Evaluation of dynamic subgrid-scale models in LES of stably stratified flow within and above an urban street canyon

Large-eddy simulation (LES) is used to simulate stably stratified turbulent boundary-layer flow over an urban street canyon. Three different subgrid-scale (SGS) models for SGS stresses and heat fluxes are tested: (a) the Smagorinsky model, (b) the Lagrangian dynamic model, and (c) the recently developed scale-dependent Lagrangian dynamic model (Stoll and Porté-Agel, 2006). Simulation results obtained with the different models are compared with measurements obtained with a laser doppler anemometer and a cold wire at the boundary-layer wind tunnel of the National Institute for Environmental Studies, Japan (Uehara et al., 2000). We find that the scale-dependent dynamic model is able to account, without any tuning, for the local changes in the eddy-viscosity and eddy-diffusivity model coefficients. It can also capture the scale dependence of these coefficients associated with flow anisotropy at the smallest resolved and sub-grid scales in regions of the flow with strong mean shear and flow stratification. As a result, the scale-dependent dynamic model yields results that are more realistic than the ones obtained with the standard Smagorinsky model and the scale-invariant Lagrangian dynamic model.