Side-entrant Jets Research Articles

Influence of cavitation on vortical structures and energy loss in a waterjet pump

Cavitation-induced vortex and energy loss are critical topics in the field of hydraulic machinery. Through a combination of experimental and numerical analysis, this paper investigates how blade loading affects vortical structures and energy loss during cavitation in a waterjet pump. The flow rate and cavitation conditions changed blade tip pressure loading, which significantly affected the trajectories of the primary tip leakage vortex cavitation and secondary tip leakage vortex cavitation. A considerable pressure gradient at the attached cavity closure region leads to a wall reentrant jet and a side-entrant jet, resulting in an attached vortex under the cavitation developing stage and severe stage. The development of the attached vortex leads to the shedding of attached cavities. Furthermore, the blade tip pressure difference loading significantly alters the distribution of tip leakage vortex and gives rise to a perpendicular leakage vortex (PLV). The PLV entrains the shedding cavities and forms a perpendicular cavitation vortex. These complex vortical structures induced by cavitation inevitably enhance enstrophy and lead to entropy production in the pump. The entropy production terms of viscous dissipation, turbulent dissipation, and the wall effect react differently to the development of cavitation. As the cavitation stages developed and became more severe, wall entropy production (S3) decreased. While turbulent dissipative entropy production S2 due to mixing losses occurring in the tip region increased sharply, which dominates the total entropy production S and results in an increase in S.

Assessment of cavitation erosion risk indicated by pressure impact exceeding material strength threshold

Cavitation erosion can seriously affect the performance of hydraulic machinery. To assess its risk, the cavitation erosion that covered a NACA0015 hydrofoil was investigated using experiment and simulation. In particular, the material strength threshold was considered in the experiment, and soft specimens were employed instead of common painting surfaces. High-speed cameras were utilized to capture cavity morphology, while pressure sensors simultaneously measured the impact intensity of cavitation collapse. Additionally, an erosion assessment method based on pressure impact was used in the numerical simulation. The results indicate that the cavitation erosion risk can be indicated by the intensity of pressure impact, which is caused by the collapse of cavity. The high erosion risk appears near 60% of chord length where shedding cloud collapses. Due to the influence of middle-entrant jet and side-entrant jets, the erosion risks are different at various span-wise locations. Additionally, the response differs for different materials exposed to cavitation. When the material strength threshold is low, the chord-wise difference of cavitation erosion is not significant, because the slight impact can also generate erosion pits. When the threshold is high, the chord-wise difference of erosion becomes larger, and erosion pits are more centralized.

Analysis of cavity shedding around the twisted hydrofoil

In order to understand the mechanism of cavity shedding and evolution, turbulent cavitating flows of the twisted hydrofoil were numerically investigated using the k-? turbulence model and the ZGB cavitation model. The results of the numerical calculation and the experimental method are basically consistent, which confirms the feasibility of the numerical calculation model. This study has obtained the following conclusions. Firstly, the cavity shedding can be summarized into six stages, and the cavity shape, pressure and velocity field at different stages are displayed, analyzed and compared in detail. Secondly, the shedding of cavity and its evolution are mainly caused by the re-entrant jet and side-entrant jet, in which the former provides the kinetic energy and the latter plays the role of guiding the direction. Thirdly, under the convective shearing action of the re-entrant jet and the main flow, a strong vortex located in the mid-back edge of the hydrofoil is formed, which promotes the transformation of the cavity shape into a U-shaped structure.

