The Influence of a Cylindrical Roughness Element on a Perturbed Laminar Boundary Layer

Abstract

Roughness induced transition in the leading edge region of airfoils is a considerable challenge in the design of highly efficient wind turbine blades especially with respect to different inflow turbulence levels. The present experiments are, therefore, devoted to the influence of a cylindrical roughness element of different heights on a quite (unperturbed) and a perturbed laminar boundary layer, respectively. For low roughness heights, which do not considerably affect the mean flow, no significant influence of the roughness on the disturbance growth in the laminar boundary layer is obvious. In contrast, for medium roughness heights the formation of two counter-rotating vortex pairs and a growth of disturbance modes near the upper branch of the neutral curve are observed in the near wake of the roughness. As the mean flow relaxes towards a spanwise uniform state, the excited disturbance modes are damped again. However, the strong disturbance growth immediately downstream of the roughness results in an excitation of low frequency modes, which are continuously amplified and, finally, initiate the breakdown to turbulence. Within the medium roughness height range, the growth of the vortical structures and of the disturbance modes is strongly nonlinear with respect to the roughness height. If the boundary layer is perturbed with single, two-dimensional disturbance modes upstream of the roughness transition is considerably shifted upstream. The most dominant spanwise modes excited by the interaction with the roughness are in the range of the roughness diameter, whereupon a significant energy transfer into higher spanwise wavenumber modes is present prior to transition. Finally, for high roughness elements boundary layer transition occurs in the close vicinity of the roughness element and is shifted towards the element with increasing roughness height. The disturbance level in the resulting turbulent boundary layer is, however, rather independent of the particular roughness height.