Tip Leakage Vortex Research Articles

Study on the blade squealer tip affecting tip leakage flow and performance of a multiphase pump

Tip clearance is the distance required between the blade tip and the pump body wall of an impeller in a helicon-axial multiphase pump, which tends to induce tip leakage flow. The tip leakage vortex formed by the interaction of tip leakage flow with the mainstream can seriously affect the performance of the multiphase pump. To minimize the adverse effects of tip leakage flow in the multiphase pump, a method to design a squealer tip on the impeller blade is proposed in this paper. The effect of the squealer tip on external characteristics, tip clearance flow characteristics, and energy dissipation of the multiphase pump is analyzed. Research results indicate that the blade squealer tip can effectively improve hydraulic efficiency of the multiphase pump. At the optimal efficiency point, the head and hydraulic efficiency of the multiphase pump with a squealer tip increased by 3.62% and 4.15%, respectively, compared with the original model. The influence of tip leakage flow in the axial rear half passage of the multiphase pump impeller is far greater than that in the axial forward half passage, especially on the back position in the middle of the impeller passage. The squealer tip can restrain the reverse of leakage flow from the pressure side to the suction side of the impeller blade, and the clearance leakage flow of the model with a squealer tip is smaller than that of the original model. The squealer tip on blade will reduce the energy dissipation caused by unsteady flow in the mainstream. The research results in this paper can provide theoretical support for effectively restraining the influence of the tip leakage vortex on the mainstream of the helicon-axial multiphase pump and contribute to engineering practice value of improving the performance of the multiphase pump.

Application and comparison of dynamic mode decomposition methods in the tip leakage cavitation of a hydrofoil case

The cavitation of the tip leakage vortex (TLV) induced by tip leakage has always been a difficult problem faced by turbomachinery, and its flow structure is complex and diverse. How to accurately extract the main structures that affect the cavitating flow of the TLV from the two-phase flow field is a key problem. In this study, the main mode extraction and low order mode reconstruction accuracy of the cavitation flow field of TLV downstream of National Advisory Committee for Aeronautics (NACA)0009 hydrofoil by two dynamic mode decomposition (DMD) methods are compared. The research shows that the main modes extracted by the standard DMD method contain a large number of noise modes, while the sparsity-promoting DMD eliminates the noise modes, showing obvious advantages in the reconstruction accuracy of the velocity field. The characteristics of cavitation signals are analyzed, and the cavitation signals are divided into four categories, which explains the reason why DMD methods have low reconstruction accuracy in cavitation. This study provides a theoretical basis and strong guarantee for the extraction of mode decomposition characteristics of the two-phase flow field. This is of great significance for accelerating the prediction of multiphase flow fields based on intelligent flow pattern learning in the future. Meanwhile, it also provides a new method and road for the introduction of artificial intelligence technology in future scientific research.

Comparative Study on the Fractal and Fractal Dimension of the Vortex Structure of Hydrofoil’s Tip Leakage Flow

Axial-flow turbomachinery is widely used in low head water transfer and electricity generation projects. As there is a gap between the impeller and casing of the tubular flow unit, the fluid will cross the gap to form tip leakage flow, which may induce intense pressure pulsation, noise and mechanical vibration, and even threaten the safe operation of the unit. In order to ensure the efficient and stable operation of hydropower units, the influence factors of tip clearance flow and its formation and development mechanism have been deeply studied in this paper. In this paper, the impact of gap width, angle of attack and inlet velocity on tip leakage flow of hydrofoil with clearance are studied by orthogonal experiment method. The results suggest that the gap width has the greatest influence on tip clearance flow, the incidence angle takes the second place, and the inlet velocity has the least effect on tip clearance flow. Then the fractal characteristics of tip leakage vortices with different gap widths are studied. The results demonstrate that the fractal dimension of tip leakage vortices in large gaps was significantly larger than that in small gaps; The fractal dimension of the leakage vortex decreases gradually along the flow direction.

Numerical and Experimental Investigations of Axial Flow Fan with Bionic Forked Trailing Edge

To improve the performance of the aerodynamic properties and reduce the aerodynamic noise of an axial flow fan in the outdoor unit of an air conditioner, this study proposed a bionic forked trailing-edge structure inspired by the forked fish caudal fin and implemented by modifying the trailing edge of the prototype fan. The effect of the bionic forked trailing edge on the aerodynamic and aeroacoustic performance was investigated experimentally, and detailed analyses of the blade load and internal vortex structures were performed based on large-eddy simulations (LES). It is shown that the bionic forked trailing edge could effectively adjust the blade load distribution, reduce the pressure difference between the pressure side and suction side near the trailing edge of the blade tip region, and weaken the intensity and influence range of the inlet vortex (IV) and the tip leakage vortex (TLV). The discrete noise caused by the vortices in the rotor tip area was also reduced, particularly at the blade passing frequency (BPF) and its harmonic frequency. The experimental results confirmed the existence of an optimal bionic forked trailing-edge structure, resulting in the maximum power-saving rate γ of 7.5% and the reduction of 0.3 ~ 0.8 dB of aerodynamic noise, with an included angle θt of 13.5°. The detailed analysis of the internal vortex structures provides a good reference for the efficiency improvement and noise reduction of axial flow fans.

