Effect of Distributed Surface Roughness on the Stability of High-Speed Flows.

A parametric variation of height and wavelength of distributed sinusoidal roughness is carried out to study the effect on the stability of a Mach 5.35 flat plate boundary layer flow. To study the effects of roughness height four different cases are considered with a relative roughness height between 10 and 44 percent of the boundary layer height. Another four cases are chosen to examine the effects of the roughness wavelength which varies between roughly 0.44 to 3.55 times the second mode wavelength. The presence of the surface roughness led to strong variations in the mean flow quantities compared to the smooth wall case causing local acceleration and deceleration of the flow. The steady mean flow field showed supersonic wave patterns generated by the roughness elements emanating into the freestream. The disturbance flow analysis in the frequency domain based on wall pressure data showed that the flow was destabilized for the fixed wavelength cases with variable roughness heights. The shortest wavelength case from the fixed roughness height cases was the only one that showed a stabilization of the flow. For the considered range of wavelengths, the strong wavelength dependence of the level of destabilization was noted. For all roughness cases, a shift towards lower frequencies was observed in the pressure amplitude plots. For the higher roughness height and larger wavelength cases, a second region of amplified modes at higher frequencies was noted which may suggest destabilization of a higher Mach mode or the occurrence of a different instability mechanism. In order to gain insight into the energy transfer mechanisms from the mean flow to the disturbance flow an energy budget analysis is also performed.