Buoyancy flows and pollutant dispersion through different scale urban areas: CFD simulations and wind-tunnel measurements

In this paper, a coupled CFD model was established to study the multiscale problems on the mixed force and buoyancy flow and dispersion passing neighborhood scale - street scale - indoor scale models, and the numerical results were validated by wind-tunnel measurements with Richardson numbers (Ri) from 0 to 4.77 with SF₆ being the tracer gas. The basic flow, heat and pollutant transfer were solved with the 3-D steady RANS (Reynolds-Averaged Navier-Stokes) equations. The results show that when Ri ≤ 0.85, the standard k-ε model (SKE) can better predict the flow and temperature fields. When Ri > 0.85, the realizable k-ε model (RLKE) performs better. For the same turbulence model equation, with the increasing Ri the effects of the two near-wall functions (standard and non-equilibrium wall functions, SWFs and NEWFs) on the flow structures and temperature distributions become more and more significant. The specific value of Sc_t has a significant effect on predicting the pollutant dispersion and the optimal value is 0.7 for the studied cases. It is also found that for indoor flow caused by an outdoor street flow there also exists the Re-independence region. For the model studied only when Re_H ≥ 7.57E+03, the wind-tunnel measured results can represent the realistic cases meaningfully for both flow and pollutant distributions in street canyon and inside the room.