On the mechanisms of pressure drop and viscous losses in hydrofoil tip-clearance flows

Local pressure drop and viscous losses due to the tip-clearance flow are key factors in the performance degradation of axial-flow hydraulic machinery. To deepen insight into the underlying mechanisms of pressure drop and viscous losses in the vicinity of the tip-gap, taking a simplified case, the present work studies the tip-clearance flow around a hydrofoil with/without foil tip-edge fillets using very large eddy simulation. A quantitative model for evaluating viscous losses and pressure drop is developed based on a detailed analysis of the conversion between pressure energy, kinetic energy, and internal energy. Results show that the turbulent loss supplied by turbulent kinetic energy (TKE) production accounts for about 90% of the total viscous losses in the tip-gap region, and TKE production is the major contributor to pressure drop. Contributions of the sub-components of the TKE production term to the TKE production of tip-leakage vortices, and the leading derivatives of the velocity for TKE production, have been explored. Additionally, it is found that the hydrofoil edge fillet promotes (suppresses) the TKE production in the tip-leakage vortex (tip-separation vortex), then enhances (weakens) the pressure drop and viscous losses, while the amount of total loss of the tip-clearance flow is not significantly affected.

Mechanism Affecting the Performance and Stability of a Centrifugal Impeller by Changing Bleeding Positions of Self-Recirculating Casing Treatment

This study aimed to investigate the influence of the bleeding position of a self-circulating casing on the aerodynamic performance of a transonic centrifugal compressor. Three types of self-circulating structures with the bleeding positions of 11% Ca (the axial chord length of the blade tip), 14% Ca and 20% Ca from the leading edge of the blade were studied by using the numerical simulation method, with the Krain impeller taken as the research object. It was found that all three types of self-recirculating casing treatments can expand the stable operating range of the impeller, and that at medium and small flow rates, the total pressure ratio and efficiency of the impeller increase gradually with the backward movement of the bleeding position. The self-circulating casing treatment can restrain the development of tip leakage vortex, reduce the blockage area, and improve the stability of the impeller by sucking low-energy fluid. The farther back the bleeding position is, the greater the bleeding mass flow rate of the self-circulating casing for the low-energy fluid in the blade-tip passage becomes. Additionally, a greater inhibition effect on the tip leakage vortex, and a better effect of improving the performance and stability of the impeller, can be obtained. The best air bleeding position is 20% Ca, but it is not directly above the blade-tip blockage center of the solid wall casing passage. Instead, it is downstream of the blockage area.

Numerical investigation on vortex control effect of pump-jet propulsion via porous blade tips

Pump-Jet Propulsions (PJPs) are widely equipped on submarines, torpedoes and other underwater vehicles. The special structure of PJP leads to complex vortex system in the working process, especially the strong Tip Leakage Vortex (TLV) in the clearance between the duct and the rotor blade tips, which seriously affects the unsteady hydrodynamic performance and acoustic characteristics of PJP. The rotor blade tips of PJP are modified with porous materials, the control effect of the porous blade tip on the TLV is investigated, and the influence of the porous blade tips with different parameters on the vortex control effect is quantitatively analyzed. Large Eddy Simulation (LES) method is applied to the flow field observation. It is concluded that the porous blade tips can weaken the TLV intensity and inhibit the backflow at the blade tip clearance. The analysis results of fluctuating pressure indicate that the negative pressure peak and pressure gradient in tip clearance can be effectively reduced after installing porous blade tips, which is helpful to the vibration and noise reduction of the PJP. Through a series of optimization work, the porous blade tip scheme with minimal propulsion performance loss and obvious vortex control effect is determined.

