Sheet Cavitation Research Articles

Large eddy simulation of micro vortex generator-controlled cavitation across multiple stages

Micro vortex generators (mVGs) control cavitation by altering the boundary layer flow structure. This study employs the wall-adapting local eddy-viscosity large eddy simulation (WALE-LES) turbulence model combined with the Zwart–Gerber–Belamri cavitation model to conduct transient numerical simulations on the National Advisory Committee for Aeronautics 0015 baseline hydrofoil and the hydrofoil equipped with mVGs under various cavitation numbers. The proper orthogonal decomposition method and experiments verify the accuracy and consistency of these simulations regarding cavity scale. The study elucidates mechanisms by which mVGs suppress cloud cavitation at low cavitation numbers and induce vortex cavitation at high cavitation numbers. Results indicate that mVGs maintain sheet cavitation characteristics at low cavitation numbers, reducing wall pressure fluctuations and enhancing flow stability. During cavitation inception, mVG-induced vortex cavitation leads to early cavitation formation. In the sheet cavitation phase, modal energy distribution is more dispersed, while in the inception phase, energy is concentrated with significant dominant modes. Moreover, the counter-rotating vortices generated by mVGs mitigate flow separation, enhance leading-edge flow attachment stability, and reduce high-frequency vibrations caused by bubble shedding. This study significantly advances the understanding of cavitation control by accurately simulating and revealing the cavitation control mechanisms of mVGs across different stages using the WALE-LES model. The findings demonstrate that mVGs can effectively stabilize cavity structures at low cavitation numbers, reducing flow instabilities and enhancing overall hydrofoil performance. These insights will have a significant impact on the design of hydrofoils and the development of cavitation control strategies.

Computational comparison of passive control for cavitation suppression on cambered hydrofoils in sheet, cloud, and supercavitation regimes

Cavitation is a transient, highly complex phenomenon found in numerous applications and can have a significant impact on the characteristics as well as the performance of the hydrofoils. This study compares the evolution of transient cavitating flow over a NACA4412(base) (NACA stands for National Advisory Committee for Aeronautics) cambered hydrofoil and over the same hydrofoil modified with a pimple and a finite (circular) trailing edge. The assessment covers sheet, cloud, and supercavitation regimes at an 8° angle of attack and the Reynolds number of 1×106, with cavitation numbers ranging from 0.9 to 0.2. The study aims to comprehensively understand the role of the rectangular pimple in controlling cavitation and its impact on hydrodynamic performance across these regimes. Numerical simulations were performed using a realizable model and the Zwart–Gerber–Belamri (ZGB) cavitation model to resolve turbulence and cavitation effects. The accuracy of the present numerical predictions has been verified both quantitatively and qualitatively with available experimental results. The present analysis includes the time evolution of cavities, temporal variation in total cavity volume, time-averaged total cavity volume, distributions of vapor volume fractions along the chord length, and their hydrodynamic performance parameters. Results demonstrate that rectangular pimples have significant impacts in the different cavitation regimes. In the sheet cavitation regime (σ=0.9), the NACA4412(pimpled) hydrofoil exhibits minimal cavity length and transient volume changes as compared to the NACA4412(base) hydrofoil. In the cloud cavitation regimes (σ=0.5), cavity initiation occurs differently, starting from the pimpled location for the NACA4412(pimpled) hydrofoil, unlike the initiation just downstream of the nose in the case of base hydrofoil. In the supercavitation regimes (σ=0.2), the cavity length remains comparable, but the NACA4412(pimpled) hydrofoil exhibits larger cavity volume evolution in both cloud and supercavitation regimes (σ=0.5 and σ=0.2) after initial fluctuations. Furthermore, hydrodynamic performance for the NACA4412(pimpled) hydrofoil shows 41%, 36%, and 17% lower lift coefficients, and 46%, 27%, and 9% lower drag coefficients in sheet, cloud, and supercavitation, respectively.

