554 0.468 40 0.741 0.611 The simulations predicted that the dimpled model would have slightly lower drag values (1015% reduction at mid-range velocities) than its smooth counterpart. As the resultant forces are quite low, this reduction of force was minimal. During testing, it was noted that the dimpled orgive shape achieved very similar results to the smooth model and that it could not be conclusively said whether the drag was increased or decreased by the dimples. Development of a Wind Tunnel Force Balance Practical Gethin Barden 39 5.2. ENG480 Projects The students of ENG480 were tasked with designing models throughout the semester to demonstrate fluid flow properties. These properties included lift and drag for the first two projects and, for the last, the power generated by a turbine. The various uses of the force balance show that the functionality and versatility is exemplary. 5.2.1. Aerofoils Aerofoils are a good method of demonstrating the concepts of lift and drag and so a number of students built aerofoils to demonstrate these fluid properties. The first project involved the construction of an aerofoil that would be suited to supersonic flow and testing it in subsonic conditions. The students expected to achieve less lift than drag for their supersonic aerofoil (Figure 19), though the difference between lift and drag would decrease as fluid velocity increased (Daley et al, 1947). This theory, when tested at the natural angle of 6o, was proved correct as can be seen in Table 8 (fluid velocity (m/s) is equal to approximately 0.76 times the fan frequency (Hz)). Figure 19 – Supersonic aerofoil fitted with pressure taps (Kelly et al, 2014) Table 11 – Data from testing of supersonic aerofoil (Kelly et al, 2014) Fan Frequency (Hz) Lift (N) Drag (N) Percentage difference between lift and drag (%) 35 1.21482 3.80756 68.09 40 2.78265 5.04995 44.95 45 4.83952 6.52868 26.15 50 7.46969 8.35844 10.84 Development of a Wind Tunnel Force Balance Practical Gethin Barden 40 5.2.2. Magnus Effect Demonstration One group of students decided to demonstrate lift and drag by creating a rotating cylinder. The purpose of this apparatus was to demonstrate the Magnus Effect. This phenomenon is where a force is produced due to the rotation of an object submerged in fluid flow. This is not unlike the sideways movement seen when a soccer ball is kicked with side spin on it or how a tennis ball dips when top spin is applied in tennis. The effect is caused by frictional forces increasing the fluid velocity on one side of the rotating object where the surface is travelling in the same direction as the fluid while decreasing it on the other side where the surface is travelling against the fluid flow (Figure 20). The increase in velocity on the one side decreases pressure, while the decrease in velocity on the other increases pressure. As a result the rotating object will experience a force towards the side where the surface is travelling in the same direction as the fluid flow (Reid, 1997). Figure 20 – Magnus effect (Aviation-for-kids, 2012) The students designed an apparatus that would use paddles to grip the air and make the cylinder spin at a certain speed (Figure 21). They calculated the rotational frequency to achieve 2N of lift to be 760rpm though they could not determine how to determine the rotational frequency during experimentation and so could not compare their results to theoretical data. The apparatus was tested and at a fluid velocity of 20m/s they achieved 0.5N of lift and that, over a range of speeds, the force was proportional with the square of the velocity as expected. The apparatus successfully proved the Magnus effect at work. Development of a Wind Tunnel Force Balance Practical Gethin Barden 41 Figure 21 – Magnus effect apparatus to demonstrate lift force 5.2.3. Wind Turbine The final project that ENG480 students undertook was the design construction of a model wind turbine. The students were to design the turbine for specific conditions power outputs then, using scaling laws, downsize their models to fit in the wind tunnel. The apparatus would then be tested to determine how much torque it produced. One group of students successfully built a model (Figure 22) and tested the apparatus. They obtained torque values by attaching a string from the outer surface of the shaft to the apparatus. By knowing the shaft diameter, and finding the force in the vertical direction, they could calculate the torque. The results (Table 9) indicate that the theoretical torque is considerably less than the measured torque for the model. The measured torque is likely to be higher as, when scaling, smaller models generally have higher efficiencies. In addition, the theoretical torque of the turbine was calculated from the power which implies that the turbine is rotating. The torque measured was with the model stationary and