Tip Leakage Vortex Cavitation Research Articles

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.

Research on Cavitation Flow Dynamics and Entropy Generation Analysis in an Axial Flow Pump

The entropy generation theory is introduced to investigate the effects of different NPSH and tip clearance size on the cavitation flow dynamics and mechanical energy dissipation intuitively and quantitatively within an axial flow pump through numerical simulations. The results indicate that main mechanical energy dissipation of the pump gathers in part impeller and diffuser, and most are turbulent dissipation. Meanwhile, the impeller is the largest place of mechanical energy dissipation of the pump under cavitation conditions, accounting for more than 50%. NPSH has significant effects on the cavitation pattern, which reflects on the field that the areas of attached sheet cavitation and tip leakage vortex cavitation around blades increase obviously with NPSH reducing under the tip clearance of 0.1% span. With NPSH decreasing, high regions of turbulent dissipation in the impeller mainly expands along blades and move downstream, with span S0.98 near the shroud having larger turbulent dissipation. Besides, high regions of turbulent dissipation are mainly distributed at the rear part of the cavity for every corresponding span of the impeller, which indicates that the turbulent dissipation has a strong relation with the cavitation pattern. In the impeller, the unstable flows cause cavity shedding at the rear of the cavity and wake flows near the blade trail induce higher turbulent kinetic energy, finally resulting in higher turbulent dissipation there. Under the same NPSH, areas of tip leakage vortex cavitation and areas of tip clearance cavitation around the tip both expand with the tip clearance increasing from 0.1% span to 0.8% span. And high areas of turbulent dissipation also are distributed at the rear of the cavity and moving downstream along the blade suction side, especially at span S0.98. Therefore, the tip clearance width mainly affects the cavitation development and turbulent dissipation distribution near the impeller’s shroud under same NPSH.

Effects of Inlet S-duct on Inducer Performance and Cavitation Instability

To investigate the effect of inlet flow distortion on two-bladed inducer performance and cavitation instability, the s-shaped duct was installed at the inlet of the inducer to induce secondary flow. Inducer cavitation performance with and without s-duct were compared while varying the cavitation number. Also, the effect of s-duct on the occurrence of alternate blade cavitation was investigated using unsteady casing pressure measurement and high-speed camera visualization. It was shown that the s-duct decreases the strength of alternate blade cavitation and lowers the head breakdown cavitation number without considerable head loss by reducing the tip leakage vortex cavitation.

Cavitating flow investigation in low specific speed axial flow waterjet pumps

Abstract In this paper, the cavitating flow in low specific speed axial flow waterjet pumps are treated at different cavitation conditions. In order to analyze the cavitating turbulent flow comprehensively, three axial flow waterjet pumps with the specific speed less than 650 min-1·m3s-1·m are chosen to compare the internal flow phenomenon. The investigations are conducted by numerical simulation, where RANS method is applied with k-ω SST turbulence model and homogeneous cavitation model. The results show that there are several types of cavitation such as tip leakage vortex cavitation, sheet cavitation attached on blade suction surface, and shedding cavity in the pumps. The vortex structure in the low specific speed waterjet pump contains tip leakage vortex, shedding vortex downstream of the sheet cavitation and perpendicular cavitation vortex. It is noted that the cavity firstly occurs at the leading edge near blade tip, and the sheet cavity covering the blade suction surface extends from leading edge to the middle part, while the tip leakage vortex cavitation develops with the decreasing cavitation number. Further analysis using relative vortex transport equation depicts that the vortex stretching term is the main contributor for the vorticity production in the pump, and the vortex dilation and baroclinic torque occur at the interface between cavity and water.