Study on the formation and characteristics of unstable sheet cavitation on hydrofoils

Hydrofoils, as fundamental components of hydraulic machinery, directly influence the performance of such machinery. This study conducted an analysis of the cavitation characteristics of hydrofoils at a + 4° angle of attack under various cavitation numbers using numerical simulation and experimental research methods. The focus of the research was to explore the phenomenon of unstable sheet cavitation and its causes, as well as to reveal the characteristics of the re-entrant jet. The large eddy simulation method was employed to calculate the cavitation morphology under three different cavitation numbers. The method is highly consistent with the experimental results in simulating the small-scale detachment at the tail of unstable sheet cavitation and the large-scale shedding of cloud cavitation. The study found that the detachment of unstable sheet cavitation is closely related to the re-entrant jet, which exhibits transient and abrupt characteristics during the unstable sheet cavitation phase. Furthermore, by applying FFT processing to the distribution of maximum reverse velocity and the spatiotemporal changes of Ux on characteristic lines, eigenfrequency of the detachment of unstable sheet cavitation were identified. The research results indicate that cavitation mainly show as sheet cavitation when the cavitation closure point does not exceed the zero-slope point. Beyond this point, it transitions to cloud cavitation. This study provides new insights into the cavitation phenomenon of hydrofoils and offers quantitative research on the phenomenon of unstable sheet cavitation.

Prediction of cavity inception speed and underwater radiated noise of a full-scale marine propeller based on a cavitation tunnel model test

Propeller cavitation is a dominant source of underwater radiated noise (URN) from shipping, and the appropriate evaluation of propeller noise mitigation measures is important for ecological ship design. In this study, a model test procedure and scaling method to predict the full-scale URN level from hydrodynamic noise sources were introduced. The cavity size based on the empirical vortex model was considered in the proposed scaling method for a quiet cruise condition with isolated tip vortex cavitation. This relation provides information on the equivalent cavitation number for the model test to have the same relative radius as that of the full-scale tip vortex cavitation. The alternative test cavitation number can also be chosen as close as possible to the point that gives the equivalent tip vortex cavity size if sheet cavitation exists in the condition for the equivalent cavity size. In this case, additional noise scaling should be performed using the ratio of the tip vortex cavity radii. The new scaling exponent introduced in this study varies according to the distance from the cavitation inception condition and shows a decreasing trend from the value at the inception condition as the equivalent cavitation number for the model test decreases. The scaled URN levels based on the proposed method showed reasonable agreement with the measurement results of a full-scale ship. Finally, the noise reduction effect of the specially designed wake control fin and propeller was investigated through a comparative model test following the proposed scaling strategy, and significant noise reductions were observed with changes in the design configuration.

Large eddy simulation investigation of micro-vortex generator control effect on early stage sheet cavitation

The wall-adapting local eddy-viscosity large-eddy simulation method is employed for the numerical simulation of a hydrofoil, with transient calculations conducted to compare and analyze the near-wall flow characteristics and cavity morphologies of both the baseline and micro-vortex generator (mVG) hydrofoil models under conditions of high cavitation numbers. High-speed photography combined with numerical analysis revealed that mVGs generate a pair of counter-rotating vortices, boosting the transfer of momentum between the boundary layer and the main flow while reducing flow separation. These vortices induce a new mixed cavity structure at the leading edge, combining vortex cavitation with attached sheet cavitation. During cavity evolution, the mVGs prevent overall tail shedding in the baseline hydrofoil, confining shedding to the sides, while the central vortex cavitation structure remains stable. It enhances hydrofoil stability by reducing pressure fluctuations and guiding cavitation toward more predictable dynamics without causing significant pressure impacts. This research elucidates the mechanism of mVGs in guiding fluid attachment, transforming the structure and shedding cycle of attached cavities, and emphasizing its effectiveness by controlling early-stage sheet cavitation.