Boundary Vorticity Analysis and Shedding Dynamics of Transient Cavitation Flow Around a Twisted Hydrofoil

AbstractThe objective of this paper is to investigate the dynamic characteristics of transient cavitating flow over a twisted NACA0009 hydrofoil. The large eddy simulation (LES) approach is selected for the computation of fluid flow and the Zwart model is used for the mass transfer due to cavitation. Moreover, the skin-friction coefficient and boundary-vorticity flux (BVF) are used to study the flow separation. Numerical results show that the attached shear layer separates from the boundary layer and then squeezes to form the separation line under the obstruction of the reentrant jet. The analysis based on the terms of vorticity transport equation demonstrates that vortex stretching and vortex dilatation terms dominate the evolution of the multiscale vortex. Moreover, the secondary shedding induced by the side-entrant jet enhances the instability of partial cavities and the underlying mechanism is comprehensively revealed. Furthermore, the feature of the pressure fluctuation indicates that high pressure generated by the cavity collapse at the tail simultaneously propagates to the leading edge and downstream of the hydrofoil. This enhances the intensity of the reentrant jet and side-entrant jet, promoting occurrences of flow separation near the suction surface and cavity shedding to a certain extent.

Numerical Investigation of Unsteady Cavitation Flow around E779A Propeller in a Nonuniform Wake with an Insight on How Cavitation Influences Vortex

In the current study, the turbulent cavitation flow around a marine propeller in a nonuniform wake is simulated with the shear stress transport (k−ω SST) turbulence model combining Zwart–Gerber–Belamri (ZGB) cavitation model. The predicted cavity evolution shows a fairly well agreement with the available experimental results. Important mechanisms of propeller cavitation flow, including side‐entrant jet and cavitation‐vortex interaction, are analyzed in this paper. Vorticity is found to be mainly located in cavitation regions and the propeller wake during propeller rotating. The unsteady behavior of cavitation and side‐entrant jet can both promote local vorticity generation and flow unsteadiness. In addition, it is indicated with the relative vorticity transport equation that the stretching term plays a major role in vorticity transportation, while baroclinic torque and Coriolis force term mainly influence the vorticity distribution along the liquid‐vapor interface.

LES investigation of sheet-cloud cavitation around a 3-D twisted wing with a NACA 16012 hydrofoil

In this paper, we sought to investigate unsteady cavitating turbulent flows around a twisted three-dimensional NACA 16012 hydrofoil using the large eddy simulation (LES) and volume of fluid (VOF) methods. The one-equation eddy viscosity model (OEEVM) was used to calculate the sub-grid scale (SGS) stress tensor. The compressive velocity VOF technique and the Kunz cavitation model were also employed. Numerical simulation was performed using the interPhaseChangeFoam solver within the OpenFOAM framework. Cavitation simulation was performed at three cavitation numbers within the cloud cavitation regime; that is to say, cavitation numbers: 0.95, 1.15, and 1.35. Afterwards, the details of flow predictions including the pressure, velocity, streamlines, volume fraction of water phase, and turbulent kinetic energy were reported. Cavity behaviours, including growth, shedding, and collapse of the cavity, were considered in detail. Three-dimensional cavity structures such as primary and secondary shedding and the shedding of the U-shaped horseshoe vortex were reported as well. The present work illustrated the side-entrant jets and the radially diverging re-entrant jet corresponding to the three-dimensional effect of the twisted wing. The cavity pattern and the shedding cycle frequency agreed well with the available experimental observations.

Research on passive control of cloud cavitation based on a bionic fin-fin structure

Purpose The purpose of this paper is to use the NACA 0015 symmetric hydrofoil as the research subject and control cloud cavitation on hydrofoils. Design/methodology/approach Based on observed distribution of caudal fin spines on fish, a bionic structure of fin-like spines is arranged on the hydrofoil suction surface, which maintains the cavitation in a quasi-steady state stage by eliminating the cyclic shedding process of cloud cavitation. Based on the modified shear stress transport k-ω turbulence model and the Zwart–Gerber–Belamri cavitation model, this paper compares and analyzes the NACA 0015 hydrofoil and the bionic NACA 0015 hydrofoil under condition of an angle of attack of 8° and a cavitation number of 0.8. Findings The results show that the average drag of the hydrofoil is reduced but the lift is decreased, and the lift-drag ratio is increased after arranging the bionic structure. The bionic structure can effectively reduce the turbulent kinetic energy and make the flow more stable; it also can effectively control the hydrofoil surface side-entrant jet and the vortex shedding process of the near wall region. Originality/value Based on the above conclusions, the bionic structure of fin-like spines can achieve a significant passive control in the hydrofoil cloud cavitation process.

