Tip Leakage Vortex Research Articles

Numerical study on effect of design parameters of the single-circumferential groove on tip leakage flow reduction in a transonic turbine

In order to investigate the impact of design parameters (depth, width, and location) of the single-circumferential groove (SCG) on tip leakage flow (TLF) reduction in a transonic turbine stage, a systematic investigation was conducted by high-precision numerical simulation method. The numerical results show that the introduction of SCG could improve the stage efficiency (η) of turbine significantly. The optimal design parameters of depth coefficient (ζ), width (W), and location (L) were ζ = 3, W = W3, and L = 20%Cax (where Cax denotes the axial chord of the blade), which could reduce tip clearance leakage rate (m*) by 28.9% and improve the η by 0.56%. The implementation of flow field display methods reveals that the introduction of SCG influences the formation and development of the vortex system at the rotor tip region. The circumferential groove vortex (CGV), induced by SCG, divided the tip leakage vortex (TLV) into two segments TLV_1 and TLV_2 due to its blocking effect. TLV_1 interacted with the passage vortex (PV) and depress the strength of PV. Meantime, TLV_2 would regenerate from the trailing edge of SCG, due to its limited development range and insufficient growth, the strength and scale of TLV_2 were weakened. Given that the TLF passing through SCG flows in an opposite direction to the original TLF, the SCG at appropriate location could lead to a fact that the net flow rate of the leakage flow reached the minimum value, thereby considerably reducing the m* and flow loss caused by TLF.

Effect of moving end wall on tip leakage flow in a compressor cascade with different clearance heights

Numerical simulations have been carried out to investigate the effect of the moving end wall on the tip leakage flow structure in a compressor cascade with four different tip clearance heights. A detailed comparison of the leakage flow’s loss; mass flow rate; and the tip leakage vortex and its roll-up, trajectory, size, and interaction with the passage flow was made. The results show that the performance of the cascade changes more severely as the tip clearance height increases with the moving end wall than with the stationary end wall. The roll-up of the tip leakage vortex has been studied by analyzing the pathline structure and vorticity distribution. An alternative explanation is proposed for the initial roll-up mechanism of the tip leakage vortex. The vorticity transport is responsible for the initial roll-up of the vortex, whereas the leakage jet/passage flow shearing is responsible for the vortex development. The smaller the clearance height is, the easier it is for the moving end wall to alter the vorticity transport from the gap to the tip leakage vortex.

Influence of tip clearance on internal energy loss characteristics of axial flow pumps under different operating conditions

Axial flow pumps possess a unique structure where there must be clearances between the impeller and the piping wall, usually not exceeding 0.1% of the impeller diameter. Despite the small size of the clearance, the internal micro-vortex structures have a non-negligible impact on the main flow field of the impeller. Under the action of the pressure difference between the suction and pressure surfaces of blades, some fluids form high-energy jets in the tip clearance area, known as tip leakage vortices (TLVs). TLV interacts with the flow of the main flow field, exerting a significant impact on the internal flow state, energy loss, and hydraulic performance of the pump. To identify the influence of TLVs on the internal flow field and energy loss of axial flow pumps, this work uses a modified partially averaged Navier–Stokes (PANS) model to perform full flow field numerical calculations for a certain axial flow pump and conducts a comparative analysis of the internal flow field energy dissipation, unsteady vortex structures, energy loss, and other characteristics under three different tip clearances: 0.2 mm (0.05%D), 0.6 mm (0.15%D), and 1.0 mm (0.25%D) based on the energy transport theory. The results indicate that at optimal operating conditions, the internal energy distribution of the fluid in each flow passage is uniform, and the energy loss is primarily caused by axial backflow in the tip area; under critical rotating stall conditions, clearance size affects the distribution state of enstrophy in the guide vane flow passage, leading to average enstrophy being highest at the rim area and the most uneven distribution of enstrophy, inducing larger energy loss in the impeller; during deep stall conditions, the unevenness of internal energy distribution is stronger than that under critical stall conditions, but the overall energy loss within the impeller flow area is lower than that under critical stall conditions, while energy unevenness is mitigated as the tip clearance size increases.

