Large Eddy Simulation of Internal Boundary Layers Created by a Change in Surface Roughness

Abstract

Turbulence in a ?-mesoscale internal boundary layer (IBL) formed by a discontinuous change in surface roughness has been investigated using a large eddy simulation (LES) model to explicitly treat turbulent transport. Two cases are examined: a rough-to-smooth transition and a smooth-to-rough transition. IBL heights are identified using two absolute criteria, one in terms of horizontal stress variation and the other in terms of vertical stress variation, and the ratio of these two heights is found to be approximately constant with fetch. The IBL growth rate with fetch is essentially the same for both transitions, which is here interpreted as reflecting self-similarity of the IBL at relatively large fetches. Parameterization of IBL growth in terms of turbulent intensity is successful when the average turbulent intensity in the IBL is employed but not if the turbulent intensity at the IBL top is utilized. The effective eddy mixing length for longitudinal velocity does not experience strong variations in the vicinity of the surface discontinuity when parameterized in terms of the local turbulent kinetic energy (TKE), but the effective mixing length is somewhat larger over the smoother surface as a result of pressure gradients induced by the horizontally inhomogeneous flow. The TKE dissipation length scale is roughly equal to the mixing length scale for longitudinal velocity, so stress and TKE are strongly coupled above the surface layer. The ?quality? of turbulence, evaluated by turbulent skewness and kurtosis, indicates that turbulence above and below the identified IBL height is associated with the upstream and underlying surfaces, respectively, demonstrating that this height is a physically relevant length scale.