  Ventilated cavities in the wake of a two-dimensional bluff body are studied
experimentally via time-resolved X-ray densitometry. With a systematic
variation of flow velocity and gas injection rate, expressed as Froude number
($Fr$) and ventilation coefficient ($C_{qs}$), four stable cavities with
different closures are identified. A regime map governed by $Fr$ and $C_{qs}$
is constructed to estimate flow conditions associated with each cavity closure
type. Each closure exhibits a different gas ejection mechanism, which in turn
dictates the cavity geometry and the pressure inside the cavity.
Three-dimensional cavity closure is seen to exist for the supercavities at low
$Fr$. However, closure is nominally two-dimensional for supercavities at higher
$Fr$. At low $C_{qs}$, cavity closure is seen to be wake-dominated, while
supercavities are seen to have wave-type closure at higher $C_{qs}$,
irrespective of $Fr$. With the measured gas fraction, a simple gas balance
analysis is performed to quantify the gas ejection rate at the transitional
cavity closure during its formation. For a range of $Fr$, the transitional
cavity closure is seen to be characterised by liquid re-entrant flow, whose
intensity depends on the flow inertia, dictating the gas ejection rates. Two
different ventilation strategies were employed to systematically investigate
the formation and maintenance gas fluxes. The interaction of wake and gas
injection is suspected to dominate the cavity formation process and not the
maintenance, resulting in ventilation hysteresis. Consequently, the ventilation
gas flux required to maintain the supercavity is significantly less than the
gas flux required to form the supercavity.
