Ventilated Cavitation Research Articles

Cavitating flow control and noise suppression using air injection

The cavitation phenomenon not only reduces hydrodynamic performance but also generates vibrations and noise, significantly compromising the operational stability of the system. In this study, we investigate the efficiency of air injection in controlling cavitation patterns and reducing noise on hydrofoil, both experimentally and numerically. The focus is to assess how the location of air injection on the suction side of the hydrofoil, the rate of air injection, and the cavitation number affect the cavitating flow. The hydrofoil has a span and chord length (C) of 100 mm. The air is injected from a column of multi-holes positioned at x/C = 0.05, 0.10, 0.30, and 0.40 separately and controlled through a flow meter. The cavitation number ranges from 3.65 to 1.62, while the air injection rates are set at 1, 3, and 5 standard liters per minute. The experiments are conducted at Chungnam National University's high-speed cavitating tunnel. Simultaneously, a high-speed camera is used to observe cavitating flow, and a pressure transducer is employed to measure noise levels. The results indicate that injecting air closer to the leading edge has the most significant impact on reducing vapor cavitation and noise. Injecting air at x/C = 0.05 reduces the length of the vapor sheet cavity by 27% compared to cases without air injection. Increasing the air injection rate increased the volume of ventilated cavitation. Noise reduction is primarily noticeable in the high-frequency region (>2 kHz) at a high cavitation number of 2.22. As the cavitation number decreases to 1.62, the noise reduction shifts mainly to the low-frequency region, and the effectiveness of air injection in suppressing noise is reduced.

Analyzing the influence of dimensions of the body behind the cavitator on ventilated cavitation

Investigating the impact of various parameters on the characteristics of supercavitation is an essential and continuously evolving matter. In the current study, the influence of the geometry behind the cavitator on the characteristics of ventilation supercavitation at different Froude numbers and at a constant ventilation coefficient has been investigated. For this purpose, at first, by using the experimental method on a model with a disk cavitator at different Froude numbers, the ventilation cavitation has been investigated and the appropriate numerical method has been validated based on the experimental results. Next, using the numerical method, the effect of filling the cavity volume by geometry with different dimensions and confined inside the cavity on the characteristics of the ventilated supercavity was investigated. Also, the geometry with unlimited length and variable diameter was investigated in order to determine the effect of increasing the diameter on the characteristics of the cavity. The results indicate that increasing the volume of the aft body confined within the supercavity does not have a significant effect on its characteristics. However, increasing the diameter of the aft body caused a significant reduction in the length of the supercavity for the geometry with an infinite body length.

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.

Investigation on natural to ventilated cavitation considering the air-vapor interactions by Merging theory with insight on air jet location/rate effect

Cavitation is of significant practical importance since unstable flow characteristics can have noticeable consequences on objects nearby. An essential approach for controlling the cavitation flow field's instability is air injection. This work aims to conduct a numerical and experimental investigation on the natural to ventilated cavitation around a Clark Y hydrofoil. Having three phases: water, vapor, and air, the cavitation model is adjusted based on the Merging theory to consider the impact of dissolved air on cavitation. Furthermore, the Density Corrected-based Method (DCM) is used to alter the turbulence model. The experimental tests are carried out in the water tunnel, which can maintain the constant water flow rate (Qwater=490 m3⁄h) and regulate the pressure level (105–180 kPa). The Clark Y hydrofoil is fixed at an angle of attack of 8° and includes injection and pressure taps. Two holes, called Tap1-injection and Tap5-injection, are alternatively used for air injection purposes.Two cavitation numbers (σ=1.1, 1.6) and three air injection rates (Q = 0, 0.5, 1 l/min) are considered current case studies. The results demonstrate the meaningful impact of location and rate of aeration on the dynamic/average characteristics of cavitation. Increasing the air injection rate results in an increase in the pressure coefficient values, a decrease in the shedding frequency, and an elongation of the cavity with an M-shaped structure. Also, during a cycle of cavity development, air injection may take place in the reversed jet, perpendicular jet, or direct jet configurations. In addition, a close agreement between numerical and experimental results is recorded.

