Examination of ventilated cavities in the wake of a two-dimensional bluff body using X-ray densitometry

Abstract

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 ( Cqs ), four cavities with different closure types are identified. A regime map governed by Fr and Cqs 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 in 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 Cqs, cavity closure is seen to be wake-dominated, while supercavities are seen to have interfacial perturbation near the closure at higher Cqs, irrespective of Fr. With the measured gas fraction, a 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 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.