Spectral Proper Orthogonal Decomposition Analysis of Turbulent Flow in a Two-Dimensional Street Canyon and Its Role in Pollutant Removal

摘要

Spectral proper orthogonal decomposition (SPOD) is applied as a post-processing technique to elucidate the relationship between turbulent motion and pollutant removal in a two-dimensional street canyon with an aspect ratio of one and a uniform roof height. A pollutant is continuously emitted from a line source set on the ground of the target canyon. First, the SPOD technique is used to decompose the velocity field obtained from a large-eddy simulation. The external large-scale coherent structures and waves caused by the Kelvin-Helmholtz instabilities are extracted and visualized using the SPOD modes. The SPOD cospectra are defined to further examine the phase relationship between the streamwise and vertical velocity components and concentrations. Based on the SPOD cospectra, the contribution of the turbulent structures at various scales to pollutant removal are quantitatively estimated. The results reveal that both the large-scale coherent structures and Kelvin-Helmholtz instabilities could cause ejection events at the canyon roof level and thus contribute to pollutant removal. Although the former occupy a large proportion of the turbulence kinetic energy, a smaller vertical turbulent mass flux is also observed. Conversely, the latter contribute to stronger ejection and sweep events with stronger vertical components. However, the shapes of the modes indicate that the external large-scale coherent structures also play a role in triggering and transporting the small-scale turbulence at the roof level, which may indirectly contribute to pollutant removal.