so does not take into account blade tip considerations which would reduce the torque. Development of a Wind Tunnel Force Balance Practical Gethin Barden 42 Figure 22 – Wind turbine secured in tunnel Table 12 – Wind turbine results Fluid velocity (m/s) Force measured (N) Torque produced Torque calculated (Nm) (Nm) 15 2.3 0.0345 0.0015 20 4.0 0.0600 0.0037 25 6.3 0.0945 0.0072 30 9.0 0.1350 0.0140 35 12.6 0.1890 0.0248 As can be seen in Table 9, the measured forces are considerably different to the calculated forces due to the reasons mentioned. The results do show polynomial behaviour which is to be expected as the force should be proportional to the square of the fluid velocity. Overall, the measured torque gives an indication of what forces are applied to a turbine in fluid flow. 5.3. Peer Testing Summary Peer testing throughout this thesis has been useful as it has incorporated a wide range of model types. The variety in models points to the versatility of the apparatus. The variety also leads to a number of deductions regarding the force balances accuracy. The best operating range for the force balance is between 15 and 30m/s. The percentage errors for all models when compared to simulated data have been less in this range. At speeds lower than 10m/s the force balance loses accuracy with speeds lower than 5m/s difficult to detect. It was also noted that at speeds higher than 35m/s that the accuracy started to decrease again. This could be caused by increased vibration at higher speeds making it difficult to gauge an accurate reading. Development of a Wind Tunnel Force Balance Practical Gethin Barden 43 The variety of models also indicated areas where improvements are required. The strut is not completely rigid and so with heavier models tends to move laterally (in the direction perpendicular to the flow). This observation has been highlighted, with others, as one area where the force balance and wind tunnel can be improved. Development of a Wind Tunnel Force Balance Practical Gethin Barden 44 6. PRACTICAL OUTLINE DESIGN The objectives of this practical are to encourage students to design, build and test their own model to be used in the wind tunnel. Students will be grouped and asked to design and build a model that is capable of meeting certain targets. They will be required to use their theoretical knowledge to determine forces that will be encountered when the model is submerged in fluid flow. These theoretical calculations will be conducted before the testing of the models. If a group is unable to build their own, they will be able to use one of the test models. Once the theoretical calculations have been obtained students can then test their models with the force balance. Students will test their models over a range of velocities and determine lift and drag coefficients from the data obtained. The students will then be asked discussion questions about the practical and the apparatus to encourage further research and original thought. The information regarding the practical exercise will be delivered to the students via a Practical Exercise Manual. The manual format has been based on existing apparatus manuals (Ebidon, 2013) used throughout the university in an effort to maintain a universal system. The manual (Appendix A) contains information and instructions regarding the following items: Detailed apparatus description  Model mounting and construction guide  Software operating instructions  Software calibration instructions  Wind tunnel operation guide  Apparatus assembly procedures  Velocity testing and coefficient derivations practical exercise method  Basic theory Development of a Wind Tunnel Force Balance Practical Gethin Barden 45 7. CONCLUSION Wind tunnels, even in this day where computer simulations are commonplace, are still an important part of research and product testing. The force balance is an important apparatus for wind tunnels as it provides the quantitative data that is required for further calculations. As such, this thesis was focussed around the construction of a force balance and a subsequent practical exercises manual. The first part of this thesis involved the construction of an apparatus to measure force. An initial design was manufactured in an attempt to directly measure the forces on the support via load cells. It was noted that the forces were not successfully decoupled and so the design was revisited and an improved design completed that successfully decoupled the forces by using a series of linkages. In addition to the physical apparatus, a software program had to be developed to convert the data to a force. Once the software was completed and statically calibrated, test models were used to determine whether the forces were being decoupled with initial testing yielded good results. Throughout this thesis, forces were calculated for each model using literature