Assessment of a one-way nesting procedure for obstacle resolved large eddy simulation of the ABL

Abstract

Large Eddy Simulation (LES) of obstacle-resolved flow over extended urban areas is still extremely costly, compared to the common practice of modeling obstacles with a rough wall-function. In the current study we investigate whether a one-way nesting procedure, consisting of a highly-resolved, small-scale LES embedded inside a low-resolution, large-scale LES, can provide a good compromise between spatial extent, accuracy, and computational requirements. The nesting is achieved by blending the fields of the coarse-grained LES in the vicinity of the nested boundaries of the fine-grained LES. Our assessment of the nesting simulation focuses on two points. Firstly, we compare the vertical profiles of mean quantities and, Reynolds stresses, along with turbulence spectra, against those from a reference LES. Secondly, we check the capabilities of the nesting procedure to facilitate regeneration of the inertial-range turbulence structure by the small-scale LES. Good agreement is found between the mean profiles, but the vertical profiles of Reynolds shear stress from the nested simulation possess an overshoot of the peak values, compared to the reference case. Despite the lack of small- and low-energy eddies transferred from the large scale model to the small scale model, the energy spectra of the nested domain and the reference domain have similar contents. We show that the distance required to reconstruct small eddies depends on the parameters used for the blending between the coarse- and the fine-grained fields. These results show a promising potential for reduction of computational costs in wind engineering applications, where high accuracy is required and where the flow can be strongly dependent on large coherent structures.