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 proposed approach is a statistical reconstruction approach based on a nonlinear forward model counting the full beam polychromaticity and applied directly to the projections without taking negative-log. Compared to the approaches based on linear forward models and the BHA correction approaches, it has advantages in noise robustness and reconstruction accuracy.

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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.

The purpose of this experimental study was to analyze a two-dimensional cavitating shear layer. The global aim of this work was to obtain a better understanding and modeling of cavitation phenomenon in a 2D turbulent sheared flow which can be considered as quite representative of cavitating rocket engine turbopomp inducers. This 2D mixing layer flow provided us a well documented test case which can be used for the characterization of the cavitation effects in sheared flows. The development of a velocity gradient was observed inside a liquid water flow: Kelvin-Helmholtz instabilities developed at the interface. Vaporizations and implosions of cavitating structures inside the vortices were observed. X-ray attenuation measurements were performed to estimate the amount of vapor present inside the mixing area. Instantaneous two-dimensional void ratio fields were acquired. The real spatial resolutions are 0.5 mm with 2000 fps and 1.5 mm with 20 000 fps. The effective time resolution is equal to the camera frame rate up to a 19% void ratio variation between two consecutive images. This seems to be sufficient in the context of the present flow configuration. The two-phase structures present inside the mixing area were analyzed at three different cavitation levels and their behaviors were compared to non-cavitating flow dynamic. Convection velocities and vortices shedding frequencies were estimated. Results show that vapor was transported by the turbulent velocity field. Statistical analysis of the void ratio signal was carried out up to the fourth order moment. This study provided a global understanding of the cavitating structure evolution and of the cavitation effects on turbulent sheared flows.

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An experimental study of the instantaneous local behavior of cavitation in turbo-pump inducers is presented in this article. Experiments held on a hydrodynamic facility equipped with an Ariane 5 inducer permitted achieving the aim. Cavitation is attained by reducing the pressure in a turbo machine having an inducer rotating at 4000 rpm. An x-ray tomography system developed specially for this aim, was used to measure the cavitation. The system employed an industrial x-ray generator and scintillation detectors. The generator/detectors system was fixed while the inducer was rotating. Vapor fraction was determined instantaneously, which confirms the applicability and the precision of the method in such type of measurements despite the constraints imposed by the geometry and the rotation speed. The dense composition of the system components introduced difficulties in the measurement especially at the inducer axis. However, prior information concerning the vapor regime as well as its stationary behavior in certain time intervals helped overcome this problem. Consequently a quantitative and qualitative evaluation of the vapor fraction is obtained. Results show a cavitation regime mostly synchronous with the rotation of the inducer thus permitting the use of tomographic reconstruction for the localization of vapor in the machine. An algebraic reconstruction algorithm (ML-EM) was used to achieve image reconstruction.

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