coefficients and computational fluid analysis software. These theoretical forces would be used as a guide as to how accurate the results from the force balance were. Results compared well with the Clark Y aerofoil test model which obtained lift forces within 5% of the theoretical values at intermediate speeds of 20 and 30m/s. The aerofoil also achieved significantly more drag which was expected due to the interference of the model mount. Excellent results were also obtained when peer testing was conducted. For the cylinder and orgive models, errors of less than 12% were obtained. These results indicate that the force balance is achieving relatively accurate results considering that they will experience some interference from the mounting system. The production of a practical exercise manual was a major deliverable in the context of this thesis. The manual was made available to ENG480 students prior to them commencing their projects and was successful in communicating the operation and constraints of the wind tunnel and force balance. Their use highlighted the versatility of the apparatus and the use over the semester showed that it was easily removed and durable. The manual will also be used by laboratory technicians who will be required to calibrate and maintain the apparatus. The force balance produced achieved all of the primary objectives in that it is versatile, removable and provides an accurate representation of forces on a model submerged in fluid flow in the wind tunnel. Development of a Wind Tunnel Force Balance Practical Gethin Barden 46 8. RECOMMENDATIONS Recommendations for future works into developing the force balance and wind tunnel include: Create a software program that enables easier calibration, better display and potential plotting against time. This will allow for the average to be taken and reduce the impact of vibrations on readings.  Alter the balance to allow for the angle of attack to be altered  Install rollers that support the force balance laterally to reduce vibration and increase stability  Alter the balance so that two struts can be used to support models which will allow for additional stability. Development of a Wind Tunnel Force Balance Practical Gethin Barden 47 9. APPENDIX Appendix A: Practical Exercise Manual Force Balance Practical Exercises Manual Contents 1. General Description of the System ............................................................................................... 48 1.1. Apparatus Description........................................................................................................... 48 1.2. Force Balance Specifications ................................................................................................. 50 1.3. Model Mount Specifications and Installation Guide ............................................................. 51 1.4. Software ................................................................................................................................ 52 1.4.1. Software Instructions ........................................................................................................ 52 1.4.2. Software Calibration.......................................................................................................... 53 1.5. 2. Wind Tunnel Operation ......................................................................................................... 54 Theory ........................................................................................................................................... 55 2.1. Force Balance Theory ............................................................................................................ 55 2.2. Coefficient Theory ................................................................................................................. 56 2.3. Pressure Distributions and Simulation Software .................................................................. 56 3. Practical Exercises ......................................................................................................................... 57 3.1. Model Construction................................................................................................................... 57 3.1.1. 3.2. Model Construction Guide and Tips.................................................................................. 57 Velocity Testing and Coefficient Derivations ............................................................................ 