Mechanisms of oscillating suction in controlling the tip leakage flow in a high-load compressor cascade

To control tip leakage flow and reduce related loss in axial-flow compressors, the oscillating suction scheme was introduced in a high-load compressor cascade with a tip clearance, and its flow control mechanisms on the tip leakage flow were studied numerically. It is found that the flow control effect of oscillating suction is the best when the unsteady excitation frequency is equal or harmonic to the feature frequency of the flow field related to the flow separation. The loss of the cascade controlled by the oscillating suction with a reduced frequency of 0.39 and excitation amplitude of 10 kPa was reduced by about 3.4% compared with the steady suction scheme, and about 11.14% less than that of the non-suction scheme. Oscillating suction discretized the concentrated tip leakage vortex into a series of scale-reduced vortex structures by the periodic unsteady excitation, which can be identified by the analysis using proper orthogonal decomposition (POD). The feature frequencies became consistent with the oscillating excitation frequency or its multiple, and the fluctuation generated by the tip leakage flow was enhanced. The energy proportion of high-order POD modes was increased by oscillating excitation, which indicated the vortex structures with weak fluctuation intensity were more active. The mixing and energy exchange among the mainstream, separated flow, endwall secondary flow, and tip leakage flow were enhanced, so that a better flow control effect could be achieved. Furthermore, flow capacity of tip passage was modified, the overall aerodynamic performance of the compressor cascade was improved thereby.

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.

Impact of complex flow structures on the turbine blade tip region mixing

Mixing of the turbine blade tip leakage and mainstream flows causes considerable aerodynamic loss. Its understanding is crucial to raise a consequential improvement of the turbine performance. In the present paper, a typical high pressure turbine rotor flow is simulated by detached eddy simulation. The complex mixing in the blade tip region is assessed by the dilution index algorithm in the streamwise direction. Influences of typical parameters on mixing are consecutively identified and analyzed. Relating the influences to flow structures, the mixing mechanism is obtained. It is revealed that the normal effective diffusion coefficient is key in mixing and is correlated with the unsteady tip leakage flow stick vortices. The latter induced by the Kelvin–Helmholtz instability can significantly enhance the local mass and energy transfer and hence diffusion. As a result, mixing is strengthened. Furthermore, the tip region is knowingly divided into the juxtaposing near and far fields. The former contains the tip leakage vortex, leakage jet, mainstream flow, and two entrainment zones. Meanwhile, the latter contains the leakage jet, mainstream flow, and entrainment zone. It was found that the entrainment zone is mixed to a high-degree, whereas the leakage jet is barely mixed.

Effect of Pre-Swirling Flow on Performance and Flow Fields in Semi-Opened Axial Fan

In this study, we tried to improve the performance by giving a pre-swirling flow to the radial inflow that occurred in the semi-opened axial fan. In addition, the flow fields of rotor outlet were clarified experimentally, and the effect of pre-swirling flow was considered. The experiment was carried out using a performance test wind tunnel with a square cross section of 880 mm. Three types of casings were prepared, in which the blade tip protruded 0%, 20%, and 40% of the meridional chord length. They were called R25, R15, and R05, respectively, in the casing bellmouth model code. Guide blades for generating a pre-swirling flow were installed on the vertical wall surface of the casing. In addition, a vertical wall was installed 60% upstream of the meridional chord length as an obstacle to prevent axial inflow. The velocity fields of the rotor outlet were measured using a hot-wire anemometer. From the results, the pre-swirling flow did not significantly affect the fan performance. When there was no obstacles wall upstream, there was a partial increase in efficiency, but the difference was not so large. When there was an obstacle wall upstream, the efficiency increased overall in the case of R15, but in the case of R05, the efficiency increased only in the low flow rate region, and conversely decreased in the high flow rate region. By observing the blade outlet flow fields when the performance was improved, it was confirmed that the influence of the tip leakage vortex was weakened.

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.

Theoretical Model and Numerical Analysis of the Tip Leakage Vortex Variations of a Centrifugal Compressor

A centrifugal compressor of a micro turbine generator system is investigated by the theoretical model and numerical analysis to explore the characteristics of the tip leakage vortex as the centrifugal compressor approaches stall. The numerical simulation results show the cross-sectional shape of the tip leakage vortex is elliptical, and its long and short axes are gradually stretched as the compressor approaches stall. Moreover, the vortex trajectory is inclined to the pressure side of the adjacent blade. In addition, the Kirchhoff elliptical vortex model is introduced to analyze the flow passage constriction effect, the passage vortex squeezing effect, and the leakage flow translation effect. Results show that there is no upper limit for the flow passage constriction effect on the tip leakage vortex. Furthermore, relative to the original vortex, the minimum constriction effect depends on the axis ratio of the elliptical tip leakage vortex. The passage vortex has an expansion effect on the tip leakage vortex rather than a squeezing effect, which is limited and also depends on the axis ratio of the ellipse. However, the effect magnitude of the leakage flow depends on the scales both of the long and short axes, which also have no upper limit.

