Large-eddy simulation of flows after an abrupt change in surface roughness

Abstract

Aerodynamic resistance changes in the atmospheric surface layers (ASLs), such as coastal zones and rural-urban interface, complicate the transport processes and lead to the internal boundary layer (IBL) development. Their effect on ASL flows is elucidated by computational fluid dynamics (CFD) calculations in this thesis. The roughness elements at the bottom are explicitly resolved by sinusoidal wavy surfaces to examine the interaction among roughness sublayers (RSLs), inertial sublayers (ISLs) and IBL.

The adjustments of mean winds and flow structure after surface transition are firstly investigated by the Reynolds-averaged Navier-Stokes (RANS) approach. Three configurations of smoother-to-rougher transition are examined to contrast the flow dynamics. After the change in (increasing) surface roughness, the flows decelerate and the momentum flux increases to overcome the augmented drag. The growth of IBL and ISL signifies that the influence from the upstream surface is being weakened while the flows are developing in equilibrium with the downstream (new) surface.

The response of flows and turbulence to surface transition is further studied by large-eddy simulation (LES). It is found that the flow adjustments initiate at the RSLs during the development of the IBL so the flows hardly establish self-similarity immediately after surface transitions. Unlike those analytical solutions, the current LES unveils a slower IBL growth in the streamwise direction, demonstrating the importance of resolving RSLs explicitly. Besides, augmented turbulent diffusion is observed over the rougher surface. The gradual adjustment of higher-order moments along with developing IBL confirms that IBL is a physically significant length scale. The spatial behaviours of skewness and kurtosis, which suggest coherent events, are visualized across the surface discontinuity.

Furthermore, wall-normal distributions of frequency spectra show that the energy-carrying turbulence shifts to higher (lower) frequency after SR (RS) transition. The shift is completed substantially on large-scale RSL turbulence, demonstrating the significance of RSLs. The budget analysis of momentum flux illustrates that RSLs complicate the transport processes. Above the RSLs, shear production and pressure-velocity interaction are dominant. Along with the IBL growth, these two terms adjust spatially that spread the impact of surface transition upward.