Experimental Investigation of Roughness Effects on Transition on Blunt Spherical Capsule Shapes

Boundary-layer transition plays an important role in aerodynamic heating of blunt capsule shapes. Transition may occur from disturbances created by surface roughness and fluctuations of the freestream. The present paper reports on a number of related experiments on Apollo-type capsule models in a range of hypersonic wind tunnels. The Adjustable Contour Expansion facility of the Texas A&M University provided Mach 6 experiments with distributed roughness at relatively low Reynolds numbers, 2.5E5 < Re-d < 5E5, with d denoting the capsule diameter. Observed boundary-layer transition data align well with current estimates of transient growth theory. Larger Reynolds numbers, 1E6 < Re-d < 3E6, could be assessed in the hypersonic Ludwieg tube Braunschweig of the Braunschweig University of Technology in Germany. Distributed roughness height values were broadly varied during these experiments, and different fluctuation levels of the freestream were considered. The results indicate roughness-induced transition for larger roughness heights, in accordance with correlations based on transient growth theory, and transition due to freestream disturbances, at subcritical roughness. Effects of total flow enthalpy on transition with isolated roughness were investigated in the High Enthalpy Shock Tunnel facility of the Japan Aerospace Exploration Agency. Here, roughness height, Reynolds number, and flow total enthalpy were varied. The paper reviews the most important results of these experiments and provides analysis, taking into account transient growth theory and numerical simulation data.