Ventilated Cavitating Flow Research Articles

Analysis of ventilated cavitation around a cylinder vehicle with nature cavitation using a new simulation method

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.

Experimental and numerical investigation of ventilated cavitating flow with special emphasis on gas leakage behavior and re-entrant jet dynamics

The objective of this paper is to investigate the ventilated cavitating flow structure by combining experimental and numerical methods. A high-speed camera technique is used to record cavity evolution patterns. The numerical simulation is performed by CFX with a free surface model and a filter-based model, and the gravity effect is considered. The results show when the gas entrainment coefficient Qv is constant, two typical mechanisms of the gas leakage exist at different Fround numbers Fr, namely toroidal vortices mode and two hollow tube vortices mode. With the increasing of Fr, the cavity would transfer from the two hollow tube vortices to the toroidal vortices. Moreover, when the Fr number keeps constant, the enlargement of the cavity causes the gravitational effect to be more significant for the case of larger value of Qv. The detail analysis of re-entrant behaviors is also conducted. One type of re-entrant flow is unsteady with air cluster being periodically rejected at the rear of the cavity. The other type of the re-entrant flow shows that the majority of the cavity is transparent, only the region at the tail of the cavity is nontransparent, due to the re-circulation of water back into the cavity.

Numerical study of ventilated cavitating flows with free surface effects

Cavitating flow is usually formed on the surface of a high-speed underwater object. When the object moves near the free surface at high speed, the cavitation signature becomes a main factor to be overcome by the sensors of a military satellite. This paper studies the free surface effect on the ventilated cavitation process. The governing equations are Navier-Stokes equations based on a homogeneous mixture model. The multiphase flow solver used here relies on an implicit preconditioning scheme in curvilinear coordinates. The cavitation model used is a new cavitation model developed by Merkle et al. (2006). Computations of free surface effects were carried out with a NACA0012 hydrofoil to enable comparisons with experimental data presented in the literature. Calculations were then performed considering the ventilated cavitation process, including the effect of a noncondensable gas with free surface effect.

Numerical Simulation on the Ventilated Cavitating Flow with High Froude Number

A numerical scheme based on the homogeneous multiphase model was proposed to model the ventilated cavitating flow by considering interaction between three phases including vapor, ventilated gas and liquid. Due to the difficulty of carrying out ventilated cavitation experiments in high-speed water tunnels, focus was put on predicting the developing process of the cavitating flow with high Froude number. The evolution processes for the ventilated cavity which transited between different gas-leakage mechanisms were successfully captured, obtaining the variation of the cavitation number along with the ventilation rate. Hydrodynamics analysis was carried out for the cavitating body with low and high Froude number respectively, thereby revealing the ventilation rate to optimize the vehicle performance. Based on the numerical results, a theoretical gas-leakage model which was adapted to a wide range of Froude number and various gas-leakage mechanisms was proposed. The model was validated through comparison against experimental data and numerical results. This research provides valuable guidance on the design for ventilated cavitating vehicles.

Numerical Simulation Study on High-Temperature Ventilated Cavitating Flow Considering the Compressibility of Gases

A numerical simulation approach of ventilated cavity considering the compressibility of gases is established in this paper, introducing the gas state equation into the calculation of ventilated supercavitating flow. Based on the comparison of computing results and experimental data, we analyzes the differences between ventilated cavitating flow fields with and without considered the compressibility of gases. The effect of ventilation on the ventilated supercavitating flow field structure is discussed considering the compressibility of gases. The results show that the simulation data of cavity form and resistance, which takes the compressibility of gases into account, accord well with the experimental ones. With the raising of ventilation temperature, the gas fraction in the front cavity and the gas velocity in the cavity increase, and the cavity becomes flat. The resistance becomes lower at high ventilation temperature, but its fluctuation range becomes larger than that at low temperature.

Numerical investigation of the ventilated cavitating flow around an under-water vehicle based on a three-component cavitation model

Based on the Reynolds-Averaged Navier-Stokes equations and mass transfer model, an approach, where a three-component cavitation model is proposed, is presented to simulate ventilated cavitating flow as well as natural cavitation. In the proposed cavitation model, the initial content of nucleus in the local flow field is updated instantaneously, and is coupled with the Rayleigh-Plesset equation to capture the cavity development. The proposed model is applied to simulate the cavitating flow around an under-water vehicle in different cavitation conditions. The results indicate that for the natural and ventilated cavitation simulation, the predicted cavitation characteristics including the cavity length, cavity diameter and cavity shape agree satisfactorily with the analytic and experimental results, for the ventilated cavitation, the proposed methods reproduce the special behavior that the axial line of the cavity bends and rises at the tail part. The study concludes that the ventilated flow rate of the non-condensate gas influences the development of natural cavitation as well as ventilated cavitation, and the vapor cavity is suppressed remarkably by the gas cavity with the increase of the gas ventilation.

Study of ventilated cavity morphology in different gas leakage regime

Based on a suite of CFD code, a homogeneous, multiphase, Reynolds averaged Navier-Stokes solver coupled with a transport cavitation model and a local linear low-Reynolds-number k – ɛ turbulence model closure was used to simulate the ventilated cavitating flows around a wedge hydrofoil. The gas leakage regimes at the aft of ventilated cavities were investigated. Three gas leakage regimes were observed, and corresponding to each regime, ventilated cavities exhibited a different morphology. The numerical results were compared to corresponding experimental results in qualitative manner. It was found that the general characteristics of the gas leakage regimes and cavity morphology were very reasonably predicted.

Numerical study of periodically forced-pitching of a supercavitating vehicle

The unsteady behaviors, such as surging, heaving and pitching motion, which often occur during the advancing of supercavitating vehicle, has significant effect on the stability of supercavitaty and the trajectory of the vehicle. This paper presents a 3-dimentional numerical simulation of periodically forced-pitching of supercavitating vehicle. Based on the finite volume method and the pressure-based segregate algorithm, in the framework of Mixture multiphase model, associated with dynamic mesh method, the Reynolds-Averaged Navier-Stokes equations are solved for the ventilated cavitating flow field in a cavitation tunnel. For both steady-state and dynamic cases, the numerical results agree with the experimental results very well. When the vehicle is pitching periodically, the profile of the supercavity doesn't vary significantly. The pressure inside the cavity fluctuates slightly during the pitching motion, while the pressure fluctuates significantly at the rear of the down-line of the vehicle. The hydrodynamic forces of the vehicle oscillated periodically, but not linear related to the pitching motion.

The Wall Effect on Ventilated Cavitating Flows in Closed Cavitation Tunnels

For ventilated cavitating flows in a closed water tunnel, the wall effect may exert an important influence on cavity shape and hydrodynamics. An isotropic mixture multiphase model was established to study the wall effect based on the RANS equations, coupled with a natural cavitation model and the RNG κ-Σ turbulent model. The governing equations were discretized using the finite volume method and solved by the Gauss-Seidel linear equation solver on the basis of a segregation algorithm. The algebraic multigrid approach was carried through to accelerate the convergence of solution. The steady ventilated cavitating flows in water tunnels of different diameter were simulated for a conceptual underwater vehicle model which had a disk cavitator. It is found that the choked cavitation number derived is close to the approximate solution of natural cavitating flow for a 3-D disk. The critical ventilation rate falls with decreasing diameter of the water tunnel. However, the cavity size and drag coefficient are rising with the decrease in tunnel diameter for the same ventilation rate, and the cavity size will be much different in water tunnels of different diameter even for the same ventilated cavitation number.