Investigation into the behaviors of ventilated supercavities in unsteady flow

Abstract

A systematic investigation of ventilated supercavitation behaviors in an unsteady flow is conducted using a high-speed water tunnel at the Saint Anthony Falls Laboratory. The cavity is generated with a forward facing model under varying ventilation rates and cavitator sizes. The unsteady flow is produced by a gust generator consisting of two hydrofoils flapping in unison with a varying angle of attack (AoA) and frequency (fg). The current experiment reveals five distinct cavity states, namely, the stable state, wavy state, pulsating state I, pulsating state II, and collapsing state, based on the variation of cavity geometry and pressure signatures inside the cavity. The distribution of cavity states over a broad range of unsteady conditions is summarized in a cavity state map. It shows that the transition of the supercavity from the stable state to pulsating and collapsing states is primarily induced by increasing AoA while the transition to the wavy state triggers largely by increasing fg. Remarkably, the state map over the non-dimensionalized half wavelength and wave amplitude of the perturbation indicates that the supercavity loses its stability and transitions to pulsating or collapsing states when the level of its distortion induced by the flow unsteadiness exceeds the cavity dimension under a steady condition. The state maps under different ventilation rates and cavitator sizes yield similar distribution but show that the occurrence of the cavity collapse can be suppressed with increasing ventilation coefficient or cavitator size. Such knowledge can be integrated into designing control strategies for the supercavitating devices operating under different unsteady conditions.

Numerical Study of the Behaviors of Ventilated Supercavities in a Periodic Gust Flow

“…Correspondingly, although the cavity shape variation diminishes with reducing g , the cavity exhibits a more prominent closure variation associated with larger cavitation number fluctuation. Such trend of cavity closure variation under different g has also been reported in experiment (Shao et al 2018) and can be explained by the theoretical analysis in Karn et al (2016) and Karn et al (2015) which connect dimensionless differential pressure ∆ ' introduced in Section 3.2 to the cavitation number. Fig.…”

“…The numerical method is first validated through the experiments under specific steady and unsteady flow conditions. It has been shown that the simulation can capture the degree of cavity Finally, it is noteworthy that the present simulation is unable to capture the cavity length change due to shed-off of gas pocket near its closure region (i.e., pulsation of the cavity) which occurs under the cases of AOA8f1 and AOA8f5 in the experiments (Shao et al 2018). As discussed in (Shao et al 2018), the transition from the wavelike undulation to the cavity pulsation is a result of the bubble breakup near cavity closure induced by the instability of the gas-liquid interface.…”

“…The ventilated supercavitation from a forward facing cavitator under periodic gust flow is numerically studied by using the Eulerian multiphase model with a free surface model. The flow unsteadiness is simulated by adding a harmonic varying vertical velocity term matching that produced from a pair of flapping hydrofoils in the experiments (Lee et al 2013, Shao et al 2018.…”

“…Although synchronized measurements of cavity shape and pressure are conducted in the past (Lee et al 2013, Karn et al 2015, and Shao et al 2018, the pressure inside the ventilated supercavity is measured at a fixed location and the information regarding to the internal velocity and pressure field is still quite scarce in periodic gust flows due to the limitation of diagnostic approaches.…”

Numerical Analysis of the Effects of Periodic Gust Flow on the Wake Structure of Ventilated Supercavities

“…They noted that a gust flow reduced the bubble condensation efficiency and an increase in the amplitude of the gust resulted in a slight monotonic increase in C q for the generation and collapse of a supercavity. Shao et al (2018) found that a supercavity was prone to becoming unstable and collapsing when its distortion exceeded its size as a result of the increase in f; further, increasing C q and the cavitator scale could inhibit the collapse of a supercavity. Due to the limits of experimental monitoring, it is relatively difficult to study the velocity distribution pattern of the entire gust flow field.…”