Application of Fast Synchrotron X-ray Imaging in Velocimetry of Cavitating Flows

Hydrodynamic cavitation is a complex two-phase flow phenomenon involving mass and heat transfer between liquid and vapor. It occurs in many widely-used hydraulic machines, such as pumps and marine propellers, and often leads to undesired effects like material erosion, noise, and vibration. To control these detrimental effects, the visualization of two-phase flow morphology inside the opaque cavity is a crucial point to improve the physical and numerical models of cavitation. The major challenge in experimental measurements of cavitating flow fields is the fact that multiple scattering and a direct reflection of visible light from phase boundaries make the flow optically opaque. In recent years, unlike traditional local measurements using various probes, the development of the third-generation synchrotron radiation sources promotes the application of Xray phase-contrast imaging, which enables clear visualization of boundaries between phases with different refractive indices. In this study, the partial cavity is formed in a convergent-divergent (Venturi) channel with a small contraction ratio where the relatively stable cavitation regime can be sustained in a wide range of cavitation numbers. The experiment performed at Advanced Photon Source (APS) of Argonne uses the short high-flux X-ray pulses emitted from synchrotron sources to capture fast dynamic events and minimize motion blur. The internal two-phase structures and bubble development dynamics inside the quasi-stable sheet cavitation can be identified. Aside from the detailed illustration of two-phase morphology, X-ray phase-contrast images were also used to perform velocimetry by tracking either seeded particles or phase interfaces inside the opaque regions. Through appropriate postprocessing to the recorded X-ray images of cavitation, the time resolved velocity and void fraction fields are obtained simultaneously. These unprecedented experimental data will be further explored in understanding fluid mechanics underneath the cavity, estimating slip velocity between fluid-vapor interactions, and reconstructing pressure fields for compressible flows.