Study of Hydrofoil Unstable Partial Cavitation Emphasizing Unstable Sheet Cavitation Phenomenon

Abstract Unstable partial cavitation phenomenon often occurs during the operation of axial flow pumps. In this paper, the hydrofoil of an axial flow pump is taken as the research object, and the cavitation morphology at the design angle of attack is obtained by high-speed photography. And the flow characteristics under the transformation cavitation number of sheet cloud cavitation are studied by SBES turbulence model. The hydrofoil has hydrodynamic phenomena such as unstable sheet cavitation and multistage cloud cavitation. The high pressure propagating upstream from the trailing edge may be the cause of secondary cloud shedding, and the closer to the hydrofoil wall surface, the greater the intensity of this pressure. The frequency of unstable sheet cavitation shedding is 166.6 Hz. The instable closure of cavitation and the hydrofoil wall may be the main cause of unstable sheet cavitation formation. The generated quasi-periodic re-entrant jet will have a greater impact on the flow in the rear of the hydrofoil. This study provides reference for the design and operation of axial flow pumps and prevention of cavitation damage.

Numerical simulation of cavitation-vortex interaction mechanism in an advanced rotational hydrodynamic cavitation reactor

Hydrodynamic cavitation (HC), a promising technology for enhancing processes, has shown distinct effectiveness and versatility in various chemical and environmental applications. The recently developed advanced rotational hydrodynamic cavitation reactors (ARHCRs), employing cavitation generation units (CGUs) to induce cavitation, have demonstrated greater suitability for industrial-scale applications than conventional devices. However, the intricate interplay between vortex and cavitation, along with its spatial-temporal evolution in the complex flow field of ARHCRs, remains inadequately elucidated. This study investigated the interaction mechanism between cavitation and vortex in a representative interaction-type ARHCR for the first time using the “simplified flow field strategy” and the Q-criterion. The findings reveal that the flow instability caused by CGUs leads to intricate helical and vortex flows, subsequently giving rise to both sheet and vortex cavitation. Subsequently, utilizing the Q-criterion, the vortex structures are identified to be concentrated inside and at CGU edges with evolution process of mergence and separation. These vortex structures directly influence the shape and dimensions of cavities, establishing a complex interaction with cavitation. Lastly, the vorticity transport equation analysis uncovered that the stretching and dilatation terms dominate the vorticity transport process. Simultaneously, the baroclinic term focuses on the vapor-liquid interface, characterized by significant alterations in density and pressure gradients. These findings contribute to a better comprehension of the cavitation-vortex interaction in ARHCRs.

Study of the effect of cavitation flow patterns in diesel injector nozzles on near-field spray atomization characteristics using a LES-VOF method

The spray atomization process plays a critical role in achieving high efficiency and reducing emissions in internal combustion engines. The fuel injection atomization process, especially the spray primary atomization process, is significantly influenced by turbulent flow and cavitation phenomena within the nozzle. In this study, based on the coupled large eddy simulation(LES) and a volume-of-fluid (VOF)method, the effects of four typical nozzle internal flow patterns on the spray primary breakup in the near-nozzle region were investigated. To realize this objective, the generation of four distinct flow patterns was accomplished through the design and construction of corresponding nozzle structures: (1) nearly single-phase flow in a tapered-orifice nozzle with low orifice location, (2) strong geometry-induced sheet cavitation flow with sheet region extending to the nozzle orifice exit in a normal cylindrical orifice, (3) vortex-induced string cavitation flow in a tapered-orifice nozzle with relative high orifice location, and (4) geometry-induced sheet cavitation is partially suppressed while it is enhanced in the orifice region close to exit in a convergent-divergent orifice. The numerical results unequivocally reveal that the initial atomization of the spray exhibits remarkable efficacy in the case of the conventional cylindrical nozzle, characterized by internal regions of substantial sheet cavitation, despite a relatively minimal flow discharge coefficient. Conversely, the normal tapered-orifice nozzle, lacking significant cavitation, demonstrates a larger flow discharge coefficient, yet its initial atomization performance is notably suboptimal.It is worth noting that the appropriate vortex-induced cavitation in the nozzle can correspond a higher flow rate with a thin string-type cavitation and simultaneously a better spray atomization quality with a strong vortex flow. Also it is concluded that a convergent-divergent nozzle with appropriate transition location can achieve strong atomization .

