Scale-dependent turbulent coherent structures and pollutant fluxes over heated urban surfaces: spectral proper orthogonal decomposition and amplitude modulation analysis

초록

Various scales of turbulent coherent structures (TCSs) individually and collectively contribute to pollutant dispersion in urban areas. Although it is well known that surface heating significantly modifies urban turbulent flows, its impact on TCSs with different scales and their interactions still needs further investigations. In this study, we examine the impact of surface heating on different scales of TCSs and their associated turbulent pollutant flux over an idealized building array. Two large-eddy simulations with and without surface heating are conducted using the PArallelized Large-eddy simulation Model (PALM). The spectral proper orthogonal decomposition (SPOD) and the amplitude modulation (AM) analysis are applied to extract different scales of TCSs and to examine their interactions, respectively. The surface heating enhances low-speed streaks at and above the roof level while it leads high-speed regions to be less organized into streaks. The surface heating preferentially enhances large-scale coherent structures and therefore substantially increases the relative importance of large-scale coherent structures for roof-level TKE and turbulent pollutant flux compared with mid-scale and small-scale coherent structures. The surface heating also enhances the modulation of small-scale turbulent momentum and pollutant fluxes by large-scale coherent structures above the roof level. This enhancement of the modulation mainly originates from the strengthened large-scale upward motions. This study emphasizes the necessity of applying modal decomposition and scale-interaction analysis together to better understand both scale-dependent characteristics of TCSs and interactions among TCSs with different scales.

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