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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 1 ¼ 8:16 at two different freestream Reynolds numbers based on the base body diameter (Re D ¼ 0:76 Â 10 6 and 3:05 Â 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 ½l=D ¼ 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 "k" shocks formed during the "collapse" phase ahead of the base body. The interaction of shedding vortices from the k-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 1 % 0:34) almost twice that of the fundamental frequency (f 1 D=u 1 % 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 À7/3 law for the low and high Re D cases, respectively.

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