Tip Leakage Vortex Trajectory Research Articles

Numerical investigation of the aerodynamic performance and loss mechanism in a low bypass ratio variable cycle engine fan

The aerodynamic performance of the variable cycle engine fan changes sharply during mode transition. Investigating the variations of flow structure and understanding the loss mechanism are helpful in providing guidance for the fan design. Three-dimensional models of single bypass and double bypass compression systems are established, and static pressure is applied at the bypass stream outlet to simulate the opening of the mode selection valve. The characteristic band of variable cycle engine fan is obtained by gradually increasing the bypass stream pressure while maintaining specific values for the core stream pressure. Results show that the overall performance of the double bypass configuration, without bypass recirculation, is almost identical to that of the conventional single bypass configuration during the throttling process. With the increase in bypass pressure, the shock wave and the trajectory of tip leakage vortex gradually move forward, thereby increasing the blockage region induced by the interaction between the shock and tip leakage vortex. In addition, the performance of fan with reverse flow is also calculated. The recirculation causes the operating point to move closer to the stability limit, reducing the isentropic efficiency. Additionally, the recirculation changes the radial distribution of axial velocity and total pressure, leading to inlet distortion in the core driven fan stage. Furthermore, the loss mechanism is clarified by modeling the splitter and conducting entropy generation analysis. The sharp expansion of bypass stream could cause severe flow separation, and reducing the curvature of casing can effectively suppress the viscous shear loss.

Numerical investigation of the effect of blade tip loading distribution on tip leakage vortex characteristics in a waterjet pump

The blade tip leakage vortex (TLV) is a common concern for waterjet pump. The TLV are closely related to the load distribution at the blade tip. This paper aims at to investigate the relationship between blade tip load distribution and TLV trajectory and size. The characteristics of tip leakage vortex in a mixed flow pump is investigated by numerical method. The SST k-ω turbulence model is used to predict TLV trajectories. The result shows a good agreement with the visualization results based on high-speed photography (HSP), which validated that the accuracy of numerical method. The numerical results show that the starting point of the TLV, as well as the angle between blade chord and TLV changes with flow rate condition, which related with the blade tip loading. Therefore, the distribution of the load at the top of the leaf necessarily influences the trajectory of the leakage vortex. Comparing the three different loading patterns at the blade tip, it can be seen that as the load moves backwards, the starting position of the leakage vortex at the blade tip moves backwards and the relative angle between the TLV and blade increases.

Accuracy Investigations of Dynamic Characteristic Predictions of Tip Leakage Flow Using Detached Eddy Simulation

The accurate prediction of tip leakage flow is the premise for flow mechanism analysis and compressor performance optimization. The detached eddy simulation (DES) method, which compromises cost and accuracy, has excellent potential for a high Reynolds flow, like a compressor.However, in the case of tip leakage flow, especially when there are multiple wall boundary layers and strong shear between the mainstream and leakage flow, the DES method exhibits accuracy deficiencies. This paper explores the resolution of the critical detailed structures using the DES method and its correlation with the accuracy of time-averaged aerodynamic parameter predictions. Based on this, we propose the necessary conditions for the DES method to accurately predict the leakage flow from the perspective of the detailed structure of the flow field. A simplified model is proposed to emphasize the characteristics of tip leakage flow with “multiple walls + narrow tip gap”, and the high-fidelity flow field of the WALE LES method is used as a benchmark. With the main fluctuation structures obtained by the SPOD method, it is concluded that the DES method is unable to resolve the Kelvin–Helmholtz instability at the initial position of the leakage, which leads to the generation of the secondary leakage vortex upstream of the leakage and the breakdown of the induced vortex, two critical flow structures, being incorrectly estimated. This can lead to misestimationsof the force direction on the tip leakage vortex and the main fluctuation on the flow field. As a result, the tip leakage vortex trajectory evolves toward the middle of the passage along the tangential direction and away from the upper wall downstream of the leakage compared with the LES results. Predictions of losses in the upstream and midstream regions are underestimated, whereas they are overestimated downstream of the leakage and outside the passage.Therefore, the accurate resolution of these two critical detailed structures is an essential prerequisite for the precise prediction of tip leakage flow using DES series methods.

