Analysis of turbulent structures around a rectangular prism building model using spectral proper orthogonal decomposition

Abstract

This paper presents a study on the turbulent structure of the wind velocity field around a rectangular prism building model, calculated using large-eddy simulation. Spectral proper orthogonal decomposition (SPOD), a technique to perform proper orthogonal decomposition (POD) in the frequency domain, was applied to extract the physical phenomenon with the largest kinetic energy from the velocity field on a plane near the ground. The obtained fluctuation modes were periodic functions, sorted by frequencies, and provided an insight into the spatiotemporal structures of various physical phenomena within the turbulence field, including the Kármán-type and arch-type vortex shedding and the flow separation phenomenon. The obtained eigenvalues depicted the kinetic energy distribution for all fluctuation modes, which was considered useful for understanding pedestrian-level wind. The results were compared for two cases (with and without fluctuation at the inflow boundary), which depicted the effect of the inflow fluctuation on the turbulent structures around the building. Additionally, for each physical phenomenon, the two modes calculated by POD were found quite similar to the real and imaginary parts of the SPOD eigenfunction. This indicates that POD needed two modes to depict each of the simple harmonic motions which were successfully detected by SPOD.