Large eddy simulation of turbulent flow over transverse variations in aerodynamic roughness length.

A great deal of literature has been published regarding the impact of complex roughness on turbulent flow. However, the topic of transverse variations in surface roughness, has received relatively little attention. In this thesis, large-eddy simulation is used to investigate the effects on turbulent, high Reynolds number flow, caused by periodic step-changes in aerodynamic roughness length, which persist in the streamwise direction. A parametric study is conducted with respect to the ratio of high to low roughness length and the transverse width of high roughness regions. Results show that lateral momentum flux across shear layers, generates secondary flows in the vicinity of transverse transitions in roughness. These secondary flows form boundary layer scale, counter-rotating vortices, which redistribute turbulence and momentum throughout the entire domain and create time-invariant regions of relatively low and high momentum, above low and high surface roughness, respectively.