Flow around a high-rise building using steady and unsteady RANS CFD: Effect of large-scale fluctuations on the velocity statistics

In this study, the performance of the unsteady Reynolds-averaged Navier-Stokes (URANS) turbulence modeling of the flow field around a high-rise building with a 1:1:2 shape was examined. The unsteady fluctuation behind the building was successfully reproduced by URANS computation using the k-ω shear stress transport (SST) model. This reproduction could not be achieved by other turbulence models, namely, the standard k-ε, renormalization group theory (RNG) k-ε, realizable k-ε, and standard k-ω models. The URANS computation using the k-ω SST model successfully contributed to the reproduction of a certain part of the large-scale unsteady flow patterns around the building, and enabled more accurate prediction of the velocity distributions behind the building compared to the steady-RANS computation. However, the URANS computation overestimated the flow separation at the building corners. A modified ε-equation was introduced into the RNG k-ε model to enable its use to reproduce the periodic fluctuation and more accurately predict the flow separation on the roof of the building. The modification took into consideration the effects of the mean-flow periodicity on the energy transfer between large-scale fluctuations and small-scale turbulence. Overall, the results of the URANS computation using the RNG k-ε model with the modified ε-equation exhibited the best agreement with experimental results.