Numerical investigation into effects on cavitation flow of inflow gust generated by pitching hydrofoils

As pressure decreases under vapor pressure, voids called cavitation form in fluid. If the pressure rises again, the voids collapse with generating large pressure. This pressure causes compressible acoustic waves and corrodes underwater devices such as pump, nozzle, propeller, etc. On the other hands, underwater vehicles and weapons utilize the cavitation phenomena to get high speed by reducing drag. Because the dynamic viscosity of cavities is lower than that of fluid, the cavitation can reduce skin-friction drag. Its effects are significant when it covers the entire surface of vehicles and weapons. Therefore, it is critical to the stable drag reduction to keep the cavitation flow covering the entire body. However, the cavitation flow pattern changes easily by external excitation such as inflow gust. In this paper, the effects of inflow gust on the cavitation flow is numerically investigated. Inflow gust is generated by using flaps consisting of the NACA0012 hydrofoil. The chord length of hydrofoil is set to be c=0.15 m. The maximum amplitude and frequency of the oscillation are 10° and 10 Hz, respectively. The rotation axis is located at the midpoint of chord line. The cavitation is generated by using a disk-type cavitator. These numerical conditions are set to be the same as the experimental conditions for the validation. The compressible Navier-Stokes equations is used as the governing equations. To simulate multi-phase flow, the homogeneous mixture model is used. The preconditioned dual-time steeping method is used for fast accurate unsteady simulations. The moving chimera grid method is used to realize the pitching motion of hydrofoils effectively. The validity of numerical results is confirmed through the comparison with the measured data in terms of time-varying pressure distribution and cavitation pattern. The numerical methods are expected to provide effective design tool for controlling cavitation flow under various unsteady inflow conditions.