Tip Leakage Vortex Cavitation Research Articles

The investigation on the flow characteristics during the continuous development of the tip leakage vortex cavitation in an axial pump-jet propulsion

The continuous deterioration and development of tip leakage vortex (TLV) cavitation in the pump-jet propulsion significantly affect propulsion performance and operational stability. Larger eddy simulation and cavitation tunnel experiment are utilized to investigate the temporal and spatial evolution characteristics of TLV cavitation under varying cavitation conditions. The results reveal that the continuous development of TLV cavitation prompts the TLV to gradually move away from the blade suction surface due to increasing pressure difference at the blade tip surface. Furthermore, the development of TLV cavitation amplifies the effect of the radial outward Coriolis force and makes the TLV even more unstable. Under the influence of the tip leakage flow, primary generation of turbulent kinetic energy (TKE) persistently migrates to the TLV core center and subsequently travels downstream. Despite the large magnitude of TKE that occurs at the TLV core center, the TKE generation remains low. With the inception of TLV cavitation, the transport of TKE between the TLV core center and the surrounding flow gradually intensifies, followed by a subsequent weakening of this transport effect. It increases again as the breakdown of TLV becomes more severe.

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.

New type of cavitation instability with peculiar frequency characteristic in liquid rocket inducer

In a liquid rocket turbopump with low pressure and thin wall tanks, cavitation inevitably occurs at the inducer which is installed at the inlet of the turbopump. The cavities at each blade oscillate periodically, and the turbopump sometimes becomes unstable, called cavitation instability. Rotating cavitation is one of the common types of cavitation instability, in which the cavities at each inducer blade oscillate respectively and appear to propagate from blade to blade. However, the new type of cavitation instability which does not follow the conventional principle has been observed in our experiment with an inducer. Converting the frequency in the inertial frame to that in the rotational frame and organizing it in the cavitation number, it was found that the frequency of unsteady cavitation increases as the cavitation number decreases, and this is the peculiar point of this instability. Additionally, during the cavitation instability, a few numbers of backflow vortex cavities were observed and moved sub-synchronously from blade to blade. In addition, the tip leakage vortex cavitation on each blade also propagates like sub-synchronous rotating cavitation but in different propagation speed from that of the backflow vortex cavities. As a result, it was supposed that the new instability is one of the types of sub-synchronous rotating cavitation related to backflow vortex cavitation, tip leakage vortex cavitation, and interaction between cavity propagation and inducer blade.

Study on the Influence of Tip Clearance on Cavitation Performance and Entropy Production of an Axial Flow Pump

The clearance existing between the impeller rim and the adjacent shroud within the pump configuration establishes conducive circumstances for the initiation of cavitation. The bubbles generated by cavitation will flow forward with the water, blocking the channel, and result in the degradation of the pump performance. When the cavitation is severe, vibration and noise will be generated. The impact formed by the collapse of the bubbles will seriously erode the blades and form pits on the blade surfaces. Drawing upon the outcomes derived from numerical simulations, this paper investigates the relationship between tip clearance and cavitation in an axial flow pump, with a specific focus on energy dissipation characteristics. The principal findings indicate that the dimensions of the tip clearance predominantly influence the spatial distribution of the tip leakage vortex (TLV) cavitation. The entropy production rate distribution at the tip correlates with both the cavitation level of the pump and the extent of the tip clearance. The shedding phenomenon of the TLV becomes more evident when analyzing the distribution of entropy production rates. During cavitation, an increased tip clearance is associated with a reduction in the dissipation of viscous entropy production within the impeller domain, and the entropy production resulting from turbulent dissipation significantly surpasses that arising from viscous dissipation.