Numerical simulation of the three-dimensional unsteady cavitating flow around a twisted hydrofoil

In this paper, unsteady cavitating flows around a twisted NACA 66 (mod) hydrofoil are simulated with both standard and non-linear k−ε turbulence models. For the standard k−ε turbulence model, no significant cavitation shedding is observed, and size of the calculated cavitation regions is much smaller than the experimental results. For the non-linear k−ε turbulence model, fair agreement with the experimental data are observed, for the cavitation region, flow structures and the primary cavitation shedding frequency. It is observed that the non-dimensional primary shedding frequency based on the maximum cavity length StLc depends linearly on the factor 1+σ. And the side-entrant jets is the inducement of the secondary shedding of the cloud cavitation. At last, a −7∕3 attenuation law, similar to other LES results, is captured in the broadband parts of the PSD curves of the pressure and hydrodynamic force.

Numerical simulations of cavitating turbulent flow around a marine propeller behind the hull with analyses of the vorticity distribution and particle tracks

The k-ω SST turbulence model was coupled with a homogenous cavitation model to simulate cavitating turbulent flow around a highly-skewed propeller behind a ship hull. The influence of the mesh resolution was investigated by an uncertainty analysis and comparisons with experimental data. The main contribution in this paper is to study the cavitating flow around the propeller behind the hull by the relative vorticity transport equation and particle trajectories. The results show that the present method can accurately simulate the transient cavitating flow around a marine propeller behind a hull. The predicted thrust coefficients, cavitation patterns and pressure fluctuations agree well with experimental data. Furthermore, the relative vorticity transport equation was used to analyze the cavitation influence on the vorticity transport. These results show that the leading edge vortex, mass transfer across the liquid-vapor interface and the side-entrant jet significantly influence the relative vorticity distribution. Moreover, particle trajectories were predicted using a three-dimensional Lagrangian technology to study the local cavitating flow structure from the Lagrangian viewpoint. The particle tracks demonstrate that the propeller rotation and the twisted geometry lead to a much more complex, distorted flow structures.

Dynamic Behaviors of Re-Entrant Jet and Cavity Shedding During Transitional Cavity Oscillation on NACA0015 Hydrofoil

Transitional cavity shedding is known as the stage of attached cavitation with high instability and distinct periodicity. In this study, we experimentally investigated the dynamic characteristics of transitional cavity (0.8≤L/c<1) shedding on NACA0015 hydrofoil with high-speed video observation and synchronous pressure measurement. In the partial cavity (0.4<L/c<0.8) oscillation, the sheet cavitation grew along the chord with good spanwise uniformity, and the middle-entrant jet played a dominant role in cavity shedding. Meanwhile, in the transitional cavity oscillation, the previous shedding cavity exhibited a prohibitive effect on the growth of sheet cavitation on the hydrofoil, resulting in concave cavity closure line. Moreover, two symmetrical side-entrant jets originated at the near-wall ends and induced the two-stage shedding phenomenon. The aft and fore parts of the sheet cavitation shed separated as different forms and eventually merged into the large-scale cloud cavity.