Numerical study of the coherent characteristics of the blade tip of a micro centrifugal compressor and its application in a new unsteady casing-treatment experiment

The tip-leakage vortex, as the dominant coherent structure of the blade tip of a compressor, is an important source of losses in centrifugal compressor rotors; it limits the stall margin and can even cause compressor surge. The miniaturization of mechanical components has attracted significant attention; however, the miniaturization of compressors—particularly centrifugal compressors—further increases the adverse effects of the tip-leakage vortex. This study sought to establish a new passive unsteady flow-control method for a micro centrifugal compressor using equal-circumferential-spacing through-holes on its casing. This approach fully exploits the unsteady characteristics of the flow field. A numerical study of the coherent characteristics of the blade tip was carried out using dynamic mode decomposition. The numerical conclusions were then applied to a physical system. The results of experimental tests of this system indicate that the improved flow-control method can increase the maximum efficiency and stall margin of the compressor by 2.5% and 9.0%, respectively, and it can also increase the maximum pressure ratio.

Vortex dynamics characteristics in the tip region based on Wray–Agarwal model

In order to solve the blockage effect and energy dissipation phenomenon caused by cavitation in the low-pressure vortex core region, this paper analyzes the spatial evolution of vorticity intensity and turbulent kinetic energy intensity under different cavitation conditions based on the Wray–Agarwal (WA) model. First, the tip leakage flow characteristics are studied, the evolution of vorticity and vorticity intensity is analyzed, then the distribution of turbulent kinetic energy distribution in the blade tip region is studied, and finally, the vorticity transport characteristics of the tip region are analyzed. It is found that the tip leakage rate is less affected by the vortex cavitation of the tip leakage, and there is a strong interaction between the leakage flow at the tip leading edge and the trailing edge, and the separation vortices and low-speed regions formed in the end-wall region cause blockage of the flow passage. Low pressure causes cavitation to cover most regions of the suction surface, inhibiting the formation and development of the tip leakage vortices. The distribution range of high turbulent kinetic energy region is almost the same as that of high-vorticity region, and there is a positive correlation between the two intensities. Severe cavitation causes the high turbulent kinetic energy region at the outlet of the flow passage to develop in the radial and axial directions of the impeller, which increases the turbulent dissipation and energy loss. The change of vorticity transport intensity caused by cavitation is mainly reflected in the expansion contraction term, and the Coriolis force term plays a dominant role in the vorticity transport process. This paper provides a reference for further improving the performance of mixed-flow pumps.

Role of wall roughness on interaction of leakage flow and main flow in a mixed flow pump with tip clearance

Due to machining precision, spraying coating, cavitation, and erosion, walls of pump flow passage components have different roughness, which will affect the flow state near the wall and even the main flow. In the present work, four types of roughness Ra = 0, 0.1, 1, and 10 μm are set to investigate the influence of wall roughness on the interaction of leakage flow and main flow in a mixed flow pump with tip clearance. Results show that the interaction area of leakage flow and main flow can be divided into three regions: (a) tip clearance zone, (b) shear flow zone, and (c) primary vortex zone. In the tip clearance zone, as roughness increases, the velocity magnitude and its derivative at the same wall distance gradually decrease, and the velocity angle at blade tip continuously increases in the inner circular boundary layer. As the roughness increases, the leakage flow gradually decreases, especially in the center chord region. In the shear flow zone, the velocity gradient rapidly decreases along the direction of pressure side to suction side, and finally, it becomes stable. As roughness increases, the velocity gradient intensity decreases, and the main reason is that the differences of velocity magnitude and velocity direction between the inner circular boundary layer and potential region decrease. In the primary vortex zone, the tip leakage vortex core indicated by the Liutex vortex method coincides with the low-pressure core location. As the roughness increases, the tip leakage vortex core becomes closer to the next blade. The pressure coefficient and vorticity of the tip leakage vortex core decrease with increasing of roughness, and the slopes remain constant on different cross sections with value of 5.9 × 103 and 2.4 × 103 m−1, respectively.

