Contribution à l'étude de l'érosion de cavitation : mécanismes hydrodynamiques et prédiction

In the field of hydraulic power plant, the leading edge cavitation is often responsible of sever erosion which may cause a premature shutdown of energy production with costly consequences. This type of cavitation is characterized by an attached vapour cavity at the leading edge of the blades. Transient vapour vortices are generated and convected by the mean ow to the pressure recovery region where they collapse violently. The resulting water hammer pressure is responsible of material damage. In order to investigate the cavitation erosion problem, many theoretical and experi- mental research has been performed in hydrodynamics, mechanical science and metallurgy. We intend in the present work to describe the hydrodynamic attack and provide new mathematical model to characterize and predict the pressure impulses induced on solid surface by repeated collapses of transient cavities. First, the dynamics of a single vortex collapse is performed in the IMHEF Cavitation Vortex Generator. High speed visualization shows systematic rebound of such cavity after the collapse. This explosive rebound is due to the dissolved gas and leads to the generation of a strong shock waves which propagates in the liquid and the solid as well. Assuming Tait'equation for water, an estimation of the shock overpressure has been performed by image processing. Overpressure as high as 2 GPa has been thus measured. Furthermore, the maximum potential energy of the cavity and the uid corresponding to the maximum volume of the cavity stands as a good basis to characterize the collapse overpressure. Investigation of the shedding process by an attached cavity is carried out in the IMHEF High Speed Cavitation Tunnel on a 2D NACA009 blade. The hydrofoil is equipped with 30 piezo resistive pressure transducers. Beside the pressure acquisition, cavitation induced vibrations as well as the main cavity dimensions are synchronously acquired. Pressure spectra and cavitation patterns analysis leads us to consider the free and the forced regime of the main cavity. The forced regime occured when the von Karman vortices frequency matches the first natural frequency of the hydrofoil. In this case, the main cavity pulsation as well as the shedding process are modulated by the blade vibration fre- quency. In the free regime, the main cavity may be stable or unstable. Stable cavitation is characterized by small amplitude of the main cavity pulsation. In this case, the tran- sient cavities have a small size compared to the cavity length and the shedding process is highly instationnary. The unstable cavitation is characterized by large amplitude of main cavity pulsation. The shedding process is modulated by the main cavity pulsation witch is governed by a Strouhal like law. The Strouhal number depends on the incidence angle and stands between 0.2 and 0.32. The cavitation induced vibrations are found to be highly modulated by the main cavity pulsations. Envelope calculation of acceleration signals allows