Effect of an end-clearance width on the gap cavitation structure: Experiments on a wall-bounded axis-equipped hydrofoil

Abstract

The paper deals with an experimental study of tip-clearance cavitation inception and development and its vortical structure coupled with dynamics of the main attached cavity on the suction side of a two-dimensional symmetric hydrofoil equipped with a rotation axis. The gap is formed by the end face of the model and a transparent sidewall of the test channel. The experiments were performed for attack angles of 3° and 9° and 0.4-, 0.8-, 1.75- and 3.75-mm gaps (or 1.8%–17% relative thickness) under various flow conditions on the cavitation number. In order to observe the tip-clearance cavitation occurrence, high-speed imaging was applied. The leakage flow velocity was measured inside the clearance by a modified Particle Tracking Velocimetry technique. It is shown that the mean velocity field of the leaking flow is split into eight distinctive zones where the flow direction and velocity magnitude substantially differ. Positions and extents of these zones are practically independent of the primary flow regime but are affected by the attack angle. Local velocity values of the leakage flow are unexpectedly found to be about 20% higher in the region of a gap cavity than the mean bulk velocity of the incoming flow. Cavitating cores of various vortices manifest themselves in the recorded images, showing that the vortex structure of the leakage flow associated with the tip-clearance cavitation is very complicated and the hydrofoil axis makes it even more complex by inducing new vortices and cavities. For the gaps considered, an increase of its size causes the tip-clearance cavitation to be initiated at higher cavitation numbers, i.e., this is favorable for its occurrence, while the development of the main cavity is hindered. In unsteady flow regimes, dynamics of the primary cavity on the hydrofoil suction side significantly influences the leakage flow direction and the tip-clearance cavitation evolution. Periods of oscillation cycles of the main and gap cavities coincide but the maximum size of the gap cavity is reached with a phase lag relative to the main one. At the small incidence angle, thicker gaps and unsteady flow conditions, extremely transient pressure waves are registered in the clearance, with their velocities ranging from 41 to 81 m/s.