Influence of the wind direction on the flow structure ...

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Proceedings of PSFVIP-4 June 3-5, 2003, Chamonix, France. F4056

Influence of the wind direction on the flow structure behind a building model 1*

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L.E. Brizzi , L. David , D. Calluaud and G. J. Poitras

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Laboratoire d'Etudes Aérodynamiques (LEA), UMR CNRS 6609, Futuroscope Poitiers, France Boulevard Pierre et Marie Curie Téléport 2, B.P. 30179 86960 FUTUROSCOPE POITIERS Cedex, France e-mail : [email protected] Tel : 0033549496969 fax : 0033549496968

Université de Moncton, Moncton (N.-B.), Canada

KEYWORDS: Main subject(s): Environmental Flow Fluid: Aerodynamics Visualization method(s): Particle Image Velocimetry (PIV) ABSTRACT : Stereoscopic Particle image velocimetry (PIV) measurements were performed downstream of a three dimensional model building (the height is 18 mm and the length and width are equal to 60 mm). The experimental investigation was done in an Eiffel type wind tunnel for a Reynolds numbers of Reh=13 200, based on the height of the models and the free stream velocity. In this study, the influence of the wind direction on the flow structure behind a building model is studied. Four values of this angle are presented (α=0°, 15°, 30° and 45°). The results show that the upwind dissymmetry of the flow modifies the flow characteristics around the model. Hence, noticeable differences of the flow structure are presented.

1. Introduction There’s a relatively extensive amount of literature concerning experimental simulations, full-scale measurements and numerical simulations of the flow around a wide variety of 3D bluff-bodies that can be considered as good models for buildings. Knowledge of the flow characteristics is useful in a variety of applications such as wind loads on structures, dispersion of pollutants, airport runway interference effects, the comfort of pedestrians walking along buildings, etc. The flow around surface mounted obstacles placed in a turbulent channel flow was experimentally characterized in previous studies ([Larousse et al. (1991)], [Martinuzzi and Tropea (1993)], [Hussein and Martinuzzi (1996)]. They have shown the complexity of the flow around and more extensively behind surface-mounted cubes. Many different characteristics of the flow were identified: o The horseshoe vortex upstream, related to the boundary layer thickness δ/h, which has a significant effect on the pressure coefficient distribution. o A secondary vortex at the upstream bottom corner. o The separation on the upstream corners of the obstacle. o A shear layer on top of the obstacle, where the maximum turbulence intensity is observed. o A reattachment of the shear layer onto the body dependant of the upstream turbulence intensity, the length and the width of the obstacle. o The recirculation zone and the arch vortex downstream where the minimum pressure is found (under the arch vortex).

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Copyright 2003 by PSFVIP-4

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o The reattachment length decreases linearly with increasing obstacle length to height ratio b/h and remains constant once reattachment on top of the obstacle occurs. o The wake flow, related to the pressure field close to the body, is affected by the upstream turbulence intensity and shear since this promotes reattachment of the separating shear layers on the sides and top of the body which results in a reduction of the cavity region. The aim of our previous studies ([Poitras et al. (2000)]) was to determine separation and reattachment patterns and to investigate the major flow differences in the mean and fluctuating velocity fields around the obstacle in order to better estimate the wind forces on buildings. We showed that large scale unsteadiness of the flow is related to the RMS pressure measurements. It is felt that these measurements will provide a better understanding of the flow and its effects on the mean pressure distribution on the model’s surfaces. For all the measurements, the thickness of the boundary layer upstream of the models was kept constant. Furthermore, these results concern a bluff body mounted on a surface with an initial wind normal to the main face of the building. However, Castro and Robin (1977) have shown that the flow around the cube is highly dependent on its orientation with respect to the wind. On the other hand, by changing the orientation of a cube in a turbulent boundary layer flow, Sakamoto and Arie (1982) measured the pressure distribution on the surface of the body. They showed that the magnitude and distribution of pressure on the cube surface vary remarkably with change in orientation and that the drag on the cube acting in the flow direction increases as the cube is positioned at different angles with respect to the incoming flow. In our current study, the influence of the wind direction on the flow structure behind a model is studied. One of the main aims is to observe how the upwind dissymmetry of the flow modifies the flow in the wake. Usually, the techniques used in these studies include surface pressure measurements, flow visualizations and localized point velocity measurements (hot-wires, pitot tubes or laser doppler anemometry). Since the flow around these bodies is quite complex, 3D-PIV measurements are necessary to investigate the three-dimensional characteristics of the flow around a surface mounted obstacle (models of low-rise buildings).

