AN EVALUATION OF AUSPLUME-PRIME WITH NEUTRAL AND STABLE FLOW WIND TUNNEL BUILDING EFFECTED DISPERSION DATA T John Taylor Engineering Air Science Pty Ltd., The Gap, 4061, Qld, Australia. Abstract This paper compares AUSPLUME PRIME with wind tunnel measurements of building effected dispersion in neutral and stable flow conditions. While AUSPLUME PRIME performed reasonably well, its performance in stable conditions was inferior to that for neutral conditions. Significant under prediction was evident immediately downwind of the recirculation region for a roof top source with the building at an oblique angle (45°) to the flow and also at large distances downwind (40 to 50 H b ) under stable flow conditions. The results would indicate there is potential for larger discrepancy further downwind. Better agreement was observed between wind tunnel and AUSPLUME predictions when dispersion was not adjusted for roughness. Keywords: Building Effects, AUSPLUME, PRIME, Wind Tunnel. 1. Introduction With the increasing dense urban landscape of the modern era, commercial offices and residential neighbour hoods are moving closer to industrial and manufacturing facilities. Often office environments and smaller industrial facilities develop in close proximity or even within the same complex. Residential developments are also encroaching on the fence lines of what were once well buffered industrial, manufacturing or agricultural facilities. The quality of the ambient air can become a critical issue for those occupying the offices or residential facilities. While the majority of problems typically arise through perceptible odour or dust impacts, unseen emissions of chemical vapours and particulate may have potential to cause more serious health related impacts. Standard practice for emissions to air from industrial activities is the use of a stack to release the pollutants at an elevated height. Emissions are typically contained or captured and exhausted into the environment through a vent or stack, often at or just above the roof height of the industrial plant or building within which the pollutants are generated. Other facilities may also rely on natural ventilation through openings such as windows doors and roof vents to provide adequate extraction of process emissions from the work environment. To quantify potential air quality impacts on the ambient environment regulatory atmospheric dispersion models are typically employed. While there are many atmospheric dispersion models used in different jurisdictions across the world, in Australia the relatively simple AUSPLUME model, CASANZ 2011 Conference - Auckland - 31 July - 2 August 2011 Paper 151 developed by the Victorian EPA, is often the most frequently used for these supposably more simple assessments. AUSPLUME is a steady state analytically based dispersion model primarily validated for near-field analysis of ambient air quality impacts. The more complex 3-dimensional models CALPUFF and TAPM may also be applied in a similar manner to AUSPLUME, and would typically provide similar predictions in the immediate vicinity of the source. With many industrially based air pollutant sources arising from operations within buildings and sheds, a critical aspect of any approach to represent the behaviour and potential impacts of the emissions is the treatment of the building, and the effect it has on plume transport and dispersion within the atmospheric boundary layer. Many regulatory atmospheric dispersion models now incorporate buildings’ modules, specifically developed to improve representation of building effects on flow and dispersion within the regulatory context. AUSPLUME, CALPUFF and TAPM, the three regulatory atmospheric dispersion models typically used in Australia and New Zealand, incorporate the Plume RIse Model Enhancement (PRIME) algorithm developed under sponsorship from the Electric Power Research Institute (EPRI) (Schulman et al. 2000). The approach of PRIME is similar to that of the UK developed ADMS Build module with the development of both tools relying heavily on the results of fluid modelling studies of flow and dispersion around surface mounted obstacles performed primarily within wind tunnels. Apart from a number of simulations conducted in the Monash University Environmental