GÉPÉSZET 2002, Proc. of the 3rd Conference on Mechanical Engineering, vol. 1, pp. 396-400
WIND TUNNEL MEASUREMENT AND NUMERICAL SIMULATION
OF WIND LOAD ACTING ON BUILDINGS
T. Lajos*, Zs. Szepesi**, I. Goricsán***, F. Paulik****, J. Suda*****, T. Régert******
Department of Fluid Mechanics, Technical University of Budapest, H-1111. Budapest, Bertalan L. u. 4-6.
*Professor, Head of dept., Tel: (1) 463 4072, e-mail: [email protected],
**Assistant Professor, Tel: (1) 463 4072, e-mail: [email protected],
***Assistant, Tel: (1) 463 3465, e-mail: [email protected],
****Ph.D. Student, Tel: (1) 463 1560, e-mail: [email protected],
*****Assistant, Tel: (1) 463 3465, e-mail: [email protected],
******Ph.D. Student, Tel: (1) 463 3187, e-mail: [email protected]
Summary
Wind load acting on buildings should be considered at their design. The standards provide
wind load figures that should be used if no data relevant to the building under consideration
are available. This approach results quite often in unnecessarily oversized and expensive
structures. So the determination of the real wind load acting on the specific building is
advantageous. This applies particularly to light and flexible building structures, like tent
roofs, where the wind load is the biggest component of the overall load and the deformation
of the surface due to the wind load influences the pressure distribution on it. The paper
presents experiences on the wind tunnel investigation of wind load acting on fabric tent roofs.
Numerical simulation of the flow past tent roofs has been launched. By using validated
numerical simulation of flow and calculation of the deformation of the tent roof an iterative
method can be developed resulting in real wind load taking the effect of deformation of the
surface of the fabric roof into consideration.
1. INTRODUCTION
The wind load expressed by pressure coefficient distribution on the surface of the
building constitutes a considerable part of the overall load acting on light building structures,
so it should be taken into consideration at strength calculations. (The shear stresses contribute
to the overall load of the structure in most cases to a relatively small extend that can be
calculated by using the relevant Standard.) In case of unusual building shapes or presence of
aerodynamically interacting surrounding buildings wind load is not available even in the
literature. The relevant Standard is not able to contain data referring to all possible building
shapes and arrangements so, obviously, it includes wind load coefficients belonging to the
most pessimistic cases. Relying on the Standard results quite often in unrealistic high wind
load and so – particularly in case of light building structures – unnecessarily oversized and
expensive structures. The application of realistic wind load values providing by wind tunnel
experiments or numerical simulation of the flow past the building considered brings about
relevant strength analyses, adequate structures an economical solutions.
Light structures like tent roofs are increasingly wider used in building structures.
Shopping centres, open air theatres, stadiums, exhibitions, fairs are typical applications. Since
the net weight of tent roofs is relatively small the biggest part of the load acting on the tent
roof is the wind load. There are three further special characteristics of the wind load acting on
tent roofs. In many cases the space covered by the tent roof communicates with the outside air
GÉPÉSZET 2002, Proc. of the 3rd Conference on Mechanical Engineering, vol. 1, pp. 396-400
through large cross sections between the lower edge of the roof and the ground. In these cases
the wind causes load not only on the upper surface of the roof, but because of the flow under
the roof also on the lower surface. So, at a given point of the roof the wind load coefficient
can be determined as difference of pressure coefficient related to the outside and inside
surface of the fabric. The effect of the internal flow is not considered by the Standard.
The second specificity comes from the relatively large influence of the wind conditions
on the load of the tent roofs: depending on the wind direction the disturbance of the flow field
caused by the neighbouring buildings and structures can cause both excessive and low local
wind loads. Thirdly, since the fabric is quite deformable the shape of the tent roof depends on
the wind load. If wind load changes the shape of the roof, also the flow field and consequently
the wind load distribution will change. So in case of wind the shape of the tent roof and the
wind load on it are influenced by complex interaction of wind forces and stresses in the
fabric. The objective of the investigations reported here is to develop an iterative method for
determination of the real wind load on and stresses in tent roofs.
