Experimental Assessment of Hypersonic Convective Heat Transfer Augmentation due to Surface Roughness

Although engineering correlations exist for the effects of small-scale roughness on heat transfer and shear stress, the complexity of flow physics for elements whose height exceeds the sonic line of hypersonic boundary layers is largely unknown. Additionally, the superposition of multiple scales of roughness in largely unexplored. This study investigates the heat transfer augmentation of two scales of patterned roughness, both individually and combined, where the large-scale roughness exceeds 20% of the boundary-layer height. These mimic technical roughnesses from the original experiments from Nikuradse low-speed experiments. The experiments were undertaken in a Ludwieg tunnel at a nominal Mach number of 5 and unit Reynolds number of [Formula: see text]. Determination of the Stanton numbers were performed by combining measurements of the freestream total temperatures with the measurement of the heat transfer from calorimeter and thin-film heat transfer gauges. Results indicate trends toward bulk heat transfer augmentation of order 20, 40, and 50% with increased [Formula: see text] for the small-, large-, and multiscale roughness patterns tested, respectively. For the large-scale roughness pattern investigated, significant heat transfer suppression was measured in the troughs between the elements; enhancement was measured to be substantial on the peaks, with notable difference distinguished between the spans and the intersections of the elements.