Numerical Study of Unsteady Cavitating Flow in an Inducer With Omega Vortex Identification

Abstract To accurately capture the behaviors of cavitation and reveal the unsteady cavitating flow mechanism, a condensate pump inducer is numerically analyzed in a separate numerical experiment with large eddy simulation (LES) at critical cavitation number σind,c under the design point. Based on the new Omega vortex identification method, the correlation between the flow structures and cavities is clearly illustrated. Besides, the pressure fluctuations around the inducer are analyzed. Special emphasis is put on the analysis of the interactions between the cavities, turbulent fluctuations, and vortical flow structures. The Omega vortex identification method could give an overall picture of the whole cavitating flow structures to present a clear correlation between the vortices and cavities. The results show that the shear cavitation dominates the cavitation characteristics under the design point. The pure rigid rotation region mainly concentrates at the edge of the cavities while the other sheet-like cavities near the casing walls are characterized by strong turbulence fluctuations. Besides, based on the analysis of the correlation between the cavities and flow structures, the rotating cavitation under the design point may mainly be attributed to the interaction between the tip leakage vortex cavitation and the next blade.

Comparative study of tip leakage vortex trajectory and cavitation in an axial flow pump with various tip clearances

Comparative study of tip leakage vortex trajectory and cavitation in an axial flow pump with various tip clearances

Effect of Tip Clearance on Helico-Axial Flow Pump Performance at Off-Design Case

To reveal the effect of tip clearance on the flow behaviors and pressurization performance of a helico-axial flow pump, the standard k-ε turbulence model is employed to simulate the flow characteristics in the self-developed helico-axial flow pump. The pressure, streamlines and turbulent kinetic energy in a helico-axial flow pump are analyzed. Results show that the tip leakage flow (TLF) forms a tip-separation vortex (TSV) when it enters the tip clearance and forms a tip-leakage vortex (TLV) when it leaves the tip clearance. As the blade tip clearance increases, the TLV moves along the blade from the leading edge (LE) to trailing edge (TE). At the same time, the entrainment between the TLV and the main flow deteriorates the flow pattern in the pump and causes great hydraulic loss. In addition, the existence of tip clearance also increases the possibility of TLV cavitation and has a great effect on the pressurization performance of the helico-axial flow pump. The research results provide the theoretical basis for the structural optimization design of the helico-axial flow pump.

THE INFLUENCE OF THE DIRECTION OF OVERHANGING GROOVES ON THE SUPPRESSION OF TLV CAVITATION AND ITS MECHANISMS

In this paper, the influence of the direction of overhanging grooves on the suppression of tip-leakage vortex (TLV) cavitation is investigated experimentally and numerically, and the mechanisms behind are discussed in detail. The results indicate that both the overhanging grooves (OHGs) with the overhanging at the suction and pressure sides show a significant suppression on TLV cavitation for a large range of gap sizes. Compared with the OHGs with overhanging at the suction side, a better suppression effect can be obtained by the OHGs with overhanging at the pressure side. Moreover, the OHGs with overhanging at the pressure side can provide a more satisfying suppression on the fluctuation of the cavity volume of TLV cavity. The experimental measurements indicate that the influence of the direction of overhanging grooves on the lift and drag coefficients of the hydrofoil is quite slight. The numerical results further indicate that the OHGs with overhanging at the pressure side can impede the flow from the pressure side to the suction side more effectively and disturb the development of TLV, which is helpful for the suppression of TLV cavitation.

