Primary Tip Leakage Vortex Research Articles

Numerical and experimental study of tip leakage vortex structures and dynamics in a mixed-flow waterjet pump

Abstract Driven by the pressure difference between pressure side and suction side, the tip leakage flow (TFL) and forms the tip leakage vortex (TLV), which interact with main flow and leads to instability of flow as well as cavitation in a mixed-flow waterjet pump. In this work, the characteristics of TLV are investigated by experiment based on high-speed photography (HSP) and the numerical simulation. The TLV trajectories predicted by numerical simulation’s shows a good agreement with the experiment test results. Cavitation of the TLV core due to low pressure forms a TLV core cavitation trajectory line. The start position of TLV trajectories and the relative angle between the TLV trajectory and the blade are significantly affected by the blade tip loading. The unsteady primary tip leakage vortex (PTLV) evolution process can be summarised in three stages of splitting, developing, and collapsing.

Tip-leakage-flow excited unsteadiness and associated control

Tip leakage flow in turbomachinery inherently generates intense unsteady features, named self-excited unsteadiness, which significantly affects the operating stability, aerodynamic efficiency, and noise but has not been well understood. A zonalized large eddy simulation is employed for a linear cascade, with wall-modeled large eddy simulation active only in the tip region. The simulation is well validated with advantages demonstrated for effectively reducing the computational cost while maintaining an equivalent prediction accuracy in the region of interest. The time-averaged and spatial-spectral characteristics of tip leakage vortex (TLV) structures are systematically discussed. The self-excited unsteady processes of TLV include the tip gap separation, the tip leakage and jet-mainstream interaction, the primary tip leakage vortex (PTLV) wandering motion, and the induced separation near end wall. The Spectral Proper Orthogonal Decomposition (SPOD) is used to examine the dominant frequencies and their coherent structures. It is found that these unsteady features change from a single high frequency to multiple lower frequencies due to the PTLV breakdown. The SPOD and correlation analyses reveal that the self-excited unsteadiness originates initially from unsteady vortex separation in the tip gap and is then fed by the interactions between the tip leakage jet and mainstream. The associated unsteady fluctuations are convected along the tip leakage jet trajectory, causing the wandering motion of PTLV core. Based on the revealed unsteadiness sources, a micro-offset tip design is proposed and shown to be an effective solution to reducing the tip flow unsteadiness. This work improves the understanding of tip-leakage-flow dynamics and informs the control of the associated unsteady fluid oscillation and noise.

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.

Energy performance and flow characteristics of a NACA0009 hydrofoil with bending shrinkage groove and tip winglet

In this work, a bending shrinkage groove and tip winglet approach is used for improving the energy performance and suppressing the tip leakage vortex (TLV) in a NACA0009 hydrofoil to enhance its characteristics for tidal energy conversion. Numerical calculations are performed by using a structured mesh to solve the Navier-Stokes equation to evaluate the effects of a bending shrinkage groove and tip winglet on hydrofoil performance under no-cavitation condition and cavitation condition. The accuracy and reliability of the numerical methods are verified by comparing with experimental results. Results show that primary tip leakage vortex generation is suppressed by the bending shrinkage groove through the impact of groove jet impingement and that the secondary tip leakage vortex is weakened by the groove breaking flow. The flow separation phenomenon at the pressure surface is suppressed, and the pressure gradient in the shape is reduced by changing the wing thickness near the tip clearance to form different bending shrinkage grooves. The results show that the vortex intensity and the volume of the TLV is suppressed by the bending shrinkage groove. For a curved tip winglet, the vortex area was decreased by up to 19.3%. The lift-drag ratio of the hydrofoils was improved by the bending shrinkage groove, with maximum increase of 22%. In addition, the curved tip winglet structure can reduce the generation of vortex and generate smooth streamline. Simulation results indicate that the performance of a NACA0009 hydrofoil is most improved with a curved tip winglet. Compared with the traditional tip clearance research, the research on the six composite tip clearance in this paper can reveal the influence of tip clearance structure on the hydrofoil performance. It lays a solid foundation for the further engineering application, such as the application of composite structure on cavitation and flow noise.