Numerical Analysis of the Effects of Different Rotor Tip Gaps in a Radial Turbine Operating at High Pressure Ratios Reaching Choked Flow

To operate, radial turbines used in turbochargers require a minimum tip gap between the rotor blades and the stationary wall casing (shroud). This gap generates leakage flow driven by the pressure difference between the pressure and suction side. The tip leakage flow is largely unturned, which translates into a reduction of the shaft work due to the decrease in the total pressure. This paper investigates the flow through the rotor blade tip gap and the effects on the main flow when the turbine operates at a lower and higher pressure ratio with the presence of supersonic regions at the rotor trailing edge for two rotational speeds using computational fluid dynamics (CFD). The rotor tip gap has been decreased and increased up to 50% of the original tip gap geometry given by the manufacturer. Depending on the operational point, the results reveal that a reduction of 50% of the tip gap can lead to an increase of almost 3% in the efficiency, whereas a rise in 50% in the gap penalty the efficiency up to 3%. Furthermore, a supersonic region appears in the tip gap just when the flow enters through the pressure side, then the flow accelerates, leaving the suction side with a higher relative Mach number, generating a vortex by mixing with the mainstream. The effects of the vortex with the variation of the tip gap on the choked area at the rotor trailing edge presents a more significant change at higher than lower speeds. At a higher speed, the choked region closer to the shroud is due to the high relative inlet flow angle and the effects of the high relative motion of the shroud wall. Furthermore, this relative motion forces the tip leakage vortex to stay closer to the tip suction side, generating a subsonic region, which increases with the tip gap height. The leakage flow at lower and higher rotational speed does not affect the main flow close to the hub. However, close to the shroud, the velocity profile changes, and the generated entropy increases when the flow goes through the tip gap.

Study on the trajectory of tip leakage vortex and energy characteristics of mixed-flow pump under cavitation conditions

To investigate the mechanism of tip leakage vortex (TLV) and energy loss caused by pump cavitation. The SST k-ω turbulence model coupled with the Reynolds-averaged Navier-Stokes equations, combined with the homogeneous flow cavitation model is employed. And the accuracy of the numerical simulation is verified experimentally. The results show that the cavitation intensity seriously affects the load distribution on suction side (SS), and the variation pattern of the low-pressure area distribution on SS is related to the cavitation location. As cavitation deteriorates, the working capacity of the impeller is weakened and the guide vane is also affected, resulting in a sharp drop in the energy performance of the pump. The malignant cavitation becomes the main reason for the decreasing head and unstable operation of the mixed flow pump. Cavitation deterioration reduces the output power of the impeller, and the power distribution characteristics are affected by the degree of cavitation. Malignant cavitation leads to increased frictional and turbulent dissipation losses and increased total energy loss, resulting in a decrease in the effective energy obtained, leading to a decrease in the overall pump performance. The results can provide theoretical reference for improving the cavitation performance of the pumps.

Aeromechanical Optimization of Scalloping in Mixed Flow Turbines

Abstract Scalloping of radial and mixed-flow turbocharger turbine rotors has been commonplace for many years as a means of inertia reduction and stress relief. The interest in turbine rotor inertia reduction is driven by transient loading requirements of turbocharged internal combustion engines, as this is a key factor in the time taken to meet transient engine torque requirements. Due to the high-density materials used in turbine rotors, any material removal from the turbine wheel has a significant impact on turbocharger inertia, and thus the transient response of the engine. It is well known that scalloping reduces not only inertia, but also efficiency. This study aimed to identify if it was possible to produce a new scallop design which reduced the scalloping efficiency penalty without increasing inertia or compromising mechanical constraints. This was carried out with the aim of developing design recommendations for scalloping where a complete minimization of inertia is not the design goal. A multipoint, multi-physics numerical optimization, with constraints on inertia and back disc stress, was carried out to determine what efficiency benefit could be realized by aerodynamically designing mixed-flow turbine scalloping. An efficiency benefit was identified across the entire turbocharger operating line, with increased benefit at low engine load, whilst not exceeding the design constraints. Scalloping losses for the baseline design were found to be greatest at low engine load, where the turbine experienced a low expansion ratio, mass flow, and speed. This explains why an aerodynamic redesign yields the greatest benefit under those operating conditions. These performance predictions were experimentally validated on the cold flow test rig at Queen's University Belfast, with good agreement between simulated and measured data. To conclude the study, a detailed loss audit was carried out to identify key loss generating flow structures and to understand how changes in geometry affected the formation and development of these flow structures throughout the passage. A large vortex which entered the passage from the scalloped region and interacted with the tip leakage vortex along the suction surface of the blade was identified as the main source of loss due to scalloping. The optimized design was found to better control the location of entry of this vortex into the blade passage, thus reducing the associated loss and facilitating a performance improvement. Geometric design guidelines were then proposed based on these findings.

