Unsteady cavitation in a Venturi-type section was simulated by two-dimensional computations of viscous, compressible, and turbulent cavitating flows. The numerical model used an implicit finite volume scheme (based on the SIMPLE algorithm) to solve Reynolds-averaged Navier-Stokes equations, associated with a barotropic vapor/liquid state law that strongly links the density variations to the pressure evolution. To simulate turbulence effects on cavitating flows, four different models were implemented (standard k-ε RNG; modified k-ε RNG; k-ω with and without compressibility effects), and numerical results obtained were compared to experimental ones. The standard models k-ε RNG and k-ω without compressibility effects lead to a poor description of the self-oscillation behavior of the cavitating flow. To improve numerical simulations by taking into account the influence of the compressibility of the two-phase medium on turbulence, two other models were implemented in the numerical code: a modified k-ε model and the k-ω model including compressibility effects. Results obtained concerning void ratio, velocity fields, and cavitation unsteady behavior were found in good agreement with experimental ones. The role of the compressibility effects on turbulent two-phase flow modeling was analyzed, and it seemed to be of primary importance in numerical simulations.

The present work was carried out in the scope of a numerical-experimental collaborative research program, whose main objective is to understand the mechanisms of instabilities in partial cavitating flow. Experiments and numerical simulations were conducted in the configuration of a 2D foil section located in a cavitation tunnel with various angles of attack. Several physical features have been pointed out by this joined approach. The role played by the re-entrant jet in the cloud shedding phenomenon was investigated at several incidences, and it was found that it is mainly responsible for the cavity break off. Moreover, a special flow pattern was evidenced for a 6° angle of attack: in that case a growth/destabilization cycle of the cavity is observed at a low frequency (~ 3.5 Hz), together with the periodic shedding of large bubble clusters (cloud cavitation).

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.

International audienceThe goal of the work is to investigate the possibility of cavitation erosion prediction using computational fluid dynamics (CFD) tools only. For that purpose, a numerical process based on a coupling between CFD and an erosion model is presented and tested in several configurations of cavitating flow on a two-dimensional hydrofoil. The CFD code, which is based on the homogeneous approach, was previously validated on numerous experiments. In the present work, the predictions of velocity and pressure evolutions in the vicinity of the hydrofoil are compared with experimentally measured data. A close agreement is systematically obtained. The erosion model is based on the physical description of phenomena from cavitation cloud implosion, pressure wave emission and its attenuation, micro-jet formation and finally to the pit formation. The coupling between CFD and the erosion model is based on the use of local pressure, void fraction and velocity values to determine the magnitude of damage at a certain point. The results are compared with the experimentally measured damage on the hydrofoil. In the experiments a thin copper foil applied to the surface of the hydrofoil was used as an erosion sensor. A pit-count method was applied to evaluate the damage. The comparison shows that it is possible to use solely CFD tools to predict time evolution of cavitation erosion, including final extent and magnitude, with a very good accuracy