57 Development of a Wind Tunnel Force Balance Practical Gethin Barden 48 1. General Description of the System 1.1. Apparatus Description The force balance unit (Figure 1) uses a series of struts to transfer the lift and drag forces encountered by a model in the wind tunnel to two external load cells to measure lift and drag. The strut system consists of the main strut (1) which links to the support strut (2) and the drag force strut (3) which connects directly to the drag load cell (4). The support strut connects to the lift strut (5) which transfers force to the lift load cell (6). The struts are 3mm thick by 25mm wide aluminium members and are connected with M5 bolts. At the end of the support strut is a counterbalance (7) to negate the weight of the model. Readings from the two load cells are transferred via four wires to the bridge (8) which transmits these to the computer via the micro-USB cable (9). In addition to the strut and load cell assembly, there are three interchangeable model mounts (Figure 2) that can be placed on the top of the main strut. The first of these is the flat plate aerofoil mount (10) for flat bottomed aerofoils or models that are to be mounted from below. The second mount is the telescopic aerofoil mount (11) which has 4 telescopic arms that clamp the model in place from its sides. The final mount is the flat rear mount (12) that allows a model to be mounted from the rear with screws or pins. Figure 1: Force balance unit Development of a Wind Tunnel Force Balance Practical Gethin Barden 49 Figure 2: Model attachment parts 1.2. Assembly The apparatus should be assembled in the following sequence. See figure 1 for diagram. 1) Attach strut 2 to the base at the pinned point ensuring the orientation is correct. 2) Screw the lift load cell (6) into position ensuring the directional arrow is pointing up. 3) Attach the lift strut (5) to strut 2 and then insert bolt through the bottom of the base, through the lift load cell and into the end of the strut. 4) Attach the drag load cell (4) onto the base support, ensuring that the arrow is pointing in the same direction as the fluid. Use a spring washer to ensure the load cell stays in position when tightened. 5) Secure the apparatus in position under the wind tunnel test chamber. 6) Attach the main support (1) to the end of strut 2, ensuring that the model support end is inside the test chamber. 7) Attach the drag strut (3) to the main support (1) and then secure the end to the drag load cell (4). 8) Connect the load cells to the bridge as per the wiring diagram in the next section. 9) Connect bridge to the laptop using the micro-USB cable provided. Adjust the screws that connect the lift and drag load cells to the struts to ensure that the apparatus is square. For disassembly, the order should be reversed. Development of a Wind Tunnel Force Balance Practical Gethin Barden 50 1.3. Force Balance Specifications Struts – the strut members in this balance are all made of 3mm by 25mm aluminium to reduce the weight of the apparatus. Bolted connections – all bolted connections are M5 galvanized steel Philips head bolts, typically 20mm in length. Load Cells – the load cells are 3133_0 Micro Load Cells (0-5kg) from Phidgets Inc. They have two M5 threaded holes at either end (40mm apart) for supporting and applying loads. Each load cell has four wires; red, black, white and green which connect to the bridge. Bridge – the bridge is a 1046 4-input Phidget bridge from Phidgets Inc. The bridge has 4 channels with each channel accepting 4 wires. Marked on the bridge for each channel are the channel number, 5V, +, - and G. Figure 3 indicates how the load cells are connected to the bridge, note the channel number indicated (channel 0 is connected) at the centre of the channel. The bridge is then connected to a computer via a micro-USB cable. Figure 3: Phidget bridge showing correct wiring for micro load cells (red to 5V, green to +, white to – and black to G); Image taken from http://www.phidgets.com/products.php?category=34&product_id=3133_0 Development of a Wind Tunnel Force Balance Practical Gethin Barden 51 1.4. Model Mount Specifications and Installation Guide Flat Plate Aerofoil Mount The flat plate aerofoil mount is an 80mm long by 40mm wide aluminium base with two M6 holes drilled 60mm apart. The model should be prepared by drilling to 6mm diameter holes at the centre of the aerofoil 60mm apart so that they match the holes in the baseplate. The model should then be placed on the baseplate and bolted into place. A second plate is present to be placed on top in order to distribute the forces better. Ensure the model is clamped tightly as not tightening sufficiently will cause the models angle of attack to change mid-testing as the bolts have room to shift. Telescopic Aerofoil Mount The telescopic aerofoil mount is used for aerofoils or objects with curved bottom surfaces that will not fit on the Flat Plate Aerofoil Mount. The mount has four telescopic arms that hold the model in place by inserting the 6mm pins into holes predrilled into the model. The arms can extend to accommodate for models ranging from 100mm to 430mm in width. There is 40mm between the base and the centre of the pins and the pins are spaced 