2. Experimental device 2-1 geometry The experimental work was performed in an Eiffel type wind tunnel (open circuit) having a transparent (glass) working section that is 400 mm high, 300 mm wide and 1300 mm long. A turbulent boundary layer was created by using a strip of sandpaper (38 mm wide and spanning the whole width of the tunnel, grain size of 310 µm) which was placed at the bottom entrance of the test chamber. A Pitot tube connected to a K type thermocouple was used to control the free-stream velocity of the tunnel and to check the temperature at the channel entrance. The model is 18 mm high (h), 60 mm long (b) and 60 mm wide (d) (blockage ratio less than 1.5%). The model was mounted on a rotating plate on the tunnel floor where it and can be oriented according to the incoming wind. The windward face of the model was placed at 420 mm (X=0 mm) from the entrance. The flow is studied for four angles (0°, 15°, 30° and 45°) of the model building. The Reynolds numbers is based on the model height and the free stream velocity: Reh= 13200 (11 m/s). The obstacle dimension variables and velocity variables used are shown on figure 1.

Fig. 1 Global experimental set-up

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2-2 Measurements Stereoscopic PIV system Various sections are investigated by 3D PIV (Z=0 mm, Y=18 mm). From the instantaneous velocity fields mean velocities, fluctuations, vorticity or Reynolds stresses are estimated. An algorithm based on direct relations between the object reference and the two reference images ([Calluaud and David (2002)]) is employed to calculate the three components of the velocity for each section of measurements. The three-dimensional calibration reconstruction presented is a method that uses a real perspective camera model without the knowledge of the geometry for the calculation of the calibration matrix. This 3D calibration method is based on an in-situ measurement of the target points whose coordinates are known. With these point locations on the two cameras, the geometric parameters of the linear stereoscopic system are calculated. The optical distortions are minimized using large fixed focal lenses and Scheimpflug arrangements. After this step, the flow images are recorded simultaneously in the two cameras and the projected 2D velocity fields are calculated by cross correlation and local adaptive window shifts. To determine for each vector obtained from a camera its corresponding vector on the other camera, a matching step is realized by interpolation of the vector fields towards the unstructured mesh of pairing. Finally, the last step is the reconstruction of the 3D velocity field from these two velocity fields by the stereoscopic system. This method is compared to the 3rd order XYZ-Polynomial imaging model supplied with the Dantec software. Furthermore, a statistical vector validation filter is suggested to calculate first and second statistic moments. To eliminate (or to minimize the effect of) residual erroneous measurements after the usual filtering (peak validation, median filter, etc ...), a new kind of filtering was used. This method is based on the hypothesis that the measured velocity vectors in time and at different characteristic points results in a Gaussian type histogram. Hence, the data are filtered for every point of measurement, for each acquisition and for all velocity components. Thus the data processing is carried out in two steps. For the first processing, each sample is validated according to a significant value of the signal/noise ratio (the threshold is fixed between 1.5 and 2.0 according the plane of measurements). This corresponds to the signal/noise ratio between the first and the second peak of the correlation function used to determine the velocity. Starting from these validated values, a velocity histogram at each point and for each velocity component is built, smoothed and filtered with an appropriate threshold. In order to estimate the mean ( V ) and fluctuating ( RMSv ) values of the velocity, we recalculate the values by using an attenuation coefficient that is based on a Gaussian histogram. These estimated values are used to obtain the boundaries of the bandwidth filter ( V ± K *RMSv ) centered on the mean velocity. For the second processing, the initial measurements are again filtered with a lower threshold than the first processing, (signal/noise ratio at 1.2). The measurements are then filtered by bandwidth re-using the results of the first processing. Supposing that the velocity histogram is Gaussian, only the values included in the interval ( V ± K *RMSv ) are validated where K is the confidence level and equal to 3. The mean values and the fluctuations for each velocity component and each point are finally calculated. After the post processing, only the statistics with more than 500 validated vectors are retained.

3. Results and discussion o

Figure 2 presents the velocity field in vector form (plane Z=0 for the configuration α=0 ). The plane is a symmetry plane since the transverse component W measurements indicate a uniform velocity field with values between ±0.25m/s. In accordance with previous work ([2], [6]) we find the same typical flow observed downstream of a cubical mounted on a plate. This is characterized on the mean field by a broad recirculation area which is closed near X=40 mm. In agreement with the work of [3] and [4], this flow behavior indicates that we are in presence of an arch vortex (cf. Fig. 3). For our configuration (Reh=13 200 and h=18 mm), the closure of the wake downstream of the bluff body occurs first vertically and then laterally. This can explain the existence of a broad recirculation zone downstream of the model. Further downstream, the flow decelerates. For the fluctuations, an area of high RMSu values that puts into evidence the location of the shear layer that develops from the top of the block and further downstream. In this area, aligned horizontally, we observe velocity fluctuations up to 2.5 m/s. These strong values of longitudinal fluctuations are linked with relatively strong values of RMSv 3


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(≈1.5 m/s). However, the strongest fluctuation rates for the vertical component are not observed in the shear layer but near the closure of the wake (X=40 and Y=10) which is characterized by a saddle point on the velocity field. Although null, the transverse velocity component W also shows significant fluctuations rates (≈1.8 m/s) downstream of the closing wake. For the second studied plane (Y=H), we find again the good flow symmetry everywhere on the global mean velocity fields (U, V and W cf. Fig. 4). Similar to previous studies, the field shows a small reduction in the longitudinal component U downstream in favor of the vertical component V (cf. Fig. 4 at X=30 mm and Z=0) because of "plunging" of the flow downstream of the recirculation area. This phenomenon increases when we move laterally (W≈-2 m/s at X=0 and Z=±17 mm) because of the presence of the arch vortex.