Cavitation–Vortex–Turbulence Interaction and One-Dimensional Model Prediction of Pressure for Hydrofoil ALE15 by Large Eddy Simulation

The cavitating flow around the asymmetric leading edge (ALE) 15 hydrofoil is investigated through large eddy simulation with the modified Schnerr–Sauer cavitation model, which considers the effect of noncondensable gas. The statistical average velocity profiles obtained by simulation and experimentation show good agreement. The time evolution of cavity shape shows that cavity growth and separation start from the short side and spread toward the long side due to a side-entrant jet. The variation frequency of the cavity length of ALE15 hydrofoil at the long side is 163.93 Hz, and the cavitation shedding frequency at the short side is 306.67 Hz, which is about twice the value of the former. The filtered vorticity transport equation is employed to investigate the cavitation–vortex–turbulence interaction. Results indicate that vortex stretching is the major promoter of cavitation development, and vortex dilatation links vapor cavity and vortices. Baroclinic torque is noticeable at the liquid–vapor interface, and turbulent stress is related to cavitation inception. Moreover, a one-dimensional model for predicting pressure fluctuation is proposed, and results show that the model can effectively predict cavitation-induced pressure fluctuation on a hydrofoil, even on a three-dimensional ALE15 hydrofoil.

Numerical analysis of the unsteady cavitation shedding flow around twisted hydrofoil based on hybrid filter model

Cavitation is a common phenomenon in components of fluid machinery and it may induce material damage and vibration. A more accurate and commercial turbulence model is required to predict cavitation. In this paper, we make a combination of filter-based model (FBM) and density correction method (DCM) to propose a new DCM FBM. Firstly, the new DCM FBM and the homogeneous cavitation model are validated by comparing the simulation result with the experiment of cavitation shedding flow around the Clark-y hydrofoil and the filter size is determined as well. Then, the cavitation pattern cycle and shedding vortex structure of the twist hydrofoil experimented by Delft University of Technology were predicted using the DCM FBM. The predicted 3-D cavitation structures and development cycle of twist hydrofoil as well as the collapsing features show a good qualitative agreement with the high speed photography results. Numerical results show that the improved turbulence model could predict the cloud cavity evolution well, including the cloud cavity generation, shedding and dissipation. It is found that the re-entrant jet induced by the by adverse pressure gradient is the main reason to generate the cloud cavity shedding. The secondary shedding is al-so observed which is result from the combination of the radially advancing re-entrant jet and side-entrant jet simulated by the DCM FBM turbulence method.

Large eddy simulation and Euler–Lagrangian coupling investigation of the transient cavitating turbulent flow around a twisted hydrofoil

In the present paper, large eddy simulations combined with the Zwart cavitation model are conducted to simulate the transient cavitating turbulent flow around a Delft Twisted hydrofoil. Numerical results show a reasonable agreement with the available experimental data. A three dimensional Lagrangian technology is developed to provide an alternative method for the analyses in cavitating flow, which is based on Lagrangian viewpoint. With this technology, the track lines of re-entrant and side-entrant jets are straightforwardly displayed and clearly indicate that collisions of the mainstream, the re-entrant jet and the side-entrant jet play an important role in the primary and secondary shedding. The evolution of U-type structures and the interactions between cavitation and vortices are well captured and discussed in detail from the Eulerian viewpoint. The numerical results show that during the stage of attached cavity, the topology of the cavity leaves an important influence on the vortex structure. Once the cavity is cut off, the vortex structure evolution will affect significantly the local cavitating flow. Further analysis demonstrates that the lift acting on U-type structures, which is induced by velocity circulation around U-type structures, significantly affects the formation and the development of U-type structures. Lagrangian Coherent Structures (LCSs) obtained with the three dimensional Lagrangian technology are used to reveal the influence of U-type structures on local flow patterns and it shows that there is a close relationship between the local flow separation and vortex structures. Our work provides an insight into the interactions of cavitation-vortex in the cavitating flow around a twisted hydrofoil and demonstrates the potential of 3D LCSs in the analyses of cavitating flow.