30mm apart. The telescopic arms are mounted on a flat plate with 4 M5 bolts and the arms are locked into place with a M5 screw. Ensure that the screws are tightened sufficiently to ensure that the model does not rotate under the forces. Rear Plate Mount This mount is for models that have a flat rear surface. This mount is a base plate with 4 holes 30mm apart horizontally and 25mm apart vertically. M5 bolts or screws are to be screwed into the back of models to hold them in place. For models that will see repeated use, it is advised that a threaded collar or a number of nuts be inserted into the model for easy attachment that will not damage the model. Development of a Wind Tunnel Force Balance Practical Gethin Barden 52 1.5. Software Software for the force balance apparatus has been developed with National Instruments LabVIEW 2011. The software takes the mV/V data obtained from the load cell and automatically converts the value to a force in Newtons (N). The image below (Figure 4) shows the interface for the balance. An executable program was created for this program however this limits the balance to its initial calibration parameters as recalibration requires access to the block diagram (Figure 5) which can only be achieved using LabVIEW. 1.5.1. Software Instructions 1. Open ForceBalanceProgram.vi on the computer. 2. Before running program ensure mini-USB cable is plugged into bridge and computer 3. On the bridge, check the channels for the DRAG and LIFT load cells and ensure that these are selected in the Channel drop menu below 4. Ensure the Enable button for each cell is ticked, Gain should be left equal to 1 5. Press the RUN button (Play arrow in top left corner) 6. If Lift and Drag forces are not zero (or near zero) refer to calibration guide or record initial values before proceeding with tests. Instructions are also written on the interface for ease of use once the program is opened. If it is difficult to read the data, the data rate can be adjusted for easier reading. The program may not work again if stopped after being run. If this occurs, save and close the program before reopening. Figure 4: LabVIEW Interface with instructions Development of a Wind Tunnel Force Balance Practical Gethin Barden 53 Figure 5: LabVIEW block diagram, note the place to add calibration formulas 1.5.2. Software Calibration To recalibrate the force balance follow the steps below: 1. Remove any models or attachments on the main strut 2. Open Force Balance Program.vi in Labview and run the program 3. Record the Bridge Value’s when there is no force applied. 4. Apply a force of 10N to the strut using the Digital Force Meter and the hook attachment in the horizontal direction (in the direction of flow) and record the Bridge Value for the drag load cell. 5. Reapply the force of 10N in the upwards direction and record the Bridge Value for the lift load cell. 6. Being a linear system, the calibration formula is equal to y=mx+c where m=10/(Bridge Value 2 – Bridge Value 1) and c can be found by inputting the data for x and y for one of the readings. 7. Once formulae have been found they can be inputted into the Labview program by opening the block diagram (Ctrl+E) and replacing the appropriate formulas. 8. Save the Labview program as once stopped the program may need to be reopened. Development of a Wind Tunnel Force Balance Practical Gethin Barden 54 1.6. Wind Tunnel Operation Follow the below instructions when operating the wind tunnel:1. Ensure the wind tunnel is closed correctly, the door at the diffuser end is open and nothing is left loose inside the tunnel. 2. Turn wind tunnel on at the power point; turn on main switch on switchboard; insert key, depress and turn. The green light on the switchboard should now be on. 3. Insert starting velocity using <RESET button to scroll and arrow buttons to change. Once done, hit enter and then run. Wind tunnel will start. Ensure someone is ready at all times to hit the emergency stop button if something comes loose in the tunnel or any other issues arise. 4. The wind tunnel does not need to be stopped to change the speed. This can be done by changing the frequency with the <RESET button and the arrows and once the selected frequency has been inputted press the enter button. Development of a Wind Tunnel Force Balance Practical Gethin Barden 55 2. Theory Lift and drag are two important concepts in engineering. Lift is created by a difference in pressure between the top and the bottom of an object. For lift to occur there must be a lower pressure on top of the object and a higher pressure underneath which pushes the object upwards. This is generally achieved in aerofoils by forcing the air flowing over the top of the foil to travel faster therefore reducing the pressure. This is why most aerofoils will have a larger curve on top so that the surface is longer. Drag is also partly