Ecoulements tridimensionnels supercavitants : traitement numérique en théorie exacte, expérimentation en tunnel hydrodynamique, analyse critique des résultats

Supercavitating three-dimensional flows. Numerical treatment in exact theory. Experiments in a hydrodynamic tunnel. Critical analysis of the results

¹ Docteur-ingénieur, Hydrodynamique
² Attaché de Recherche au C.N.R.S.

Abstract

Technical progress based on the use of supercavitating wings with wetted upper surface is expected. An essential difficulty for the correct use of this type of wing is the determination of the free field of carvitation. This field requires knowledge of the pressure coefficients in the region of the leading edge of the wing ; this can be obtained from experiments or calculation. With regard to calculation, the fact that the cavity is of unknown shape, while determination of the pressure coefficient at the leading edge requires a non-linear procedure, creates a considerable problem. The calculation method used is that of discretized particularities. If we assume initially that the geometry of the cavity is known, we can solve the problem by distributing a single layer on the wing, a double layer on the wing, the cavity and the wake, and distributing linear pits behind the cavity. Solution of the problem leads to non-uniform pressure in the cavity because of the arbitrary nature of the geometry chosen to start off the calculation. When the cavity pressure is made uniform by an iterative procedure, the length of the cavity can be adjusted in terms of the span of the wing. We then start again with this shape of cavity, and the iterations are continued to detail the shape in the three space dimensions, in order that the slip condition may be satisfied everywhere on it. Experimentally, investigations on wings of limited span require a large installation in order to minimize the influence of the side walls, the effects of the boundary layer, and the effects of the housing, as well as the errors arising from the geometrical definition of the wing itself. The series of tests reported on here were carried out on a wing of fixed profile whose sweep-back was varied by rotation around an axis at the root, and whose aspect ratio was varied by successive cutting back. The tests were carried out in the TH2-TH8 hydrodynamic tunnel of the NEYRTEC Company, whose smallest water-vein permits studies on three-dimensional flows under acceptable conditions. The theoretical and experimental results were subjected to critical analysis of the influence of the principal geometrical parameters and the area of non-cavitation. In particular, when then aspect ratio of a ventilated-base wing is reduced, a study of the slope of the curve, the coefficient of lift in terms of the angle of attack, and of the spread of non-cavitation reveals a significant increase in the spread of non-cavitation. This points to the advantage of using wings of very low aspect ratio in order to increase the possible incidence deflections of the wing. Tith regard to the localization of the points where cavitation appears, for swept-back wings cavitation on the upper surface appears at about two thirds of the span ; on the under surface, cavitation appears first at the wing root. This result, confirmed by experience, shows that there may be an advantage in using a twisted wing, with the a smaller angle for the end sections of the wing than for those at the root.