TRANSITION MECHANISMS IN SHEET TO CLOUD CAVITATION

Vaca Revelo, Diego

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TRANSITION MECHANISMS IN SHEET TO CLOUD CAVITATION

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Cavitation refers to the formation of vapor when the pressure in liquid falls below vapor pressure. It occurs in a wide variety of situations, such as in valves, orifices, and propulsor blades. The formation of vapor is often followed by a growth of the vapor cavity and its violent collapse under a high-pressure environment. The physical consequences of this collapse include noise, vibration, and surface erosion. Sheet cavitation and its transition to cloud cavitation, a class of hydrodynamic cavitating flows, are of great practical interest since this highly unsteady flow can induce significant fluctuations in the thrust and torque of marine propellers. The collapse of the cloud also causes material damage to the blades. Hence, a fundamental understanding of this phenomenon is necessary to mitigate and control the detrimental effects of sheet to cloud cavitation. The transition of a sheet cavity to a cloud cavity occurs primarily due to two mechanisms: liquid re-entrant jet and condensation shock. While the role of the liquid re-entrant jet in sheet to cloud cavitation has been investigated by many authors in detail, the fundamental knowledge behind the condensation shock mechanism has not been explored in detail yet. Hence, this study aims to conduct a systematic parametric investigation of the sheet to cloud cavitation through high-fidelity numerical simulations, focusing especially on the condensation shock mechanism and the physical conditions that lead to the transition between the re-entrant jet mechanism and the condensation shock mechanism. A wedge configuration was chosen based on the experiments of Ganesh et al. (2016) at Reynolds number of 200,000 and several cavitation numbers ranging from 1.44 to 2.18. The multiphase medium is described using the homogeneous mixture model, and the governing equations are the compressible Navier-Stokes equations for the liquid/vapor mixture, along with a transport equation for the vapor mass fraction. The simulations use the algorithm developed by Gnanaskandan and Mahesh (2015) for cavitating flows on unstructured grids. The numerical approach is first validated by comparison with the experimental measurements reported by Ganesh et al. (2016) showing acceptable agreement, and then, a parametric investigation is carried out. The numerical results confirm the presence of re-entrant jet and condensation shock as dominating mechanisms under different conditions. In general terms, a shedding cycle starts when a sheet cavity is initiated at the apex of the wedge; then, it grows up to a critical length after which a re-entrant jet mechanism or a condensation shock mechanism takes place, resulting in a cloud shedding. In this study, two cases in the condensation shock regime and two cases in the re-entrant jet regime are investigated; the similarities and differences between the two regimes are elucidated. The Rankine-Hugoniot jump conditions for a homogeneous mixture system are used to verify the presence of shock fronts in the condensation shock regime. Then, the results are used to propose a new description of the condensation shock mechanism, which suggests that this mechanism can be initiated by a re-entrant jet that transforms into a shock front as it travels upstream through the vapor cavity.

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