LES INVESTIGATION ON THE INFLUENCE OF CAVITATION ON THE EVOLUTION AND CHARACTERISTICS OF TIP LEAKAGE VORTEX

In the present paper, the tip-leakage vortex (TLV) cavitation around a NACA0009 hydrofoil is numerically investigated with large eddy simulation method combined with a new Euler-Lagrangian cavitation model, which takes into account the effect of nuclei on tip-leakage cavitating flow. A satisfying agreement between the numerical and experimental results is obtained. Based on the numerical data, the evolution behavior of TLV cavitation with different gap sizes and its influence on the strength and radius of TLV, the nuclei distribution in the vortex core and the distribution of tangential velocity are discussed in detail. The results show that once cavitation occurs, the intensity of TLV is mainly affected by the evolution behavior of the sheet cavitation, while the tip-leakage cavitating flow has little effect on the strength of TLV. In addition, the results suggest that a smaller gap size will result in a more unstable sheet cavity, and the strength of TLV in turn presents corresponding quasi-periodic fluctuation as influenced by the evolution of the sheet cavitation. As the tip clearance progressively increases, the strength of the sheet cavitation gradually decreases with the reduction of instability, and the intensity of TLV gradually returns to the level without cavitation, together with its fluctuation level. Cavitation has a more significant influence on the nuclei distribution in the vortex core, and its degree of influence depends on the spatial stability of TLV and the intensity of TLV cavitation after the occurrence of cavitation. Moreover, the numerical results obtained by our simulations also indicate that cavitation has a significant influence on the radius of TLV. After cavitation occurs, the radius of TLV increases to a certain extent, and a ``rigid body rotation'' tangential velocity distribution is formed on the periphery of cavitation region, which is mainly caused by the expansion process induced by cavitation growth and the viscous effect of flow.

Numerical investigation of the effects of free surface on tip-leakage vortex cavitation behaviors over a NACA0009 hydrofoil

Tip-leakage vortex (TLV) cavitation is complex and challenging multiphase flow problem in propulsion equipment. In this paper, the large-eddy method and the Schnerr–Sauer cavitation model are combined to simulate a NACA0009 hydrofoil near the free surface. The numerical method used is validated by comparing the characteristics of TLV cavitation with available experimental data. The results show that the free surface affects the evolution of the cavitation and the characteristics of the hydrodynamic load. The lift and drag coefficient of the TLV hydrofoil decreases for the near free surface case, but the pulsation frequency of the lift coefficient increases. Meanwhile, the mechanism of the TLV near the free surface is found by comparing that case to a “no free surface” scenario. Moreover, an improved dynamic mode decomposition (DMD) method is used to investigate the unsteady cavitation flow characteristic of the TLV. The results show the main flow features of the TLV cavitation including the coherent structure and energy distribution are closely related to the existence of free surfaces by analyzing the first two modes based on the DMD method.

A review of cavitation in tip-leakage flow and its control

The tip-leakage vortex (TLV) cavitation is a challenging issue for a variety of axial hydraulic turbines and pumps from both technical and scientific viewpoints. The flow characteristics of the TLV cavitation were widely studied in the past decades, but the knowledge about the tip-leakage cavitating flow is still limited. The present paper reviews the progresses in the researches of the TLV cavitation, including the numerical methods for the TLV cavitation, the flow characteristics of the TLV, the influences of the TLV cavitation on the local flow field and the control strategies of the TLV cavitation. It is indicated that the non-condensable gas may play an important role in the development of the TLV cavitation, and this fact should be considered during a careful simulation of the TLV cavitation. It is also suggested that the development of the TLV cavitation will significantly influence the distributions of the vorticity and the turbulence kinetic energy. Due to the complexity of the TLV cavitation, it is still an open question how to suppress the TLV cavitation in a simple but effective way. Finally, based on these understandings, some advanced topics for the future work are suggested to further promote the study of the TLV cavitation, for a deeper knowledge about the TLV cavitation.

LES of tip-leakage cavitating flow with special emphasis on different tip clearance sizes by a new Euler-Lagrangian cavitation model

In the present paper, a new cavitation model proposed by Cheng et al. (2020a) combined with LES is utilized to simulate the tip-leakage cavitating flow around a NACA0009 hydrofoil, which considers the effect of non-condensable gas on cavitation. The predicted tip-leakage vortex (TLV) cavitation agrees well with available experimental observations. Based on the numerical results, the influence of different tip clearance sizes on tip-leakage cavitating flow is discussed in detail. It is shown that with the increase of tip clearance size, the intensity of tip-separation vortex (TSV) cavitation decreases, while the intensity of TLV cavitation increases first and then decreases. Their fusion position moves downstream with the increase of tip clearance size. Moreover, the numerical result implies that the size of tip clearance also has an impact on characteristics of sheet cavitation, such as its length and shape. Finally, the vorticity transport equation is utilized to investigate mechanisms of vortex development around TLV cavitation. As tip clearance size increases, the effect of stretching term decreases, while that of dilatation and baroclinic torque terms increases first and then decreases. However, it is found that the stretching term is dominant in all investigated cases regardless of tip clearance size.

