•An unsteady natural cavitating flow around an axisymmetric projectile is numerically studied using fully compressible homogeneous multiphase approach.•Good agreement is obtained regarding the cavity ...structures and dynamics evolution with the experimental data.•Detailed mechanisms of the cavity shedding with periodic cavity–vortex–pressure interaction behaviors are revealed.•The cavity shape tends to be mushier with increasing water temperature under the same reference cavitation number and reynolds number.
Here, an unsteady natural cavitating flow around an axisymmetric projectile is computationally studied using a homogeneous multiphase approach. The numerical models used are based on a dual-time preconditioning method, an interface-capturing scheme, and a modified cavitation model, to capture unsteady cavitation structure behaviors. Full compressibility of two phases and an energy equation are used to determine the effects of temperature on a cavitating flow. First, the experimental data were validated at a cavitation number of σ = 0.435. Both quantitatively and qualitatively good agreements were achieved, including the evolution of the cavity shape, cavity length, and cavity thickness. Then, the key characteristics of an unsteady cavitation structure regarding the cavity growth, re-entrant jet, cavity shedding, and collapse were analyzed. In particular, the mechanism of the re-entrant jet inside the cavity, causing the periodic cavity shedding, was explored. Furthermore, detailed mechanisms of cavity shedding with periodic cavity–vortex–pressure interaction behaviors were analyzed. Finally, the influences of free-stream temperature on the cavitation structure behaviors were carefully investigated. It was found that both the cavity length and thickness increased with the free-stream temperature under the atmospheric pressure condition. The most sensitive effects occurred when the free-stream temperature approached its boiling point. In case of the same reference cavitation number and Reynolds number, the cavity shape tended to be mushier with increasing water temperature.
Evolution of the cavity shape of unsteady natural cavitating flow around an axisymmetric projectile with the cavitation number σ= 0.435. Special characteristic motions including cavity incipient, growth, re-entryre-entrant jet, shedding, and collapse are well captured with experimental images. Display omitted
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Cavitating flow around a hydrofoil is simulated by a compressible solver in OpenFOAM.•A new dynamic cubic nonlinear sub grid-scale model is applied in cavitating flow.•The pressure wave ...characteristics and its origin are demonstrated.•The difference of the re-entrant jet and the condensation shock are compared.
Cavity shedding mechanisms, such as those due to re-entrant jets (RJ) and condensation shocks (CS), are important but challenging topics in cavitating flows. This study investigates unsteady cavity shedding around a NACA66 hydrofoil using a self-defined compressible cavitation solver based on OpenFOAM with a dynamic cubic nonlinear subgrid-scale model. The predictions give satisfactory agreement with experimental data for the quantitative pressure evolution and the cavity shedding behavior induced by the re-entrant jet and the pressure wave. Moreover, the numerical data provides deeper insight into the pressure wave characteristics (e.g. propagation speed, wave intensity, shock Mach number, etc.) during cavity contraction. Multi-perspective analyses demonstrate that the pressure wave is a condensation shock which is responsible for the attached cavity abruptly disappearing. Additionally, the different influences of the re-entrant jet and the condensation shock on the local flow patterns are compared in detail in terms of the duration, average motion velocity, cavity behavior, formation mechanism and pressure intensity. The results show the condensation shock is characterized by fast propagation speed, short duration and high pressure pulse, which differs from the re-entrant jet features. This research provides an improved understanding of the cavity shedding mechanism around a cavitating hydrofoil and demonstrates the effectiveness of the current compressible cavitation solver.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The objective of this paper is mainly to investigate the ventilated cavitating flow characteristics around an axisymmetric body with a focus on three-dimensional unsteady behaviors. A high-speed ...camera technique is used to record the ventilated cavitating flow patterns. A homogeneous free surface model coupled with Filter-based turbulence model is used to simulate the time-evolution process of the unsteady cavitating flows. Numerical results show a reasonable agreement with the experimental data. The evolution of the ventilated cavity quasi-periodic unsteady shedding is well captured and discussed. The asymmetric and three-dimensional unsteady behaviors are impressive in the ventilated cavitating flow. Further analysis demonstrates that the re-entrant jet which originates from different circumferential positions at the closure region of the cavity moves upstream with different speeds. Lagrangian coherent structures (LCS) methods are applied to reveal the influence of re-entrant jet on the U-type cavity shedding and it shows that there is a close relationship between unsteady behaviors and the re-entrant jet flow motion consisting of moving upstream and circumferential. In addition, the effect of asymmetry on unsteady shedding behaviors of ventilated cavitating flow at different angles of attack have also been investigated combined with LCS methods.
•The ventilated cavitating flow characteristics around an axisymmetric body with focus on the three-dimensional unsteady behaviors is investigated.•Lagrangian coherent structures (LCS) methods are applied to reveal the influence of re-entrant jet on the U-type cavity shedding.•The typical ventilated cavitating flow structures at different angles of attack have been studied.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
In the present study, interactions between two parallel bubbles after the gas jets inject into a liquid cross flow are studied experimentally. For bubble coalescence on the surface of a moving body, ...the water layer between the twin parallel bubbles possesses initial momentum and bubble coalescence is difficult to conduct. Based on the verification of the repeatability of the experiments, the coalescence mechanism of the twin parallel bubbles subjected to a cross-flow is studied and the effect of re-entrant jet is analyzed, which can reduce the initial momentum of the water layer and promote the coalescence of bubbles. After that, the influences of the initial pressure inside the air chamber and motion speed of the body have been analyzed. The bubble length and its growth rate are proportional to the motion speed of the body and the initial pressure inside the air chamber has little influence on the growth rate of the bubble length. Moreover, only after the stream-wise length of the water layer reaches critical value, bubble coalescence can occur. According to parametric studies, the critical value of stream-wise length of the water layer depends on the motion speed of the body and a correlation has been estimated.
