The unsteady structure of cavitating flows is investigated by coupled experimental and numerical means. Experiments focus on the structure and dynamics of sheet cavitation on the upper side of a two-dimensional foil section in the ENSTA cavitation tunnel. Various flow conditions are investigated by varying the pressure, the flow velocity, and the incidence of the foil section. High-frequency local measurements of volume fractions of the vapour phase are performed inside the liquid/vapour mixture by a X-ray absorption method. The numerical approach is based on a macroscopic formulation of the balance equations for a two-phase flow. The assumptions required by this formulation are detailed and they are shown to be common to almost all the models used to simulate cavitating flows. In the present case we apply a single-fluid model associated with a barotropic state law that governs the mixture density evolution. Numerical simulations are performed at the experimental conditions and the results are compared to the experimental data. A reliable agreement is obtained for the internal structure of the cavity for incidence varying between 3° and 6°. Special attention is paid to the mechanisms of partial and transitional instabilities, and to the effects of the interaction between the two sides of the foil section.

The internal structure and the dynamics of two-dimensional (2D) sheet cavitation on the suction side of a 2D foil section were investigated experimentally. Experiments were conducted in a cavitation tunnel and situations ranging from steady sheet cavitation to unsteady cloud cavitation were obtained by varying the foil incidence and the cavitation number. Using a novel endoscopic technique, coupled with x-ray attenuation measurements, the two-phase morphology and the void fraction within the sheet cavitation were investigated. Supplemental information on the instantaneous shape of the sheet cavity and its instability frequency were also obtained by visualization and pressure measurements, respectively. The investigations focused on (a) the void fraction distribution and (b) the frequency of the cavity oscillations. It was found that the void ratio can reach as much as 50% depending on the conditions of operation, and the Strouhal numbers are around 0.25 in the case of partial cavity instability and 0.12 in the case of transitional sheet cavitation. Finally, the visualization of the vapor structures within the sheet cavity for various stations along the chord gives a qualitative understanding of the process of vapor production and condensation.