created by a pressure difference. Lower pressures will exist behind an object as pressure at the front increases as the fluid encounters the object and is slowed down. Once behind the object it accelerates again to produce a lower pressure. An easier explanation is that the inertia of the fluid impacting the object will create a force that pushes the object in the direction of the free flowing fluid. In addition to the drag due to the pressure difference, there will also be a force due to the friction created as the fluid flows over the object. A number of different methods exist to find the drag and lift forces. Firstly, research has been conducted on many different objects and shapes. This research leads to the development of lift and drag coefficients which can be used to find lift and drag forces for different dimensions and fluid velocities. Secondly, for lift especially, the pressure difference can be found between the top and the bottom (or front and back for drag) of the aerofoil. Thirdly, with the rise of computers, simulation software exists that will use algorithms to calculate forces on an object in fluid flow and finally, there is the use of a balance or force measuring device to directly measure the forces. 2.1. Force Balance Theory The force balance works by physically putting a model in the fluid flow and measuring the forces upon it. The purpose of the strut system seen in this force balance takes the resultant force and separates it into the two constituent lift and drag forces. These forces are then read by the load cells and converted to a force value by the software. The load cells work by measuring the deformation of a specimen of known dimensions using a strain gauge. The strain gauge measures the resistance of the strain gauge and, as the wires of the gauge are stretched due to the deformation of the specimen, the resistance will change. This changes the amount of voltage returning through the circuit. From this point, the strain gauge transmits the return voltage to the bridge and thru to the computer where it can be converted and manipulated. The balance will have an adverse effect on the drag performance of the model. A small part of this will be due to the strut being directly exposed to the fluid, however as the strut is 3mm thick and only 25mm long (in the direction of fluid flow) this drag will be minimal. The drag caused by interference of the strut on the model however will be significant. When two objects join, fluid flowing around each object is forced through smaller areas meaning it accelerates which uses additional energy and creates added Development of a Wind Tunnel Force Balance Practical Gethin Barden 56 turbulence. This is particularly evident when the join is sudden and so designs are often altered to accommodate this or reduce its effects. 2.2. Coefficient Theory Perhaps one of the simplest methods of calculating the lift and drag forces for various models is by using lift and drag coefficients. These coefficient are tabulated in various forms of literature. The coefficients, however, are dependent on the Reynolds number and so a coefficient for a model at low speeds may be different to the model’s coefficient at higher speeds. As well as being reliant on the Reynolds number, the angle of attack is also very important to the coefficient, particularly in aerofoils. As the angle changes, so does the profile of the object and therefore the amount of force produced. As a result, coefficients are generally tabulated against the angle of attack or alternatively against each other (ie Drag coefficient vs Lift coefficient). Once the coefficients for a model is found at a certain Reynolds number and angle of attack, it can be used in the coefficient formula. The formula for drag can be seen below where ρ is the density of the fluid, V is the fluid velocity and As is the cross sectional area (area perpendicular to flow for drag and parallel for lift):- The force (F) and coefficient (C) can be changed with the respective lift values without altering the formula (the area will also change due to the direction of the force changing). These values, presuming the model is dimensionally accurate, should give close representations to the true force acting on the model. 2.3. Pressure Distributions and Simulation Software Other methods of calculating forces include finding the pressure distributions across a model. Once found, the overall pressure can be calculated and then multiplied by the respective area to find the force. This method can be done experimentally with a manometer or pressure measuring apparatus. Simulation software can also be used to calculate the forces on a model. Using algorithms, the software creates a mesh over an object and then calculates the pressure distribution. This distribution is then converted to a resultant force. These simulations are generally good representations of actual forces and will be a lot more precise than an experimental pressure distribution method. Development of a Wind Tunnel Force Balance Practical Gethin Barden 57 3. Practical Exercises 3.1. Model Construction The aim of this practical exercise is to develop a model that demonstrates the properties of lift and drag. This model can then be produced and tested as per section 3.2 of this manual. 