La prévision de la cavitation en fonction de l'état de nucléation de l'eau et des conditions hydrodynamiques. Cas de l'écoulement autour d'une aile bidimensionnelle

Prediction of cavitation as a function of water nuclei content and hydrodynamic conditions (case of the flow around a two-dimensional hydrofoil)

Institut de mécanique de Grenoble

Abstract

Prediction of cavitation as a function of water nuclei content and hydrodynamic conditions (case of the flow around a two-dimensional hydrofoil) Numerous tests have been carried out recently on the hydrodynamic tunnel of the "Institut de mécanique de Grenoble" to investigate the coupled effects of nuclei and viscosity on cavitation. Several water nuclei contents are tested by means of the injection of air microbubbles in variable quantity upstream of the test section. The spectra of injected cavitation nuclei, characterized by a histogram of critical pressures, is measured by means of a venturi. The influence of the water nuclei content is analysed for different types of boundary layers obtained in varying the angle of attack of a two-dimensional Naca 16209 hydrofoil: laminar boundary layer with laminar separation, transition boundary layer, turbulent boundary layer, laminar separation bubble. For each configuration, we study the conditions of inception of cavitation and the type of cavitation which develops - travelling bubbles or attached cavity - as a function of the water quality.

In addition, we studied the transition from an attached cavity to travelling bubble cavitation which may occur when the nuclei concentration exceeds a critical value. Several rapid films allowed us to analyse in details the influence of nuclei explosion on the boundary layer and on the cavity, and to point out the physical mechanisms responsible for the transition from one type of cavitation to the other one.

All these tests permit now to predict the conditions of inception and development of cavitation on a two-dimensional hydrofoil as a function of the water nuclei content and the hydrodynamic conditions, and particularly of the state of the boundary layer which develops on the foil. The present paper is devoted to the presentation of the criteria on which the prediction is based.