Ventilated Cavitating Flow Research Articles

The experimental and numerical studies of formation and collapse processes of ventilated supercavitating flow

The aim of this paper is to investigate the ventilated cavitating flow behavior, through experimental and numerical methods. The experiments are performed for a test model placed in a water tunnel and a high-speed camera is used to image the formation and collapse of the supercavity. For the numerical simulations, Ansys CFX commercial software is employed to solve the Reynolds-averaged Navier–Stokes (RANS) equations and additional transport equation for the liquid volume fraction in the unsteady condition. Both the experimental observation and numerical prediction show a hysteresis behavior for ventilated supercavity in which the value of the air entrainment coefficient required to maintain the supercavity in the formation process is less than the amount required to form it. The good agreement observed between the numerical prediction and experimental data, revealed the accuracy and capability of the numerical scheme. Also, the numerical simulation shows that the variations in the supercavity length, in addition to the air entrainment, are a function of the air leakage regime from the closure region of supercavity.

Large eddy simulation of the cavity shedding characteristics of ventilated cavitation around the underwater vehicle

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.

Experimental and numerical study on ventilated cavitation of high-speed projectile

In this study, ventilated cavitating flow characteristics around an axisymmetric projectile are investigated by combining experiments and numerical simulations. Experiments were carried out with a Split–Hopkinson pressure bar launch system and the pressure-equaling exhaust technology. Modular projectiles are designed to experimentally investigate the influence of head shape and ventilatory volume on flow characteristics. Large eddy simulation model is applied to obtain more flow field information. Compared with the conical head projectile, the hemispherical head projectile has a thinner attached cavity and more local detachment of the cavity. The statistical structure of the velocity and pressure fluctuations are analyzed by combining histograms and Q–Q diagrams. The results show that the pressure drag is dominant in the total drag and the periodic pulsation of the tail cavity and the stable vortex structure at the tail cause the variation of drag. The larger cavity volume changes the actual shape of the projectile, making the drag of the conical head projectile higher. The evolution characteristics of the cavitating flow field around the projectile with different ventilatory volumes are obtained, and the relationship between pressure fluctuation and chamber volume is derived. It is found that the reentrant jet causes a reverse flow at the nozzle, which leads to local pressure rise at the same interval. The above research work could contribute to the design and flow control of the ventilated cavity body.

Flow pattern regime and unsteady characteristics of ventilated cavitating flow around the axisymmetric bodies with different headforms

This paper presents an experimental investigation of the flow pattern regime and unsteady characteristics of ventilated cavities with different headform shapes. The test model consists of a removable headform with three different forebodies (the conical, the blunt, and the hemispherical) and a common cylinder rear body. Experiments are conducted in a closed-loop cavitation tunnel. First, the flow pattern regimes on the ventilated cavity for different headforms are discussed in detail, and the dimensions of several flow patterns are measured. The results show that the cavity dimension and the regime are strongly dependent on the headform shape, and all typical flow patterns are introduced by schematic illustrations. Second, the ventilation hysteresis that happened during the flow pattern transition is pointed out. A quantitative gas leakage model is employed to explain the cause of hysteresis and flow pattern transition, and the results show the Strouhal number for different headforms is approximately ∼0.21. However, the blunt presented stronger gas leakage with a large constant parameter, K = 0.80. Finally, the unsteady characteristics of the ventilated cavity around different tested headforms are involved through descriptions of developments of the recirculating vortex and transparent cavity. In addition, an estimated cavitation number is applied to investigate the unsteady characteristics, and the maximum cavitation number and the strongest characteristic are obtained by the blunt headform due to the large drag coefficient and strong flow separation.

Experimental analysis on dynamic/morphological quality of cavitation induced by different air injection rates and sites

Ventilated cavitating flow features resulting from the air injection at the hydrofoil surface are characterized based on experimental investigation. The experiments have been conducted in the cavitation tunnel at the Silesian University of Technology. The main focus of this work is to investigate how both the location of the injection hole at the surface of the hydrofoil (so-called injection site) and the injection rate have an impact on the cavitating flow in various flow conditions (i.e., different cavitation numbers). The Clark Y hydrofoil is fixed at an 8° angle of attack. In addition, three cavitation numbers, σ = 1.1, 1.25, and 1.6; five air injection rates, Q = 0, 0.25, 0.5, 0.75, and 1 l/min; and two injection sites at the surface of hydrofoil (Tap1-injection and Tap5-injection) are selected for the case studies. Furthermore, the level of dissolved air in water is kept constant at 11.7 mg/l. The unsteady measurements and high-speed imagining declare that, regardless of the injection rate, the injection site has a significant effect on the cavitation dynamic features and morphology. Moreover, it is shown that the effectiveness of air injection depends on the flow conditions.