Energy dissipation mechanism of tip-leakage cavitation in mixed-flow pump blades

Tip leakage flow is one of the significant factors influencing the internal flow stability of mixed-flow pumps, and in severe cases, it can lead to channel blockage and energy loss. In order to gain a deeper understanding of the energy dissipation mechanism induced by tip leakage vortex cavitation, this study is based on the Wray–Agarwal (WA) turbulence model and the homogeneous flow model, investigating the cavitation flow characteristics of mixed-flow pumps. Additionally, the entropy production theory is employed to evaluate the energy losses within the mixed-flow pump and analyze the components of energy loss in the impeller and guide vanes. The research results reveal that with increasing cavitation intensity, the low-pressure region at the leading edge of the blade extends toward the trailing edge, influencing the static pressure distribution on the blade's pressure side. Leakage flow and the spatial distribution of leakage vortices move closer to the suction side of the blade with increasing cavitation intensity. Cavitation primarily affects the energy losses in the impeller region, with turbulent dissipation being the main source of energy loss. High turbulent dissipation zones are concentrated at the trailing edge of the blade, correlating with recirculation vortices and trailing-edge vortices. This study provides theoretical insights with practical implications for enhancing the cavitation performance of mixed-flow pumps, offering valuable guidance for design and operation.

Cavitation inception noise excited by a tip leakage vortex with various gap sizes: A Eulerian–Lagrangian investigation

In this paper, the noise characteristics of tip leakage vortex cavitation (TLVC) inception are numerically investigated using a previously developed hybrid Eulerian–Lagrangian model [Wang et al., “Numerical investigation of how gap size influences tip leakage vortex cavitation inception using a Eulerian–Lagrangian method,” Phys. Fluids 35, 012113 (2023).] and the porous Ffowcs-Williams and Hawkings analogy method. A gap size significantly affects the far-field noise, which is louder for large gaps than small gaps, and for high-frequency band than low- and medium-frequency bands. Wavelet and theoretical analyses show that the main broadband noise source is the multiple collapses of cavitating bubbles and that the noise intensity is positively related to the maximum bubble radius. The maximum bubble radius depends on the unsteady pressure evolution at the TLV center, where the mean pressure is mainly influenced by the tip leakage velocity and the pressure fluctuation results from the boundary layer roll-up. This paper reveals the hydroacoustic mechanism of TLVC inception, which can provide insight into noise reduction techniques.

Numerical investigations on the mechanisms of the tip leakage vortex cavitation development in a cryogenic inducer with large eddy simulation

Liquid hydrogen is considered clean energy and is usually pressurized by cryogenic pumps in various industries. To ensure the safe operation of cryogenic pumps, the inducer is installed in front of the pump to improve the impeller inlet pressure but causes cavitation instabilities. This paper aims to investigate the mechanisms of the tip leakage vortex (TLV) cavitating flow in a cryogenic inducer with liquid nitrogen. The large eddy simulations model was used to analyze the thermodynamic effects on the tip leakage vortex cavitation (TLVC). The cavity structure and the pulsation mechanisms of the TLVC were analyzed through the flow characteristics and the vorticity transportation process. The predicted cavitation performance is in good agreement with the experimental measurements. The numerical results showed that the TLVC is suppressed and forms the separation point between the primary TLVC and the secondary TLVC due to the thermodynamic effects. The inhibition rate of the vapor volume fraction at the leading edge is 30%. The pressure fluctuations are caused by the propagation pattern of the detached cavity interacting with the adjacent blade periodically. The velocity triangles near the detached cavity were proposed to reveal the development of the TLVC. It indicates that TLVC instability is caused by the periodic coupling effect of the cavity development, the flow rate magnitude, and the local incidence angle variation. The vorticity transport equation is utilized to investigate the interaction of cavitation and vortex. Comparing the three terms reveals that the stretching and bending term dominates in the vorticity production of the TLV cavitating flow. The dilatation term controls the transportation of vorticity inside the TLV cavity, while the contribution of the baroclinic torque term is negligible in comparison to the other terms. This study provides a reference for optimizing the TLV cavitating flow and instabilities for designing the cryogenic pump.

