The Effect of Near Wall Disturbances on a Compressible Turbulent Boundary Layer

This study investigates the effects of near wall disturbances in the form of roughness on a compressible turbulent boundary layer. The studies were carried out using numerical methods which directly solve the Navier-Stokes equations. This provides for unique opportunities to investigate three dimensional structures of the flow as well as avoid the loss of physical fidelity with turbulence modeling. Three cases were ran, a smooth wall case, and two rough wall cases with different heights of the roughness elements between the cases. The results are first visualized with different approaches. Then statistical methods were used to characterize the flow.

The idea of wall-similarity is common in rough wall studies. This theory assumes that at high enough Reynolds numbers, the difference of length scales between those relevant to the near-wall flow and those relevant to the outer layer are separated enough that the effect of roughness is localized to a region of the boundary layer known as the roughness sublayer. Some studies have suggested that the disruption of commonly seen coherent streaks of low and high-speed fluid near the wall will disrupt ejections and sweeping events, affecting the outer layer of the boundary layer. This study found that the presence of the roughness elements seemed to make the near wall streaks less visible when plotting planes that cut through roughness elements, but did not disrupt the presence of the vortices associated with the streaks. The presence of the streaks was also found to be better observed when plotting a plane of constant wall-normal distance. This is consistent with the statistical findings that wall-similarity is generally observed. One exception is a shift in the Reynolds stress profiles in the outer region of the rough wall cases from the smooth wall case. This may suggest that there is a need in compressible flow studies for another scaling parameter involving the roughness geometry. These findings are consistent with recent compressible flow rough wall studies.