Unsteady pulsating flowfield over spiked axisymmetric forebody at hypersonic flows

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-infinity = 8.16 at two different freestream Reynolds numbers based on the base body diameter (Re-D = 0.76 x 10(6) and 3.05 x 10(6)). 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 Re-D cases are calculated. A maximum drop of 98.24% in the pressure loading and fluctuation intensity is observed between the high and low Re-D cases. Due to the low-density field at low Re-D case, almost all image analyses are done with the high Re-D 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 "lambda " shocks formed during the "collapse " phase ahead of the base body. The interaction of shedding vortices from the lambda-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 (f(2)D/u(infinity) & AP; 0.34) almost twice that of the fundamental frequency (f(1)D/u(infinity) & AP; 0.17). The observed frequencies are invariant irrespective of the Re-D 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 Re-D cases, respectively.