Numerical study of two types of rough groove in suppressing the tip clearance cavitation

Tip clearance cavitation (TCC) which is consisted of tip leakage vortex (TLV) cavitation and gap cavitation (occurs inside the gap region) is a common phenomenon in the gap flow filed in axial turbomachinery, and can trigger a series of negative effects. Based on the suppression mechanism of a type of prototypical horizontal casing groove, two types of rough groove structures are proposed in this paper aiming at enhancing the suppression effect on the TCC by utilizing the Coanda effect of the gap leakage jet. The gap flow field is generated via a NACA0009 hydrofoil and a solid wall. The gap size is fixed as 2 mm. The numerical simulation method is adopted. The results show that combining the detached-eddy simulation (DES) approach, SchnerrSauer cavitation model and a type of varRhoTurbVOF method on the OpenFOAM platform, accurate numerical results of cavitating gap flow fields under the small gap conditions can be obtained. The gap flow field analysis reveals that the control ability of the grooves on the leakage flow is enhanced via divergent–convergent flow passage in the roughness groove cases. The leakage rate is reduced while the local turbulence intensity is increased, which weakens the TLV and the tip separation vortex (TSV) dramatically. The rough casing grooves show a superior suppression effect on the TCC. The present work can inspire new casing treatment method for improving the anti-cavitation performance in the axial-flow hydraulic machinery.

Experimental investigation of characteristics of tip leakage vortex cavitation-induced vibration of a pump

Cavitation and vibration are complicated and critical characteristics in pumps, which has a negative influence on the reliability and stability of nuclear power plant systems. In this paper, a synchronous experimental system including a high-speed camera and sensors is established to investigate the characteristics of blade tip leakage vortex (TLV) cavitation, cavitation-induced pressure fluctuation, and vibration. The results show that the formation of the TLV cavitation cloud significantly deteriorated the pump's performance, changed the pressure distribution, and increased the pressure fluctuation and vibration. Based on the Hilbert-Huang transformation (HHT) method, transient and non-stationary vibration signals are decomposed into Intrinsic Mode Functions (IMFs) by Empirical Mode Decomposition (EMD). The RMS values of first-order IMF in vibration signals increase rapidly as the TLV cavitation cloud develops. The properties extracted from the Hilbert marginal spectrum can be used to detect the pump's cavitation development stages.

Effects of Tip Leakage Vortex Cavitation on Flow Field under Cavitation Instability

Cavitation instabilities can induce axial and circumferential vibrations, as well as noise in turbopump inducers. Therefore, the purpose of the present study is to investigate the mechanism of cavitation instability. The flow field near the two-bladed inducer leading edge under alternate blade cavitation was experimentally investigated using particle image velocimetry (PIV). It was found that the tip leakage vortex cavitation draws the flow toward its region of collapse and induces a negative change in the incidence to the adjacent blade. Moreover, this blade-to-blade interaction was identified as the main cause of alternate blade cavitation. Furthermore, it was demonstrated that this blade-to-blade interaction is strongest when the cavity collapse occurs in the inducer throat area, where the leading edge of the following blade is located.

Method of bionic wavy tip on vortex and cavitation suppression of a hydrofoil in tidal energy

The tip leakage vortex (TLV) has a detrimental effect on the energy performance and operation stability of bulb turbines, which is the core component of energy conversion in tidal power stations. A hydrofoil with tip clearance is selected as the research object, the novel method of the bionic wavy tip on vortex and cavitation suppression is proposed, then the vortex structure, streamline, pressure and cavitation are analyzed numerically. The results show that the vortex structure mainly includes the tip separation vortex and TLV. A low-pressure region appears in the tip wall and suction surface (SS) and induces cavitation. The bionic wavy tip can increase the resistance of the tip leakage flow passing through the tip clearance by increasing the vertical wall shear stress at the blade tip. The large-scale TLV is divided into small-scale TLVs, which dissipate quickly. The pressure at the center of the vortex core increases, and the low-pressure region decreases. In addition, the bionic wavy tip can effectively suppress cavitation, and the cavitation volume decreases by 44%. The TLV cavitation almost disappears. However, the bionic wavy tip increases the pressure on the SS, resulting in an increase in drag and a decrease in the lift-drag ratio.