Impact of the ending position of controllable speed casing on the flow stability in a transonic compressor rotor

The structure of casing treatment has a significant influence on the stability expansion effect. The ending position of the rotatable ring is one of the important structural parameters in the controllable speed casing. The impact of ending position of the rotatable ring rotating at the same speed as the rotor on the stability expansion and stability expansion mechanism was studied in this paper. The results show that, when the ending position of rotatable ring expands downstream in the range of 20% to 60% axial chord length, because of the improvement in the tip leakage vortex trajectory and main flow/leakage flow interface, the stability expansion effect rises gradually. The further downstream expansion of ending position from 60% axial chord length, however, has a slight impact on tip flow, bringing out a steady stability expansion effect around 52.5%. The best stability expansion effect of ending position occurs at 60% axial chord length where the stable operating margin increases by 53.5%.

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.

Influence factors and prediction model of enstrophy dissipation from the tip leakage vortex in a multiphase pump.

In a multiphase pump, tip clearance is the required distance between the blade tip and the pump body wall of the impeller, forming tip leakage vortex (TLV), causing unstable flow and energy dissipation. In the present work, the enstrophy dissipation theory is innovatively applied to quantitatively study the energy dissipation of the TLV. The flow rate, tip clearance, and inlet gas void fraction (IGVF) play a crucial role in affecting the enstrophy dissipation of the TLV. The results show that increasing flow rate, tip clearance, and IGVF significantly exacerbate the TLV pattern and raise the TLV scale, which gradually raises volume enstrophy dissipation and decreases wall enstrophy dissipation. As the flow rate increases, the separation angle between the primary TLV trajectory and the blade gradually decreases, and widely dispersing the enstrophy dissipation near the shroud. However, as the tip clearance increases, the tip separated vortex scale increases and extends to the suction surface, raising the velocity gradient. Besides, as the IGVF increases, the secondary TLV develops from a continuous sheet vortex to a scattered strip vortex, increasing the significantly increasing the enstrophy dissipation. Considering the flow rate, tip clearance, and IGVF as independent variables, simple and multiple nonlinear regression models have the ability to predict the enstrophy dissipation of the TLV accurately.

Comparative study of unsteady flow mechanisms for swept and radially stacked rotors in a dual-stage counter-rotating compressor

Counter-rotating (CR) compressor is a potential configuration to increase the thrust-to-weight ratio of aero-engines. The present study numerically investigated a dual-stage CR compressor by solving unsteady Reynolds-averaged Naiver–Stokes equations. The unsteady flow mechanisms were analyzed and compared for swept and baseline (radially stacked) CR rotors. Both of these rotor configurations were numerically simulated for the entire design speed line. The unsteady flow field near the mid-span and blade tip characterized by tip leakage vortex (TLV) was analyzed in detail for both near peak efficiency (NPE) and near stall (NS) conditions. According to the results, blade loading of the second rotor (R2) was much higher than that of the first rotor (R1). Moreover, the blade loading was higher in the swept rotors in comparison with the baseline CR rotors. In both baseline and swept CR compressors, the potential effect of R2 on the static pressure distribution of R1 was more significant compared to the opposite effect. Under the NS condition, the potential effect was confined in the rear zone of R1 where the potential flow of R2 passed. Despite the obvious impact of wake-induced negative jet on the flow field of R2 blade passage, the static pressure of R2 blade exhibited no evident variation except at 90% spanwise position. Although the blade loading of the swept CR compressor was increased, the potential effect of R2 on R1 was considerably reduced due to growth of axial gap between R1 and R2. Since the blade sweep improved the flow field, the wake passing effect on R2 was more moderate compared to that of the baseline CR compressor. The perturbation velocity magnitude of the swept CR compressor was approximately half of that of the baseline CR compressor. The TLV of R2 was periodically disturbed, and therefore, low axial velocity patches were observed on the TLV trajectory of R2. In addition, the swept CR compressor exhibited smaller low axial velocity patches of TLV due to weaker aerodynamic interaction between R1 and R2.