Experimental Investigation of the Transient Patterns and Pressure Evolution of Tip Leakage Vortex and Induced-Vortices Cavitation in an Axial Flow Pump

Abstract Cavitating flow is extremely complex in axial and mixed flow pumps, resulting in several adverse effects on pump performance. In this paper, the tip leakage vortex (TLV) cavitation patterns in an axial flow pump model were studied based on high-speed photography and transient pressure measurements. The TLV cavitation morphology and transient development of the induced suction-side-perpendicular cavitating vortices (SSPCVs) were investigated at multi-operating conditions. The time-domain of the transient pressure was employed to clarify the relationship between the tip cavitation and the pressure field. The results showed that cavitation inception occurred earlier with an unstable TLV cavitation shape at part-load conditions. Cavitation was more intense with a decrease of the cavitation number, presenting a larger area of triangular cavitation with the shedding of SSPCV. The inception of SSPCV was attributed to the tail of the shedding cavitation cloud originally attached to the suction surface (SS) of the blade, moving in the direction of the adjacent blade perpendicular to the SS, resulting in a flow blockage. With a further decrease in pressure, the SSPCVs grew in size and strength, accompanied by a rapid degradation in performance of the pump. The cavitation images and the corresponding circumferential pressure distributions showed that the lowest pressure point coincided with the SS corner. After this position, the pressure fluctuated as the cavitation intensity changed. The transient characteristics of SSPCV are a basis for revealing the instability mechanism of its evolution in the axial flow pump.

Suppressing tip-leakage vortex cavitation by overhanging grooves

In the present paper, a new control method for tip-leakage vortex (TLV) cavitation is proposed. This method, which combines the effects of grooves and winglets, is made of overhanging grooves (OHGs) fitted at the tip of a hydrofoil. Experimental and numerical investigations are conducted to evaluate the performances of OHGs in terms of TLV cavitation suppression. The results are systematically compared with the baseline, conventional grooves (CGs) and anti-cavitation lip (ACL) and a significant improvement of TLV cavitation suppression is obtained with the OHGs. We also carried out a primary optimization design of the OHGs and a more effective suppression on TLV cavitation is obtained for small gaps and TLV cavitation is almost suppressed for middle and large gaps. The underlying reasons for TLV cavitation mitigation are discussed in detail with the help of numerical simulations. It indicates that for small gap sizes, OHGs can effectively weaken the strength of both TLV and tip-separation vortex (TSV) and mitigate TLV cavitation. For middle and large gap sizes, it is found that OHGs will induce an increase in the TLV core size, which further increases local minimum pressure. The influence of OHGs on the performance of hydrofoil is also examined, which indicates that the fluctuation of TLV cavitation can be effectively suppressed by OHGs and no significant alteration of time-averaged drag and lift is induced by OHGs. Our work shows that the OHGs can effectively suppress TLV cavitation with limited influence on the performances of hydrofoil in a large range of the gap sizes, which is a promising method for the control of TLV cavitation in hydraulic machinery.

