Numerical investigation of positive effects of ventilated cavitation around a NACA66 hydrofoil

Abstract

Cavitation is of great practical interest because unsteady flow features can induce negative effects such as mechanical erosion and noise. Air injection is an important way to adjust the instability of the cavitation flow field. In the present study, we numerically simulate a NACA66 hydrofoil with a particular emphasis on understanding the effects of ventilation on cavitation characteristics such as cavity dynamics, vortex structure, and the wake field. Cavitation is modeled by using the Schnerr-Sauer cavitation model, and the Large Eddy Simulation (LES) method is used to calculate the unsteady natural and ventilated cavitation flow. The results of the numerical simulation are in good agreement with experiment, which verifies the effectiveness of the numerical method. The evolution of cavity dynamics and vortex structure are analyzed to clarify how multi-scale vortex structures evolve in the flow field. The results show that the shedding speed of the ventilated cloud cavity is faster than the natural cavitation. In addition, ventilation improves the pressure pulsation on the cavitation hydrofoil surface. The rotational effect in ventilated cavitation is more significant. Furthermore, ventilation causes strong, large-scale, pulsating eddy currents to rotate into small-scale eddies. An analysis of the vorticity transfer equation shows that the gas injection increases the velocity gradient and enhances the conversion of the two gas-liquid phases. The results in the wake of the hydrofoil show that ventilation can effectively reduce the turbulence intensity and turbulence integral scale, which indicates that ventilation can improve the velocity pulsation and instability. Interestingly, increasing the ventilation rate helps to improve the pressure peak on the hydrofoil surface and increases the lift and drag coefficient. Moreover, gas injection is responsible for cavity pulsation and the re-entrant jet enhances the fluctuation intensity.