POD Analysis of Unsteady Turbulent Flow and Dispersion around a Model Building Using Scale-Adaptive Simulation

Several studies have been performed on computational fluid dynamics (CFD) predictions based on unsteady Reynolds-averaged Navier-Stokes equations (URANS) models for flow and dispersion around buildings. But it has been reported that URANS models only contribute to reproduction of a certain part of large-scale unsteady flow patterns and give information on low-frequency contents around model buildings. Scale-Adaptive Simulation (SAS) approach is an improved URANS formulation, which allows the resolution of turbulent spectrum in unstable flow conditions. Nevertheless, very few studies have evaluated the performance of SAS in modelling flow and dispersion. The purpose of this study is to evaluate the relative performance of SAS in modelling unsteady concentration and flow fields around a model building and to clarify the mechanisms for its discrepancy in relation to other transient simulations such as large eddy simulation (LES) and URANS. For this purpose, in the present paper the transient behavior and dominant structures of flow field predicted by SAS are evaluated by proper orthogonal decomposition (POD) technique. Results highlight the outstanding performance of SAS in comparison with the URANS computation based on SST modelling not only for instantaneous distributions of concentration and velocity, but also for time-averaged ones. This tendency is directly related to the accurate reproduction of unsteady fluctuations around the model building by SAS. The quantitative and qualitative agreements for the shapes and magnitudes of POD modes between SAS and LES confirm the LES-like behavior of SAS in the wake region.