ABSTRACT

In the present work, the development of sheet cavitation and the shedding of cloud cavitation around hydrofoil NACA0015 are simulated in RANS and LES method. Three kinds of turbulence models -- SST k-omega, modified SST k-omega and Smagorinsky model are used in this paper. The simulating abilities of sheet and cloud cavitation with those three turbulence models are compared in cavitation shape, shedding frequency and so on. It is found that when simulating at the cavitation number σ = 1, Smagorinsky and modified SST k-omega turbulence models perform better at the aspects of cavitation shape and shedding frequency. The numerical results also show that the vortex near the wake of sheet cavitation on the suction side is the primary reason for the pinch-off and shedding of sheet cavitation.

INTRODUCTION

Cavitation is a dynamic phase-change phenomenon that often occurs in the flow over rudders, propellers, pumps and other fluid machinery. It is recognized that cavitation occurs when the local pressure drops below the saturated vapor pressure, and collapses in the area where the pressure recovers to large enough, which may lead to several problems such as vibration, erosion and noise on the surface of fluid machinery. Therefore, accurate simulation of cavitation flow becomes more and more significant in the section of propeller design.

The researches on cavitation have been conducted in the last half century. In the early study, experiment was the most effective method which often used stroboscope, pressure sensor and high-speed camera to carry out the prediction in the model scale. Rouse and Mcnown (1948) investigated the cavitation on the cylinders with different head shapes including hemispherical shape, blunt shape, ellipsoidal shape and so on at 0 degree angle of attack. Their cases were introduced as soundness check for cavitation flows by Kunz et al. (2000) and repeated by several authors in the next decades, e.g. Senocak and Shyy (2004), Ahuja et al. (2001) and so on. Kjeldsen et al. (2000) observed the flow around an hydrofoil NACA0015 in a cavitation tube. The results showed that the characteristics of the cavitation are influenced by the angle of attack (AoA) and the cavitation number. Besides, they found that the lift force of the hydrofoil fluctuated violently in a small cavitation number. The case of this hydrofoil at 6 degree angle of attack has been often used as a test case. Amromin et al. (2006) obtained a new hydrofoil OK-2003 by modifying the suction surface of NACA0015 and compared them in experiments. It was found that the cavitation on the suction surface of OK-2003 can effectively reduce the resistance of the hydrofoil, and the amplitude of lift and drag force.

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