contribution a l'étude de l'érosion de cavitation - Infoscience - EPFL

Abstract
In the eld 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 anattached 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 experimental
research has been performed in hydrodynamics, mechanical science and metallurgy.
We intend in the present work to describe the hydrodynamic attack andprovide 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 suchcavity 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 rst natural frequency of the hydrofoil. In this case, the main
cavity pulsation as well as the shedding process are modulated by the blade vibration frequency.
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 transient
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 to identify the shedding
frequency. This result is validated in a centrifugal pump model. In this case, vibration
signal is found to be modulated by the blade passing frequency. Furthermore, we have
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