Investigation of Tip Leakage Vortex Structure and Trajectory in a Centrifugal Pump with a New Omega Vortex Identification Method

Semi-open centrifugal pumps are widely used in various fields. However, the tip leakage vortex (TLV) has a malign effect on the impeller flow field. The structure and trajectory of a TLV under different discharge conditions were simulated and are discussed herein. Then, the characteristics of the TLV were analyzed using a new omega vortex identification method. The external characteristic and pressure fluctuation of the simulation and experiment were consistent. A secondary leakage vortex near the blade outlet was formed under the high-discharge condition. A leading-edge overflow phenomenon under the low-discharge condition and led to the formation of a reverse-flow vortex. The interface between the main flow and tip leakage flow moved toward the impeller upstream with decreased discharge. As a result, the peak of the entropy production curve moved upstream, and leading-edge overflow and reverse flow occurred. The tip leakage flow changed the blade pressure distribution, resulting in a decrease in the blade load.

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 geometry on the performance of low-speed axial flow fan

Low-speed axial flow fans are widely used especially in air conditioning and ventilation systems. The performance of axial flow fans is negatively affected by the blockage and loss resulted from the tip leakage vortex (TLV) and the reverse flow. This paper investigates the development of TLV in a partially shrouded fan used in the outdoor unit of an air-conditioning system. It was found that the TLV follows the blade tip chord, due to the radial inflow in the initial unshrouded part of the blade, unlike the fully covered fan. Thus, the TLV trajectory can be manipulated by modifying the blade tip chord shape. It was also found that the reverse flow from the outlet near the shroud is driven by the negative component of TLV. The blade tip chord was modified to be in the axial direction before reaching the shrouded part to reduce the blockage and negative component of TLV. Comparing the results for the modified blade shows that the reverse flow and TLV blockage are effectively reduced by adjusting the TLV trajectory to be along the fan's rotational axis. The maximum flow rate increased up to 12% accompanied by a noticeable reduction in the loss by the tip modification.

Numerical evaluation of suppression mechanism of shark-inspired riblet on tip leakage vortex of a NACA0009 hydrofoil with tip clearance

The tip leakage vortex (TLV) formed by tip clearance has adverse effects on the operation stability of bulb turbines in tidal power stations. In this paper, the NACA0009 hydrofoil is selected as the research object, a method of suppressing TLV by shark-inspired riblet is proposed, and the suppression mechanism is numerically studied. The results show that the shark-inspired riblet effectively suppresses the TLV, decreases the high-vorticity region generated by TLV, and increases the pressure in the centre of the vortex core. The area of the low-pressure region on hydrofoil tip decreases, with a decreasing trend moving downstream and to the suction surface. The tip leakage flow is controlled by reverse rotating vortex inside the riblet. The leakage velocity decreases, which leads to a reduction in the driving force of the tip leakage flow. In addition, the leakage rate declines 5.9%. Moreover, the TLV trajectory is close to the suction surface, the vertical distance of TLV movement is reduced, and the maximum TLV intensity decreases by 7%. With increasing riblet height, the suppression effect of shark-inspired riblet on TLV is more obvious, but the lift-to-drag ratio of hydrofoil decreases rapidly when the riblet height is greater than a critical value.

