On the interaction between the sheet and tip vortex cavitation over an elliptical hydrofoil

This study investigates the unsteady oscillation cycle of sheet and tip vortex cavitation over an elliptical NACA 66 Subscript 2 <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"> <mml:msub> <mml:mn>66</mml:mn> <mml:mn>2</mml:mn> </mml:msub> </mml:math> $66_2$ -415 hydrofoil using high-speed imaging and time-resolved tomographic particle image velocimetry. Synchronised measurements of radiated noise are conducted. The oscillation cycle consists of three phases, involving growth and collapse of the sheet and tip vortex cavitation, followed by intermittent rebounding of the tip vortex cavity. The collapse of the sheet cavity is triggered by a side-entrant jet, leading to the formation of the cloud and secondary vortex cavitation. The interaction between the secondary and tip vortex cavities further promotes the collapse of the latter. The three-dimensional instantaneous flow organisation indicates that the evolution of cavitation affects the centre displacement, deformation and breakdown of the tip vortex. Spanwise vortices emerge and interact with the tip vortex after the growth phase, causing violent fluctuations of the tip vortex during the collapse phase. The intense interaction between these vortices enhances the perturbation growth and promotes tip vortex breakdown. Proper orthogonal decomposition analysis reveals that the dominant unstable modes near the hydrofoil tip correspond to the displacement and deformation types. In addition to the suction-side shear layer, the vortex interaction region also exhibits intense production of turbulent kinetic energy. The temporal variation of sound pressure over an oscillation cycle indicates a strong correlation with the cavity morphology.