Large Eddy Simulation and investigation on the flow structure of the cascading cavitation shedding regime around 3D twisted hydrofoil

Cavitating flows are full of complex interactions between cavity evolution and vortical structures difficult to be measured in experiments. In the present work, the flow structure and dynamics of a kind of cascading cavitation shedding regime around a 3D twisted hydrofoil in full-scale water tunnel were numerically investigated using Large Eddy Simulation (LES). A homogeneous cavitation model of vapor fraction transport and a random perturbation generator based on spectral method are adopted. The mesh feasibility was carefully checked to provide an adequate capturing of the intrinsic vortical structure and the turbulence energy spectrum. The predicted cavitation behavior agrees with the experiment in primary aspects such as the cavity topology in periodic evolution, the frequency characteristics and the interlaced spanwise and streamwise vortices. Special emphasis was put on the analysis of flow details including the vortex-cavitation interaction and the pressure wave propagation. The observed convex cascading shedding is found to be caused by the particular re-entrant and side-entrant jet dynamics formed on the twisted surface. The jets reflected on the cavity closure are found to first concentrate then diverge along the minimal local slope of the foil. Another significant phenomenon is that the lift or drag has two peaks of similar magnitude in each cycle, which is analyzed to be a result of the cascading break-up and the ultimate collapse of the cavitation. An approximately linear relationship is predicted between the second-order time derivative of cavity volume and its induced pressure disturbance. The physical mechanism is clarified using a simplified analytical model.

Combined experimental observation and numerical simulation of the cloud cavitation with U-type flow structures on hydrofoils

Sheet/cloud cavitation is an important topic that is a very common type of cavitation in turbo-machinery and marine propeller. Up to now we still have limited understanding of the cavitation shedding dynamics and cloud cavity formation and development. The present study used experimental and numerical studies to gain a better understanding of the complex physics involved in this problem. A series of experimental observations around hydrofoils are carried out in the cavitation tunnel of the China Ship Scientific Research Center (CSSRC) to illustrate the spatial–temporal evolution of the cloud cavity in detail. The results demonstrate that U-type flow structures are common in cloud cavities and can be divided into three stages and the closure line in a sheet cavity often has a convex–concave profile. Reentrant flows occur in the convex region with the jet direction normal to the contour edge so the shedding is mainly caused by the converging reentrant flows. Further analysis demonstrated that there was a striking difference with the cavity growth suppressed substantially in the twisted hydrofoil case if compared with straight hydrofoil and the effect of side entrant jets might make the cavity more uniform across the span. Numerical simulations were used to simulate the formation and development of the cloud cavity. The results show that the strong adverse pressure gradient in the stagnation region at the downstream end of the attached cavity forces the re-entrant flows into the vapor structure with a radially-diverging re-entrant jet and a pair of side-entrant jets, which causes the cavity shedding. Further analyzes of the local flow fields show that the interactions between the circulating flow and the shedding vapor cloud may be the main reason for the formation of the U-type cloud cavity structures.

Numerical analysis of unsteady cavitating turbulent flow and shedding horse-shoe vortex structure around a twisted hydrofoil

Cavitating turbulent flow around hydrofoils was simulated using the Partially-Averaged Navier–Stokes (PANS) method and a mass transfer cavitation model with the maximum density ratio (ρl/ρv,clip) effect between the liquid and the vapor. The predicted cavity length and thickness of stable cavities as well as the pressure distribution along the suction surface of a NACA66(MOD) hydrofoil compare well with experimental data when using the actual maximum density ratio (ρl/ρv,clip=43391) at room temperature. The unsteady cavitation patterns and their evolution around a Delft twisted hydrofoil were then simulated. The numerical results indicate that the cavity volume fluctuates dramatically as the cavitating flow develops with cavity growth, destabilization, and collapse. The predicted three dimensional cavity structures due to the variation of attack angle in the span-wise direction and the shedding cycle as well as its frequency agree fairly well with experimental observations. The distinct side-lobes of the attached cavity and the shedding U-shaped horse-shoe vortex are well captured. Furthermore, it is shown that the shedding horse-shoe vortex includes a primary U-shaped vapor cloud and two secondary U-shaped vapor clouds originating from the primary shedding at the cavity center and the secondary shedding at both cavity sides. The primary shedding is related to the collision of a radially-diverging re-entrant jet and the attached cavity surface, while the secondary shedding is due to the collision of side-entrant jets and the radially-diverging re-entrant jet. The local flow fields show that the interaction between the circulating flow and the shedding vapor cloud may be the main mechanism producing the cavitating horse-shoe vortex. Two side views described by iso-surfaces of the vapor volume fraction for a 10% vapor volume, and a non-dimensional Q-criterion equal to 200 are used to illustrate the formation, roll-up and transport of the shedding horse-shoe vortex. The predicted height of the shedding horse-shoe vortex increases as the vortex moves downstream. It is shown that the shape of the horse-shoe vortex for the non-dimensional Q-criterion is more complicated than that of the 10% vapor fraction iso-surface and is more consistent with the experiments. Further, though the time-averaged lift coefficient predicted by the PANS calculation is about 12% lower than the experimental value, it is better than other predictions based on RANS solvers.