3.1.1. Model Construction Guide and Tips In order to construct a model that can be analysed the following tips should help produce a model that will perform as expected.  Use a shape that lift and drag forces can be calculated for.  Set a goal for the model to achieve. One method of doing this is to create a real life scenario and then use scaling laws to create a model that will represent the scenario.  Keep model weight down. Models should be as lightweight as possible to take stress of the apparatus.  Keep the maximum model width/height to 360mm. Good wind tunnel practice is to ensure that models are not interfered with by the sides of the tunnel. The accepted maximum width for a model is 80% of the test chambers width.  Aerofoil models should be relatively large to reduce the effect that the apparatus will have on the force readings.  Allow for pressure taps to be inserted so that pressure distributions can be taken in other practical exercises  If using the flat plate mount and an angle of attack is desired, create a wedge to clamp between the bottom plate and the model. 3.2. Velocity Testing and Coefficient Derivations The purpose of this exercise is to test a model at a range of different velocities and use the data obtained calculate the coefficients of lift and drag for the model at its natural angle of attack. 1. Create model (this should be completed well in advance to the practical), preferably based on a shape that has literature values for lift and drag coefficients available. 2. Mount model on the most applicable mount (see section 1.3) and bolt mount to the main strut. 3. Open the LabVIEW program and commence with the instructions seen in section 1.4.1 or on the program interface. 4. Once setup, start the wind tunnel (see section 1.5 for basic wind tunnel operation procedures) at a low fluid velocity (10m/s or 12.5Hz is a good starting speed). 5. Increase the speed to the first desired fluid velocity. Allow the flow to stabilise by waiting 30 seconds and record the lift and drag force displayed. Development of a Wind Tunnel Force Balance Practical Gethin Barden 58 6. Once measurements are completed, turn off wind tunnel, open the test chamber and remove the model. 7. Using the lift and drag data obtained, calculate the lift and drag coefficients. Compare the calculated and literature coefficient values for your model and plot against each other. Discuss the results and any discrepancies or trends in the data. 8. (Optional) Create a 3D model of your object using a 3D CAD program and use Computational Fluid Dynamics software to find lift and drag forces for comparison with the lift and drag obtained in the testing. Development of a Wind Tunnel Force Balance Practical Gethin Barden 59 Appendix B: Design Calculations Plexiglass properties Yield Strength = σy=72MPa; Elastic Modulus = E=3.3GPa (Evonik Industries, n.d.) Average shear stress Safety factor = 4; F=300N 𝐹 300 𝜏𝑦 = 𝐴 = 0.01∗𝑥 = 72 000 000 4 Therefore minimum thickness to avoid failure by shear is x = 1.67mm Maximum bending stress 𝜎𝑦 = 𝑀𝑦 , 𝜎𝑦 = 18 000 000𝑃𝑎 𝐼 𝑏ℎ3 0.01 ∗ 𝑥 3 𝐼= = ; 12 12 𝑀𝑦 = 𝐹𝐿 = 300 ∗ 0.3 = 90𝑁𝑚 𝜎𝑦 = 𝑀𝑦 12 ∗ 𝑥 ∗ 90 = ; 𝐼 2 ∗ 0.01𝑥 3 𝑥 = 0.0548𝑚 Minimum thickness to avoid failure from bending stress is 54.8mm Deflection at x=55mm I = 1.4*10-7m4 𝛿= 𝐹𝐿3 300 ∗ 0.33 = = 0.0058𝑚 3𝐸𝐼 3 ∗ 3.3 ∗ 109 ∗ 1.4 ∗ 10−7 5.8mm deflection considered to be too much so x was increased to 70mm. This yielded a deflection of 2.9mm which was considered suitable. Development of a Wind Tunnel Force Balance Practical Gethin Barden Appendix C: Force Balance Drawing 10.REFERENCES AirfoilTools.com 2013, Clark Y Airfoil, AirfoilTools.com, viewed 10/10/2013, <http://airfoiltools.com/airfoil/details?airfoil=clarky-il> Ahn, SH, Montero, M, Odell, D, Roundy, S, Wright, PK 2002, �Anisotropic material properties of fused deposition modelling ABS’, Rapid Prototyping, vol 8, iss 4 accessed via Emerald Anderson, JD 2001, Fundamentals of Aerodynamics, McGraw-Hill Science, New York, NY Arney, GD & Harter, WT 1964, A Low-Load Three-Component Force Balance For Measurements In A Low-Density Wind Tunnel, Von Karman Dynamics Gas Facility, ARO Inc, Tennessee, USA, accessed 10/07/2013, <www.dtic.mil/cgibin/GetTRDoc?AD=AD0453130> Ashby, MF & Jones, DRH, Engineering Material I – An Introduction to Properties, Applications and Design, 3rd edn, Elsevier Ltd., Oxford, UK Aviation-for-kids 2012, The Magnus Force, Aviation-for-kids.com, accessed 