•Experiments on bubble coalescence suffering a cross flow were conducted.•Coalescence mechanism of the twin parallel bubbles subjected to a cross-flow was studied.•The influences of the motion speed and the initial pressure inside the air chamber on bubble interactions were studied.•A parameter characterizing bubble coalescence threshold was proposed and a correlation had been estimated.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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.
•Ventilated cavitating flow structure is studied by experimental and numerical methods.•The method with filter-based model is proposed to simulate the ventilated cavity.•The influence of Qv and Fr on the gas leakage behavior is discussed.•The mechanism of two types of re-entrant behaviors is illustrated.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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•Diffuse illumination enables fluid dynamics observation inside cavitation bubbles.•Dynamics of laser-induced bubbles near a 90° sharp solid-liquid boundary is studied.•Bubble-driven ...overflow of cliff-like solid edge forms a fixed-type secondary cavity.•Re-entrant injection of surrounding liquid into the bubble is detected at the edge.•Characteristics of re-entrant jet depend on liquid viscosity and surface tension.
Laser ablation in liquids is growing in popularity for various applications including nanoparticle production, breakdown spectroscopy, and surface functionalization. When laser pulse ablates the solid target submerged in liquid, a cavitation bubble develops. In case of “finite” geometries of ablated solids, liquid dynamical phenomena can occur inside the bubble when the bubble overflows the surface edge. To observe this dynamics, we use diffuse illumination of a flashlamp in combination with a high-speed videography by exposure times down to 250 ns. The developed theoretical modelling and its comparison with the experimental observations clearly prove that this approach widens the observable area inside the bubble. We thereby use it to study the dynamics of laser-induced cavitation bubble during its expansion over a sharp-edge (“cliff-like” 90°) geometry submerged in water, ethanol, and polyethylene glycol 300. The samples are 17 mm wide stainless steel plates with thickness in the range of 0.025–2 mm. Bubbles are induced on the samples by 1064-nm laser pulses with pulse durations of 7–60 ns and pulse energies of 10–55 mJ. We observe formation of a fixed-type secondary cavity behind the edge where low-pressure area develops due to bubble-driven flow of the liquid. This occurs when the velocity of liquid overflow exceeds ~20 m s−1. A re-entrant liquid injection with up to ~40 m s−1 velocity may occur inside the bubble when the bubble overflows the edge of the sample. Formation and characteristics of the jet evidently depend on the relation between the breakdown-edge offset and the bubble energy, as well as the properties of the surrounding liquid. Higher viscosity of the liquid prevents the generation of the jet.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•A self-sustained oscillating cavitation pocket developing along a Venturi is computed and compared with experimental results.•One fluid compressible RANS simulations are performed based on a void ...ratio transport equation model.•The importance of traveling pressure waves in the physical mechanism is put in evidence.•The importance of considering a non-equilibrium state for the vapour phase is exhibited.
Unsteady partial cavitation is mainly formed by an attached cavity which presents periodic oscillations. Under certain conditions, instabilities are characterized by the formation of vapour clouds convected downstream the cavity and collapsing in higher pressure region. Two main mechanisms have been identified for the break-off cycles. The development of a liquid re-entrant jet is the most common type of instabilities, but more recently, the role of pressure waves created by the cloud collapses has been highlighted. This paper presents one-fluid compressible simulations of a self-sustained oscillating cavitation pocket developing along a Venturi geometry. The mass transfer between phases is driven by a void ratio transport equation model. The importance of traveling pressure waves in the physical mechanism is put in evidence. Moreover, the importance of considering a non-equilibrium state for the vapour phase is exhibited.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
In order to understand the mechanism of cavity shedding and evolution, turbulent cavitating flows of the twisted hydrofoil were numerically investigated using the k-? turbulence model and the ZGB ...cavitation model. The results of the numerical calculation and the experimental method are basically consistent, which confirms the feasibility of the numerical calculation model. This study has obtained the following conclusions. Firstly, the cavity shedding can be summarized into six stages, and the cavity shape, pressure and velocity field at different stages are displayed, analyzed and compared in detail. Secondly, the shedding of cavity and its evolution are mainly caused by the re-entrant jet and side-entrant jet, in which the former provides the kinetic energy and the latter plays the role of guiding the direction. Thirdly, under the convective shearing action of the re-entrant jet and the main flow, a strong vortex located in the mid-back edge of the hydrofoil is formed, which promotes the transformation of the cavity shape into a U-shaped structure.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The objective of this study was to understand better the ventilated cavitation flow structure around an underwater ventilated vehicle. A high-speed camera system was used to observe the cavity ...evolution of unsteady cavitation flow, and a dynamic pressure measurement system was used to measure the instantaneous pressure during cavity growth. The numerical simulation is presented using the secondary development of computational fluid dynamics code CFX with a filter-based turbulence model. The results indicate that the ventilated flow rate of the gas influences the development of ventilated cavitation, and the pressure fluctuation is suppressed remarkably by the ventilated cavity evolution. The results also indicate that the proposed method can effectively capture the unsteady cavitation structure in accordance with the quantitative features observed in the experiment. It can therefore be concluded that the pressure fluctuations are induced by the vortex because of its periodic shedding toward downstream. The vortex shedding causes changes in the pressure distribution on the vehicle surface. Some secondary pressure oscillations can be observed that are attributable to the shedding of secondary vortex structures near the vehicle surface. These findings provide an important basis for facilitating the better understanding of the unsteady ventilated cavitation flows.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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