Experimental and numerical study of free surface effect on the ventilated cavitating flow around a surface vehicle model

In the current study, the ventilated cavitating flow for a model near the free surface is investigated experimentally and numerically. The experiments are performed in a towing tank equipped with a ventilation system, drag measurement interface and a photography system. The numerical simulation is performed by commercial CFD code ANSYS-CFX with the volume of fluid (VOF) method, the RNG k-ε turbulence model, and effectiveness of the gravity. The ventilated supercavity and drag coefficient of the model under free surface effect is studied at different submergence depths and velocities. Both the experimental observations and numerical predictions show that with increasing the Froude number (Fr), the ventilated supercavity size increases and the drag coefficient decreases. The good agreement observed between the numerical predictions and experimental data, in terms of both the supercavity size and drag coefficient, has revealed the accuracy and capability of the applied numerical model. The numerical results reveal that with increasing the submergence depth, the drag coefficient increases due to existence of the cavitator at the front of the vehicle. Also, in the regions near the free surface, the variation of supercavity length with Fr is different from that of the regions away from the free surface.

Numerical investigation of ventilated cavitating flow from high to low cavitation numbers

Cavitation occurs when an underwater vehicle moves at a high speed, causing the water to change from the liquid phase into the vapour phase in the low-pressure region. As one of the most efficient drag reduction technologies for high-speed underwater vehicles, the artificially ventilated cavitation flow needs to be studied. In this paper, the ventilated cavitating flow over a high-speed underwater vehicle is analysed with a multi-phase solver from high to low natural cavitation numbers. The large eddy simulation turbulence model, volume of fluid method and Kunz cavitation model are used in the numerical calculation to simulate and capture the three-phase (air, liquid water, and vapour) cavitation flow. The miscibility of the air and vapour phases inside the cavity is also considered in the simulation. To validate the solver, two benchmark cases with air ventilated into a cavitating flow are simulated. Then the characteristics of the ventilated cavitating flows from a high to low natural cavitation number are analysed to study the development of the ventilated cavity with miscible air and vapour inside it. The results show that air ventilation can prevent pressure fluctuations at the surface of an underwater vehicle caused by cloud cavitating flow at a moderate natural cavitation number. Compared to no ventilation, drag reduction of ventilated cavitating flow can reach up to about 85% at a low cavitation number.

Mode vortex and turbulence in ventilated cavitation over hydrofoils

In this paper, the complex interaction of gas-vortex-turbulence in ventilated cavitation over a NACA0015 hydrofoil is investigated by using large eddy simulation. The proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) are used for analyzing the three-dimensional fluctuation modes of gas-liquid interfaces induced by turbulent coherent structures in ventilated cavitation. The results show that among the first four dominant POD modes possessing 99.5% modal kinetic energy in the ventilated cavitating flow, and the turbulent POD mode 2 contains the multi-scale coherent structures and acts as a bridge for the energy scale transition from mode 1 to mode 3 through the gas-liquid interface oscillations. The DMD modal structures are reconstructed from the roller-shaped energetic structure captured by POD mode 2, which further reveal the process of breaking down near the hydrofoil leading edge, changing into the hairpin shape and finally stretching into the horseshoe shape above the hydrofoil trailing edge for the turbulent coherent structures on the cavity interfaces. Such low-frequency wake structures are energetic but sustained by means of the high-frequency turbulent fluctuations, with the interfacial oscillation amplitude changed through the formation and transition of the multi-scale coherent structures. Further, the modal turbulent kinetic energy transport theory is introduced to describe the ventilated coherent structures. It is worth noting that the large-scale energetic structures in POD mode 2 are mainly drawing energy from mean flows through the Reynolds stress, and the modal turbulent energy stored by shedding vortex and air cavities in the ventilated wake flow is redistributed and balanced due to the turbulent and pressure diffusion effects. Besides, the uneven spatial distribution of turbulent energy caused by the vortex structures of different modal scales is proved to promote the spatial oscillation level of the ventilated cavity interfaces. This study is helpful to understand the interaction mechanism between turbulence and gas-liquid interfacial fluctuations for the ventilated wake flow.

