EXPERIMENTAL STUDY OF SCALE EFFECTS ON THE SCALING LAW FOR TIP VORTEX CAVITATION NOISE

Novel scaling law for the tip vortex cavitation (TVC) noise is derived from the physical basis of TVC, employing the Rankine vortex model, the Rayleigh-Plesset equation, the lifting surface theory, and the number of bubbles generated per unit time (N-o). All terms appearing in the scaling law have physical or mathematical grounds except for N-o. Therefore, to experimentally validate the N-o term, experiments are designed to keep the same TVC patterns as velocities and dimensions vary. Optimal shooting conditions with a velocity and size variation are determined from the scaling exponents, cavitation numbers and Reynolds numbers at each condition. To avoid wall effects and flow field interaction, two hydrofoils are optimally arranged by using computational fluid dynamics (CFD) for size variation. Images taken by a high speed camera are used to count N-o, considering similitude of the spectra of nuclei. Scaling exponents curve fitted from five velocities and cavitation inception numbers have an exponent value of 0.371, which is closely placed on scaling exponents curve deduced from Schlichting's friction coefficients fitting with Reynolds number. The tendency that N-o is proportional to a velocity and inversely proportional to a size can be confirmed by this study.