Unsteady RANSE Simulations of Surface Piercing Hydrofoils in Different Ventilated Flow Regimes

Ventilation if a fluid dynamic on high speed surface piercing hydrofoils is a complex fluid physics that exhibits violent instabilities and bifurcation or hysteresis. It involves interaction between boundary layer separation,turbulence, free surface flow with waves and sprays and vortex shedding. In this respect, it represents a truly challenging hydrodynamic problem to simulate for any multi-phase CFD solvers. Here we test the capabilities of OpenFOAM v-18.12 multiphase unsteady RANSE solver, to study the steady and unsteady characteristics of super-cavitating ventilated hydrofoils, operating at low/moderate Froude numbers. In particular, we employ interFoam solver for the solution of the pressure and the velocity equations. The turbulent flow characteristics have been described using a k-ω SST model, while the solution of the free surface, representing one of the most relevant challenges of this fluid dynamic problem has been achieved using a surface capturing approach where the Volume of Fluid scalar has been numerically solved using the MULES algorithm. The finite volume numerical problem has been described using an unstructured grid having higher resolution across the predicted free surface as well as close to the hydrofoil surface in order to obtain higher accuracy in the description of the ventilation effect as well as better resolution in the description of the boundary layer flow.

When the surface-piercing hydrofoil reaches yaw angles higher than the critical value, the separated wake induces a pressure drop triggering a progressive intrusion of air until a portion of or the whole suction side of the hydrofoil is enveloped into a pocket of air, as captured in the example of figure 1. Ventilation can be caused by different phenomena depending on the hydrofoil shape, the angle of attack, as well discussed in [Young et al., 2017]. Different flow regimes can be classified according to the extension of the ventilated cavity:fully wetted, partially ventilated and fully ventilated flow regimes. When the fully-ventilated (super-cavitating) regime occurs, a sharp drop of lift forces is experienced which may be dangerous for the operation of marine crafts dynamically supported or controlled by hydrofoils. The transition between these regime can be abrupt or gradual and the same unsteady transition applies to the pressure distribution and hydrodynamic forces acting on the hydrofoil.Obviously, being able to accurately predict the bifurcation points between the various regimes is essential for the design of marine vehicles equipped with surface piercing hydrofoils.

We present the main results of a study aimed to the development of a computational framework, based on OpenFOAM libraries, capable of reproducing the steady and unsteady physics and accurately predicting the hydrodynamic characteristics of surface piercing hydrofoils operating in different ventilated flow regimes,corresponding to different points of the operational domain in terms of angles of attack and Froude number.Simulation results are compared and validated against experimental observations performed on towing tank test models of surface piercing struts by [Harwood et al., 2016].