Spatial-temporal evolution of tip leakage cavitation with double-hump in a mixed flow pump with tip clearance

Tip clearance in pump induces tip leakage vortex (TLV), which interacts with the main flow and leads to instability of flow pattern and decrease in pump performance. In this work, a closed-loop experimental rig with high-speed observation for a mixed flow pump is established, and the numerical simulation on tip leakage cavitation is conducted with experimental validation. A new double-hump pattern of tip leakage cavitation is first observed in the mixed flow pump. Results show that primary tip leakage vortex cavitation develops from a linear pattern to a banded pattern and, finally, to a double-hump pattern. The spatial–temporal evolution of tip leakage cavitation with double-hump can be classified into four stages: incepting stage, growing stage, merging stage, and propagating stage. The inception of the front hump is found to correspond to the periodic evolution of the specific passage vortex-tip leakage vortex (PV-TLV) pair. Affected by the propagation of the high-pressure area at the passage outlet, the PV wanders in the downstream passage, which greatly affects the intensity of tip leakage flow. The away motion of the PV from the tip clearance brings stronger tip leakage flow and results in the intensified TLV and the lower core pressure. As a result, the front hump incepts, grows, and travels downstream. Finally, it merges with the rear hump and together propagates to the downstream passage.

Research on Cavitation Wake Vortex Structures Near the Impeller Tip of a Water-Jet Pump

Cavitation can cause noise in the water-jet pump. If cavitation occurs in the water-jet pump, the hydraulic components in the pump are prone to erosion. The surface erosion reduces energy delivery efficiency and increases maintenance costs. The decline in pump performance will lead to the instability of the entire energy system. In this paper, the cavitation flow structure of the water-jet pump is studied by the method of numerical simulation and experiment, which provides a reference for the prediction and improvement of cavitation. Based on the closed test platform, in order to reveal the physical process of cavitation evolution, high-speed photography is used to capture the complex cavitation flow phenomenon in the pump. After that, the cavitation vortex structure was further explored by numerical simulation. Through the simulation of the impeller blade tip leakage flow and the Tip Leakage Vortex Cavitation (TLVC) characteristics under different cavitation conditions, the flow mechanism of the impeller blade tip leakage flow and the separation vortex induced by the cavitation region under different cavitation conditions were revealed. The main factors affecting the development of the cavitation wake vortex structures were summarized.

Numerical investigation of how gap size influences tip leakage vortex cavitation inception using a Eulerian–Lagrangian method

This paper investigates the effect of gap size on the inception of tip leakage vortex cavitation (TLVC) with a hybrid Eulerian–Lagrangian model. Good agreement is achieved between the simulation results and experimental data for velocity distributions around the TLV, bubble motion, and its size oscillations. It is found that the minimum pressure criterion is not accurate enough for the prediction of TLVC inception due to the significant effect of pressure fluctuation and increased concentration of nuclei in the TLV core region. The pressure fluctuation in the TLV core is noted to be a non-negligible factor, while the corresponding effect on nuclei dynamics in the TLV core is still unclear. To deal with this problem, the inducement of this excited turbulence is further analyzed and discussed in detail, which shows a close relationship with the TLV instability raised by the vortical interaction between TLV and tip-separation vortex/induced vortex. Our work provides an insight into the mechanism of TLVC inception through the flow characteristics in the TLV core region, which is helpful for controlling TLVC inception in engineering designs.