Numerical Study of Tip Leakage Vortex Around a NACA0009 Hydrofoil

Abstract In the tip clearance flow, the dominant vortex is the tip leakage vortex (TLV), which has a significant impact on the hydraulic and cavitation performance of axial flow machineries. In order to reveal the impact mechanism of the gap size on the TLV, gap flows with two gap sizes, i.e., τ=0.2 (2 mm) and τ=1.0 (10 mm), are numerically investigated. A NACA0009 hydrofoil is selected to create the gap flow, with an incoming velocity of 10 m/s and an attack angle of 10 deg. The results show that the two flow cases are significantly different in terms of vortex feature and the leakage flow distribution. In the small gap, a type of jet-pattern flow appears, whereas a type of rolling-pattern flow passes over the large gap. The vertical velocity gradient of the leakage flow has a decisive influence on the TLV trajectory. In addition, for the large gap, the axial velocity in the vortex center exceeds the incoming flow. This jet-like state of axial velocity can be maintained for a long distance, making the vortex more stable. However, the axial velocity in the case of τ=0.2 cannot stay at the jet-like state and rapidly switches to a wake-like state.

Effects of Tip Clearance Size on Energy Performance and Pressure Fluctuation of a Tidal Propeller Turbine

Unavoidable tip clearance between blade tip and casing shroud plays an important role in the performance and characteristics of a tidal propeller turbine. In this work, the tip-leakage vortex (TLV) induced in the end-wall region was numerically illustrated by using the shear-stress transport (SST) k–ω turbulence model at various flow conditions and different tip-clearance sizes (TCSs). The swirling strength criterion was employed to visualize the tip-leakage vortex trajectory and investigate vortex evolution according to clearance size change. Although TLV occurs in both design and off-design conditions, vortex intensity develops strongly under excess flow rate with increased tip gap. The extreme influence of TCS on the turbine’s generated power and efficiency was predicted in steady simulations for four TCS cases, namely, δ = 0%, 0.25%, 0.5%, and 0.75%. With the extension of the tip gap, turbine performance was drastically reduced because of vigorous turbulent leakage flow combined with considerable volumetric loss. The effect of TCS on pressure fluctuation intensity were also explored on the basis of the transient simulation statistic. Maximal pressure variation amplitude and dominant frequency were presented in spectrum analysis utilizing fast Fourier transform.

Tip leakage vortex trajectory and dynamics in a multiphase pump at off-design condition

Because of the clearance and pressure difference between the pressure side (PS) and the suction side (SS), the tip leakage flow developed and evolved into tip leakage vortex (TLV). The relationship between the swirling strength of TLV and the pressure difference between the PS and the SS was analyzed. The velocity distribution in the tip clearance was also investigated in a multiphase pump. The reliability of numerical method was verified using high-speed photography. The trajectory of TLV was found to be distributed in a wavy shape and the swirling strength of TLV was positively correlated with the pressure difference between the PS and the SS. The tip separated vortex was formed when the tip leakage flow entered the tip clearance, and the TLV was formed when tip leakage flow moved out of the tip clearance. The low-pressure region in the tip clearance consisted of two parts. The part near the PS was caused by the tip separated vortex, and the other part near the SS was caused by the TLV. In addition, the flow separation phenomenon occurred at the radial coefficient of 0.97 and led to the increase of the turbulent energy loss of the pump.

Effect of the inlet gas void fraction on the tip leakage vortex in a multiphase pump

Inlet gas void fraction (IGVF) played an important role on the flow characteristics in a multiphase pump. To reveal the effect of inlet gas void fraction on the flow characteristics in the tip clearance, a combination of numerical simulation and experiment was carried out and the reliability of numerical method was verified by comparing with the experimental data of the flow field by using high-speed photography. The results showed the accumulated gas was mainly at the impeller inlet near the pressure side (PS), tip clearance near the tip and the suction side (SS). When the IGVF increased, there was an obvious stratified structure and the separated vortex in the tip clearance. Compared to the water case, the gas caused the tip leakage flow velocity to decrease from the blade inlet to the streamwise coefficient of 0.2, and to increase from the streamwise coefficient of 0.2 to the blade trailing edge. At the same time, the IGVF had a significant influence on the tip leakage vortex (TLV) structure and trajectory, and the streamlines and vorticity distribution corresponding to the wake and the TLV were changed. Moreover, the flow characters and the structure of the TLV were more complicated under gas-liquid condition.