Wind Tunnel using an inverted modelling technique (Melbourne & Taylor 1994), the majority of fluid simulations were conducted under neutral atmospheric flow conditions. Generally, studies investigating flow around surface mounted obstacles, and primarily performed to develop understanding of building effects on atmospheric flow and dispersion, have ignored the effect that stable stratification may have. With a lack of experimental data relating to stably stratified conditions, buildings module algorithms have considered prediction schemes derived from neutral studies as general and equally applicable for both neutral and stably stratified flow conditions. To address this issue an experimental investigation into the effects of stable stratification on flow and dispersion around a surface mounted obstacle was undertaken. Experiments focusing on the region beyond the near wake, a region of very limited previous investigation, were conducted within the meteorological wind tunnel within the Environmental Flow Research Centre (EnFlo) at the University Of Surrey, UK, (Maré 2003). The current investigation utilises the results of these wind tunnel studies to assess the performance of PRIME within the AUSPLUME dispersion model with respect to predicted ground level concentrations (GLC’s) for pollutant releases from building height and above, under neutral and stably stratified flow conditions. 2. Background 2.1. Dispersion Model AUSPLUME is a steady-state Gaussian plume dispersion model designed to predict ground level concentrations or dry deposition of pollutants released into the atmosphere. Developed by the Victorian Environmental Protection Authority, it has a mathematical basis derived from the “Plume Calculation Procedures“ (EPAV 1985), which is an extension of the US EPA Industrial Source Complex (ISC) model (Bowers et al. 1979). The model’s primary purpose is for the regulatory assessment of the impact of emissions from ‘individual’ industrial sources. The latest versions of AUSPLUME include the PRIME algorithms as their most sophisticated method of accommodating the effect of a building in the vicinity of a point source, adjusting the plume dispersion for the effect buildings may have on the flow and turbulence. The PRIME algorithms have been incorporated into many dispersion models including the US models ISC, AERMOD and CALPUFF, and the CSIRO model TAPM, with numerous validation studies performed using laboratory and field based data sets. The data set CASANZ 2011 Conference - Auckland - 31 July - 2 August 2011 Paper 151 used in this analysis is independent of the previous validation studies. PRIME uses the Building Profile Input Program (BPIP) to analyse point source and building configurations and provide effective building parameters for each 10 degree segment of the compass. Dispersion is then determined based on the effective building parameters determined for the appropriate 10 degree segment for the wind direction of concern. The PRIME model breaks the region downwind of the building into two primary regions: Near Wake, being the high turbulence recirculation region in the immediate lee of the building defined by the location of streamline reattachment. In this region PRIME assumes uniform concentrations based on the plume fraction entrained into the near wake region; Far Wake, being the region of enhanced turbulence downwind of the near wake. A two source model is used whereby a virtual ground level source is used to represent the plume originating from the near wake, and an elevated source represents the remaining plume. 2.2. Wind Tunnel Experiments Wind tunnel experiments investigating the effect of stable stratification on flow and dispersion around a cube were conducted in the meteorological wind tunnel at the Environmental Flow Research Centre (EnFlo), University of Surrey (Maré 2003). The tunnel has a working section 20 m long by 3.5 m wide and 1.5 m high. Barrier walls, vorticity generators and surface roughness elements are used to create drag and generate a model simulation of the natural atmospheric boundary layer under neutral conditions as in a boundary layer wind tunnel. To create stratified boundary layer flow, a density profile is generated through the use of heating tubes, controlling the