A Numerical Study on the Flow Characteristics in step-up street canyons

In this study, we analyzed the flow characteristics in strep-up street canyons using a computational fluid dynamics (CFD) model. We considered building height ratios (H_u/H_d = 0.33 and 0.6, H_u and H_d respectively indicate the upwind and downwind building heights) and building-length ratios (L/S = 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0, L and S respectively indicate the building length and street-canyon width). The simulated results were compared with the wind-tunnel data (Hu/Hd = 0.33, 0.6 and L/S = 1, 2, 3, and 4) for the model validation. The CFD model reproduced the clockwise-rotating and counterclockwise-rotating vortices in the street canyons, recirculation zones above the downwind buildings reasonably well. Furthermore, the flow characteristics in the step-up street canyons were analyzed in detail, mainly based on the numerical results. We suggested the effects of different building-height and building-length ratios on flow characteristics in the step-up street canyon. In the case with H_u/H_d = 0.33, one clockwise-rotating vortex was formed in the street canyon and its center was slightly tilted toward the wall of the upwind building. However, in the case with H_u/H_d = 0.6, two clockwise-rotating vortices were formed in the upper and lower layers. A clockwise vortex and a counterclockwise vortex were stabilized as the building width ratio increased.