Investigation into turbulent coherent structures and pollutant dispersion over radiatively heated/cooled urban surfaces using large-eddy simulation

초록

Turbulent coherent structures (TCSs) play important roles in momentum, pollutant, and heat transports. While TCSs over urban areas have been widely investigated assuming neutral conditions or constant surface heat fluxes, TCSs over urban surfaces heated/cooled by realistic radiative processes are little investigated yet. This study examines TCSs and their roles in pollutant dispersion and heat transport over radiatively heated/cooled urban surfaces using the PArallelized Large-eddy simulation Model (PALM) including radiative physics. A cubical building array is considered. The radiative processes result in heterogeneous temperature distributions on roof, road, and wall surfaces during daytime and nighttime. During daytime, low-speed streak appears up to well above the roof level while high-speed streak is found only near the roof level. During nighttime, the sizes of TCSs are markedly small and streaks are not found. The spectral proper orthogonal decomposition (SPOD) analysis is conducted to identify coherent structures of different scales and quantify their contributions to pollutant dispersion and heat transport. During daytime, coherent structures of various scales appear near and above the roof level. Relatively large (small) coherent structures exhibit relatively large (small) contributions to turbulent kinetic energy (TKE) but show relatively small (large) contributions to vertical turbulent pollutant and heat fluxes at the roof level. During nighttime, coherent structures are comprised of small irregular vortices. Coherent structures having relatively large (small) contributions to TKE show relatively large (small) contributions to vertical turbulent pollutant and heat fluxes above the roof level but exhibit relatively small (large) contributions at the roof level.

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