inlet air temperature, and floor cooling panels assisting to develop and maintain a stratified flow along the length of the wind tunnel. Flow and dispersion data is provided in three boundary layer flows of differing stability, one neutral and two stable flows, considering dispersion in ‘undisturbed’ and building effected conditions for two source heights: Building height (H b ), and 1.5 H b . Two building scenario configurations considered: are Upwind face normal (90°) to the flow; and Cube at 45° oblique angle to the flow. The experiments utilised a streamlined horizontal non-buoyant source with the exit velocity matched to that of the local freestream flow to limit any effect the source configuration may have on the flow and dispersion. 3. Model Configuration 3.1. Wind Tunnel The modelling of wind effects on square form structures, including effects on dispersion, is relatively straight forward, as the location of flow separation is distinctly defined by the sharp edges of the obstacle. However, the limited size of a wind tunnel can restrict the downwind and crosswind distance over which reliable measurements can be obtained, as well as restricting larger eddy sizes, limiting representation of meander behaviour. Flow conditioning devices are used to enhance generation of a scaled representation of the atmospheric boundary layer flow. Typically, a region of reasonably horizontally homogeneous flow is developed through the middle and later sections of the wind tunnel working section in which the simulations are performed. A summary of the wind tunnel boundary layer and model parameters is provided in Table 1. A cube of dimensions 100 mm was used as the bluff body obstacle, representing a building within the experiments. Table 1: Model and flow parameters summarising the wind tunnel model scenarios. Wind Tunnel Model Neutra Stable 1.3 Stable 1.1 l H b (m) z o (m) z o /H b δ (m) Max x/H b Max x (m) U b (m/s) u* (m/s) L MO (m) 0.1 0.001 0.01 1.06 50 5 1.45 0.14 ∞ 0.1 0.001 0.01 0.6 50 5 0.82 0.04 0.133 0.1 0.001 0.01 0.6 50 5 0.69 0.03 0.081 3.2. AUSPLUME Regulatory dispersion models including AUSPLUME are developed to emulate field configurations of actual atmospheric flow and dispersion. They cannot be directly applied to the smaller scale model configuration of wind tunnel experiments of atmospheric flow and dispersion. Hence, the experiments require scaling to a size representative of possible field scenarios. The scaling can be somewhat arbitrary, within the limits of what would be considered a realistic real-world scenario. Consideration should also be given to the CASANZ 2011 Conference - Auckland - 31 July - 2 August 2011 Paper 151 eddy size within the wind tunnel flow and that appropriate to the simulated atmospheric conditions. The length scale of the wind tunnel experiments was adjusted by a scaling factor of 300 to represent a realistic field scenario. This provided for a cubic building of 30 m dimensions. The building and flow parameters used in the AUSPLUME configuration are summarised in Table 2. As the model plume within the wind tunnel experiments was released horizontally, with normalised momentum and no buoyancy, the wind speeds used for the AUSPLUME configuration can also be somewhat arbitrary, particularly as the concentration results are normalised by the corresponding model velocities to enable direct comparison. The model was configured assuming the meteorological measurements were obtained at the building height (30 m). As AUSPLUME, like most regulatory dispersion models, cannot represent horizontal emissions, a vertical stack with a small exit velocity was employed to limit momentum plume rise to virtually zero. Buoyancy induced rise was also minimised by the use of a release temperature equivalent to that of the ambient flow. Table 2: Summary of the AUSPLUME configuration used to represent the wind tunnel simulation on a scale of 300:1. Scale 300 H b (m) z o (m) z o /H b δ (m) Max x/H b Max x (m) U b (m/s) P-G Cat. AUSPLUME Model Stable Neutral Stable 1.1 1.3 30 0.3 0.01 318 50 1500 5 D 30 0.3 0.01 180 50 1500 3.1 E 30 0.3 0.01 180 50 1500 1.6 F AUSPLUME is an older generation model with dispersion primarily based on stability class (A to F) and a power law velocity profile used to estimate