High resolution urban large-eddy simulation studies from street canyon to neighbourhood scale

Abstract

Urban turbulence characteristics are investigated at street canyon and neighbourhood scale. Three high resolution urban large-eddy simulation (LES) studies are performed using the urban version of the parallelized LES model PALM. Validation shows that the urban PALM version is in line with experimental and previous LES results, i.e. superior to the faster/cheaper conventional Reynolds-averaged (RANS) models. Two studies focus on quasi-2D urban street canyons driven by perpendicular flow. First is a parametric study of turbulence characteristics and flow dynamics within the canyon. The main results are: (1) Integral vertical turbulence profiles in deep canyons scale with canyon width. This is relevant for urban canopy parameterizations in larger-scale meteorological models. (2) A new concept of a “cavity shear layer” complements classical free shear layer concepts. (3) For the first time in urban LES Kelvin–Helmholtz instabilities are identified at the top of the urban street canyon. This is relevant for modelling urban dispersion, because the street canyon circulation is more intermittent than suggested by previous RANS results. Second, an Eulerian dispersion case study shows that differences in canyon flow dynamics are reflected in canyon dispersion characteristics compared to a previous RANS study. Third is a neighbourhood scale urban LES feasibility study: a passive tracer Lagrangian dispersion animation of Shinjuku, downtown Tokyo reveals turbulent flow features, upstream flow and intermittency. The main implications are always to use 3D models for turbulence simulations even in quasi-2D geometries, and not to underestimate the intermittency of turbulent flow. Standard deviations of velocity components within the canyon should not be treated as constant for perpendicular ambient wind but may be parameterized conveniently based on a vertical scaling with canyon width in deep canyons.