Instability and transition on the HIFiRE-5 in a Mach-6 Quiet Tunnel

A 2:1 aspect-ratio elliptic cone with a blunt nosetip was tested in the Boeing/ AFOSR Mach-6 Quiet Tunnel to investigate the effects of freestream noise level, surface roughness, angle of attack, and freestream Reynolds number on laminar-to-turbulent boundary layer transition on the windward surface. The cone had a minor axis half-angle of 7° and a nose radius of 0.95 mm. Temperature-sensitive paint enabled a global measurement of the temperature distribution and detection of the transition front. Transition apparently arising from two mechanisms was observed: transition along the centerline suspected to arise from the amplification of second-mode waves in the inflected boundary layer, and transition roughly halfway between the centerline and leading edges probably due to the breakdown of crossflow vortices. Reducing noise level from conventional (root-mean-square pressure 3% of the mean) to quiet (root-mean-square pressure less than 0.1% of the mean) substantially delayed transition due to both mechanisms. Increasing the angle of attack from 0° to 4° delayed the crossflow transition mode on the windward side. Transition moved forward as freestream unit Reynolds number increased from 2.6 · 106 /m to 11.9 · 106 /m. PCB fast-response pressure transducers installed along the model centerline detected apparent instabilities at frequencies from 50 to 150 kHz prior to transition under noisy flow; quiet-flow results are less clear. Mass flux profiles along the centerline were measured with a calibrated hot wire