On ventilated supercavities moving horizontally near free surface

By means of open source CFD tool OpenFOAM platform, a multiphase solver capable to deal with two phases flows is used to investigate ventilated supercavities. The VOF (volume of fraction) method is employed in order to capture cavity interface and the free surface. The main objective of this work is to investigate ventilated supercavity generated after a disk-cavitator which moves horizontally near a free surface. Most of the work in this research has been focused on the shape of supercavity and its asymmetry. Furthermore, two distinct cavity closure types, i.e. the re-entrant jet (RJ) and the twin vortex (TV) and are discussed separately to compare their gas leakage mechanisms at different submergence ratio. Numerical results indicate that the air-water free surface will have a significant influence on the cavity surface curvature and gas shedding process. Under the influence of free surface, the length diameter ratio of ventilated cavitation increases with H‾ decrease with decrease. The influence of free boundary on the upper surface of cavitation interface is greater than that on the lower surface. When Fr number and ventilation volume are constant, when approaching the free surface, the interface curvature of ventilation cavitation will increase. The deflection of cavitation axis is mainly affected by gravity and free surface. When the Fr number is small, the cavitation axis will be slightly disturbed. When the Fr number is large, the cavitation axis deflects reversely. These may give meaningful references for the design of surface vehicles.

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 investigation of pulsating energy evolution in ventilated cavitation around the NACA0015 hydrofoil

The pulsation characteristics of the ventilated cavitation are investigated in this paper with the Large Eddy Simulation method. As the ventilation rate increases, the air entrainment effect aggravates the instability of vapor cavity, and the re-entrant jet further accelerates the separation of attached vapor cavity from the suction surface. The suppression of cloud vapor cavity and the formation of stable ventilated supercavity can significantly reduce the time-averaged pulsation level of pressure and velocity. The pulsating energy of cavitating flows essentially comes from the time-averaged flow in natural cavitation and ventilated cavitation. The gradient diffusion effect tends to transfer the turbulent kinetic energy carried by cavities to the mainstream region, while the redistribution of pulsating enstrophy through the gas-liquid interface can be significantly suppressed by the injected air. The viscous dissipation term cannot be ignored because it does perform some fluctuating strip-like structures near the interface of large-scale vortex in ventilated cavitation. Two fluctuation stages and mechanisms of the ventilated cavity interface on a hydrofoil are revealed, moreover, the transportation of pulsating enstrophy on the surface of ventilated cavity located above the suction surface is clarified as an important mechanism to maintain the interface fluctuating state.

Analysis of the re-entrant jet to twin vortex flow regimes transition in ventilated cavitation

The combination of ventilated cavitation (injection of a non-condensable gas in a liquid flow) and natural cavitation (flow vaporization due to a local pressure drop) is studied with a numerical model based on a homogeneous multiphase flow approach, using two transport equations for vapor and air. Ventilated cavitation behind an axisymmetric body is simulated and the results compare well with previous experimental data for both the reentrant jet and the twin vortex flow patterns. The transition between the two regimes is found to be primarily related to a critical shift of the vortices generated behind the body, due to air injection. It is also shown that the critical value of σc⋅Fr (product of ventilated cavitation number with the Froude number) at the transition depends strongly on the shape of the body. The influence of the baseline flow without air injection (Reynolds number and natural cavitation) on the transition is also analyzed. When natural cavitation is negligible, only the twin vortex regime is obtained for a Reynolds number Re < 8 104, while for a higher Re, the ventilation air flow coefficient at transition CQt varies almost linearly with Ln (Re). In presence of additional natural cavitation, (σ < 1), a decrease of CQt is observed. This phenomenon is amplified when natural cavitation increases. This drop of CQt can be directly related to the amount of vapor generated in the flow field, as it is shown that it corresponds to the maximum positive flux of vapor behind the body.

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.

Numerical study on effect of brash ice on water exit dynamics of ventilated cavitation cylinder

This study numerically examines the evolution of cavitation, trajectory, and impact load during the process of water exit of a submerged and ventilated cavitation cylinder in a brash ice zone by coupling the computational fluid dynamics (CFD) method with the discrete element method (DEM). A two-way coupling scheme is used to study the influence on the fluid due to the brash ice. The ice conditions off Wilkes Land in the Antarctic marginal ice zone in late winter were used to formulate a DEM particle model without failure limitation. The brash ice zone was established according to random positions of the ice generated by self-programming. The CFD-DEM coupling method was verified through comparisons with both experimental results and empirical formulae in the literature. The results show that the cavity quickly broke while crossing the water surface, and a prominent formation of spikes was observed in brash ice conditions. The displacement and deflection angle of the cylinder had different features in the two directions perpendicular to the axis. Furthermore, strong alternating loads were observed when the water surface was crossed in comparison with those in ice-free conditions.

