Ventilated supercavitating hydrofoils for ride control of high-speed craft

An investigation into the hydrodynamic performance and flow field characteristics of a novel high-speed supercavitating hydrofoil concept proposed by Elms (1999) is presented. The hydrofoil is wedge-shaped with a supercavity detaching from geometric discontinuities at its trailing edges. Lift is generated by the asymmetry of the cavity/flow field created by trailing edge forwardand backward-facing steps. In this way bi-directional lift can be created from a symmetric hydrofoil. To ensure establishment and maintenance of a stable supercavity air is introduced by external ventilation via the hydrofoil base. The formation of the trailing edge steps would be practically realised by the deflection of a trailing flap. At zero incidence and flap deflection there would be no supercavity formed and no lift produced. The cavity formation from a hydrofoil by this mechanism is analogous to the separated flow over an 'interceptor' device fitted to the transom of a high-speed hull for trim and/or steerage control. Due to this similarity the concept has been termed an 'intercepted hydrofoil'. This hydrofoil configuration is analysed using a potential based 2-D nonlinear boundary element method. For a given cavity length, the resulting cavity surface velocity and shape are determined in an iterative manner under prescribed constant pressure and flow tangency boundary conditions. Both infinite and confined flow domain cases of the boundary element analysis are presented. The latter case is of interest in providing blockage correction information for a future companion physical experimental program. An optimum base-ventilated supercavitating hydrofoil profile is a compromise between limiting of the pressure minimum at the leading edge and maintaining stable cavity detachment from the trailing edges. These are both necessary so as to maintain the hydrofoil surfaces in a wetted condition, thereby ensuring that the generated forces remain steady and predictable. The greatest efficiency is obtained by using the smallest thickness to chord ratio with a sufficient margin against cavity breakdown allowing for variance in operating conditions. Hydrodynamic performance of the 'interceptor' in isolation from the foil, i.e. cavitating flow over a wall-mounted fence, is also presented. Classical analytical, boundary element and Reynolds-Averaged Navier-Stokes equation based computational fluid dynamics methods were used for this analysis. The 'ideal' optimum hydrodynamic performance obtained from potential flow analysis is compared with the viscous flow numerical results.