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

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

Unsteady Behavior of Tip Leakage Vortex on Rotating Instability in an Axial Compressor

Unsteady Behavior of Tip Leakage Vortex on Rotating Instability in an Axial Compressor

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.

Research on the suppression mechanism of a tip leakage vortex over a hydrofoil with double-control-hole structure

Tip leakage vortex (TLV) usually exists at a small tip clearance in axial-flow energy conversion machines, which may induce flow loss, vibration, noise and vortex cavitation. In this work, the double-control-hole (DC hole) at the leading edge of a hydrofoil is used to suppress TLV by inducing a passive jet. The SST-CC model by adding a rotation-curvature correction (CC) term into the original shear stress transport (SST) k-ω and Zwart-Gerber-Belamri (ZGB) model were applied to investigate the controlling effect under different Reynolds numbers (Re). The results show that TLV is decomposed and new vortices are generated when changing Re. By guiding the high-pressure flow from the pressure side (PS) to the suction side (SS) of hydrofoil vertically, the TLV core pressure is increased and local cavitation is suppressed. When the inflow velocity gradually increases to 7.5 m/s, 10 m/s and 12.5 m/s, the velocity swirl intensity at the TLV core is suppressed by nearly 50%, 36.4% and 28%, respectively, showing a downward trend. Based on the vorticity equation, it is observed that the distribution of TLV vortex bending terms is changed. Also, the vortex bending term and the vortex stretching term are suppressed in specific directions. Further analysis of the suppression mechanism of DC hole indicates that this structure can increase the turbulent kinetic energy of TLV, which destabilizes the flow field. Besides, the velocity circulation in the downstream region of TLV can be reduced, which can suppress the TLV effectively.

Partial tip air injection to extend the stall margin in an axial flow compressor with circumferential inlet distortion

As a typical non-axisymmetric inflow, circumferential pressure inlet distortion was experimentally investigated in an axial flow compressor. Based on the obvious decreasing trend of stall margin with the enhancement of distorted intensity, the stall mechanism under the condition of maximum distorted intensity, namely, 24.57 %, was studied in detailed. The casing static pressure over the rotor blade tip along the chord and circumferential direction were measured using a collection of dynamic transducers. Results show that under the circumferential inlet distortion condition, the pressure disturbance is always generated downstream from the distorted region along the rotational direction, and circumferentially propagates until it disappears in the upstream of the distorted region. With the continuous throttling, if the pressure disturbance that has not completely disappeared in the undistorted region enters the distorted region again, its energy will be enhanced, finally induces the stall. Using the above understanding as a foundation to apply partial tip air injection to suppress the disturbance caused by the circumferential inlet distortion, and resist its adverse effect on stall margin to ensure the safe operation of the compressor. It demonstrated that the tip air injection applied in the downstream of the distorted region can obtained the largest stall margin on the premise of the same injected momentum. For the strongest distortion case (DC (60) equals to 24.57 %), the injectors fixed downstream the distorted region can reduce the number and scale of the spike disturbances, suppress the local tip leakage vortex, and decrease the amplitude of the pressure disturbance energy, thus a maximum stall margin improvement of 3.18 % is obtained. However, tip air injection in the undistorted region cannot contribute the stall margin improvement that it leads to stronger tip leakage vortex and earlier stall inception. This study provides a guideline to design the non-axisymmetric tip air injection for circumferential inlet distortion with a help of injected mass flow and energy reduction.