stack height wind speed when it is different from that of the meteorological data. A wind speed at the building height (H b = 30 m) was provided in the meteorological input file in this investigation, and thus a correction was only applied for the higher stack simulations with H s = 1.5H b . AUSPLUME was initially run in a ‘default’ regulatory mode to provide a comparison of ground level concentration predictions for a typical operational configuration. In addition to investigating the performance of PRIME, predictions for isolated stacks, without the effect of buildings, were also compared. Subsequent comparisons concentrated on improved similarity in predicted ground level concentration profiles for the isolated stack scenario, and the corresponding effect this had when building effects were considered. 4. Centreline GLC comparison The wind tunnel measurements (Maré, 2003) investigated a number of aspects of the dispersion downwind of the idealised model building, however the most significant relevant to regulatory applications is typically the potential maximum ground level concentrations that occur on the downwind centreline. It is these predictions that are likely to provide the impacts on which regulatory decisions are made. Concentrations are presented in a non-dimensional format as detailed in Equation 1 below: C U b H b2 Q Equation 1 Here C is the concentration, Q the emission rate and U b and H b the mean velocity and height at the top of the building respectively. The downwind distance x, is also normalised by H b. 4.1. Default configuration The results for a comparison using the default ‘regulatory’ configuration of AUSPLUME, based on the measured wind tunnel boundary layer parameters as summarised in Table 2, are presented in Figure 1. A summary of a statistical analysis is presented in Table A- 1. From visual inspection of Figure 1 it is immediately evident that the main features representing plume behaviour, such as the location of initial plume touchdown and maximum concentration and the general decay of the concentration with downwind distance, are in best agreement for the neutral flow condition. As the stability of the boundary layer increases, the similarity in these main features between the wind tunnel results and the predictions of AUSPLUME is observed to reduce significantly. Under stable flow conditions and for the isolated stacks in particular, the location of maximum concentration is observed significantly closer to the source for the AUSPLUME predictions than the wind tunnel results would indicate. The stable flow wind tunnel measurements would indicate the maximum GLC has not been achieved for either isolated stack or for the building configuration with the higher stack when the cube is at 90° to the wind direction. The measured concentrations would also suggest the region over which the peak concentration extends is increased, and the rate of reduction reduced under the stable flow conditions, leading to higher concentrations observed further CASANZ 2011 Conference - Auckland - 31 July - 2 August 2011 Paper 151 downwind (40 – 50 H b ) in the wind tunnel measurements than predicted by AUSPLUME. The other significant variation is the over prediction of GLC’s by AUSPLUME PRIME closer to the buildings (x < 15H b ) when the flow is at 90° to the building. The effect is more pronounced under the stable flow conditions, with concentrations over estimated by up to a factor of 3, or more for the stable flow configuration with H s = 1.5H b . There is also a general under prediction downwind of the near wake (3H b <x<15H b ) when the flow is at an oblique angle of 45° to the building, with the PRIME model reducing the concentration beyond the near wake more rapidly than observed in the wind tunnel when H s = H b , particularly for the stable flows. When the stack is higher (H s = 1.5H b ), PRIME over predicts the rate of entrainment to the ground, and thus the GLC for the more stable flow configuration. 