An improved laminar–turbulent transition cavitation model using the IDDES method

Prediction of the detachment positions of sheet cavitation remains an unsolved problem, being not only dependent on local pressure but also closely related to the boundary layer transition. For hydrofoils, traditional cavitation models tend to predict premature cavity detachment positions. In the study reported here, to obtain cavity detachment positions that match experimental results, improvements are made to both turbulence models and cavitation models. A scale-resolving transition turbulence model is established by incorporating transport equations to represent boundary layer transition that are based on the improved delayed detached eddy simulation (IDDES) method. The intermittency γ, representing the probability of turbulence at a specific point in space, is introduced into the homogeneous cavitation model to account for the influence of the laminar–turbulent transition on the sheet cavitation detachment position. Numerical results show that the IDDES-based transition turbulence model combines transition and scale-resolving capabilities. The improved turbulence transition cavitation model proposed here is shown to be able to accurately predict the detachment position of sheet cavitation around two-dimensional hydrofoils and a typical axisymmetric ITTC body.

Interaction mechanism between cloud cavitation and micro vortex flows

Micro vortex generators (micro-VGs) could affect cavitation, but the control mechanism is not yet fully understood. Simulating and analyzing the effect of micro vortex generators (micro-VGs) on cloud cavitation is crucial as it can help in suppressing the collapse of cavitation clouds, which is beneficial for marine machinery. In this work, micro-VGs were installed parallel to the initiation of the attached cavity on a NACA0015 hydrofoil. A compressible simulation method was used to obtain the cavitating flows around both the smooth hydrofoil and the one with micro-VGs. Subsequently, the generation of micro vortex flows and the effect of micro-VGs on transient evolution of cloud cavitation were analyzed. The results indicate that the installation of triangular micro vortex generators alters the flow vectors around them, inducing micro vortex flows with opposite rotational directions behind adjacent vortex generators. These vortex generators potentially affect cavitation on the hydrofoil by modifying pressure distribution, vortex structures, and turbulent kinetic energy. Under the specified cloud cavitation operating condition, the shedding and collapse of clouds on the smooth hydrofoil are replaced by the partial separation and collapse of sheet cavitation on the hydrofoil with micro-VGs. Despite a minor loss in time-averaged lift, the lift-to-drag ratio is improved, and the stability of the flow field is enhanced. After installing micro-VGs, the pressure fluctuation is primarily caused by the partial collapse and separation of sheet cavitation. Installing micro VGs at the hydrofoil's leading edge suppresses pressure pulsation.

Numerical investigation of cavitation vortex dynamics in different cavitation patterns coupled implicit large eddy simulation and boundary data immersion method

The objective of this paper is to investigate the flow characteristics of different cavitation flow patterns around a NACA (National Advisory Committee for Aeronautics) 66 hydrofoil by applying the BDIM (boundary data immersion method) and ILES (implicit large eddy simulation) with an artificial code. Meanwhile, an artificial compressibility method is also employed to consider the effects of compressibility on cavitating flow. The results present that the numerical method can effectively capture different cavitation patterns, which agrees well with the previous experimental data. Subsequently, the detailed analysis of vortex structures and dynamics for the non-cavitation (σ = 3.0), sheet cavitation (σ = 2.0), and cloud cavitation (σ = 1.6) cases with the Liutex method and the vortex enstrophy transport equation have been investigated. When cavitation occurs, the degree of turbulence and the enstrophy in the flow field have been enhanced, due to the disturbance of the velocity field. For sheet cavitation, complex vortex structures appear in the attached cavity region with high-intensity enstrophy causing by the highly intense velocity and density gradient. As the cavitation pattern transits from the sheet cavitation to the cloud cavitation, more complex vortex structures can be observed in the cavitation region. Furthermore, the value and the fluctuation amplitude of enstrophy intensity increase significantly under the effect of reentrant jet. Analysis of the enstrophy transport equation indicates that the vortex stretching term and dilatation term for cloud cavitation increase relatively significantly with the movement of the reentrant flow and are highly dependent on the cavitation evolution. In addition, the region affected by the baroclinic torque also increases.