Numerical analysis of ventilated cavitating flow around an axisymmetric object with different discharged temperature conditions

Nowadays, with the widespread use of modern technology in underwater vehicles, particularly in naval applications, the artificial gas ejected from ventilated holes is normally heated because of engine operations. However, the dynamics of ventilated hot gas have not yet been fully studied. In this study, the cavitating flow around an axisymmetric body with different ventilated temperatures was numerically investigated. A fully compressible mixture model based on a homogeneous multiphase approach was employed. A high-order accuracy flow solver based on the numerical models of a dual-time preconditioning technique, a sharp interface-capturing scheme, and an enhanced cavitation model was used to examine the effects of temperature on the cavitating flow. First, the numerical results were validated by comparison with available experimental data of cavitating flow around a conical cavitator. Reasonable agreements on the cavity shape, cavity length, and cavity thickness were achieved. The solver was then applied in simulations to analyze the development of the ventilated cavity, including the cavity formation at the early stage, the re-entrant jet phenomenon, and the cavity shedding mechanism. In addition, the effects of ventilated temperatures on the cavity length and cavity thickness were studied. Further insight into the distributions of the flow field inside the cavity, and the instability of the gas-water interface when temperature increases were also provided.

Computational investigation on ventilated supercavitating flows and its hydrodynamic characteristics around a high-speed underwater vehicle

Notwithstanding the effectiveness of ventilated cavitation as a safe way to accelerate and achieve natural supercavitation, detailed physics of ventilated cavitating flows and hydrodynamic effects on underwater vehicles have been rarely explored. The present work deals with computational investigation on ventilated supercavitating flows around a high-speed underwater vehicle and its hydrodynamic characteristics. The homogeneous mixture model is adopted for describing the cavitating flow field composed of water, water vapor, and a ventilated gas. After extensively validating the flow solver with experiments of ventilated cavitating flows, we carry out a series of computations of supercavitating flows around a high-speed underwater vehicle with four control fins, and investigate the flow physics and the vehicle’s hydrodynamic characteristics. The computations are performed under different freestream velocities, ventilation rates, and angles of attack. Furthermore, the unsteady effects of turbulence are examined by comparing the computed results based on unsteady Reynolds-Averaged Navier–Stokes (URANS) simulation and Detached Eddy Simulation (DES).The computational results show that the cavitating flows cause substantial nonlinear characteristics in cavity shapes and hydrodynamic forces such as lift, drag, and pitching moment, due to the presence of natural and ventilated cavitations and their interaction. This can be beneficially exploited in understanding and predicting ventilated supercavitating flows around a high-speed underwater vehicle, and establishing an accurate dynamic model of the vehicle’s motion.

Physical and numerical study on the transition of gas leakage regime of ventilated cavitating flow

The objective of this paper is mainly to investigate the ventilated cavitating flow around an axisymmetric body with a focus on the gas leakage mechanisms via combining experimental and numerical methods. A high-speed camera technique is used to record the ventilated cavitating flow patterns. The numerical simulation is performed with a free surface model and Filter-based turbulence model (FBM). Good agreement can be obtained between the experimental and numerical results. In the formation of a ventilated cavity, there are three typical gas leakage types with different Froude numbers Fr and gas entrainment coefficients CQ, including RJ closure mode with gas leakage of toroidal vortex shedding (TVS-RJ), TV closure mode with gas leakage of twin-vortex tube entrainment (TVTE-TV), and the coexistence of toroidal vortex shedding and twin-vortex tube entrainment (TVS-TVTE). The internal flow characteristics revealed three distinct regions with the ventilated cavity, including the ventilation influence region, the internal shear layer and the reverse flow region. With the increase of CQ, the incline angle of the cavity bottom surface α gradually decreases. When the incline angle α decrease to a critical value of ∼17.1°, the flow separation disappears completely and the flow regime migrates completely to TV cavity from RJ cavity.