Dynamic response and acoustic characteristics of composite hydrofoil under cavitation-induced vibration

Nonconstant characteristics of cavitating flow can cause adverse effects, including vibration, noise, and cavitation. With the application of composite materials, the vibration and deformation of hydrofoils are becoming increasingly obvious, and the fluid–solid coupling problem is becoming very important. Herein, the cavitating flow field of National Advisory Committee for Aeronautics 0012 hydrofoil composed of bronze alloy and composite material is numerically investigated. The hydro-elastic response of a hydrofoil is obtained via a fluid–structure coupling method under a tight coupling strategy. The Schnerr–Sauer model is used to describe the cavitation process, and the large eddy simulation method is used to solve the turbulence problem. Additionally, the finite element method is used to determine the structural deformation. A comparison of the numerical calculation results of the fluid–solid acoustic coupling under two operating conditions shows that the composite materials can suppress the sheet cavitation attached to the hydrofoil suction surface and the tip leakage vortex (TLV) cavitation and can decrease the flow field pressure pulsation and increase the lift-to-drag ratio of the hydrofoil. Additionally, the composite material significantly improves the wake field turbulence, reducing the turbulence intensity and integration scale, and nearly eliminates the large-scale vortex. Moreover, the composite material changes the vortex structure evolution at the gap flow field, results in smoother TLV development, and enhances the flow field velocity gradient effect. Finally, the monitoring of the flow noise radiation characteristics shows that the composite material effectively reduces the sound pressure level of the self-noise and far-field radiation noise.

Investigation on leakage vortex cavitation and corresponding enstrophy characteristics in a liquid nitrogen inducer

A modified thermal cavitation model was employed to study the thermodynamic cavitation and effects of tip leakage vortex (TLV) on hydraulic losses in a three-blade inducer. Comparative analyses for TLV cavitation under different temperatures were presented. The tip leakage flow was spirally distributed, and its shape was negatively correlated with the nitrogen temperature. Thermodynamic effects reduced the vapor volume in the cavitation region and vorticity primarily at the leading edge, thus inhibiting the development of TLV. Enstrophy analysis was proposed to reveal the energy losses on TLV, and the enstrophy dissipation rate showed good predictability for TLV evolution. With the increasing temperature, the intensity of energy losses was reduced due to the thermal effects combined with weak cavitation intensity. Moreover, the change of temperature mainly influenced the primary tip leakage vortex (PTLV) but had minimal impact of the secondary tip leakage vortex. The energy losses were positive correlated with TLV, and PTLV was offset to the hub of the inducer due to the thermodynamic effects. Thus, the tip leakage can be considered as a key parameter to minimize the occurrence of thermodynamic cavitation in a liquid nitrogen inducer toprovide a reference for the design of the inducer.

Suppression effect on TLV cavitation of overhanging grooves with different tab orientations: An experimental investigation

In this paper, a high speed camera and endoscopic particle image velocimetry system are utilized to investigate the influence of tab orientations of overhanging grooves on the suppression effect of tip-leakage vortex cavitation around a truncated NACA0012 hydrofoil. Four kinds of overhanging grooves (OHGs) with different tab orientations, namely OHGs_1–4, are tested, with the original hydrofoil as a comparison. The results suggest that compared with the baseline, all the overhanging grooves tested in our experiments show the suppression effect on TLV cavitation. However, the tab orientations of overhanging grooves leave significant influence on its effect. OHGs_2 with tabs extending beyond the pressure side surface shows the best suppression effect for all the measured gap sizes, followed by OHGs_1 with tabs extending beyond both the suction and pressure side surfaces. For OHGs_3 and OHGs_4, due to the lack of tabs extending beyond the pressure side surface, the suppression effect is not so good as OHGs_1 and OHGs_2. Further analysis indicates that the reason for the better suppression effect of OHGs_2 is the interference of tabs extending beyond the pressure side surface on the fusion of tip-leakage vortex and tip-separation vortex, which decreases the tip-leakage vortex circulation and increases its radius.

