Influence of roughness-generated streaks on laminar separation bubbles

Abstract

This experimental investigation studies the impact of streaks on two-dimensional laminar separation bubbles forming over an aerofoil. Streaks are introduced into the boundary layer using cylindrical roughness elements, and the resulting mean and unsteady flow fields are measured using hot-wire anemometry. The observed streaks generated by roughness exhibit analogous behavior to those generated by freestream turbulence, significantly altering the mean-flow characteristics of the bubble, including reductions in its length, height, and the introduction of spanwise velocity gradients. These mean flow modifications have a damping effect on convective disturbance growth. The experiments suggest the coexistence of modal instability due to the laminar separation bubble and non-modal instability due to streaks. We perform linear stability analysis at the fore position of the laminar separation bubble and find that by incorporating a spanwise wavenumber into the calculations, the experimental findings are reasonably modeled, indicating an oblique modal instability. To investigate the combined effect of roughness and the presence of freestream turbulence, we increase the turbulence level from the baseline in the presence of a roughness forcing configuration. We find that increasing the turbulence intensity leads to an enhancement of non-modal instability, accompanied by distinctive chordwise disturbance growth compared to lower freestream turbulence intensity levels. Moreover, increasing the turbulence intensity level in the presence of roughness not only bypasses the modal instability observed without the addition of turbulence but also suppresses laminar separation.