Unsteady behavior of ventilated cavitating flows around an axisymmetric body

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.

Numerical investigation of ventilated cavitating flow in the wake of a circular cylinder

The characteristics of ventilated cavitating flow behind a circular cylinder is investigated by numerical simulation and the cavity interface is captured by the coupled level-set and volume of fluid method. The interaction between the ventilated cavitation and vortex structures are investigated. In particular, the bubble identification method is applied to study the bubble distribution and bubble behaviors in the wake.

Ventilated cavitating flow over a bluff body with special emphasis on the vortex-cavitation interaction

The objective of this paper is to investigate the vortex-cavitation interaction in ventilated cavitating flow over a bluff body at Reynolds number Re = 6.7 × 104 by numerical method. The results present that in the active cavitation area, the turbulent kinetic energy mainly distributes in the leading edge of the cavity and the gas-liquid interface of the shedding vortex. The intensity variation is closely related to the cavity morphology and the ventilated cavitation development. With the increase of the gas entrainment coefficient Qv, the ventilated cavitation reduces the turbulence intensity. Three well-employed vortex identification methods, ωz criterion, Q criterion and the improved Ω method, are applied to identify the vortex structures. The comparison shows that the improved Ω method can well capture the vortices of different intensity in the cavitating flow field without using the threshold value, especially for the formation of the vortices. As for the ventilated cavitation vortex dynamics, strong rotation appears with the shear layer rolling up and relatively strong shear occurs with the entrainment of vortices during the vortex formation. With the increase of Qv, both the rotation and shear in the active cavitation area tend to become weaken owing to the reduction of turbulent intensity.

Experimental investigation of critical air entrainment in ventilated cavitating flow for a forward facing model

Ventilated supercavitation is a favorable method for skin friction reduction of underwater bodies, whereby a cavity is generated through artificial injection of air to cover the entire body. In the current study, several experiments are carried out to investigate ventilated supercavity behavior under different flow conditions. A closed-wall water tunnel and air injection system are provided to form ventilated supercavities around a model which was mounted in a forward facing setup. The present research concentrates on the formation and collapse processes of supercavities in order to determine the critical values of air entrainment coefficients which are important parameters for the ventilation demand of a high-speed supercavitating vehicle. Our experiments show that there are critical air entrainment coefficients in which the supercavity length suddenly increases or decreases, due to the change of closure mode between the re-entrant jet regime and twin vortex regime at the rear portion of the supercavity. The results reveal that the formation and collapse air entrainment coefficients vary with Froude number. Moreover, some pressure sensors are positioned in the model and are used to measure the pressure distribution inside the supercavity. The most novelty of our work is to present measurements of the supercavity pressure for a forward facing model, especially at closure region of the supercavity. The pressure measurements show that the pressure increases slightly along the test body surface and is accompanied by a sharp increase at the closure region of the supercavities.

Physical and numerical study on unsteady shedding behaviors of ventilated partial cavitating flow around an axisymmetric body

The objective is to investigate the unsteady ventilated partial cavitating flow characteristics with focus on the unsteady shedding behaviors via combined experimental and numerical methods. Experimental results are presented for an axisymmetric body in a water tunnel with the flow pattern recorded by a high speed camera. The numerical simulation is performed with a filter-based turbulence model. Good agreement can be obtained between the experimental and numerical results. The ventilated cavity around the axisymmetric body experiences the rapid growth stage, the growth with small pulsation stage and the periodic shedding stage. The typical ventilated cavitating flow patterns at different angles of attack have also been studied to further investigate the effect of the asymmetry on the unsteady shedding behaviors of ventilated partial cavitating flow. With the increase of the angle of attack, the ventilated cavity develops asymmetrically and the non-transparent gas-liquid mixture region is gradually concentrated on the opposing stream surface along with the transparent cavity region increasing on the confronted stream surface.

