  Flow topology and unsteady cavitation dynamics in the wake of a backward
facing step was investigated using high speed videography and time-resolved
X-Ray densitometry, along with static and dynamic pressure measurements. The
measurements are used to inform the understanding of underlying mechanisms of
observed flow dynamics as they are related to shock wave induced instabilities.
The differences in cavity topology and behaviour at different cavitation
numbers are examined to highlight flow features such as the pair of cavitating
spanwise vortices in the shear layer and propagating bubbly shock front. The
shock speeds at different cavitation conditions are estimated based on
void-fraction measurements using the X-T diagram and compared to those computed
using the one-dimensional Rankine Hugoniot jump condition. The two
speeds(computed and measured) are found to be comparable. The effect of
compressibility of the bubbly mixture is determined by estimating the local
speed of sound using homogeneous `frozen model' and homogeneous `equilibrium
model'. The current estimation of sound speed suggests that expression used to
determine the local speed of sound is closer to the classic `frozen model' than
the `equilibrium model' of speed of sound.
