Large eddy simulation of cloud cavitation and wake vortex cavitation around a trailing-truncated hydrofoil

The cavitation has received considerable attention for decades because of its negative influence on the performance and the safety of the hydraulic machinery. In this study, a large eddy simulation is carried out to numerically investigate the unsteady cavitating flow around a trailing-truncated NACA 0009 hydrofoil for determining the underlying physical mechanisms. Two types of cavitation morphologies are identified: The large-scale bubble cluster and the von Kármán vortex cavity, named as the cloud cavitation and the wake vortex cavitation, respectively. It is shown that the velocity profiles obtained over the hydrofoil suction surface are in good agreement with the experimental data, indicating the accuracy of the current simulation. The dynamic evolution of the sheet/cloud cavity is also well reproduced, covering the sheet cavity breakup, the sheet/cloud transformation, and the collapse of the cloudy bubble cluster. The wake-vortex cavitation is caused by the blunt geometry at the hydrofoil trailing edge, where pairs of vortex cavities are induced. Both the cloud and vortex cavities significantly affect the lift oscillation, which makes it difficult to decompose the components. The fundamental shedding mechanisms of the wake vortex cavitation are discussed based on the finite-time Lyapunov exponent field. Specifically, the suction-side bubble grows and squeezes the giant pressure bubble away from the trailing edge. After the pressure bubble detaches, a new counterclockwise vortex or a new bubble appears at the pressure side, thus lifting the ridge towards the suction trailing edge and generating a strong vortex eye that pinches off the trailing portion of the suction-side bubble.