Les écoulements autour des ailes ventilées

Ventilated hydrofoil flow

¹ Professeur
² Chargée de Recherche au C.N.R.S.
³ Ingénieur au C.N.R.S. Institut de Mécanique de Grenoble

Abstract

Increasing requirements for high-speed navigation have been focussing interest on such designs as hydrofoils for which cavitation or ventilation are acceptable in that they do not result in an excessive reduction in aspect ratio. Results of present experimental and theoretical research on base-ventilated hydrofoils at the Grenoble Institute of Fluid Mechanics are presented. Investigation of hydrofoil behaviour in two-dimensional flow is considered to be a good starting-point, as it lends itself to more convenient experimental investigation and more rewarding analytical model simulation, e.g. conformal representation, linearisation or connected asymptotic expansion. Comparative reference is made to the frequently significant effect of prototype hydrofoil behaviour which, in the case of thin hydrofoils causing minor deviation of the initially undisturbed flow, is invariably marked by a lack of symmetry (effect of surface and bed boundaries, gravity or limited span). Main flow-governing parameters include hydrofoil and flow channel geometry, physical fluid properties, gravity, velocity at infinity and boundary pressures. Thus, experimental and theoretical investigations come under two categories, one relating to overall cavity geometry, forces and general properties (for which the non-dimensional difference between the reference pressure at infinity and cavity pressure is most significant), and the other in which cavity pressure is investigated as a parameter responding to initial data (mainly ventilation flow). A satisfactory explanation has been found for the first case, but the second will require further research to elucidate the air entrainment processes for which the geometrical and dynamic "micro-structure" of the flow appears to be significant. Recent experimental and theoretical lift research has shown the effect of a free surface on incidence to determine a distinct minimum on the lift vs. relative depression characteristic (Fig. 2). In addition, the theoretical data show calculated lift to depend very closely on cavity geometry, and therefore, the need for a more "refined" analytical model. Marked model simulation progress has been made by allowing for existing momentum deficiency after the cavity ("negative overspeed" in the wake). With a model improved to allow for this factor, cavity geometry and variation with relative depression can be determined more exactly, and therefore, more realistic simulation of specific hydrodynamic properties achieved. An analytical study of experimentally-confirmed gravity effect (fig. 6 and 7) on a first-order perturbation model (now in progress), is expected to provide an estimated order of magnitude for this factor. An experimental study of three-dimensional flow past hydrofoils of finite span has provided extensive data for use in establishing appropriate theoretical schemes. Though the existence of variegated forms of cavity (median and marginal cavities associated with circulation around the hydrofoil span) singularly complicates flow description, some general data have emerged, e.g. reduction of the lift/incidence ratio (dC_z/dx), correlative non-cavitating leading-edge ranges tending to improve hydrofoil stability.