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

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

Numerical investigation of the tip-vortex-induced ventilation formation mechanism for a surface-piercing hydrofoil

Recent experiments have demonstrated that tip vortices can trigger the ventilation formation around a surface-piercing hydrofoil. However, the influence of this ventilation on transient flow structures and vortex evolution remains unresolved. This paper numerically investigates the tip-vortex-induced ventilation formation for a surface-piercing hydrofoil at a stalled yaw angle. The predicted unsteady ventilated cavities with tip vortices and pressure-side spray are in reasonable agreement with experimental observations. The ventilation formation process can be divided into three stages: base ventilation, tip-vortex ventilation, and suction-side ventilation. It is indicated that ventilation has a greater impact on the lift coefficient than the drag coefficient. The lift coefficient increases during the base ventilation and tip-vortex ventilation stages due to the expansion of the low-pressure stalled flow, but decreases in the suction-side ventilation stage because of the gradual replacement of this low-pressure region by an aerated cavity. Tip-leakage and tip-separation vortices initially exist independently at the hydrofoil tip, then expand and merge through air ventilation, ultimately forming a strongly stable tip vortex. Furthermore, ventilation promotes vortex generation, with the major contributors being the vortex stretching and baroclinic torque terms.

Low-frequency fluctuation propagation of rotating stall in the centrifugal compressor and pipe system

The characteristic signal judgment of rotating stall in a centrifugal compressor is necessary to avoid the compressor becoming instability in operation. This study investigates rotating stall in a centrifugal compressor and pipe system using experimental and numerical simulation methods. In the experiment, a low-frequency (approximately 10% of the impeller's rotational frequency) pressure fluctuation is observed in the inlet and outlet pipes as the pressure ratio curve declined at small flow rate. The frequency spectrum results of different measuring locations suggest that the low-frequency disturbance is in the flow direction within the pipe system during rotating stall. To further analyze this pressure fluctuation, a transient numerical simulation of the centrifugal compressor with plenum model is conducted. Rotating stall can be captured by the numerical model, and the low-frequency pressure fluctuation is also observed in the transient simulation, aligning with the experimental results. The periodic evolution of the tip leakage vortex influences the flow in the impeller passage, causing a fluctuation in the flow direction that propagates upstream and downstream as revealed by flow field analysis. The low-frequency pressure fluctuation in the inlet and outlet pipe system is a characteristic signal, which can be a new stall judgment of rotating stall in the operation of centrifugal compressor.

A novel single-fluid cavitation model with gas content and slip velocity, applied to cavitating tip leakage vortex

A novel single-fluid cavitation model with gas content and slip velocity, applied to cavitating tip leakage vortex

Analysis of vortex characteristics and energy losses in a cryogenic hydrogen turbo-expander for a 5 t/d hydrogen liquefier

The cryogenic hydrogen turbo-expander (CHTB) is the most core component which determines the dependability and efficiency of hydrogen liquefiers. In present study, CFD method are adopted to simulate the internal flow field inside a CHTB for a 5 t/d hydrogen liquefier. The Ω method and entropy generation method are implemented to identify vortex structures and calculate energy losses. The results show tip leakage vortex is the primary vortex and causes energy losses through strong fluid shear but not rotation effect. The turbulent dissipation is the main energy dissipation. The total entropy generation of CHTB impeller is significantly higher than nozzle because of tip clearances. As tip clearances increase from 0% to 1.6%, total entropy generation increases by 83%, and output shaft power and isentropic efficiency reduce by 9% and 6% respectively. Working conditions mainly affect the inlet angle of attack and significantly impact vortex characteristics and energy losses in the CHTB.

Leading-edge flow prediction in Wells turbine under the influence of tip leakage flows

Monitoring of aerodynamic parameters of the blade helps to enhance the operating capabilities of the Wells turbine. In this study, a few discrete pressure points were used to estimate the pressure patterns by fitting the exact potential-flow solution for flow over a parabola. The sectional aerodynamics was assessed by the leading-edge flow sensing (LEFS) algorithm. A characteristic parameter was employed to deduce leading-edge flow status. Moreover, the influence of rotor speeds and tip gaps on the vortex boundary was discussed in terms of the Lagrangian frame. For the rotor speed larger than 1000 rpm at the incipient stall point, there is no boundary layer separation at the suction side of the blade tip leading edge. At the same rotor speed, a large tip gap reduces the axial shear of the suction flows and the axial stretching of the tip leakage vortex, which helps to enhance the interaction time and strength of the leakage vortex and the suction flows. For the Wells turbine with attached flows, the LEFS algorithm can accurately evaluate the pressure distribution and suction parameters near the leading edge. The LEFS-derived dimensionless parameter can act as an equivalent angle of attack under different tip gaps, providing the possibility for stall warning of Wells turbines.