PIV measurement of cavitating flow around a pitching hydrofoil

Particle Image Velocimetry (PIV) and hydrodynamic measurements were applied to investigate the cavitating flow of a pitching hydrofoil in the form of triangular wave motion (mean incidence α 0=10°, amplitude Δα=5° and pitching frequency f*=2Hz). The Reynolds number is Re=4.5×105. The cavitation number is set to σ=2.3, 1.36. Different typical cavity patterns are observed by PIV images in the whole pitching period, including sub cavitation, inception cavitation, sheet cavitation and cloud cavitation. As the σ decreases, the cavity area increases. And the the corresponding time and position of cavitation patterns also changed significantly. In the instantaneous velocity field of an oscillating hydrofoil, the low velocity zone (LVZ) and the mainstream zone (MZ) are observed. The LVZ corresponds to the cavitation region and changes with the change of cavitation morphology. Meanwhile, the LVZ has a large velocity gradient. The velocity gradient in the MZ is larger along the direction parallel to the incoming flow, and the velocity tends to increase first and then decrease gradually downstream. With the decrease of cavitation number, the area of LVZ increases. And the distribution of LVZ is more uneven, which is mainly caused by oscillation motion and uneven water vapor mixing.

Numerical investigation on vortex dynamics characteristics of a large vertical centrifugal pump operating in the hump region under cavitation conditions

The large vertical centrifugal pump (LVCP) is widely used in major hydraulic engineering such as cross-region water transfer. This paper focuses on the hydraulic stability of LVCP under extremely operating conditions. The transient numerical simulations of cavitation in LVCP in the hump region are conducted based on delayed detached eddy simulation coupled with a homogeneous cavitation model. The main research is the effect of cavitation on the flow pattern and the vortex dynamics of the LVCP under stall conditions. The results of the numerical investigations show that the variation of the pump performance parameters is consistent with the experimental data under cavitation conditions. Under deep stall conditions, the H drop due to cavitation in the pump can happen earlier. Under the critical cavitation of the stall condition, backflow vortex cavitation, sheet cavitation and separation vortex cavitation can be observed at the impeller inlet. The vortex structures generated at the cavitation tail can result in worse flow patterns in the impeller. The vortex dynamics analysis shows that severe cavitation in the impeller deteriorates the inlet conditions of the vaned diffuser and induces sheet cavitation at the LE of vanes. The cavitations in the vaned diffuser accelerate the production and development of stall vortices and result in increased flow instability. The research in this paper can be helpful to improve the operation stability of LVCP.

Stabilize cloud cavitation with an obstacle near hydrofoil's trailing edge and conduct local entropy production analysis

Cloud cavitation always causes severe damage to the efficiency and stability of the hydraulic machinery, resulting in extra energy losses in the system. We have observed an effective and simple way to prevent cloud cavitation formation by placing an obstacle near the hydrofoil's trailing edge. Cavitating flows around four different types of hydrofoils were simulated using the stress-blended eddy simulation turbulence model: the National Advisory Committee for Aeronautics (NACA) 66 hydrofoil and the NACA 66 hydrofoil with a 1 ×1 mm2 obstacle at 0.3c, 0.5c, or 0.7c. Sheet cavitation is the predominant mode of cavity flow when the obstruction is positioned at 0.7c. To find out why the cloud cavitation growth can be stopped when the obstruction is positioned at 0.7c, the velocity field, vorticity in the Z direction, and vortex structure of the Q-criterion were computed. To study the energy loss of the cavity flow and comprehend how obstacles affect it, the local entropy production rate was computed. It was discovered that the vorticity downstream of the obstacle, positioned at 0.7c, is restructured, which helps manage the flow separation upstream of the obstacle. Consequently, the hydrofoil's suction surface vorticity nearly rotates in the same direction as the obstacle at 0.7c, and the direction of Vx upstream of the obstacle is in the positive direction of the X axis, indicating that the reentrant flow has been controlled upstream of the 0.7c obstacle. Furthermore, cavitation shedding and the entropy production rate are strongly correlated, and regulating cloud cavitation growth is advantageous for energy conservation.

