The effect of upstream turbulence on a tip-vortex

Abstract

A tip-vortex in the wake of a finite length lift generating surface has a low pressure region near the axis of the tip-vortex. This low pressure region can trigger tip-vortex cavitation especially in ship propellers which has adverse effects by causing vibration and underwater noise production. One way of minimising the tip-vortex cavitation is by reducing the propeller blade loading but at a cost of lower operating efficiency of the propeller. An alternative passive approach, to delay the onset of cavitation, by increasing the minimum pressure in the core of the tip-vortex through modification of the tangential velocity profile of the tip-vortex is investigated in this thesis. In this thesis, the effect of upstream turbulence on the peak tangential velocity of the tip-vortex from a NACA 662-415 hydrofoil is investigated through stereo-Particle Image Velocimetry measurements. The turbulence is produced by a passive grid placed in the upstream of the hydrofoil. The tangential velocity profile at three different turbulent intensities were compared with measurements from a baseline no grid case that had nearly uniform and steady inflow to the hydrofoil. The tangential velocity profile of the vortex exhibits a reduction in the average peak tangential velocity under the influence of the grid generated turbulence compared to the no grid case. The measurements taken at two downstream locations, 0.5 and 1.3 chord length downstream from tip of the hydrofoil, showed that a significant reduction in the magnitude of the average peak tangential velocity of the tip-vortex is observed only at the latter position. Also, on examining the axial velocity profiles, a significant reduction in the magnitude of the average peak axial velocity in the core of the tip-vortex was observed under the influence of grid generated turbulence. An apparent effect of the upstream turbulence on the tangential velocity of the tipvortex is seen to appear only when sufficient time is given for a secondary vortex structure, from the upstream turbulence, to form around and be intensified by the primary tip-vortex. At 1.3 chord length downstream from the tip of the hydrofoil, the reduction in the magnitude of the peak tangential velocity is accompanied by increase core radius of the tip-vortex.