When superhydrophobicity can be a drag: Ventilated cavitation and splashing effects in hydrofoil and speed-boat models tests

Superhydrophobic surfaces are expected to reduce the hydrodynamic drag on marine vessels due to the lubrication effects of the naturally sustained thin air-layer plastron. By conducting model-tests with a hydrofoil-boat and a speed-boat, we demonstrate that the application of a nanoparticles-deposition-based superhydrophobic coatings on marine vessels can also lead to hydrodynamic effects that significantly increase the drag. In the case of the hydrofoil boat, the use of superhydrophobic coating resulted in plastron-enhanced ventilated cavitation and the formation of water jets that reduce the speed of the boat by more than 30%. In the case of a towed speed-boat, the use of nanoparticle-deposition superhydrophobic coating on the hull affected how the boat splashes water but did not change the net drag on the boat during the transition to the high-speed planing mode of operation. The use of a superhydrophobic coating on the speed-boat propeller was found to inhibit its surface piercing and prohibit the transition to the planing mode, resulting in up to three-time lower speed. These novel effects of the superhydrophobic coating should be accounted for together with the anticipated reduction in friction drag in the design of advanced marine vessels.

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.

Numerical modeling and investigation of the effect of internal waves on the dynamic behavior of an asymmetric ventilated supercavity

In this paper, a realizable two-layer k−ε turbulence model and the Schnerr-Sauer cavitation model are used to simulate dynamic behavior of asymmetric ventilated supercavies while considering the effects of internal wave s. The effectiveness of the numerical method is verified via comparison with the available experimental data and the analytical solution of the internal wave velocity. The results show that the presence of internal waves affects the supercavity closure patterns and shedding periods. Compared to a system with no internal waves, the cavity shedding and pressure pulsation intensity increase and the shedding period is closely related to that of the internal wave. Internal waves also play an important role in the distribution of ventilated cavitation patterns inside the cavity. The local cavitation number is shown to reflect the pressure change during cavity evolution and indicates that the internal wave affects the flow field inside the cavity. In addition, the effects of internal waves of various amplitudes on cavitator hydrodynamic performance are studied. As the internal wave amplitude increases, the cavity pulsation evolution becomes more intense. Furthermore, the ventilated cavity intensity, lift, and drag are closely related to the amplitude of internal waves.

Numerical investigation on the formation mechanism of ventilated cavitation with gas jet cavitator

The complexity of the formation mechanism of ventilated cavities makes it difficult to be explored experimentally. In this study, the formation mechanism of cavity regimes around a gas jet cavitator were first numerically predicted using the partially averaged Navier–Stokes (PANS) and homogeneous free surface models. The numerical framework was validated by comparing the numerical predictions with available experimental data. The numerical results showed that the cavity evolves across four different regimes with increasing ventilation rate, that is, bubbly flow, stable cavity, unstable cavity, and jet cavity. Moreover, the gas jet length in the front of the nozzle continues to increase, and the vortex structure in the cavity transitions from a streamwise vortex to a vortex filament. Furthermore, the correlation between the adverse pressure gradients and the characteristics of the cavity was analyzed, and the results demonstrated that the adverse pressure gradient leads to stagnation of the gas jet and cavity closure.

Numerical Simulation of Ventilated Cavitation Evolution with an Insight on How Ventilation Influences Pressure Fluctuation and Cavitation Noise

