The present paper is devoted to an analysis of tip vortex cavitation under confined
situations. The tip vortex is generated by a three-dimensional foil of elliptical planform,
and the confinement is achieved by flat plates set perpendicular to the span, at an
adjustable distance from the tip. In the range of variation of the boundary-layer
thickness investigated, no significant interaction was observed between the tip vortex
and the boundary layer which develops on the confinement plate. In particular, the
cavitation inception index for tip vortex cavitation does not depend significantly
upon the length of the plate upstream of the foil. On the contrary, tip clearance 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 the tip clearance is reduced to a few millimetres, the
two extrema of the velocity profiles increase. This increase results in a decrease of
the minimum pressure in the vortex centre and accounts for the smaller resistance to
cavitation observed when tip clearance is reduced. For smaller values of tip clearance,
a reduction of tip clearance induces on the contrary a significant reduction in the
maxima of the tangential velocity together with a significant increase in the size
of the vortex core estimated along the confinement plate. Hence, the resistance to
cavitation is much higher for such small values of tip clearance and in practice, no tip
vortex cavitation is observed for tip clearances below 1.5 mm. The cavitation number
for the inception of tip vortex cavitation does not correlate satisfactorily with the
lift coefficient, contrary to classical results obtained without any confinement. Owing
to the specificity introduced by the confinement, the usual procedure 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 velocity and the core size, which
proved to be better correlated to the cavitation inception data.
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