Re-entrainment around a low-rise industrial building: 2D versus 3D wind tunnel study

We investigate the re-entrainment of pollutants around a low-rise industrial building under opposing cross winds through experimental means in a wind tunnel. Two scaled models of an industrial building for electrowinning metal extraction were tested. The first model was a two-dimensional simplified segment of the building with a scale ratio of 1:40, while the second was a 1: 100 three-dimensional model of the full building. Particle image velocimetry was adopted to provide the planar velocity measurements that illustrated the flow distribution around the building. Flame ionization detection with propane tracer gas was used to measure the concentration distribution. The results of the 2D model show that the exhaust plume interacted with the opposing wind in two different stages, namely ground attachment and bent-over. The ground attachment stage occurred under low wind speeds, whereby the exhaust plume exhibited Coanda attachment with the ground surface before being lifted off by the cross wind and circulated to the leeward wake cavity. Upon further increase in the wind speed, the bent-over stage occurred with the exhaust plume being detached from the ground and deflected upward over the roof, before entrained by the wake. The re-entrainment ratio decreased with the increase in wind speed within the range of wind speeds tested, indicating that the range did not include the critical wind speed. Results from the 3D model painted a somewhat different picture and pointed to the significance of the end conditions. The maximum re-entrainment always occurred near the two ends of the building, where the pollutants mostly flowed around the ends rather than over the roof. The re-entrainment ratio was consistently higher at the two ends compared to the central sections. Finally, a building re-entrainment index, (K-R)(infinity) is proposed to characterize the re-entrainment performance of a specific building geometry. (C) 2004 Elsevier Ltd. All rights reserved.