  Shock related unsteadiness over axisymmetric spiked body configurations is
experimentally investigated at a freestream supersonic Mach number of 2.0 at
0$^\circ$ angle of attack. Three different forebody configurations mounted with
a sharp spike-tip ranging from blunt to streamlined (flat-face, hemispherical,
and elliptical) are considered. Steady and unsteady pressure measurements,
short-exposure high-speed shadowgraphy, shock footprint analysis from $x-t$
plots, and identification of dominant spatiotemporal modes through modal
analysis are carried out to explain the unsteady flow physics. The present
investigation tools are validated against the well-known events of `pulsation'
associated with the flat-face case. The hemispherical case is characterized by
the formation of a separated free shear layer and associated localized shock
oscillations. The cycle of charging and ejection of fluid mass from the
recirculation zone, confined between the separated shear layer and the spiked
body, is identified to drive the flow unsteadiness. Such an event triggers the
out-of-phase motion between the separated and reattachment shocks. In the
elliptical case, the overall flow field resembles that of the hemispherical
case, except with dampened unsteadiness. The value of the cone angle
($\lambda$) associated with the recirculation region is found to be responsible
for the fluctuations from the charging and ejection of fluid mass. Thereby it
controls the extent of out-of-shock phase motion. In the elliptical case,
$\lambda$ is observed to be smaller and exhibits a reduction in shock
unsteadiness. \hlt{Based on the gathered results and understanding, the
reduction in unsteadiness associated with the aerodisk mounted on the
hemispherical forebody is explained via the almost complete elimination of the
out-of-phase shock motion.