Nonaxisymmetric Study of Tip Leakage Flow in a Centrifugal Compressor With a Volute During Stall Process

Abstract Tip leakage flow (TLF) patterns, which affect compressor performance, are closely related to compressor stability. To date, minimal attention has been given to circumferential nonuniformity of the TLF in a centrifugal compressor with a nonaxisymmetric volute structure. In this study, the circumferential difference of the TLF in a centrifugal compressor with a volute during the stall process is analyzed. The circumferential nonuniformity of tip leakage vortex (TLV) trajectories, loading distribution near the tip, and distance between the TLV core and the leading edge (LE) of splitter blades were also investigated. It is shown that in the circumferential direction, there are two peaks associated with the angle (α) between the TLV trajectory of the seven main blades and the axial direction. As the stall process progresses, the blade whose LE is affected by the high static pressure band (PP) induced by the volute tongue (VT) loses its work capacity first and the α difference between this blade and the other blades increases. In addition, the tip loading and TLF velocity of the blade whose LE is affected by the high static pressure band induced by the VT are at a minimum, and the flow loss in the tip clearance is higher. There is a phenomenon of the TLV breakdown. When the blade trailing edge (TE) is located in the low static pressure region, TLV streamlines appear as a significant turn at the breakdown point. However, the TLV streamlines at other circumferential positions do not exhibit this phenomenon.

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.

One-dimensional modeling for tip clearance leakage vortex trajectory and stall-onset prediction in subsonic centrifugal impellers

Two one-dimensional models are established for the tip leakage vortex trajectory and rotating stall-onset point prediction respectively for subsonic centrifugal impellers. The goal of modeling is to supply an effective estimation strategy of the stall-onset point for use in the one-dimensional performance prediction stage. The tip leakage vortex trajectory prediction is a critical part of the stall-onset prediction. The proposed one-dimensional model (one-dimensional tip leakage vortex trajectory model) to predict the tip leakage vortex trajectory is based on blade loading, i.e. the velocity difference between the pressure and suction surfaces. The loading function considers the effect of radial rotation, blade turning, and passage width variation. Compared with the computational fluid dynamics results, the current model shows reasonable accuracy, with an average relative error below 12.35%. The one-dimensional prediction model (Model II) is developed to determine the stall-onset point, where the interface between the tip leakage flow and the main flow spills from the blade leading edge. In this model, the momentum balance analysis is applied to identify the position of the interface. The parameter of the tip leakage vortex trajectory in Model II is determined by one-dimensional tip leakage vortex trajectory model. The effective origin of the tip leakage flow is correlated with the rotational speed and tip clearance. The effectiveness of Model II is validated with the experimental and computational fluid dynamics results using three impellers. Compared with the conventional model (Model I), Model II shows better accuracy, with a maximum error of about 7.42%.

Effect of circumferential static pressure nonuniformity caused by volute on tip leakage flow in a centrifugal compressor

For a centrifugal compressor with volute, the flow field is circumferentially nonuniform because of the volute asymmetrical structure and leads to a circumferential difference in the tip leakage flow. In this work, the compressor performance and the casing wall static pressure distribution are measured, and the results are compared with the time-averaged results of the unsteady calculation to verify the reliability of the simulation. The results show a relationship between the tip leakage vortex trajectory and the high static pressure region in the diffuser, based on which a prediction model is established for the reverse propagation of pressure waves caused by a volute tongue. Influenced by the volute asymmetric structure, the trajectory, shape, and strength of the tip leakage vortex at different circumferential positions differs significantly. The tip leakage vortex trajectory affected by the high static pressure is more inclined to a circumferential direction because the tip leakage flow velocity flowing out of the suction surface is reduced, and the tip leakage flow with low velocity is subjected to the high-pressure gradient in a passage. Moreover, the tip leakage vortex breakdown in different passages differs significantly. A tip leakage vortex core more inclined towards the streamwise direction is more likely to break down than a tip leakage vortex core inclined towards the circumferential direction because of the larger reverse pressure gradient.

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.