The Influence of Favorable Pressure Gradients upon the Coherent Motions in a Mach 5 Turbulent Boundary Layer

The effects of streamline curvature-induced favorable pressure gradients on a Mach 4.9 turbulent boundary layer were investigated. The goal was to provide a physical explanation for the response of the Reynolds stresses, by examining the distortion, re-orientation, and attenuation of the turbulent structures. Pre-multiplied power spectra of the streamwise component displayed a peak at Λx + = 200 – 300 for all cases below y/δ = 0.1. The strong pressure gradient showed a greater relative amount of energy concentrated at that wavelength, suggesting that this length scale plays a more significant role in the SPG case. The swirling strength criteria, weighted by the sign of the vorticity to differentiate vortex orientations, was applied to each vector field. An initial inspection of the instantaneous distribution of turbulent structures shows strong similarities with incompressible flow, suggesting that the same hairpin packet model is applicable to the flowfields in question. Conditional averaging of the velocity fields was performed using prograde and retrograde vortices as the conditional event. The strong pressure gradient re-oriented the interface region to 20°, reduced the intensity of the induced low-speed fluid, and produced a more isotropic velocity distribution around the vortex. These observations suggest that the pressure gradient is attenuating the fluctuating velocity associated with the turbulent structures, thereby limiting their contribution to the Reynolds stress.