Numerical analysis of the three-dimensional cloud cavitating flow around a twisted hydrofoil

Unsteady sheet cavitation on a three-dimensional twisted hydrofoil was studied using a Reynolds-averaged Navier–Stokes simulation technique based on a cell-centered finite volume method. As a verification test of the computational method, the leading edge and mid-chord cavitating flows over a two-dimensional modified NACA66 foil section were simulated for various cavitation numbers and validated against existing experimental data. The cavitation model parameters and numerical schemes were determined by a comparison with the measured pressure distribution. Non-cavitating and cavitating flows around a three-dimensional twisted hydrofoil were simulated with the selected computational method. The computed pressure on the foil and the cavity shedding patterns were validated by comparing these results with existing experimental data. The cavity shedding dynamics due to a re-entrant jet and a side entrant jet were investigated in terms of the cavity shedding cycles. The computed lift force and Strouhal number were compared against existing experimental data.

Numerical Simulation of Cavity Shedding from a Three-Dimensional Twisted Hydrofoil and Induced Pressure Fluctuation by Large-Eddy Simulation

Simulation of cavity shedding around a three-dimensional twisted hydrofoil has been conducted by large eddy simulation coupling with a mass transfer cavitation model based on the Rayleigh-Plesset equation. From comparison of the numerical results with experimental observations, e.g., cavity shedding evolution, it is validated that the unsteady cavitating flow around a twisted hydrofoil is reasonably simulated by the proposed method. Numerical results clearly reproduce the cavity shedding process, such as cavity development, breaking-off and collapsing in the downstream. Regarding vapor shedding in the cavitating flow around three-dimensional foils, it is primarily attributed to the effect of the re-entrant flow consisting of a re-entrant jet and a pair of side-entrant jets. Formation of the re-entrant jet in the rear part of an attached cavity is affected by collapse of the last shedding vapor. Numerical results also show that the cavity shedding causes the surface pressure fluctuation of the hydrofoil and the force vibration. Accompanying the cavity evolution, the wave of pressure fluctuation propagates in two directions, namely, from the leading edge of the foil to the trailing edge and from the central plane to the side of the hydrofoil along the span. It is seen that the large pressure fluctuation occurs at the central part of the hydrofoil, where the flow incidence is larger.

On the Collapse Structure of an Attached Cavity on a Three-Dimensional Hydrofoil

A three-dimensional twisted hydrofoil with an attached cavitaty closely related to propellers was observed with a high-speed camera at the University of Delft Cavitation Tunnel. Reentrant flow coming from the sides of the cavity aimed at the center plane—termed side-entrant flow—collided in the closure region of the cavity, pinching off a part of the sheet resulting in a periodic shedding. The collapse of the remainder of the sheet appears to be a mixing layer at the location of the colliding reentrant flows. Collision of side-entrant jets in the closure region of a cavity is identified as a second shedding mechanism, in addition to reentrant flow impinging the sheet interface at the leading edge.