Unsteady pulsating flowfield over spiked axisymmetric forebody at hypersonic flows

Abstract

The paper gives experimental observations of the hypersonic flow past an axisymmetric flat-face cylinder with a protruding sharp-tip spike. Unsteady pressure measurements and high-speed schlieren images are performed in tandem on a hypersonic Ludwieg tunnel at a freestream Mach number of M∞=8.16 at two different freestream Reynolds numbers based on the base body diameter (ReD=0.76×106 and 3.05×106). The obtained high-speed images are subjected further to modal analysis to understand the flow dynamics parallel to the unsteady pressure measurements. The protruding spike of length to base body diameter ratio of [l/D]=1 creates a familiar form of an unsteady flowfield called “pulsation.” Pressure loading and fluctuation intensity at two different ReD cases are calculated. A maximum drop of 98.24% in the pressure loading and fluctuation intensity is observed between the high and low ReD cases. Due to the low-density field at low ReD case, almost all image analyses are done with the high ReD case. Based on the analysis, a difference in the pulsation characteristics is noticed, which arises from two vortical zones, each from a system of two “λ” shocks formed during the “collapse” phase ahead of the base body. The interaction of shedding vortices from the λ-shocks' triple-points, along with the rotating stationary waves, contributes to the asymmetric high-pressure loading and the observation of shock pulsation on the flat-face cylinder. The vortical interactions forming the second dominant spatial mode with a temporal mode carry a dimensionless frequency (f2D/u∞≈0.34) almost twice that of the fundamental frequency (f1D/u∞≈0.17). The observed frequencies are invariant irrespective of the ReD cases. However, for the high-frequency range, the spectral pressure decay is observed to follow an inverse and −7/3 law for the low and high ReD cases, respectively.

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