PIV experiments on a rough-wall hypersonic turbulent boundary layer

In the aerodynamics design of hypersonic vehicles, it is necessary to understand the behavior of turbulence with complex boundary conditions. Surface roughness is one such boundary condition that significantly affects turbulent hypersonic boundary layers. Experiments using Particle Image Velocimetry were performed to study the effects of surface roughness on a hypersonic (M _e = 7.3), low Reynolds number (typically Re_θ = 5500), turbulent boundary layer. Two different types of roughness elements on a flat plate model were used: diamond mesh and d-type square bars. Two different heights of roughness elements ranging from partially rough (about 38 viscous units) to fully rough (about 136 viscous units) regime with varying roughness density were tested. The van Driest transformed mean velocity profiles for rough walls showed the expected response to roughness, which is the shift of the velocity profile below the standard log-law, without any change in shape. In the region closer to the wall, the streamwise fluctuation was influenced by the size of the diamond mesh roughness. However, the fluctuations were influenced throughout the boundary layer in case of square bar elements. The wall-normal fluctuations indicated no apparent dependence on the size of roughness in case of mesh elements. Some dependence was observed in case of square bars elements. The size and shape of the roughness elements significantly altered the Reynolds shear stress. Also, significant compressibility effects were demonstrated by the roughness elements at this high Mach number.