Numerical investigation on the inhibition mechanisms of unsteady cavitating flow around stepped hydrofoils

Abstract

In this work, the influence of obstacles on the control of unsteady cavitating flows is investigated numerically within the framework of homogeneous flow. The predicted Strouhal number of the cavity shedding around the smooth hydrofoil is close to the experimental value, indicating the accuracy of the numerical simulation. Four installation sites are examined, including 10%, 20%, 30%, and 40% of the chord length from the leading edge. The instantaneous and time-averaged hydrodynamics in conjunction with the unsteady cavity behaviors are discussed in detail. The results indicate that all cases display similar flow patterns within one shedding cycle, including the development of the attached sheet cavity, sheet/cloud transformation, and cloud cavity collapse. The hydraulic lift is governed by the partial cavity oscillation but could be significantly modified by the trailing wake vortices, especially after installing the obstacle. The recommended optimal position of the obstacle is X/L=0.4, and the corresponding pressure fluctuations and lift-to-drag ratio achieve the best performance. However, the mean vapor volume is significantly reduced when the obstacle is located at X/L=0.3, indicating the minimum risk of cavitation erosion. Installing the obstacle at different locations along the suction surface will provide different effects on oscillation based on the analyses of the Lagrangian coherent structures. The corresponding primary frequency varies in a narrow range between 106.67 Hz ∼ 121.33 Hz. Proper orthogonal decomposition (POD) analyses of the vapor fraction reveal the contribution of flow morphology to the partial cavity oscillation. Installing the obstacle could significantly limit the size of each pattern, especially for X/L=0.4.