The unsteady turbulent cloud cavitation around a NACA66 hydrofoil was simulated using the filter-based density corrected model(FBDCM). The cloud cavitation was treated as a homogeneous liquid-vapor mixture and the effects of turbulent eddy viscosity were reduced in cavitation regions near the hydrofoil and in the wake. The numerical results(in terms of the vapor shedding structure and transient pressure pulsation due to cavitation evolution) agree well with the available experimental data, showing the validity of the FBDCM method. Furthermore, the interaction of vortex and cavitation was analyzed based on the vorticity transport equation, revealing that the cavitation evolution has a strong connection with vortex dynamics. A detailed analysis shows that the cavitation could promote the vortex production and flow unsteadiness by the dilatation and baroclinic torque terms in the vorticity transport equation.
The free/open source software as OpenFOAM,Salome meshing and python language under Debian Linux system are evaluated to model unsteady cavitating turbulent flow around a NACA0015 hydrofoil. Based on the cavitation model proposed by Kunz and large eddy simulation(LES) method, we denote the benefits of free software and open source tools as an alternative to proprietary software of computational fluid mechanics, and provided a modified cavitation model to improve numerical accuracy.The simulation results of typical break-off cycle for cavitation shedding are compared to available experimental data, and validated using image processing to find percentage of similarities. The sheet cavity frequency of 7.752 Hz was obtained based on cavitation evolution and pressure fluctuations. The study gives relevant information for CFD software development in the future.
Large Eddy Simulation （LES） was coupled with a mass transfer cavitation model to predict unsteady 3-D turbulent cavita- ting flows around a twisted hydrofoil. The wall-adapting local eddy-viscosity （WALE） model was used to give the Sub-Grid Scale （SGS） stress term. The predicted 3-D cavitation evolutions, including the cavity growth, break-off and collapse downstream, and the shedding cycle as well as its frequency agree fairly well with experimental results. The mechanism for the interactions between the cavitation and the vortices was discussed based on the analysis of the vorticity transport equation related to the vortex stretching, volumetric expansion/contraction and baroclinic torque terms along the hydrofoil mid-plane. The vortical flow analysis demonstrates that cavitation promotes the vortex production and the flow unsteadiness. In non-cavitation conditions, the streamline smoothly passes along the upper wall of the hydrofoil with no boundary layer separation and the boundary layer is thin and attached to the foil except at the trailing edge. With decreasing cavitation number, the present case has O＂ = 1.07, and the attached sheet cavitation beco- mes highly unsteady, with periodic growth and break-off to form the cavitation cloud. The expansion due to cavitation induces boun- dary layer separation and significantly increases the vorticity magnitude at the cavity interface. A detailed analysis using the vorticity transport equation shows that the cavitation accelerates the vortex stretching and dilatation and increases the baroclinic torque as the major source of vorticity generation. Examination of the flow field shows that the vortex dilatation and baroclinic torque terms in- crease in the cavitating case to the same magnitude as the vortex stretching term, while for the non-cavitating case these two terms are zero.
In the present paper, a new simulation method is developed for unsteady cavitating flow with air ventilation,which is very useful for alleviating the pressure oscillation in hydroturbine draft tube and reducing the drag force on an underwater vehicle. Because the fluid includes three components, i.e., the water, vapor, and air, the interactions between water–air and water–vapor are treated by applying the level set method, and the effect of surface tension is taken into account in governing equations. Further, the phase change between the water and the vapor is modeled by a homogeneous model,where the effect of air injection is considered by applying the air volume fraction in the mass transport equation. For calibration, the cavitating flows around a cylinder vehicle were simulated using the proposed method, and the numerical results were compared with the experimental data at three different ventilation conditions. The good agreement of cavitation evolutions between the simulation and the experiment indicated that the proposed method was acceptable for the simulation of ventilated cavitating flows with nature cavitation and would be usable for various engineering applications.Moreover, the vorticity analysis depicts that the vortex is closely related to cavitation evolution, and air injection much changes the vorticity production in cavitating flow. It was also revealed that vorticities only occurred in regions with high vapor/air volume fraction and the vortex stretching term created the most vorticities.
The present paper focuses on the erosive cavitation behavior around a plane convex hydrofoil. The Zwart-Gerber-Belamri cavitation model is implemented in a library form to be used with the OpenFOAM. The implicit large eddy simulation （ILES） is app- lied to analyze the three-dimensional unsteady cavitating flow around a plane convex hydrofoil. The numerical results in the cases under the hydrodynamic conditions, which were experimentally tested at the high speed cavitation tunnel of the l^cole Polytechnique F6d＆ale de Lausanne （EPFL）, clearly show the sheet cavitation development, the shedding and the collapse of vapor clouds. It is noted that the cavitation evolutions including the maximum vapor length, the detachment and the oscillation frequency, are captured fairly well. Furthermore, the pressure pulses due to the cavitation development as well as the complex vortex structures are reasona- bly well predicted. Consequently, it may be concluded that the present numerical method can be used to investigate the unsteady cavitation around hydrofoils with a satisfactory accuracy.