2. THE WIND TUNNEL
The wind tunnel experiments are carried out in the test section of the horizontal wind
tunnel of the Department of Fluid Mechanics. This is a recirculating (Goettingen type) wind
tunnel. The nozzle diameter is 2.6 m, the length of the test section is 5.7 m. The maximum
wind velocity is 60 m/s. The turbulence intensity in the empty test section is 0.6%.
There are two options for the characteristics of the flow approaching the building
model: a) uniform, low turbulence flow, b) flow corresponding to the atmospheric turbulent
boundary layer at the future place of the building. In the first case the model is fixed on a
horizontal plate simulating the ground, so, disregarding the thin boundary layer developing on
the surface of the plate, the velocity distribution in front of the model is quite uniform, the
turbulence intensity is low. In case b) a grid system in the wind tunnel nozzle and roughness
elements on the fixed plate simulating the ground produces a flow field corresponding to that
of atmospheric boundary layer [1]. The turbulent (length scale) in the test section of this wind
tunnel make the use of models of 1:250 -1:750 scales possible [2], [3]. At both types of
approaching flow the model is fixed to the turntable of 2m in diameter integrated in the fixed
plate of test section by rotating of which the relative wind direction can be changed
.
3. MEASUREMENT OF WIND LOAD
The wind load is usually investigated by using 1:50 - 1:250 scale building models of
characteristic length 1-1.5 m. The models are in most cases prepared of synthetic resin
strengthened with fibreglass. The thickness of the wall of roof models is about 2 mm. The
wind load, i.e. the difference of the distributions of mean pressure coefficient on the outside
and inside surface of the model roofs or walls is determined by pressure measurements
separately on outside and inside surfaces of the model. The models are provided with about
60-120 pressure taps of 1-2 mm in diameter. If the interior of the building does not
communicate with the ambient air, the internal pressure is regarded constant and in general
equal to the pressure in the undisturbed flow. In this case the pressure distribution
measurements occur only on the outside surface of the building model. If the internal pressure
distribution can change (i.e. at the fabric roof of a stadium or open air theatre) both the
outside and the inside pressure distributions are measured.
GÉPÉSZET 2002, Proc. of the 3rd Conference on Mechanical Engineering, vol. 1, pp. 396-400
The pressure taps are connected via a plastic tubing system and a SCANIVALVE
pressure switch to a pressure transducer. The stepping of the pressure measuring place
(pressure taps) by SCANIVALVE as well as the data acquisition and processing are
controlled by a PC. After change of the pressure tap connected with the pressure transducer
few seconds time is given for decay of transients. Subsequently data acquisition occurs during
10 - 30 sec. with 200 Hz frequency. So 2000-6000 pressure data are collected in each
measuring point. The mean pressure and for getting information about the local wind
condition also the pressure fluctuation is calculated in each measuring points from these data.
(The pressure fluctuation values are only indicative, because correct pressure fluctuation
measurement needs pressure transducer placed close to the pressure taps [4].)
4. INVESTIGATION OF REYNOLDS NUMBER EFFECT
Before the measurement of pressure distributions investigations are carried out in order
to eliminate the Reynolds number effect. Since the model scale is 1: 50 – 1:250 and the wind
velocity in the wind tunnel corresponds to a high wind velocity in full-scale case, the
Reynolds number is nearly two orders of magnitude smaller in wind tunnel than in full scale.
The Reynolds number can be related to the undisturbed wind velocity (v [m/s]) and as a
characteristic size (l [m]) to the width of the full scale roof and the model: Re = v·l / ν where
ν [m2/s] is the kinematic viscosity of air. The characteristic Reynolds number at full scale roof
and model is Re ≈ 5·107 – 1.5·108 and Rem ≈ 5·105 – 1.5·106, respectively.