Experimental and numerical investigation of tip leakage vortex cavitation in an axial flow pump under design and off-design conditions

The interaction of tip leakage flow and main flow in an axial flow pump can induce the tip leakage vortex, which causes the unstable flow and complex cavitation structures. In the present research, the modified shear stress transport k–ω turbulence model was utilized to predict the cavitating flow of the model pump under design and off-design conditions. Validations were carried out using high-speed photography techniques. Results show that the simulation results about cavitation performance based on the modified shear stress transport k–ω turbulence model agree well with the experimental results. Cavitation inception occurs more possible at part-load conditions, and with the increase of the flow rate, the starting point of tip leakage vortex gradually moves to the back edge of the blade chord. Both the sheet cavitation and the triangular cavitation cloud that formed in the blade tip grow in size and intensity gradually with the decrease of cavitation number. Distributions of the cavity area fraction and axial velocity show that the tip leakage vortex cavitation locates at the radial coefficient r* = 0.95–1.0 with peaks at about r* = 0.97, while the sheet cavitation is found at r* = 0.5–0.9 on the suction side (SS). The cavitation structures lead to a significant decrease in the axial velocity, especially in the tip region of the blade.

Numerical investigation of turbulent cavitating flow in an axial flow pump using a new transport-based model

With the aim to enhance the capability of predicting cavitating flows for conventional cavitation models, a developed alternative numerical model was proposed based on an alternative truncated Rayleigh-Plesset equation and the homogeneous flow assumption. Particularly, the effect of vortex on mass transfer was accounted in the formulation of the proposed model. Turbulent cavitating flows under various flow rates in an axial flow pump with a specific speed ns = 692 were computed and compared by the proposed and the Schnerr-Sauer models, for which the experimental results were also presented for guidance. The results show that the cavitation performance predicted by the proposed model agrees better with the experiments than that by the Schnerr-Sauer model. The effect of vortex on mass transfer results in different patterns of the tip leakage vortex (TLV) cavitation near the tip leakage. Further, the solution of the proposed model reveals the corner vortex cavitation, shear layer cavitation and TLV cavita-tion could be integrated into a cloud vapor at critical cavitation number, and the cloud cavity sheds and collapses periodically near trailing edge of blade.

Experimental and Numerical Investigation on the Tip Leakage Vortex Cavitation in an Axial Flow Pump with Different Tip Clearances

The tip gap existing between the blade tip and casing can give rise to tip leakage flow and interfere with the main flow, which causes unstable flow characteristics and intricate vortex in the passage. Investigation on the tip clearance effect is of great important due to its extensive applications in the rotating component of pumps. In this study, a scaling axial flow pump used in a south-north water diversion project with different sizes of tip clearances was employed to study the tip clearance effect on tip leakage vortex (TLV) characteristics. This analysis is based on a modified turbulence model. Validations were carried out using a high-speed photography technique. The tip clearance effect on the generation and evolution of TLV was investigated through the mean velocity, pressure, and vorticity fields. Results show that there are two kinds of TLV structures in the tip region. Accompanied by tip clearance increasing, the viscous loss in the tip area of the axial flow pump increases. Furthermore, the tip clearance effect on pressure distribution in the blade passage is discussed. Beyond that, the tip clearance effect on vortex core pressure and cavitation is studied.

Numerical analysis of the groove effect on the tip leakage vortex cavitating flow

In this paper, the groove effect on the tip leakage vortex cavitating flow characteristics of a simplified NACA0009 hydrofoil with tip gap is studied. Considering local rotation characteristics and curvature effects of the tip leakage vortex flow, the rotation-curvature corrected shear-stress-transport turbulence model is applied to simulate the time-averaged turbulent flow. The Zwart–Gerber–Belamri cavitation model is used to simulate the cavitating flow. The results show that the groove could affect the tip leakage vortex cavitating flow. The groove enhances the interaction between the tip leakage flow and main flow, and then it affects the cavitation of the tip leakage vortex. Compared with the non-groove case, for groove cases of αgre ≤75°, the tip leakage vortex cavitating flow is suppressed, the flow pattern in the gap is improved, and the mean leakage velocity Vlk < 0.8. The region of high leakage velocity is eliminated and the distribution of the pressure is more uniform. The tip leakage vortex cavitation area is reduced, and the maximum decrease is 72.90%. While for groove cases of αgre≥90°, neither the tip leakage vortex cavitating flow nor flow pattern in the tip gap is ameliorated, the mean leakage velocity Vlk lies the range from 0.90 to 0.96. The region of high leakage velocity still exists and even the tip leakage vortex cavitation area is increased. Based on three-dimensional streamlines and vorticity transport equation, the interaction between the tip leakage flow and main flow leads to the variation of the tip leakage vortex cavitating flow. This paper aims for a useful reference to mitigate the tip leakage vortex cavitation and control the influence of the tip leakage vortex cavitating flow for the hydraulic machinery.