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.

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.

Three-dimensional spatial-temporal evolution and dynamics of the tip leakage vortex in an oil–gas multiphase pump

To explore the spatial-temporal evolution and dynamics of the tip leakage vortex (TLV) in an oil–gas multiphase pump, the TLV was captured accurately and vortex structures were analyzed in detail under different operating conditions. Results revealed that the TLV structures included the leading edge vortex, tip separation vortex, primary tip leakage vortex (PTLV), secondary tip leakage vortex (STLV), and trailing edge vortex. In one impeller rotation period, the three-dimensional spatial-temporal evolution of the TLV could be divided into three stages: splitting, shrinking, and merging. In this process, the spatial-temporal evolution of the PTLV and STLV was closely correlated. In addition, the relative vorticity transport equation was used to analyze the TLV near the tip clearance region of the impeller. Results showed that the relative vortex stretching item (RVS), Coriolis force (CORF), and viscous diffusion (VISD) jointly controlled the spatial-temporal evolution of the TLV and were the dynamic sources of variation in the vorticity and trajectory of the TLV. In particular, the gas phase changed the distributions of the RVS, CORF, and VISD on the intensity isosurface of the TLV and had a significant effect on the spatial-temporal evolution of the TLV.

Numerical simulation on hydraulic performance and tip leakage vortex of a slanted axial-flow pump with different blade angles

The blade angle has a great effect on hydraulic performance and internal flow field for axial-flow pumps. This research investigated the effect of the blade angle on hydraulic performance and tip leakage vortex (TLV) of a slanted axial-flow pump. The hydraulic performance and the TLV are compared with different setting angles. The dimensionless turbulence kinetic energy (TKE) is used to investigate the TLV. A novel variable fv is utilized to analyze the relation among the TLV, strain tensor and vorticity tensor. The proper orthogonal decomposition (POD) method is used to analyze TLV structure. The results show that with the increase of the blade angle, the pump head is getting larger, the flow rate of the best efficiency moves to be larger, and both the primary TLV (P-TLV) and the secondary TLV (S-TLV) are getting stronger. The P-TLV often exists in the outer edge of TKE distribution and S-TLVs often exist in the largest value area of TKE. This phenomenon is more evident with blade angle increasing. Through POD method, it shows that the first six modes contain more than 90% of TKE. The reason why the TKE value near the region of S-TLV is high is that the tip leakage flow is a kind of jet-like flow with high kinetic energy. The main structure of the P-TLV is shown in modes 4−6, resulting in a reflux zone but not with the highest TKE.

Effect of Tip Clearance Size on Tubular Turbine Leakage Characteristics

To study the effect of tip clearance on unsteady flow in a tubular turbine, a full-channel numerical calculation was carried out based on the SST k–ω turbulence model using a power-plant prototype as the research object. Tip leakage flow characteristics of three clearance δ schemes were compared. The results show that the clearance value is directly proportional to the axial velocity, momentum, and flow sum of the leakage flow but inversely proportional to turbulent kinetic energy. At approximately 35–50% of the flow direction, velocity and turbulent kinetic energy of the leakage flow show the trough and peak variation law, respectively. The leakage vortex includes a primary tip leakage vortex (PTLV) and a secondary tip leakage vortex (STLV). Increasing clearance increases the vortex strength of both parts, as the STLV vortex core overlaps Core A of PTLV, and Core B of PTLV becomes the main part of the tip leakage vortex. A “right angle effect” causes flow separation on the pressure side of the tip, and a local low-pressure area subsequently generates a separation vortex. Increasing the gap strengthens the separation vortex, intensifying the flow instability. Tip clearance should therefore be maximally reduced in tubular turbines, barring other considerations.