4.1.1. Modified dispersion configuration With concern the higher observed than predicted concentrations for the building effected stable flow scenarios at the greater downwind distances may arise due to the poor agreement of the base dispersion (isolated stack) case, the standard roughness adjustment to dispersion within AUSPLUME was selectively decommissioned. This improved the AUSPLUME representation of the wind tunnel results, particularly for the stable scenarios. The adjustments that gave best agreement between the wind tunnel dispersion measurements and the AUSPLUME predictions for the isolated stack scenarios are summarised in Table 3. The adjustment for surface roughness reduced the magnitude of the dispersion parameter used within AUSPLUME. Graphical results comparing the wind tunnel measurements with the predictions of AUSPLUME with modified dispersion are presented in Figure 2. Corresponding statistical parameters are presented in Table A- 2. The improved representation for the isolated stack scenarios under stable flow and also for the neutral case with H s = 1.5H b are evident. However, the features discussed above in relation to the higher observed concentrations than predicted at the larger downwind distances under the building affected stable flow scenarios are still evident, and even more pronounced the Category E stability. It is also evident that AUSPLUME PRIME is in better agreement with the wind tunnel measurements for building effected dispersion under neutral flow conditions than stable flow conditions. This was particularly the case when the building was at an oblique angle of 45° to the flow, with PRIME having a tendency to under estimate GLC’s, particularly at distances further downwind. Table 3: Summary of AUSPLUME dispersion parameter adjustments used to improve agreement with the wind tunnel isolated stack scenarios. Neutral Vertical dispersion adjustment only Stable 1.3 No dispersion roughness adjustments Stable 1.1 Horizontal dispersion adjustment only H s = 1.5 H b Hs = Hb 0.08 0.07 0.045 Neutral WT Neutral AUSPLUME Stable 1.3 WT Category E AUSPLUME Stable 1.1 WT Category F AUSPLUME 0.04 0.035 0.06 0.03 0.05 0.04 0.025 0.02 0.03 0.015 0.02 0.01 0.01 0.005 0 0 0 10 20 30 40 50 60 0 10 20 x/H b 0.9 0.8 0.7 30 40 50 60 x/H b 0.14 Neutral WT Neutral AUSPLUME Stable 1.3 WT Category E AUSPLUME Stable 1.1 WT Category F AUSPLUME 0.12 Neutral WT Neutral AUSPLUME Stable 1.3 WT Category E AUSPLUME Stable 1.1 WT Category F AUSPLUME 0.1 0.6 0.08 0.5 0.4 0.06 0.3 0.04 0.2 0.02 0.1 0 0 0 10 20 30 40 50 60 0 10 20 x/H b 0.7 0.6 30 40 50 60 x/H b 0.25 Neutral WT Neutral AUSPLUME Stable 1.3 WT Category E AUSPLUME Stable 1.1 WT Category F AUSPLUME 0.2 Neutral WT Neutral AUSPLUME Stable 1.3 WT Category E AUSPLUME Stable 1.1 WT Category F AUSPLUME 0.5 0.15 0.4 0.3 0.1 0.2 0.05 0.1 0 0 0 10 20 30 40 50 60 x/H b 0 10 20 30 40 50 60 x/H b Figure 1: Downwind centreline non-dimensional ground level concentrations measured in the wind tunnel and predicted by AUSPLUME in ‘default’ configuration. H s = H b - left, H s = 1.5H b - right, isolated stack - top, cube at 90° - centre, cube 45° - bottom. 5. Conclusions AUSPLUME PRIME predicted GLC’s downwind of a building are compared to wind tunnel results for neutral and stable flow conditions. The results demonstrate AUSPLUME PRIME provides more accurate predictions of downwind GLC’s under neutral flow conditions than stable conditions. For stable conditions AUSPLUME PRIME had a CASANZ 2011 Conference - Auckland - 31 July - 2 August 2011 Paper 151 tendency to under predict GLC’s, particularly for wind directions oblique to the building and for distances greater than 30 to 40 building heights downwind. While generally AUSPLUME PRIME performed well, there is some concern of its ability to represent dispersion from the more elevated stack with respect to the location of the maximum GLC’s. Higher impacts occurred at distances from the source under stable conditions, than otherwise predicted by AUSPLUME. Building module improvement could be achieved through greater consideration of flow angle relative to the building along with improved understanding of the effect of Hs = Hb 0.12 0.1 stability on flow and dispersion around buildings, particularly at extended distances downwind where building effects under neutral conditions have typically dissipated. H s = 1.5 H b 0.045 Neutral WT Neutral AUSPLUME Stable 1.3 WT Category E AUSPLUME Stable 1.1 WT Category F AUSPLUME 0.04 0.035 0.08 0.03 0.025 0.06 0.02 0.04 0.015 0.01 0.02 0.005 0 0 0 10 20 30 40 50 0 60 10 20 x/H b 0.9 