Magnetic non-destructive monitoring of a ship's propeller blade after long-term operation

This study examines the potential of magnetic non-destructive techniques for monitoring surface damage to ship propeller blades made of stainless steel. The propeller considered in this investigation operated for 35 years in the Danube river. Sheet and vortex cavitations, as well as random mechanical impacts on the trailing and leading edges, were detected as the major mechanisms of damage to the blade. Non-destructive monitoring of this damage was based on the Barkhausen noise and Helmholtz feritscope techniques. The blades investigated here had a composite structure in which the ferrite stainless body was mixed with the deposition of austenite on the trailing and especially the leading edges. Damage to the ferritic phase was mostly associated with sheet cavitation and a decreasing magnitude of Barkhausen noise as a result of repetitive plastic deformation. Conversely, vortex cavitation and random impacts to the edges in the austenite regions were associated with increasing Barkhausen noise due to strain-induced martensite transformation, a result that was also confirmed by data obtained from a Helmholtz feritscope. Information about plastic deformation and the fraction of strain-induced transformation was also confirmed by X-ray and electron backscatter diffraction techniques.

A panel method for the hydrodynamic analysis of a cavitating propeller with sheet and developed tip vortex cavitation

In this article, we present a boundary element method (or panel method) applied to a cavitating propeller to investigate its performance with/without cavitation. The present method models both sheet cavitation and developed tip vortex cavitation concurrently so they can coexist in the flow induced by a propeller. The sheet cavitation can be either partial cavitation or supercavitation depending on the imposed loading on the propeller, and the developed tip vortex cavitation considers loading from the blade tip to determine its sectional volume in space and time. Elaborate numerical schemes are introduced to model both cavities, and it is shown that the present method can predict cavity patterns closely following experimental observations at various loading conditions and cavitation numbers. With the predicted cavities considered, cavitating propeller performance is predicted in potential flow and compared with experimental data, results from a viscous flow solver, and fully wetted performance to verify the accuracy and feasibility of the present method. Steady flow is considered in this study, although the method can be readily extended to unsteady problems.

Investigation of cavitation evolvement in a pump inducer by proper orthogonal decomposition analysis of video images

The aim of this study is to develop a contour extraction method for the investigation of the temporal and spatial characteristics of cavitation evolution in high-speed inducers. Cavitation experiments were conducted, and corresponding snapshots were captured using a high-speed camera subsequently. The raw images were processed while using threshold value filtering techniques to extract only relevant bubble information. Proper orthogonal decomposition was employed to explore the evolving characteristics in a quantitative manner. Results show that the cavitation evolution process is an unsteady process when viewed from the side. In high net positive suction head (NPSH) environments, multiple small-scale spatial modals contribute significantly to the effective proportion of the cavitation structure. The attached sheet cavitation exists in the blade edge which is the main embodiment of cavitation area evolvement. Under low NPSH conditions, the primary flow feature can be reflected by a low-mode flow pattern, associated with the inducer’s rotation effect. The evolving regulation of cavitation area in developed stage is closely submitted to the trigonometric function. And the exhibited spatial coefficient exposes the hidden reflux vortex feature. The proposed image separation techniques enable the quantification of morphological features, which can provide technical details for the digital design of inducers.

