Ventilated cavitation is utilized for reducing drag and improving the headway of underwater vehicles. The ventilated cavitating flow around the axisymmetric body is simulated using the Large Eddy ...Simulation (LES) method in this study. Three cavity morphologies with different ventilation rates (Cq) are observed: Foamy Cavity (FC), Continuous Transparent and Foamy Cavity (CTFC) and Super Cavity (SC). The cavity shedding characteristic of the three different cavity morphologies is revealed. For partial cavity, the periodic cavity shedding is dominated by the local high pressure, re-entrant jet and vortex. The distributions of the turbulent kinetic energy and the pulsation enstrophy exhibit different migration mechanisms during the cavity shedding. The energy characteristic is reflected by three terms of the entropy generation. And the entropy production rate by direct dissipation occupies a dominant position, accounting for about 80% of the total entropy production, while the entropy production rate by viscous dissipation only accounts for 4%. For super cavity, gas leakage is dominated by a re-entrant jet induced by the intense adverse pressure gradient. And the results also demonstrated the velocity and vorticity pulsations around the test body wrapped in a super cavity are not intense.
•The typical ventilated cavitating flow structures at different ventilated rates have been studied.•The pulsation characteristics and the energy characteristics during the cavity shedding are revealed.•The energy loss composition during the cavity shedding is quantified.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
To analyze the effect of skew on propeller cavitation performance, single-blade propellers with different amounts of skew are investigated. Simulations are preformed using the Large Eddy Simulation ...(LES) and the Schnerr–Sauer cavitation model. The computational domain is meshed by cutting cells. The rotation of the propellers is simulated using Moving Reference Frame (MRF). Present study also examines the roll-up of the cavitating tip vortex and re-entrant jet with increasing skew. The results suggest that the hydrodynamic performance and cavitation vary as the skew increases. The circulation of the tip vortex cavitation fluctuates with diameter oscillations of the tip vortex. The re-entrant jet is observed at the suction side of the blade. Furthermore, there is a strong radial velocity component in the fluid flow near the re-entrant jet.
•Cavitation in single-blade propellers with different amounts of skew was studied.•Large eddy simulation (LES) and the Schnerr–Sauer cavitation model were used.•The hydrodynamic performance and cavitation vary as skew increases.•Tip vortex cavitation circulation fluctuates with tip vortex diameter oscillations.•A strong radial velocity component exists in the fluid flow near the re-entrant jet.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In this study, the numerical simulation of the NACA66(modified) hydrofoil is carried out by using the cavitation flow solution method considering compressibility. The cavities shedding behaviors and ...pressure fluctuation characteristics are discussed. The simulation results reproduce the cavities shedding process, which is in good agreement with the experimental results. The results show that cavities are affected by the re-entrant jet and shock wave successively. The large-scale cavities shedding caused by re-entrant jet provides the premise for the generation of shock waves. The phenomenon of detached cavities collapses induces the formation of shock waves. The pressure distribution of the monitoring points shows that the shock wave propagates upstream from the trailing edge of the hydrofoil, which is the main reason for the rapid shedding and collapse of the residual cavities. The shock wave propagation speed shows that there is an acceleration in shock wave propagation process. The increase of driving pressure caused by the residual cavity collapses is the main reason for accelerated propagation of shock wave.
•The shock wave phenomenon is captured effectively by the compressible method.•The large-scale shedding of the cavity caused by the re-entrant jet leads to the generation of shock waves.•The collapse of residual cavity is the main reason for the acceleration of shock wave propagation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The main purpose of this work is to shed light on the physics involved in the two distinct cavity cloud shedding mechanisms in cloud cavitating flows, namely re-entrant jet mechanism (RJM) and shock ...wave propagation mechanism (SWM). A compressible cavitating flow solver, which considers the compressibility effects of both liquid and vapor, is used to account for the wave dynamics in cavitating flows. The compressible Navier-Stokes equations, coupling the mass, momentum energy equations, and phase fraction transport equation, along with the thermodynamic equations of state for both liquid and vapor, Saito cavitation model and the SST-SAS turbulence model, are solved. Numerical results are presented for the conditions of both the re-entrant jet mechanism and shock wave mechanism around a NACA66 (mod) hydrofoil (Leroux et al., 2005), respectively, with emphasis on the process of re-entrant jet development and shock wave formation and propagation. The results show that the re-entrant jet can cause the attached cavity sheet breakup for both the two cavity cloud shedding mechanisms in both high and low angle of attack, while the shock wave formation and propagation process only occurs under shock wave mechanism at low angles of attack. Pressure evolution illustrates that in the re-entrant jet mechanism, cavity cloud collapse will induce high pressure load, while no shock wave and thus the corresponding rebound phenomenon are observed. In shock wave mechanism, during the whole process of the shock wave dynamics, namely generation, propagation and rebound process, pressure fluctuations increase sharply along with the generation of the pressure peaks with large amplitude and short time interval. Further study on the compressible characteristics involved in the two cavity shedding mechanisms illustrates that vapor fraction and mass transfer have a significant effect in sonic speed and Mach number characteristics. The average and standard derivation of maximum Mach number (Mamax) in shock wave mechanism is lower than that in re-entrant jet mechanism. Cavitating flows are characterized by low sonic speed value in the cavitation region and high sonic speed value in the pure liquid region and a sonic speed boundary layer exists between the two regions, the thickness of which is about the size of the local attached cavity sheet on foil surface. In the process of shock wave generation and rebound, cavity volume and cavity volume rate experience large fluctuations, showing strong cavitation instabilities in shock wave mechanism.