0.8 0.7 30 40 50 60 x/H b 0.25 Neutral WT Neutral AUSPLUME Neutral WT Neutral AUSPLUME Stable 1.3 WT Category E AUSPLUME Stable 1.3 WT Category E AUSPLUME Stable 1.1 WT Category F AUSPLUME Stable 1.1 WT Category F AUSPLUME 0.2 0.6 0.15 0.5 0.4 0.1 0.3 0.2 0.05 0.1 0 0 0 10 20 30 40 50 0 60 10 20 x/H b 0.7 0.6 30 40 50 60 x/Hb 0.25 Neutral WT Neutral AUSPLUME Neutral WT Neutral AUSPLUME Stable 1.3 WT Category E AUSPLUME Stable 1.3 WT Category E AUSPLUME Stable 1.1 WT Category F AUSPLUME Stable 1.1 WT Category F AUSPLUME 0.2 0.5 0.15 0.4 0.3 0.1 0.2 0.05 0.1 0 0 0 10 20 30 40 50 60 x/H b 0 10 20 30 40 50 60 x/H b Figure 2: Comparison of downwind centreline non-dimensional ground level concentrations measured in the wind tunnel and predicted by AUSPLUME with modified dispersion. H s = H b - left, H s = 1.5H b - right, isolated stack top, cube at 90° - centre, cube 45° - bottom. 6. References Bowers, J. F., Bjorkland, R. J., & Cheney, C. S., 1979. "Industrial source complex model users guide" report EPA-450/4-79-030. US-EPA. EPAV, 1985. "Plume Calculation Procedures". Publication 210. Melbourne: EPA Victoria. CASANZ 2011 Conference - Auckland - 31 July - 2 August 2011 Paper 151 Maré, C., 2003. Effects of Stratification on Flow and Dispersion Around Obstacles in Turbulent Boundary Layers. PhD Thesis, University of Surrey, School of Engineering. Melbourne, W. H., & Taylor, T. J., 1994. Plume Rise and Downwash Wind Tunnel Studies: Combustion Turbine Unit 4 Sayreville. TR-235274 Draft Final Report, Monash University, Department of Mechanical Engineering, Clayton. Schulman, L. L., Strimaitis, D. G., & Scire, J. S. (2000). Developemnt and evaluation of the PRIME plume rise and building downwash model. Journal of Air & Waste Management Association , 8, 708‐717. CASANZ 2011 Conference - Auckland - 31 July - 2 August 2011 Paper 151 Appendix A Table A- 1: Statistical summary of comparison of downwind centreline non-dimensional ground level concentrations measured in the wind tunnel and predicted by AUSPLUME in ‘default’ configuration. Undisturbed H b Neutral Normalised Mean Stable 1.3 Undisturbed 1.5H b Stable 1.1 Neutral Stable 1.3 Stable 1.1 0.82 0.95 3.06 0.55 1.29 2.04 2.05E+02 1.34E+01 1.22E+02 8.62E+10 2.17E+08 5.38E+10 Fractional Bias (FB) 0.19 0.05 -1.02 0.57 -0.25 -0.69 F1.5 0.75 0.25 0.06 0.06 0.31 0.00 F3 0.88 0.81 0.25 0.75 0.75 0.31 NMSE Cube 90 H b Neutral Stable 1.3 Cube 90 1.5H b Stable 1.1 Neutral Stable 1.3 Stable 1.1 Normalised Mean 1.42 1.37 2.42 0.96 1.21 2.67 NMSE 0.10 0.29 0.58 2.72 0.65 3.94 -0.35 -0.31 -0.83 0.04 -0.19 -0.91 F1.5 0.88 0.38 0.31 0.88 0.25 0.19 F3 1.00 0.94 0.75 0.88 0.81 0.38 Fractional Bias (FB) Cube 45 H b Neutral Stable 1.3 Cube 45 1.5H b Stable 1.1 Neutral Stable 1.3 Stable 1.1 Normalised Mean 0.82 0.60 0.75 0.65 0.68 1.79 NMSE 0.08 0.52 0.22 0.34 0.85 0.64 Fractional Bias (FB) 0.20 0.50 0.28 0.42 0.38 -0.57 F1.5 0.81 0.19 0.50 0.25 0.38 0.25 F3 1.00 0.94 1.00 0.94 0.75 0.81 CASANZ 2011 Conference - Auckland - 31 July - 2 August 2011 Paper 151 Table A- 2: Statistical summary of comparison of downwind centreline non-dimensional ground level concentrations measured in the wind tunnel and predicted by AUSPLUME in ‘default’ configuration. Undisturbed H b Neutral Normalised Mean Stable 1.3 Undisturbed 1.5H b Stable 1.1 Neutral Stable 1.3 Stable 1.1 1.31 1.29E+0 2 0.94 1.38 0.88 0.95 0.45 7.71E+07 5.27E+09 5.44E+10 1.98E+24 2.92E+28 -0.26 0.06 -0.32 0.13 0.05 0.76 F1.5 0.75 0.63 0.19 0.69 0.50 0.19 F3 0.88 0.75 0.63 0.81 0.56 0.38 NMSE Fractional Bias (FB) Cube 90 H b Neutral Normalised Mean Stable 1.3 Cube 90 1.5H b Stable 1.1 Neutral Stable 1.3 Stable 1.1 1.67 1.48 2.44 1.44 1.45 2.48 2.38E-01 1.15E-01 6.09E-01 2.31E+00 6.37E-01 3.24E+00 -0.50 -0.39 -0.84 -0.36 -0.37 -0.85 F1.5 0.56 0.63 0.31 0.63 0.25 0.31 F3 1.00 0.94 0.75 0.88 0.75 0.50 NMSE Fractional Bias (FB) Cube 45 H b Neutral Normalised Mean Stable 1.3 Cube 45 1.5H b Stable 1.1 Neutral Stable 1.3 Stable 1.1 0.93 0.64 0.76 0.89 0.78 1.75 1.35E-01 3.21E-01 1.96E-01 2.07E-01 3.90E-01 5.69E-01 Fractional Bias (FB) 0.07 0.44 0.28 0.12 0.25 -0.54 F1.5 0.75 0.25 0.63 0.75 0.44 0.25 F3 1.00 1.00 1.00 0.94 0.88 0.81 NMSE CASANZ 2011 Conference - Auckland - 31 July - 2 August 2011 Paper 151
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