Large eddy simulation of the tip-leakage cavitating flow with an insight on how cavitation influences vorticity and turbulence

• The tip-leakage cavitating flow is simulated by LES with Cartesian cut-cell mesh. • Three types of cavitating vortical flows, including TLV, TSV and IV, are analyzed from Euler/Lagrangian viewpoints. • Three stages for the spatial–temporal evolution of tip-leakage cavitating flow are discussed. • The physical mechanism for cavitation-turbulence interaction is clarified. Cavitation within a tip leakage flow remains a challenging issue in a variety of axial hydraulic machines. It is still not possible nowadays to predict cavitation occurrence in such a flow with acceptable accuracy. In the present study, we have carried out numerical simulations of a tip leakage cavitating flow, generated by a straight NACA0009 hydrofoil. We have used the LES method combined with the Schnerr–Sauer cavitation model. The numerical results agree well with experimental data. The evolution of the tip leakage cavitating flow, involving tip-leakage vortex (TLV), tip-separation vortex (TSV) and induced vortex (IV), is analyzed from Eulerian and Lagrangian viewpoints. The results show that the spatial evolution of the tip leakage cavitating flow can be divided into three stages: Stage Ⅰ, Independent development of the TLV and TSV; Stage Ⅱ, Fusion of the TLV and TSV; and Stage Ⅲ, Development of the IV and dissipation of the TLV. The Lagrangian coherent structures (LCSs) obtained from the numerical results indicate that the TLV cavitation significantly influence the local flow patterns. The vorticity transport equation was then used to further analyze the influence of the cavitation on the vortices. The results demonstrate that the stretching term dominates the TLV evolution and the dilatation term is responsible for the vorticity reduction inside the TLV cavity. The results also show how the cavitation influences the local turbulence and that the transport term in the turbulent kinetic energy equation influences the turbulence distribution near the TLV cavity.

Numerical investigation of the blade tip leakage vortex cavitation in a waterjet pump

The blade tip leakage vortex (TLV) cavitation is a concern for hydraulic machineries. The underlying mechanism of the TLV cavitation is complex due to the interactions between the vortical flow and cavities, leading to the necessity of improving the numerical methods. In consideration of the influence of the angular momentum on the vortex cavitation, the Zwart's cavitation model is improved by adjusting the condensation coefficient based on the vortex intensity. The modified cavitation model is used to investigate the TLV cavitation from a waterjet pump. The energy characteristics of this pump and the cavitation phenomena near the blade tip are validated by the referenced experiment. The blade tip leakage flow and the TLV cavitation features are revealed by present simulation under different cavitation conditions. It can be found that both the TLV and its cavitation reduce the leakage flow rate through the rotor tip. According to the analysis of the pressure fluctuation characteristics, it shows that the TLV cavitation especially its terminal region enhances the pressure pulsation. With the decreasing of the cavitation coefficients, the amplitude of the pressure fluctuation is increasing and the corresponding positions with the maximum pulsation migrate to the downstream. • The blade tip leakage vortex cavitation characteristics for an axial waterjet pump are numerically studied. • An improved Zwart's cavitation model is used and evaluated. • The vortical flow features near the blade tip are investigated on different cavitation conditions. • The tip leakage vortex cavitation enhances the pressure fluctuation at the rotor casing.