Effects of cavitation on the hydrodynamic loading and wake vortex evolution of a pre-swirl pump-jet propulsor

The purpose of this study is to investigate the effects of cavitation on the hydrodynamic loading and wake vortex evolution in a pre-swirl pump-jet propulsor, and also the cavitation-vortex interaction mechanism. The cavitating flow is simulated by delayed detached eddy simulation coupled with a homogeneous cavitation model. Based on available experimental validation, the cavitation patterns, hydrodynamic loadings, the tip leakage vortex (TLV) evolutions and trailing edge vortex interactions are orderly investigated under two typical cavitation conditions. Results show that the blade sheet cavitation, TLV cavitation and tip clearance cavitation are regard as the main cavitation types of the rotor, where the sheet cavity on adjacent blades is non-uniformly distributed under the perturbations of the stator wakes and phase effects. The interaction between the thickening sheet cavity and stator wakes causes the shift of dominant frequencies of rotor loading from the rotor blade passing frequency fBPF and its harmonics to stator blade passing frequency fs and its harmonics. The relative vorticity transport equation is used to analyze cavitation-vortex interaction of TLV. The TLV cavitation promotes the vorticity production of TLV at the incipient stage and increases its intensity downstream. The instability of TLV is triggered earlier when cavitation is heavy due to the enhanced mutual interaction between consecutive spirals of the TLVs and their interaction with sheet cavity induced shedding vortices. The trailing edge vortices of stator and rotor mutually interacts with blade sheet cavity, which accelerates the breakdown of trailing vortex system downstream.

Unsteady characteristics of tip leakage vortex structure and dynamics in an axial flow pump

The aim of this work is to investigate the unsteady characteristics of tip leakage vortex (TLV) structure in an axial flow pump experimentally and numerically. Experiments were carried out to study the performance of an axial flow pump under cavitating cavitation. The numerical simulation was conducted by employing the Large Eddy Simulation turbulence and Zwart cavitation models, and the results show a good agreement with the experimental ones. The evolution mechanisms and influencing factors of the transient TLV morphology was revealed, and the correlation between pressure difference and leakage velocity were investigated in a systematic way. The TLV evolution appears periodic development, coinciding with the variation of leakage velocity driven by the pressure difference. The cylindrical coordinate system is more suitable for analyzing the TLV dynamics, due to the impeller rotation. The vorticity in the tip region is dominated by tangential vorticity ωθ and axial vorticity ωz. The transport terms of ωθ and ωz present a great relationship with TLV and TLV-induced cavitation. Relative vortex stretching dominants the vorticity generation in the tip region, and relative vortex dilation and Coriolis force are also important sources. In contrast, baroclinic torque has the least influence on the vorticity in the flow passage.

Method of variable-depth groove on vortex and cavitation suppression for a NACA0009 hydrofoil with tip clearance in tidal energy

Tidal energy has attracted great attention due to its massive potential and eco-friendly nature. Performance of ducted turbines used in tidal energy conversion is significantly influenced by tip clearance and associated leakage flow. In this work, a method of variable-depth (VD) groove is developed to suppress the tip leakage vortex (TLV) and the tip leakage vortex cavitation (TLVC) around a NACA0009 hydrofoil, and different variable-depth laws are proposed to improve the suppressing effect. The flow pattern around the hydrofoil is investigated on basis of numerical method, which is validated by experiment results. Results show that this groove method not only effectively suppresses primary TLV by high-velocity flow impingement, but also reduces the primary TLVC by increasing local pressure in vortex core regions. In addition to the intrinsic primary TLV and secondary TLV, some local small-scale vortices are witnessed in vicinity of the groove. Among the VD grooves, the divergent groove shows an optimal effect on suppressing vortex and cavitation in general. Increasing the divergent groove depth at hydrofoil pressure side causes the shrinkage of the primary TLV but a larger scale of the additional vortices in vicinity of the groove. The suppressing effect of the divergent groove on the TLVC reaches a peak when the pressure side depth is increased to 50% of the tip clearance size. The lift coefficient of the hydrofoil is sensitive to the groove pressure side depth, by decreasing which can effectively improve the lift-drag performance of the hydrofoil.