Cavitation occurred in hydraulic machines can generate severe pressure fluctuations and induce high-pitched noise. Ventilated cavitation (inject air into the cavitating flow) is one of the most effective ways to control cavitation and then alleviate the noise and fluctuations. Thus, the evolution of ventilation cavitation around a NACA0015 hydrofoil was numerically investigated with a modified model. The results indicated that the ventilated cavitation consists of two parts: the attached cavity which attached to the leading edge of the hydrofoil and the detached cavity which detached from the hydrofoil surface. With the air injection increased, the detached cavity becomes larger. Besides, the ventilated cavity evolves periodically along with two opposite vortexes which fall off in turn near the tailing edge of the hydrofoil. Among three ventilation volumes, an air injection of 250 L/min presents the best alleviation on pressure fluctuation induced by cloud cavitation. The acoustic analysis indicated that air injection is an effective way to alleviate the cavitation induced noise. With air injected into the flow, two new types noises induced by the ventilated cavitation has been detected by monitoring points along the upper side and behind of the hydrofoil: the lower frequency noise induced by the waving of attached cavity and the higher frequency noise induced by the shedding of the detached cavity. While with the air injection increased, both of the two types noises increased. The acoustic and dynamic patterns under different air injection conditions are able to provide guidance in engineering application.

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.

Large Eddy simulation of ventilated cavitation with an insight on the correlation mechanism between ventilation and vortex evolutions

In this paper, the unsteady natural cavitation and ventilated cavitation are investigated with the large eddy simulation method. The predicted cavity morphology and evolution process before and after ventilation agree well with the available experimental data. The results indicate that in natural cavitation, the formation of a U-type cavity is closely related to the separation of a re-entrant jet and the effect of vortex evolution, while the local high pressure region plays an important role in shaping the appearance of the primary and secondary U-type cavity in the process of moving downstream. During ventilated cavitation, the deflation effect of the air drives the pair of vortices to shed periodically, accompanied by the downstream transportation of vorticity, and promotes the formation of arched air cavities. In the anticlockwise vortex shedding region, the vortex dilatation term has a concentrated distribution on the air-liquid interface which is due to the intense mixing of the phases. Further analysis demonstrates that, in the air cavity fluttering area, the magnitude of the velocity pulsation produces a significant downward tendency due to the gradual weakening of vortex intensity. Moreover, the interaction between vortices and turbulence shows a high degree of consistency in the wake flow.

Numerical Analysis of Multi-Phase Flow around Supercavitating Body at Various Cavitator Angle of Attack and Ventilation Mass Flux

Cavitation flow is an important issue in many areas of mechanical engineering. In this study, the natural and ventilated cavitation analyses were performed using the developed code for the cavitation flow analysis. The governing equation is the Navier-Stokes equation based on a homogeneous mixture model. This model assumes that all fluids are in an equilibrium state of momentum. The momentum equations are solved using the homogeneous mixture phase, although the continuity equations are solved in the liquid, vapor, and gas phases, separately. Computational analysis was performed for the different injection conditions and inflow velocity conditions under the same conditions as the experiments. The comparison between the cavitation shape and the drag showed good agreement with the experiments. Based on this, this study predicted the change of cavitation shape according to the change of cavitator angle of attack.

Numerical investigation of positive effects of ventilated cavitation around a NACA66 hydrofoil

Cavitation is of great practical interest because unsteady flow features can induce negative effects such as mechanical erosion and noise. Air injection is an important way to adjust the instability of the cavitation flow field. In the present study, we numerically simulate a NACA66 hydrofoil with a particular emphasis on understanding the effects of ventilation on cavitation characteristics such as cavity dynamics, vortex structure, and the wake field. Cavitation is modeled by using the Schnerr-Sauer cavitation model, and the Large Eddy Simulation (LES) method is used to calculate the unsteady natural and ventilated cavitation flow. The results of the numerical simulation are in good agreement with experiment, which verifies the effectiveness of the numerical method. The evolution of cavity dynamics and vortex structure are analyzed to clarify how multi-scale vortex structures evolve in the flow field. The results show that the shedding speed of the ventilated cloud cavity is faster than the natural cavitation. In addition, ventilation improves the pressure pulsation on the cavitation hydrofoil surface. The rotational effect in ventilated cavitation is more significant. Furthermore, ventilation causes strong, large-scale, pulsating eddy currents to rotate into small-scale eddies. An analysis of the vorticity transfer equation shows that the gas injection increases the velocity gradient and enhances the conversion of the two gas-liquid phases. The results in the wake of the hydrofoil show that ventilation can effectively reduce the turbulence intensity and turbulence integral scale, which indicates that ventilation can improve the velocity pulsation and instability. Interestingly, increasing the ventilation rate helps to improve the pressure peak on the hydrofoil surface and increases the lift and drag coefficient. Moreover, gas injection is responsible for cavity pulsation and the re-entrant jet enhances the fluctuation intensity.