Development of a multiphase cavitation solver and its application for ventilated cavitating flows with natural cavitation

The aim of this article is to develop a cavitation solver capable of dealing with the cavitating flow where more than three phases coexist based on the OpenFOAM platform. A new phase change class coupled with newly reported cavitation models and turbulence models has been implemented in the solver. Firstly, validations have been carried out using cases of a two-dimensional cavitating hydrofoil and a three-dimensional projectile near the free surface. The pressure distribution and the gas-liquid interface profile agree well with the experimental data. The solver is then applied for a base-ventilated cavitating hydrofoil with different angle of attack where natural cavitation could occur simultaneously on the hydrofoil surface. The cavity shape as well as the transient cavity shedding process are correctly predicted using the PANS turbulence model. The resulting velocity and pressure distributions are clearly analyzed, giving great insights into the formation and deformation mechanisms of the ventilated cavity. The developed solver is proved to be accurate and efficient in solving the details of turbulent multiphase flows with phase change occurring and has great potential for future applications.

Numerical modeling and simulation of the shedding mechanism and vortex structures at the development stage of ventilated partial cavitating flows

Abstract Ventilated partial cavitation is a complex multi-phase turbulent flow due to the strong interactions between gas and liquid. In the present work, we specially focus on the numerical modeling and simulation of the shedding mechanism and vortex structures. The Reynolds Averaged Navier–Stokes (RANS) method combined with a filter-based turbulence model (FBM) is proposed to explore the physical mechanism of the ventilated partial cavitating flows. Experimental results of cavity evolution and pressure are utilized to assess the prediction ability of the proposed method. Good agreements are observed between experimental measurements and numerical predictions, including the ventilated cavity growth, break off, shedding and the transient dynamic pressure inside the cavity. Based on the model strategy, the cavity dynamic evolution and shedding mechanism are analyzed. The results indicate that the re-entrant flow gives birth to the gas leakage at the cavity interface and is responsible for the ventilated cavity shedding. In addition, streamline vortex is presented to reveal the ventilated cavity shedding characteristics. Moreover, based on the vorticity transport equation, the influence of velocity gradient, fluid volumetric expansion/contraction, pressure gradient and the viscous dissipation factors on the vortex production in ventilated cavitating flows is examined. The present study can provide important basis to better understand the shedding mechanism and vortex structures on the development stage of ventilated partial cavitating flows.

Numerical investigation of ventilated cavitating vortex shedding over a bluff body

The objective of this paper is to investigate ventilated cavitating vortex shedding dynamics over a bluff body at Re = 6.7 × 104 with large eddy simulation (LES) model. The finite-time Lyapunov exponent (FTLE) and Lagrangian coherent structures (LCS) methods are applied to investigate the formation, evolution and shedding of ventilated cavitating vortices. The results show that ventilated cavitation plays an important role in the vortex structures and the vortex shedding process. Comparing with non-cavitating flow, the Strouhal number St corresponding to vortex shedding increases and the length of formation region decreases when the gas entrainment coefficient is Qv = 0.0231. While with further increase of gas entrainment coefficient, the St reduces gradually and the formation region expands. Based on the Lagrangian analysis of vortex dynamics, it can clearly be seen that the turbulent wake can be divided into two parts: near wake (the formation and development of vortices) and far wake (vortex street). The variation of the size of near wake is the same as that of the formation region. In the far wake, the vortices stretch and distort, and the rotation of vortices in the ventilated cavitating flow is not evident as that in non-cavitating flow.

Experimental investigation of ventilated partial cavitating flows with special emphasis on flow pattern regime and unsteady shedding behavior around an axisymmetric body at different angles of attack

The objective of this paper is mainly to investigate the ventilated partial cavitating flow structure at different angles of attack with experimental methods. A high-speed camera technique is used to record cavity evolution patterns. The numerical simulation is performed by CFX Solver with a free surface model and filter-based turbulence model. Three flow pattern maps are constructed to depict the flow pattern and structures at different angles of attack α, i.e. the range of Froude number Fr and gas entrainment coefficient CQ. The maps show that the angle of attack α has a significant effect on ventilated cavitating flow structures. With the increasing value of α, the asymmetry of ventilated cavity becomes obvious and the angle between the axis of the body and the closure line θ is decreased. The value of the included angle θ is found to vary with Fr number and CQ. The cavity break-off may be mainly determined by the momentum and the direction of re-entrant jet flow.