The Influence of Geometric Model Complexity on Near-Wall Flow Phenomena and their Resolution in Large-Eddy Simulations

Abstract

Large-eddy simulation models are increasingly used to study the urban climate. Technological advances in the field of high performance computing enable modelers to calculate urban flow phenomena at increasingly high resolutions. Thus, the focus of urban climate studies using large-eddy simulations is moving gradually closer to the wall. It is particularly in these near-wall regions where two integral components of large-eddy simulations no longer function reliably. Surface boundary conditions, on the one hand, cannot account for heterogeneity in surface roughness. Furthermore, sub-grid scale models are often based on heuristic assumptions about eddy geometries. In the course of this work, two wind-tunnel experiments are designed and carried out at the Environmental Wind Tunnel Laboratory using the WOTAN wind tunnel. Results of these experiments are then compared to the results of corresponding large-eddy simulation studies carried out with the contemporary urban climate model PALM. The aim of the comparison is to validate the replication of near-wall turbulence at rough surfaces in large-eddy simulation models. Findings from both experiments indicate an underestimation of wall-normal turbulent fluxes by PALM with respect to the wind-tunnel results. This difference can be significantly eliminated either by artificially increasing the roughness lengths in the surface boundary condition, or by increasing the grid resolution, or by choosing scale-dependent sub-grid scale models. Furthermore, the wind-tunnel flow exhibits near-wall anisotropy of the turbulent motions as a function of surface roughness. The large-eddy simulation results do not show any anisotropy of the turbulent motions near the wall. Future sub-grid scale models must be able to address these differences. Until then, modelers need to cultivate an awareness of the uncertainties of the large-eddy simulation technique especially in the near-wall region.