  Our study provides the first experimental investigation of the internal flows
of ventilated partial cavitation (VPC) formed by air injection behind a
backward-facing step. The experiments are conducted using flow visualization
and planar particle image velocimetry (PIV) with fog particles for two
different cavity regimes of VPC, i.e., open cavity (OC) and two-branch cavity
(TBC), under various range of free stream velocity (U) and ventilation rates
(Q). Our experiments reveal similar flow patterns for both OC and TBC,
including forward flow region near the air-water interface, reverse flow
region, near-cavitator vortex, and internal flow circulation vortex. However,
OC internal flow exhibits highly unsteady internal flow features, while TBC
internal flow shows laminar-like flow patterns with a Kelvin-Helmholtz
instability developed at the interface between forward and reverse flow regions
within the cavity. Internal flow patterns and the unsteadiness of OC resemble
those of turbulent flow separation past a backward-facing step (BFS flow),
suggesting a strong coupling of internal flow and turbulent external
recirculation region for OC. Likewise, internal flow patterns of TBC resemble
those of laminar BFS flow, with the presence of unsteadiness due to the strong
velocity gradient across the forward-reverse flow interface. The variation of
the internal flow upon changing U or Q is further employed to explain the
cavity regime transition and the corresponding change of cavity geometry. Our
study suggests that ventilation control can potentially stabilize the cavity in
the TBC regime by delaying its internal flow regime transition from
laminar-like to highly unsteady.