26/05/2015, <http://www.aviation-for-kids.com/the-magnus-force.html> Barlow, JB, Rae, WH & Pope, A 1999, Low-Speed Wind Tunnel Testing, 3rd edn, John Wiley & Sons, Inc, Massachusetts, USA Bychkov, V 2013, Fluid Dynamics; Computer laboratories using COMSOL 4.3a, Umea University, Sweden, accessed 10/10/2013, <http://www.umu.se/digitalAssets/120/120985_instructions-forfluiddynamics_comsol_v4.3a.pdf> Daley, BN, Humprey, MD & Lindsey, WF 1947, The Flow and Force Characteristics of Supersonic Airfoils at High Subsonic Speeds, National Advisory Committee for Aeronautics, Washington, USA, accessed 25/05/2014, <http://naca.central.cranfield.ac.uk/reports/1947/naca-tn-1211.pdf> Ebidon 2013, UCPCN-UB Base Unit Manual, Ebidon International, Madrid, Spain Evonik Industries 2013, Technical Information: Plexiglass, Evonik Industries, Darmstadt, Germany, accessed 20/10/2013, <http://www.plexiglas.de/sites/dc/Downloadcenter/Evonik/Product/PLEXIGLASSheet/PLEXIGLAS/211-1-PLEXIGLAS-GS-XT-en.pdf> Fail, R, Lawford, JA & Eyre, RCW 1959, Low-Speed Experiments on the Wake Characteristics of Flat Plates normal to an Air Stream, Aeronautical Research Council – Ministry of Supply, London, UK, accessed 25/05/2014, <http://naca.central.cranfield.ac.uk/reports/arc/rm/3120.pdf> Foss, J, Tropea, C & Yarin, A 2007, Springer Handbook of Experimental Fluid Mechanics, Springer Reference, ISBN: 978-3-540-33582-5. 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Theses, Blacksburg, Virginia, USA, accessed 10/07/2013, <http://scholar.lib.vt.edu/theses/available/etd-0808200013180002/unrestricted/02Thesis_Miranda.pdf> Munson, BR, Young, DF, Okiishi, TH & Huebsch, WW 2009, Fundamentals of Fluid Mechanics, 6th edn, John Wiley & Sons, Inc, Massachusetts, USA NASA refer to National Aeronautics and Space Administration National Aeronautics and Space Administration 2010, What is Drag?, National Aeronautics and Space Administration, Ohio, USA, viewed 16/09/2013 <www.grc.nasa.gov/WWW/k12/airplane/drag1.html> National Aeronautics and Space Administration 2011, Drag Measurement, National Aeronautics and Space Administration, Ohio, USA, viewed 16/09/2013 <www.grc.nasa.gov/WWW/K-12/airplane/dragdat.html> National Aeronautics and Space Administration 2012, External Force Balance, National Aeronautics and Space Administration, Ohio, USA, viewed 16/05/2014 <http://www.grc.nasa.gov/WWW/k-12/airplane/tunbalext.html> National Aeronautics and Space Administration 2012, Model Mounts, National Aeronautics and Space Administration, Ohio, USA, viewed 16/05/2014 <http://www.grc.nasa.gov/WWW/k-12/airplane/tunbalmnt.html> National Research Council of Canada 2009, High-Speed Wind Tunnel, National Research Council Institute of Aerospace Research, Ontario, Canada, viewed 16/10/2013, <http://archive.nrc-cnrc.gc.ca/eng/multimedia/highspeed-wind-tunnel.html> Onera, 2009, Model supports, Onera, France, accessed 21/09/2013, <http://windtunnel.onera.fr/model-support> Phidgets Inc 2012, 3133_0 – Micro Load Cell (0-5kg), Phidgets Inc, Alberta, Canada, accessed 16/05/2014, <http://www.phidgets.com/products.php?category=34&product_id=3133_0> Phidgets Inc 2013, 1046 User Guide, Phidgets Inc, Alberta, Canada, accessed 16/10/13, <http://www.phidgets.com/docs/1046_User_Guide> Ried, F 1997, �The Magnus Effect’, Parabola Online, vol 33, iss 1, pp3-6, accessed via University of New South Wales, Australia Reis, MLCC, Castro, RM & Mello, OAF 2013, �Calibration uncertainty estimation of a straingage external balance’, Measurement, vol 43, pp 24-33, accessed via ScienceDirect Development of a Wind Tunnel Force Balance Practical Gethin Barden 63 Schaefer, WT 1965, Characteristics of Major Active Wind Tunnels At The Langley Research Center, National Aeronautics and Space Administration, Virginia, USA, accessed via Defence Technical Information Center Silverstein, A 1935, Scale effect on Clark Y airfoil characteristics from N.A.C.A Full Scale Wind-Tunnel Tests, National Advisory Committee, viewed 19/10/2013, <http://www.aerodesign.ufsc.br/teoria/artigos/aerodynamics/nr0502.pdf> Smiadak, DM 2008, Lift Coefficient for an Airfoil, Grand Valley State University, Michigan USA, accessed 10/10/2013, <claymore.engineer.gvsu.edu/~smiadakd/EGR365-Lab9.pdf> The Engineering Toolbox, Drag Coefficient, EngineeringToolbox.com, viewed 19/10/2013, <http://www.engineeringtoolbox.com/drag-coefficient-d_627.html> University of Iowa, 2002, Measurement of Pressure Distribution and Lift for an Airfoil, Iowa, USA, accessed 16/10/2013, <www.engineering.uiowa.edu/~cfd/pdfs/57-020/lab4.pdf> Zimmerman, CH 1933, Characteristics of Clark-Y Airfoils of Small Aspect Ratio, National Advisory Committee of Aeronautics, viewed 19/10/2013, <http://naca.central.cranfield.ac.uk/reports/1933/naca-report-431.pdf> Development of a Wind Tunnel Force Balance Practical Gethin Barden
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