Mechanically Distorted Supersonic Boundary Layers
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The effects of coupling locally generated distortions associated with a periodic diamond shaped roughness pattern (ks+ 100) with globally acting distortions driven by favorable pressure gradients generated by streamline curvature on high-speed (M = 2.86), high Reynolds number (Re 60,000) supersonic turbulent boundary layers were examined. Large localized distortions (d = -0.5 to 0.4) were generated from the shock and expansion structure produced by diamond roughness elements. Weak (dmax = 0.05) and strong global favorable pressure gradients (d max = 0.25) were studied. The results were compared to similar flows with canonical surface patterns (smooth and square roughness). The measurements included planar contours of the mean and fluctuating velocity, Pitot pressure profiles, pressure sensitive paint and schlieren photography. The canonical flows followed established trends. However, their inclusion provides (1) a basis for comparison for the non-canonical flows and (2) new high-speed experimental data with turbulence. The diamond roughness element produced substantially different flows that were characterized by strong local distortions (d = -0.5 to 1.8 across the element) and highly varying turbulence properties, where the shear stress levels varied by -100%. The present data showed that combining the global pressure gradient to the local gradients associated with the diamond roughness element produced regions of flow over a rough surface with turbulence levels reduced to 70% of the undisturbed zero pressure boundary layer. These data and trends have important implications in controlling the turbulence in high-speed boundary layers.
