  The paper gives experimental observations on the hypersonic flow past an
axisymmetric flat-face cylinder with a protruding sharp-tip spike at a
freestream Mach number of $M_\infty = 8.16$ at two different freestream
Reynolds numbers based on the base body diameter ($Re_D = 0.76 \times 10^6$,
and $3.05 \times 10^6$). Furthermore, modal analysis is done on schlieren
images to understand the flow dynamics parallel with the unsteady pressure
measurements. The protruding spike of length to base body diameter ratio of
$[l/D]=1$ creates a familiar unsteady flowfield called 'pulsation.' Pressure
loading and fluctuation intensity at two different $Re_D$ cases are calculated.
A maximum drop of 98.24\% is observed in both parameters between the high and
low ReD cases. Based on the analysis, a difference in the pulsation
characteristics are noticed, which arise 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 form the second dominant spatial
mode with a temporal mode carrying a dimensionless frequency ($f_2D/u_\infty
\approx 0.34$) almost twice that of the fundamental frequency ($f_1D/u_\infty
\approx 0.17$). 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 $Re_D$ cases,
respectively.
