Effect of Gap Size on Tip Leakage Cavitation Inception, Associated Noise and Flow Structure

Abstract

This paper focuses on the onset of tip-leakage cavitation on a fixed hydrofoil. The objectives are to investigate the effect of gap size on the flow structure, conditions of cavitation inception, the associated bubble dynamics and cavitation noise. The same hydrofoil with three tip gap sizes of 12%, 28%, and 52% of the maximum tip thickness are studied. Controlled cavitation tests are performed after de-aerating the water in the tunnel and using electrolysis to generate cavitation nuclei. The experiments consist of simultaneously detecting cavitation inception using a 2000 fps digital camera (visual) and two accelerometers (“acoustic”) mounted on the test section windows. Good agreement between these methods is achieved when the visual observations are performed carefully. To obtain the time-dependent noise spectra, portions of the signal containing cavitation noise are analyzed using Hilbert-Huang transforms. Rates of cavitation events as a function of the cavitation index (σ) for the three gap sizes are also measured. The cavitation inception index decreases with increasing gap sizes. The experiments demonstrate that high-amplitude noise spikes are generated when the bubbles are distorted and “shredded”—broken to several bubbles following their growth in the vortex core. Mere changes to bubble size and shape caused significantly lower noise. High-resolution particle image velocimetry (PIV) with a vector spacing of 180 μm is used to measure the flow, especially to capture the slender tip vortices where cavitation inception is observed. The instantaneous realizations are analyzed to obtain probability density functions of the circulation of the leakage vortex. The circulation decreases with increasing gap sizes and minimum pressure coefficients in the cores of these vortices are estimated using a Rankine model. The diameter of the vortex core varied between 540–720 μm. These coefficients show a very good agreement with the measured cavitation inception indices.