Fig. 2 : Velocity field (Z=0 ; α=0°).

Fig. 3 : Scheme of the flow from Martinuzzi et al. 1993. For the second and third studied configuration (α=15° and α=30°), the vertical plane Z=0 is not a symmetry plane for the flow (cf. Fig.5 for α=15°). That produces logically non-zero values for transverse velocity component W (cf. Fig.6). The maximum mean value, now close to 4 m/s, is reached downstream of the geometry (X= 20 mm; Y= 5 mm). This puts into evidence the “feeding” of the wake by the flow that bypasses the model. As in the previous case, the extreme fluctuation rates for the U and V components are present in the shear layer that starts from the top of the building. However, contrary to the configuration α=0°, the shear layer is not horizontal but is clearly directed towards the wall. This shows that the flow goes more towards the lower wall compared to the configuration α=0°. Indeed we find, in the measurements for the Y=h plane, more significant V values along the axis Z=0. There are however some similarities between the two flows: we find that the strongest fluctuation rates of the W component are located downstream of the closing wake.

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Fig. 4 : Vertical component V (Y=h ; α=0°).

Fig. 5 : Velocity field (Z=0 ; α=15°)

Fig. 6 : Transverse component W (Z=0 ; α=15°) For the α=45° configuration, the Z=0 plane becomes again a symmetry plane for the flow (cf. Fig.7). As in the case α=0°, the transverse component W is uniform and almost null (±0.25 m/s). For the remainder of the flow, the two configurations show appreciably different characteristics. Contrary to the α=0° case, the α=45° configuration does not have a swirling structure downstream but rather has 5


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a large return flow zone characterized by a significant vertical component. The shear layer that develops over the top of the model presents a little slope towards the wall. As previously, this zone presents the most significant fluctuations rates for U and V (3.2 m/s for U and V). We can also notice that, for α=45°, the thickness of this shear layer is much finer than for the α=0° configuration. This seems related to the presence (or not) of a structure on the top wall of the model. For the α=0° case, previous studies ([3], [4]) show that a flat structure with an arch form exists on top of the block. This structure favors the growth of the shear layer by causing a more significant separation. For the configuration α=45°, this separation does not seem as significant.

Fig. 7 : Velocity field (Z=0 ; α=45°)

4. Conclusion and prospects Stereoscopic Particle Image Velocimetry (PIV) measurements were performed downstream of a three dimensional model building for four different angles. The results presented in this study confirm the very strong influence of wind direction on the flow structure around a bluff body block mounted on a wall. Indeed, the four angles studied show, from velocity measurements, a very different topological behavior. Further measurements (pressure, 3C-PIV) nearer the wall should confirm the differences for this kind of high 3-dimentionnal turbulent flow.

References [1] Calluaud D, David L., 2002. Backward projection algorithm and stereoscopic particle image velocimetry measurements of the flow around a square cylinder. 11th International Symposium on Applications of Laser. [2] Castro I. P. and Robins G., 1977. The flow around a surface-mounted cube in uniform and turbulent streams. Journal of Fluid Mechanics, Vol. 79, Part 2, pp. 307-335. [3] Hussein H. J., Martinuzzi R. J., 1996. Energy balance for turbulent flow around surface mounted cube placed in a channel. Physics and Fluids, Vol. 8, No 3, pp. 764-780. [4] Larousse A., Martinuzzi R. and Tropea C. Flow around surface-mounted, three-dimensional obstacles. 8 th Symposium on Turbulent Shear Flows, TU-Munich/Germany, 1, pp. 14-4-1/14-4-6, 1991. [5] Martunizzi RJ; Tropea C, 1993. The flow around surface-mounted, prismatic obstacles placed in a fully developed channel, J. Fluid Engineering 115: 85-92. [6] Poitras G. J., Brizzi L. E., Pecheux J., Gagnon Y. 2000. The study of fluid flows in the immediate vicinity of building models. Proc 9 th International Symposium on Flow Visualization, paper 246.1246.10. [7] Sakamoto H. and Arie M.,1982. Flow around a cubic body immersed in a turbulent boundary layer. Journal of Wind Engineering and Industrial Aerodynamics, Vol. 9, pp. 275-293. 6