In order to obtain pressure coefficient distributions from the wind tunnel experiments
that are valid for the full scale the most important task is to bring about the laminar-turbulent
transition of the boundary layer on all parts of the model. This transition can be generated on
the model either by increase of Reynolds number (increase of wind velocity) or by use of
turbulence generator disturbing the flow in the boundary layer [5]. This is a wire of 1-2 mm in
diameter that is fixed on the model surface facing the wind, upstream of the curvature of the
surface (e.g. in front of an edge or other cylindrical part of the roof). Pressure distribution
measurements without and with transition wires at various velocities (various Reynolds
number) have shown that the use of turbulence generator makes the pressure coefficient
practically independent from the Reynolds number in the investigated range. From the results
of these measurements the conclusion can be drawn that pressure coefficient distributions
determined by model experiments can be regarded valid for full-scale building. (The pressure
∆p mean
where ∆pmean [Pa] is the time average of
coefficient is defined by c pmean =
ρ / 2 ⋅ v 2ref
difference of local pressure and the pressure in undisturbed flow, ρ [kg/m3] is the air density
and vref [m/s] is the reference velocity, in most cases the undisturbed wind velocity.) In
general, the wind load measurements are carried out with turbulence generator and at
relatively high Reynolds number (Re = 106), which corresponds to nearly 100 km/h wind
velocity.
5. RESULTS OF WIND LOAD MEASUREMENTS
Wind load measurement has been performed in uniform low turbulence flow on a 1:60
model of a tent roof of an ice-stadium of 62 m length, 34 m width and 15 m height. The
distance between the lower edge of the tent roof and the ground is 4.1 m, so the wind can
cause internal flow under the fabric roof. That is why pressure distribution measurements on
GÉPÉSZET 2002, Proc. of the 3rd Conference on Mechanical Engineering, vol. 1, pp. 396-400
both sides of the fabric roof have been performed. Fig. 1. shows the shape of the tent roof.
Fig. 2. includes the pressure coefficient distributions on outside surface when the direction of
approaching flow is perpendicular to the longitudinal symmetry plane of the roof. Fig. 2.
shows the vertical projection of the roof and the cpmean = const. lines.
Fig.1. The shape of the investigated tent roof
Bild 6. Druckbeiwerte an äusserer Oberfläche ( Cpa [-] )
Windrichtung 0°
1:300
0°
Fig.2. The pressure coefficient cpmean distribution on the outside surface of tent roof model
On the lower part of the outer surface facing the wind slight overpressure (cpmean≤ 0.4)
can be observed, while on all other parts depression (pressure lower than the ambient). It can
be seen that areas of excessive depression (cpmean≈ - 1) develop over the side of the coneshaped part of the roof, perpendicular to the wind direction. On the rear part of the roof the
pressure increases downstream, indicating flow deceleration.
On the internal surface of the model the pressure coefficient values are in general much
smaller than that on outside surface ⎟cpmean⏐< 0.3. On large part of the roof the effect of the
internal pressure distribution on the overall wind load is relatively modest. Relative
significant influence can be observed on the lower part of the windward side of the roof
(where the curvature and so the stability of the roof is relatively modest): here both the
outside overpressure and the inside depression increase the wind load of the roof (wind forces
vertical downwards). The inside pressure distribution increases the wind load also behind the
cone-shaped part of the roof: here both the outside depression and the inside overpressure
increase the deformation of the fabric.
GÉPÉSZET 2002, Proc. of the 3rd Conference on Mechanical Engineering, vol. 1, pp. 396-400
6. NUMERICAL SIMULATION OF THE FLOW PAST THE ROOF
Numerical simulation of the flow past tent roof has been carried out by using finite
volume code FLUENT 6. The calculated pressure coefficient (cpmean) distribution is shown on
the outside surface of the fabric by Fig.3.
Fig.3. The pressure coefficient cpmean distribution on the a) outside and
b) inside surface of tent roof model
The comparison of the two distributions shows on the majority of the roof a quite good
agreement. Only on small areas on the sides of the cone-shaped parts of the roof,
perpendicular to the wind direction can be observed relatively large differences: the calculated
pressure is smaller than the measured. Further investigations are needed to clarify this finding.
After validation of the numerical simulation the iterative stress and flow calculation can yield
the final shape of the tent roof and the stresses caused by the wind.
Authors acknowledge the support of OTKA T030116 as well as FKFP 0624/2000 and
0075/2001.
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