An experimental insight into the effect of confinement on tip vortex cavitation of an elliptical hydrofoil

The present paper is devoted to an analysis of tip vortex cavitation under confined situations. The tip vortex is generated by a three-dimensional foi l of elliptical planform, and the confinement is achieved by flat plates se t perpendicular to the span, at an adjustable distance from the tip. In the range of variation of the boundary-layer thickness investigated, no signif icant interaction was observed between the tip vortex and the boundary laye r which develops on the confinement plate. In particular, the cavitation in ception index for tip vortex cavitation does not depend significantly upon the length of the plate upstream of the foil. On the contrary, tip clearanc e has a strong influence on the non-cavitating structure of the tip vortex and consequently on the inception of cavitation in its core. The tangential velocity profiles measured by a laser-Doppler velocimetry (LDV) technique through the vortex, between the suction and the pressure sides of the foil, are strongly asymmetric near the tip. They become more and more symmetric downstream and the confinement speeds up the symmetrization process. When t he tip clearance is reduced to a few millimetres, the two extrema of the ve locity profiles increase. This increase results in a decrease of the minimu m pressure in the vortex centre and accounts for the smaller resistance to cavitation observed when tip clearance is reduced. For smaller values of ti p clearance, a reduction of tip clearance induces on the contrary a signifi cant reduction in the maxima of the tangential velocity together with a sig nificant increase in the size of the vortex core estimated along the confin ement plate. Hence, the resistance to cavitation is much higher for such sm all values of tip clearance and in practice, no tip vortex cavitation is ob served for tip clearances below 1.5 mm. The cavitation number for the incep tion of tip vortex cavitation does not correlate satisfactorily with the li ft coefficient, contrary to classical results obtained without any confinem ent. Owing to the specificity introduced by the confinement, the usual proc edure developed in an infinite medium to estimate the vortex strength from LDV measurements is not applicable here. Hence, a new quantity homogeneous to a circulation had to be defined on the basis of the maximum tangential v elocity and the core size, which proved to be better correlated to the cavi tation inception data.