Flow pattern regime of ventilated cavity around an axisymmetric body under a varying incoming flow velocity: an experimental study

The time-varying velocity is one of the interfaces when a physical underwater vehicle is moving, but there are relatively few studies expounding the effect of varying incoming flow velocity on ventilated cavity flow physically yet now. In this case, it is experimentally investigated with visible photographs. Four flow patterns are observed under varying velocities, and the increasing velocity makes flow patterns transit from pulsating cavity (PC) to twin vortex tubes (TV), re-entrant jet (RJ), and intermittent and transitional cavity (ITC), successively. Increasing velocity produces an increase in adverse pressure gradient (APG) and gas leakage at the cavity rear, and the cavity size is increased to overcome the increase of APG in TV. Until the cavity reaches its maximum under the fixed ventilation, the further increases of velocity and APG transform flow pattern into RJ and the inclined angle of the bottom cavity surface. Then, ITC is obtained behind RJ, and the cavity is consisting of an intermitted transparent cavity and a recirculating region at the rear which causes the periodic foamy shedding. A quantitative gas leakage model with the constant parameter K (∼0.575 and ∼0.552) and St (0.21) based on the gas volume balance is established with the size and frequency of the shedding, to give an explanation of the formation and disintegration of ITC. On the other hand, the decreasing velocity leads to reverse developments of flow patterns from ITC to PC, and the APG at the cavity rear is weakened gradually with the enhanced gas leakage. A phenomenon of ventilation hysteresis is obtained that the critical velocity for forming ITC is significantly greater than that for destroying ITC, which is related to the gas leakage model of ITC.