Multi-scale characteristics investigation of sheet/cloud cavitation

Sheet and cloud cavitation involve the formation of large-scale cavities and microbubbles across a wide range of length scales. In this study, a two-way coupling Eulerian-Lagrangian algorithm was utilized to simulate the cavitating flow around a NACA66 hydrofoil at an incidence of 8 degrees and a cavitation number of 1.2. The large eddy simulation (LES) and volume of fraction (VOF) methods are used to capture the large scale vapor structures in Eulerian frame. Meanwhile, the discrete bubble model (DBM) and simplified Rayleigh-Plesset equation are implied to solve the dynamic of Lagrangian microbubbles smaller than local grid scale. The results indicate that large-scale cavities are periodically shed downstream, influenced alternatively by the re-entrant jet and shock wave mechanisms, resulting in periodic variations of both the number and Sauter mean diameter of microscale bubbles. During the quasi-periodic evolution process of sheet/cloud cavitation, the microbubbles scatter around the large scale cavities where have strong turbulence intensity and high vorticity, and the probability density function of discrete bubble diameter similarly conforms to Gamma distribution with the dominant bubble size between 30 and 70μm. During the growth of an attached cavity, the number density of bubbles follows a power-law scaling with a value of -5/3 on radius. For the other stages, the bubble size spectrum of smaller microbubbles exhibits a -2/3 power-law scaling when the diameter of the microbubble is less than 200 μm. For larger bubbles with a diameter greater than 300 μm, the bubble density is proportional to the bubble radius to the power of -6.

Qualitative and quantitative analysis of interaction between cavitation patterns and vortices of a pitching hydrofoil from Lagrangian viewpoint

The evolution of transient flow structures and mass transport in cavitating flow around a pitching hydrofoil is investigated qualitatively and quantitatively, and the interaction between cavitation patterns and vortices is elucidated from Lagrangian viewpoint. First, turbulence effects are estimated by the density-corrected k–ω model to account for the local compressibility of the multiphase flow at Reynolds number Re=6.4×105. Then, the formation and evolution of vorticity structures during the whole pitching cycle are analyzed using Lagrangian averaged vorticity deviation method. By comparing the flow structures and hydrodynamic properties at varying angles of attack, the cavitating flow is divided into two distinct stages, namely multi-scale cloud cavitation phase from α+=10° to α−=8°, and traveling sheet cavitation phase from α−=8° to α+=10°. Specifically in cloud cavitation, the formation of the cavitation pattern is closely related to the development of the main vortex. Furthermore, the quantitative analysis method based on Lagrangian flow network is developed to deeply analyze the transport and mixing processes. Importantly, the coherence ratio and the mixing parameter are proposed as transport indicators to precisely quantify the spatial connectivity behavior. Finally, the correlations between vapor fraction, codelength, global coherence ratio and global mixing parameter are evaluated. As the conclusion, it is shown that Lagrangian methods are powerful tool for both qualitative and quantitative analysis, and the results obtained could provide a key and important understanding of the flow structure and changing mechanism between cavitation and vortices in marine hydro and propulsion systems.

Computational investigation of cavitating flow on 3D NACA4412 hydrofoil with arc dimple

Cavitation is an unsteady and complicated process. When the cavity develops at the leading edge of hydrofoil, it expands to the trailing edge. As cavitation travels downstream, a cloud or supercavitation phenomenon may take place, which makes the flow to be more unstable. In this study, the influence of the cavitation number on the performance of a NACA 4412 hydrofoil with a dimple at an angle of attack of 12° has been explored. The simulation has been carried out in commercial software ANSYS Fluent. Zwart Gerber-Belamri (ZGB) cavitation model along with Realizable κ-∊ turbulence model has been utilised for the computation. It has been observed that performance parameters of hydrofoils are associated with cavitation number. For an angle of attack of 120 moving from the supercavitation zone (σ = 0.3) to the sheet cavitation zone (σ = 2.5), the increase in cavitation number results in a decrease in vapor cavity volume.