Numerical investigation on vibration and pressure fluctuation characteristics in a centrifugal pump under low flow rate

Vibration characteristic is an important factor in evaluating operation stability of centrifugal pump. The vibration of main shaft was measured using a laser vibrometer, internal flow field was simulated via the shear stress transport turbulence model, and distribution law of vibration and pressure fluctuation in the impeller were analysed to explore the induction factor of vibration and the inherent relationship with pressure fluctuation in a semi-open centrifugal pump under low flow rate condition. Results of the numerical simulation are consistent with the experimental data. In addition to rotation frequency caused by impeller rotation, vibration frequency also includes characteristic frequency with high amplitude induced by unstable flow. The complex vortex in the impeller is composed of tip leakage vortex (TLV), reverse flow vortex, passage vortex and tip separation vortex. The primary tip leakage vortex (PTLV) formed by the streamline spills from 0 to 0.2λ where λ is the dimensionless distance from leading edge to trailing edge collides with tip leakage flow, the leading edge overflow and reverse flow vortex at the frequency of 1.6 fn ( fn is the rotating frequency) and 2.2 fn appear, respectively. The tip separation vortex formed in the tip clearance induced a frequency of 1.2 fn. The frequency of unstable flow phenomenon was consistent with the vibration frequency of main shaft, which induced the vibration of centrifugal pump.

Influence of rotating speed on tip leakage vortex in a mixed flow pump as turbine at pump mode

Tip clearance results in tip leakage flow from blade pressure side to suction side, and further deteriorates the energy performance and operation stability. In the present work, the influence of rotating speed on tip leakage vortex in a mixed flow pump as turbine at pump mode is investigated. As rotating speed increases, the pump head increases according to the similarity law and the best efficiency point shifts towards large flow rate. Meanwhile, the tip leakage flow and axial momentum also increase. A prediction model for separation angle of tip leakage vortex is proposed, and its relative error is below 5%. As rotating speed increases, the oscillating frequency of primary tip leakage vortex increases from 201.92 Hz to 215.60Hz, and the high pressure fluctuation region develops from leading edge to nearly the whole blade-to-blade passage.

Blade rotation angle on energy performance and tip leakage vortex in a mixed flow pump as turbine at pump mode

The energy performance and tip leakage vortex in a mixed flow pump as turbine (PAT) at pump mode is investigated by numerical method validated by experiment measurement. With the blade rotation angle α increasing, the head drops at pump mode, but it rises at turbine mode. Meanwhile, the best efficiency point shifts towards a larger flow rate at both modes. The tip leakage vortex (TLV) evolution is categorized into three stages, namely the splitting stage, the developing stage and the merging stage in an impeller rotation period at both α = −4° and α = 4°. An intensive oscillating characteristic of the TLV is observed at α = −4°, with an oscillation frequency of 8.2 fi (fi is the impeller rotation frequency). The analysis based on the vorticity transportation equation illuminates that the relative vortex stretching item related to velocity gradient, especially its component in axial direction, dominates the splitting, stretching and merging processes of the primary tip leakage vortex (PTLV). Furthermore, the impeller rotation has a crucial effect on the secondary tip leakage vortex (STLV) evolution, and further contributes to the formation and merging process of the vortexes. The TLV oscillation characteristic occurring at α = −4° generates a severe impact on pressure fluctuation in the impeller, which is almost 10 times that at α = 4°.

Influence of C groove on energy performance and noise source of a NACA0009 hydrofoil with tip clearance

Abstract In present work, the SST k-ω turbulence model and broadband noise source models are adopted to analyze the effect of the C groove on the energy performance and sources generated by dipoles and quadrupoles around a NACA0009 hydrofoil. Then the correlation mechanism between tip leakage flow (TLF) characteristics and its induced noise is investigated. Results show that the C groove significantly reduces both the area of the primary tip leakage vortex (PTLV) and the turbulent kinetic energy. It can be deduced that the closer the C groove is to the leading edge, the smaller the area of high decibel value of the dipole noise on the surface near the leading edge, but the area of the high decibel value of the dipole noise emitted from the blade tip becomes larger. Furthermore, compared with the original hydrofoil, the C groove reduces the area of quadrupole noise with maximum decrease of 29% for 50 dB. The turbulent kinetic energy in the flow region has a great influence on the quadrupole noise.

