  Flow oscillations in the near-wake region of a 2D circular cylinder are
experimentally investigated at Mach 6 over the Reynolds number range
$2.3\times10^5$ to $5\times10^5$. The oscillation frequency is obtained by
spectral proper orthogonal decomposition of high-speed schlieren data. The
Strouhal number based on the length of the near-wake shear layers is found to
exhibit universal behavior. This corroborates experimental findings at Mach 4
from recent literature, and further, the universal behavior is also seen to
hold with respect to Mach number. Time-resolved pressure measurements at the
flow separation points on the cylinder aft surface show that coherent
oscillatory activity occurs with a phase difference of $\pi$ radians between
the two statistically-symmetric halves of the flow. This aspect of the flow
dynamics at high speeds is in common with its low-speed counterpart, i.e. the
canonical problem of cylinder wake in an incompressible flow.