•Dynamics of compressible cavitating flows associated with cavity shedding are numerically studied using a compressible solver.•Pressure fluctuations involved in different cavity shedding dynamics are illustrated.•The cavitation compressibility characteristics, including sonic speed and Mach number are demonstrated.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•We present an efficient passive control method to stabilize the cloud cavitation instabilities using Cylindrical Cavitating-bubble Generators (CCGs).•Addition of the PANS Model in the OpenFOAM ...package.•Combination of the PANS method with Schnerr-Sauer cavitation model for modeling the unsteady cloud cavitating flow.•Using our passive control method a notable reduction in cavitation-induced vibration and high wall-pressure peaks on the solid surface was observed.
Unsteady cloud cavitation phenomenon is an important subject due to its undesirable effects in various applications such as ship propeller, rudder and hydraulic machinery systems. We present an efficient passive control method to control the cavitation instabilities which may be caused by the shedding of cavity structures in the vicinity of the solid surface of an immersible body. We proposed a passive control method so called Cylindrical Cavitating-bubble Generators (CCGs) on the surface of a benchmark hydrofoil and analyzed the effects of this passive controller on the dynamics of the unsteady cloud cavitation. First we modeled the unsteady cavitating flow around the hydrofoil without CCGs using a hybrid URANS model which was implemented in an open source code. Next, we studied the effect of CCGs on the mechanism of the unsteady cloud cavitation. The results show that using this method, the unsteady cavity structure was changed to a quasi-stable cavity structure compared with the cloud cavity shedding in the case without CCGs. We observed that the instability behavior of the unsteady cloud cavitation was mitigated and only small-scale cavity may be shedded from the hydrofoil in the free stream flow away from the hydrofoil surface. A notable reduction in cavitation-induced vibration and high wall-pressure peaks on the solid surface was observed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The problem of jet in cross flow has been studied for many decades. However, the investigation on gas jet in liquid cross flow is still rare. In addition, although ventilated cavity has been widely ...studied in ship drag reduction where the gas jet is injected into a cross flow vertically, the investigation on the dynamic behaviors of gas jet injected into cross flow horizontally is scarce. In this paper, an experimental device was designed to study dynamic characteristics of the ventilation bubbles injected from a vertical moving body. The bubble dynamic behaviors were captured by a high speed camera. Based on the experimental system, a series of experiments of the air exhaust process from a vertical moving body is studied under different motion states of the vertical moving body and different initial pressure ratio. The air accumulation effect and low pressure region on front of the ventilation holes causing the front of the ventilation bubble moves toward the head. Also, due to the Coandă effect and pressure difference the closure mode of re-entrant jet is presented. In addition, three types of flow patterns of the ventilation bubble were discovered.
•The unsteady bubble dynamic behaviours near the venting holes are observed and analyzed.•Morphological transformation and position of the re-entrant jet have been studied.•Pressure evolutions inside the air chamber with different conditions are presented.•Three flow patterns of interaction between adjacent bubbles have been discovered.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
This paper investigates the ventilation elimination mechanisms during the deceleration process of a surface-piercing hydrofoil using the unsteady Reynolds-averaged Navier-Stokes (RANS) method ...together with a Volume of Fluid (VOF) model. The numerical results are in good agreement with the experimental data. The ventilation elimination mechanism of the surface-piercing hydrofoil is analyzed from the perspectives of the hydrofoil hydrodynamic performance, the ventilated cavity evolution, vortex structures, and re-entrant jets. The results indicate that the ventilation elimination includes three stages, i.e. a decrease in the ventilated cavity, washout, and reattachment. The decrease in the ventilated cavity is due to the hydrofoil speed decrease in the FV flow. Washout is the transition from fully ventilated to partially ventilated flow, and reattachment is the transition from partially ventilated to fully wetted flow. The underwater vortex structures around the surface-piercing hydrofoil are composed of a tip vortex, an unstable vortex induced by the shear layer, and a Karman vortex caused by the vortex shedding from the trailing edge of the hydrofoil. Ventilation stability strongly depends on the re-entrant jet. When Φ (the angle between the flow direction and the closure line of the ventilated cavity) is greater than 45°, the re-entrant jet impinges on the ventilated cavity's leading edge and destabilizes the ventilated cavity.
•Ventilation elimination mechanisms are revealed during the hydrofoil deceleration.•Ventilation elimination includes three stages, i.e., cavity reduction, washout and reattachment.•Vortices include a tip vortex, a vortex induced by the shear layer and a Kaman vortex.•Ventilation stability strongly depends on the re-entrant jet.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Sheet-to-cloud cavitation transitions are very complex owing to the simultaneous appearance of the re-entrant jet and shock waves. The objective of this paper is to investigate the physics of ...compressible cavitating flows with emphasis on the simultaneous existence of the re-entrant jet and shock waves. A compressible cavitating solver, which considers the compressibility effects of both the liquid and the vapour, was used to account for the different shedding characteristics induced by the re-entrant jet mechanism (RJM), the shock wave mechanism (SWM), and both the re-entrant jet and the shock waves (RJM-SMW). The solver used Large Eddy Simulations (LES) and the Schnerr-Sauer cavitation model. The numerical results are first compared with available experimental data 1 which showed satisfactory agreement for various aspects of the flow, including the pressure distribution, cavity shapes, condensation front speed, and shedding frequency. The results first show three different cavity shedding processes induced by the re-entrant jet and the shock waves. Frequency analyses show that the shock waves produce a wider frequency distribution which indicates that shock waves induce more shedding than the re-entrant jet. The quasi-periodic characteristics of the transitions between the various cavity shedding characteristics are mainly induced by the variations of the pressure, residual cavity volume and medium compressibility. Further analyses show that the different pressure, residual cavity volume and medium compressibility amplitudes determine the cavity shedding characteristics in the subsequent shedding cycle. When the conditions favour both the re-entrant jets and the shock wave effects, both the re-entrant jet and the shock waves impact the cavity shedding.
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