The groove at blade tip designed for suppression of tip-leakage vortex may bring the risk of inducing new cavitation

Previous research showed that slotting at the tip section of a rotating machinery blade could suppress the tip-leakage vortex (TLV) by forming a new groove flow, while the possible adverse effects caused by the discontinuous tip section have not fully been studied. In this Letter, unfavorable effects due to an extra cavitation caused by the groove found in the standard incipient cavitation experiments are reported. Then, this anomaly is clarified by using large eddy simulation that the grooves cause step-like flows and induce low-pressure areas behind the groove near the pressure surface. This increased risk of inducing new cavitation deserves special attention when the medium is water.

Unsteady Structure of Compressor Tip Leakage Flows

Abstract Direct numerical simulations (DNS) are performed of a cantilevered stator blade to identify the unsteady and turbulent flow structure within compressor tip flows. The simulations were performed with clearances of 1.6% and 3.2% of chord. The results show that the flow both within the gap and at the exit on the suction side highly unsteady phenomena controlled by fine-scale turbulent structures. The signature of the classical tip-leakage vortex is a consequence of time-averaging and does not exist in the true unsteady flow. Despite the complexity, we are able to replicate the flow within the tip gap using a validated quasi-three-dimensional (Q3D) model. This enables a wide range of Q3D DNS simulations to study the effects of blade tip corner radius and Reynolds number. Tip corner radius is found to radically alter the unsteady flow in the tip; it affects both separation bubble size and shape, as well as transition mechanisms in the tip flow. These effects can lead to variations in tip mass flow of up to 10% and a factor of 2 variation in dissipation within the tip gap.

Characteristics and mechanisms of the tip leakage cavitating flow around a NACA66(mod) hydrofoil under different cavitation states

The purpose of the present study is to shed light on the characteristics and mechanisms of tip leakage cavitating flow under different cavitation states, especially for the tip leakage vortex (TLV) and tip leakage vortex cavity (TLVC). The large-eddy simulation and a newly proposed Burgers vortex cavitation model have been used for the numerical simulations. Three modes of TLV have been summarized according to the development features of the TLV circulation Γ with different cavitation states. At the ascending stage, Mode I has 30.5% and 229.5% more averaged values of Γ than Mode II and Mode III, respectively. However, at the declining stage, the averaged values of Γ for Mode I are 33.2% and 1461.3% larger than those of Mode II and Mode III, respectively. Detailed analysis of the relative axial velocity and the pressure gradient in the TLV core shows that the adverse pressure gradient has a huge effect on the stability of the TLVC. The interaction between the cavitation and vortex was investigated by using the vorticity transport equation. It is found that stretching and dilatation terms are the most critical factors during the development of the TLVC, however, they play different roles in different positions.

Effects of Clearance and Operating Conditions on Tip Leakage Vortex-Induced Energy Loss in an Axial-Flow Pump Using Entropy Production Method

Abstract Tip leakage flow (TLF) is a typical flow phenomenon in the internal flow of axial-flow pumps that has a serious impact on their safety and stability. In this study, numerical simulations are performed to investigate the influence of various tip clearances and operating conditions on the characteristics of the tip leakage vortex (TLV) and energy loss of a prototype of a vertical axial-flow pump. First, based on entropy production theory, the TLV-induced energy loss is quantitatively studied. The entropy production rate caused by turbulence dissipation (EPTD), which is caused by pulsating velocity, contributes the most to the total energy loss. The EPTD at the impeller is principally distributed on the leading edge of the blade due to the influence of the tip clearance. Then, the spatial shape and trajectory of the core of the TLV are discussed, and their correlations with pressure and vorticity are investigated to reveal the spatial distribution characteristics and formation mechanism of TLVs. With increasing tip clearance, the trajectory of the vortex core extends radially outward, and the low-pressure area near the blade tip is consistent with the trajectory of the core of the TLV, which accompanies high vorticity. Fundamentally, pressure gradients and flow separation at the leading edge are the root causes of the TLVs. Lastly, the spatial evolution of TLVs under different calculation schemes is discussed by utilizing the vorticity transport equation, demonstrating that the Coriolis force (CORF) is the main factor that affects the location of a TLV, whereas the vorticity stretching term (VST) has a greater influence on the vorticity variation rate of the TLV than the CORF and plays a predominant role in the spatial development of the TLF.