EXPERIMENTAL FLOW CHARACTERIZATION AND HEAT FLUX AUGMENTATION ANALYSIS OF A HYPERSONIC TURBULENT BOUNDARY LAYER ALONG A ROUGH SURFACE

Abstract

Surface roughness increases skin friction drag and convective heat transfer along high-speed flight vehicles. Although the corresponding heat flux augmentation is usually lower compared to increased friction, careful consideration in the prediction of the resulting heat loads is required to define suitable margins in the design of thermal protection systems. In the present study, the response of a hypersonic turbulent boundary layer to a smooth and rough surface along a sharp right-circular cone is examined. Tests were conducted at an inflow of Ma = 6 and Re = 16 Million per meter in the hypersonic wind tunnel H2K at DLR Cologne. The testing time was in the order of 20 seconds. The model consisted of three segments with exchangeable parts to consider smooth or rough surfaces. The roughness topology consisted of square bar elements to enable comparisons to previous experimental campaigns. The roughness-element wavelength was four times the depth of the elements. The model was made of a specific material with low thermal conductivity, in order to measure the surface temperature distribution by means of global quantitative infrared thermography and to avoid lateral heat dissipation. The flow field along the smooth and rough cone was measured in selected regions of interest by Particle Image Velocimetry (PIV). This technique was successfully applied for the first time in the high-speed environment of the H2K. The data is compared and discussed based on comparison to analytical and numerical predictions. The analytical calculations include classical turbulent smooth cone relations as well as correlations for rough surfaces. The data for numerical comparisons was derived by a boundary layer code and full CFD. In case of the boundary layer code a modified Krogstad model was applied to account for the rough wall.