Dynamic mode decomposition and reconstruction of tip leakage vortex in a mixed flow pump as turbine at pump mode

In the present work, the tip leakage vortex (TLV) in a mixed flow pump as turbine at pump mode is decomposed and reconstructed by Dynamic Mode Decomposition (DMD) for the first time. The flow field is solved by the k-ω SST turbulence model with validation of experimental data. The unstable primary tip leakage vortex (PTLV) can be mainly classified as two parts, oscillating PTLV-A and shedding PTLV-B. The evolution frequency of PTLV is 8.4fi (fi is the impeller rotating frequency). Results of DMD show that DMD can exactly decompose the dominant frequency of TLV evolution and its harmonic frequencies. The coherent structures captured by DMD mainly distribute near the trajectory of PTLV. Meanwhile, DMD can successfully reconstruct the flow field on basis of mean flow mode and the first mode. Proper Orthogonal Decomposition (POD) can also capture the main coherent structures of PTLV, due to the dominant frequencies of the most energetic modes contain the characteristic frequency of TLV.

Influence of C groove on suppressing vortex and cavitation for a NACA0009 hydrofoil with tip clearance in tidal energy

Tidal energy was one of the renewable and sustainable energy forms under rapid development in the past decades. In tidal energy conversion, the hydraulic machinery is the key work component. However, the tip leakage vortex and cavitation have negative influence on the energy performance and operation stability of hydraulic machinery. In the present work, a C groove method is developed to improve the energy performance and suppress the tip leakage vortex and cavitation around a NACA0009 hydrofoil in tidal energy conversion. The optimization mechanism of the C groove on the energy improvement and vortex suppression is that the groove can suppress the primary tip leakage vortex (PTLV) by the groove jet impingement, and suppress the secondary tip leakage vortex (STLV) by the groove channel flow. The numerical simulations under non-cavitation and cavitation conditions are conducted to investigate the effect of C groove with different groove depths. The results demonstrate that the C groove can improve the lift coefficient by a maximum rise of 1.04% under 50% δ, and improve the lift-drag ratio by a maximum rise of 2.85% under 25% δ, respectively. The C groove most effectively reduces the TLV volume and swirling strength under 50% δ. Meanwhile, the C groove makes the vortex core shrink and far away from the foil surface, and most effectively suppresses the TLV cavitation under 50% δ. Therefore, the groove depth of 50% δ is recommended as the optimal depth on suppressing the TLV cavitation.

Theoretical Prediction Model of Tip Leakage Vortex in a Mixed Flow Pump With Tip Clearance

Abstract Tip clearance results in the leakage flow from blade pressure side to suction side, which will further cause the tip leakage vortex (TLV). Moreover, the flow pattern in an impeller is seriously deteriorated due to the TLV and its interaction with the main stream. In this work, the TLV in a mixed flow pump is investigated by numerical simulation validated by experiment measurement. The primary tip leakage vortex (PTLV) trajectory is specially studied with consideration of the tip clearance size δ, the impeller blade number Zi, and the impeller rotational speed n. The results show that δ slightly shifts the separation point (SP) of the PTLV but rarely affects the separation angle α. The increase in Zi and the decrease in n both lead to the shift of the SP toward the blade trailing edge and the decrease in α. Furthermore, a theoretical prediction model is proposed to predict the PTLV trajectory, by which the axial position and radial position of PTLV trajectory versus the rotation angle can be predicted. The proposed model is verified to be accurate to predict the PTLV trajectory, especially for the PTLV trajectory in the main flow passage. The dynamic evolution of TLV under different tip clearance sizes can all be classified into the same three stages: splitting stage, developing stage, and merging stage. Meanwhile, the dynamic evolution frequency fe is the same as the impeller rotational frequency fi.

