Benefits of precursor simulations with a drag-porosity model as inlet conditions for LES of atmospheric flow within urban canopy

The study of wind flow characteristics within urban environments is of prior interest for climate, meteorology and air pollution issues. Previous works have shown that within the urban areas the turbulent transfers of momentum, heat and mass between the Atmospheric Boundary Layer (ABL) and the urban canopy are governed by the interactions between large- and small-scale turbulent structures. Hence, to accurately compute flow characteristics at the neighbourhood scale, it is required to compute canopy-generated turbulence, as well as the turbulent structures prevailing in the ABL. This wide range of scales presents a major issue when performing unsteady simulations of turbulent flows such as Large-Eddy Simulations (LES), since the flow field specified at the inlet conditions the downstream development of the flow. In ABL numerical models, the most common method is to use a multi-scale approach based on successive grid-nesting from mesoscale to the local scale. When performing multi-scale nesting to study urban canopy flows, the urban obstacles are generally explicitly taken into account in the smallest high-resolution domain only, larger domains being not refined enough for a realistic representation of the canopy elements. However, it has been shown that adding roughness elements in the larger domain had a noticeable impact on the flow dynamics and statistics within the small domain (Wiersema et al., Mon. Wea. Rev., 2020).

This study addresses the problem of the generation of realistic unsteady atmospheric inflow conditions on an idealised urban-like canopy consisting of a staggered array of cubes of constant height and packing density. Here we investigate the performance of using drag-porosity based simulations as precursor calculations for obstacle-resolving LES. The objective is to show that by using a precursor simulation with a less-costly canopy modelling method as inlet, LES of ABL flows within and above a canopy of explicitly resolved obstacles is possible with minimum flow adjustment in the inlet region.

The LES atmospheric model ARPS (Advanced Regional Prediction System) is used with an Immersed Boundary Method (IBM) to model the flow over building-like obstacles. In order to assess the proposed approach, three inflow conditions are explored and compared : (1) a periodic precursor calculation over a large domain covered by cube array modelled with IBM, (2) a synthetic turbulence inflow obtained from purposely deteriorating flow information of the periodic case (without modifying moments and spectra), and (3) a drag-porosity precursor calculation (in which the urban canopy is modelled as a porous media depending on canopy averaged morphological characteristics). To the authors’ knowledge, the use of an urban-designed drag model as precursor calculation for an obstacle-resolving simulation is the first to be performed within a single atmospheric solver.