Experimental and numerical investigation of the blade tip-related aeroacoustic sound source mechanisms of a ducted low-speed axial flow fan

A combined aerodynamic and aeroacoustic study of a ducted low-speed axial flow fan at various operating points is presented herein, focusing on the blade tip-related noise sources. The investigation includes upstream and downstream phased array microphone measurements and computational fluid dynamic simulations. The downstream acoustic measurements are carried out by using an acoustically transparent duct. The resulting flow fields and noise source maps are evaluated simultaneously. The results show that at higher flow rates, the dominant noise source is the turbulent boundary layer – trailing edge noise. At lower flow rates, the dominating noise sources can be found at the blade tip’s leading edge. These noise sources are found to be tip leakage flow related. Also, downstream microphone array measurements indicate strong noise sources at the blade tip’s trailing edge. These are determined to be a combination of the turbulent boundary layer - trailing edge noise, tip leakage vortex impingement noise, and double-leakage noise.

Role of tip leakage flow in an ultra-highly loaded transonic rotor aerodynamics

Tip clearance size is a very critical design parameter for the modern transonic rotor. This investigation attempts to reveal the role of tip leakage flow in an ultra-highly loaded low reaction transonic rotor (loading coefficient equals to 0.782) aerodynamics. Flow fields in the rotor with varied tip clearance sizes, ranging from 0 to 3 times of the datum case, are discussed at length. Both performance variation and stall mechanisms involved are elaborated. As a result, the rotor performance presents a continuous decline with the increased tip clearance size as a result of the additional secondary flow loss induced by the tip leakage flow. While for stall margin, an optimal tip clearance size exists, and it happens to be the datum case. The tip leakage flow is found to have the ability to blow away the low-energy flow accumulated in the rotor tip suction side. Specially, the forward portion of tip leakage flow plays the dominate role in extending the rotor stall margin. However, as the tip clearance size exceeds the datum case, the shock-tip leakage vortex interaction causes the tip leakage vortex breakdown and the stall margin is hence narrowed.

Experimental investigation of characteristics and influence of tip leakage vortex wandering in an axial compressor cascade

Vortex wandering is one of the most basic unsteady flow characteristics of the tip leakage vortex (TLV) in compressors. In this study, stereo particle image velocimetry (stereo-PIV) has been conducted in compressor cascades with various tip clearances to investigate the characteristics and influence of TLV wandering. The most effective vortex identification method for stereo-PIV data has been clarified. The wandering characteristics of the TLV are statistically analyzed, and the relationship between the vortex wandering and the dominant proper orthogonal decomposition mode is identified. The results reveal that TLV preferentially wanders along the pitchwise direction in the blade channel and gradually loses the dominant wandering direction downstream of the blade. The large displacement of the vortex center around its mean location is characterized by higher probabilities at small tip clearance size (1% chord length C). The spatial distribution characteristics demonstrate similarly concentric isocontour around the mean vortex center location for large tip clearances (3% and 5% chord length C). The effect of TLV wandering on the secondary velocity distribution, the tip flow blockage, and the distribution of Reynolds stress is explored using the vortex wandering corrected technique. The analysis verifies that the elevated turbulence kinetic energy in the core of the time-averaged uncorrected TLV is caused by vortex wandering rather than vortex deformation. The discussions of the current paper will enhance our knowledge of TLV wandering. Regardless of the reference to TLV, the interpretation of other swirling flows can benefit from the discussions presented here.