Spatial–Temporal Evolution of Tip Leakage Vortex 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 of pump performance. In this work, the characteristics of TLV in a mixed-flow pump are investigated by the numerical simulation using shear stress transport (SST) k–ω turbulence model with experimental validation. The trajectory of the primary tip leakage vortex (PTLV) is determined, and a power function law is proposed to describe the intensity of the PTLV core along the trajectory. Spatial–temporal evolution of the TLV in an impeller revolution period T can be classified into three stages: splitting stage, developing stage, and merging stage. The TLV oscillation period TT is found as 19/160 T, corresponding to the frequency 8.4 fi (fi is impeller rotating frequency). Results reveal that the TLV oscillation is intensified by the sudden pressure variation at the junction of two adjacent blades. On analysis of the relative vorticity transport equation, it is revealed that the relative vortex stretching item in Z direction is the major source of the splitting and shedding of the PTLV. The dominant frequency of pressure and vorticity fluctuations on the PTLV trajectory is 8.4 fi, same as the TLV oscillation frequency. This result reveals that the flow instability in the PTLV trajectory is dominated by the oscillation of the TLV. The blade number has significant effect on pressure fluctuation in tip clearance and on blade pressure side, because the TLV oscillation period varies with the circumferential length of flow passage.

Development of a Tip-Leakage Flow Part 2: Comparison Between the Ducted and Un-ducted Rotor

The vortical flow in the tip region of a three-bladed rotor was examined using particle imaging velocimetry (PIV). The vortex forming at the tip of the un-ducted propeller was compared to the tip-leakage vortex of the ducted rotor. The planar flow fields were used to identify regions of concentrated vorticity and determine instantaneous vortex properties, revealing the presence of a primary tip-leakage vortex surrounded by a number of secondary vortices. Comparison between the ducted and un-ducted rotor indicated that the presence of the duct reduced the relative strength of the primary tip vortex, making its strength a smaller fraction of the overall shed circulation near the tip. The weaker tip-leakage vortex then became closer in strength to the other secondary vortices in the tip-flow region. However, for the rotor tip geometry considered here, the radius of the primary vortex core did not vary substantially between the ducted and un-ducted cases. The variability of the flow was larger for the ducted case, in terms of the primary vortex position, its identified circulation, core size, and inferred core pressure. This variability was also observed in the scaled velocity fluctuations near the core of the vortex.

Instantaneous and time-averaged flow fields of multiple vortices in the tip region of a ducted propulsor

The instantaneous and time-averaged flow fields in the tip region of a ducted marine propulsor are examined. In this flow, a primary tip-leakage vortex interacts with a secondary, co-rotating trailing edge vortex and other co- and counter-rotating vorticity found in the blade wake. Planar particle imaging velocimetry (PIV) is used to examine the flow in a plane approximately perpendicular to the mean axis of the primary vortex. An identification procedure is used to characterize multiple regions of compact vorticity in the flow fields as series of Gaussian vortices. Significant differences are found between the vortex properties from the time-averaged flow fields and the average vortex properties identified in the instantaneous flow fields. Variability in the vortical flow field results from spatial wandering of the vortices, correlated fluctuations of the vortex strength and core size, and both correlated and uncorrelated fluctuations in the relative positions of the vortices. This variability leads to pseudo-turbulent velocity fluctuations. Corrections for some of this variability are performed on the instantaneous flow fields. The resulting processed flow fields reveal a significant increase in flow variability in a region relatively far downstream of the blade trailing edge, a phenomenon that is masked through the process of simple averaging. This increased flow variability is also accompanied by the inception of discrete vortex cavitation bubbles, which is an unexpected result, since the mean flow pressures in the region of inception are much higher than the vapor pressure of the liquid. This suggests that unresolved fine-scale vortex interactions and stretching may be occurring in the region of increased flow variability.