Experimental Investigation of Ventilated Supercavitation Under Unsteady Conditions
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Ventilated supercavitation is a promising technique to achieve high-speed transport underwater by generating a gas bubble enclosing a moving object through ventilation. However, implementing this technique requires clear understanding and precise control of supercavity behaviors under unsteady conditions in practical applications. In this study, we present the systematic investigation of ventilated supercavity behaviors over a broad range of unsteady conditions. The experiments are conducted in the high-speed water tunnel at Saint Anthony Falls Laboratory. The unsteady conditions are generated using a gust generator consisting of two flapping hydrofoils mounted upstream of a forward-facing cavitator. We use high-speed imaging to capture the variation of cavity dimension and flow patterns along with simultaneous pressure measurements. Measurements are conducted under fixed tunnel speed and ventilation rate with the flapping hydrofoils operating under varying angle of attack (AoA) and frequency (fg). The visualization and pressure signals reveal five distinct states of supercavity under unsteady conditions, referred to as stable state, wavy state, pulsating state I, pulsating state II and collapsing state. The stable state occurs under low AoA and fg, the supercavity only exhibits small amplitude of oscillation without appreciable deformation of the cavity interface. When the supercavity is at wavy state (moderate AoA and high fg), the supercavity displays clear periodic wavelike deformation of its surface with cavity pressure varies periodically at two times of fg. Under high AoA with low fg, pulsating state I of the cavity shows significant fluctuations in length with intermittent shed-off of gas pockets at the rear part of the cavity. With increase of fg, the cavity exhibits enhanced pulsating behavior and a sharp of difference between cavity pressure and test section pressure (i.e. pulsating state II). At the highest AoA (i.e. AoA 10°) and fg above 2 Hz, the cavity collapses. The transition across different supercavity states under a broad range of unsteady conditions is summarized in the supercavity state map, showing the dependence of supercavity states on the characteristic scales of unsteadiness in comparison to longitudinal and lateral dimensions of the supercavity.