Jet-wake Flow Research Articles

A Free Streamline Model for Fully Separated Turbomachine Flows

Abstract This paper describes the use of the free streamline approach to determine flows in fully separated cascades, and to thereby provide a rational basis for estimating the performance of axial turbomachinery at severely off-design conditions. On the assumption of two-dimensional, incompressible flow, the free streamlines that bound the separated region are determined using the hodograph method, in which the methods of complex analysis are used to define a tractable numerical problem. The resulting jet-wake flow profile at cascade exit is then presumed to undergo mixing at constant-area, from which performance parameters for the cascade can be determined. The proposed method is verified by comparison against flat plate cascade solutions and by checks against momentum conservation. Validation of the free streamline approach is carried out by comparison against several cascade configurations for which experimental data and simulation results are available. In general, the predictions based upon the present theoretical model are found to be good, especially for negative incidences such as those that occur in the compressor section of an engine operating at sub-idle conditions. This suggests that the present approach can be used in the estimation of turbomachinery performance maps far from the design point, especially during early design phases.

Numerical study on jet-wake flow and its evolution in a centrifugal pump with alternating stall

Stall and jet-wake flow are two typical forms of unstable flow phenomena in centrifugal pumps, significantly affecting their stability. This paper investigates the interaction mechanisms between stall and jet-wake flow in a centrifugal impeller under different working conditions. The unsteady numerical study utilizes the partially averaged Navier–Stokes (PANS) model with a new dynamic fk expression derived from the rotation-corrected energy spectrum. The results reveal four stages in the flow field evolution of the centrifugal pump under different working conditions. In stage I, no stall vortices are present, and the jet-wake flow occurs. The velocity distribution at the impeller outlet depends on the pressure difference distribution between the pressure and suction sides within the flow passage. As the flow rate decreases, the pressure difference between the two sides increases, intensifying the jet-wake phenomenon. Under part-load conditions (stages II, III, and IV), the presence of stall vortices becomes the main factor affecting the jet-wake flow. These stall vortices influence the local and downstream flow fields, thus altering the distribution of the jet-wake. When the stall vortex is on the pressure side, it reduces the velocity near the pressure side, weakening the intensity of the jet-wake. On the other hand, when the stall vortex is on the suction side, it reduces the velocity near the suction side, enhancing the intensity of the jet-wake.

Investigation on energy dissipation mechanism in a double-suction centrifugal pump based on Rortex and enstrophy

Since double-suction centrifugal pumps consume quantities of energy, revealing critical factors for energy dissipation is helpful for energy-saving design. This study aims to investigate the relationship between energy dissipation and vortex based on Rortex method in a double-suction centrifugal pump. Detached eddy simulation was applied to obtain the flow field. Enstrophy was used to present the strength of the local rigid vortex and shear. The results indicate that the local shear dominates energy dissipation in the pump. Owing to jet flows, the energy loss on blade leading edges (LE) and trailing edges (TE) were 102-103 times that of the middle region at 0.4Qd and ten times at 1.4Qd. The energy dissipation on pressure sides (PS) was ten times greater than that on suction sides (SS) at the TE, while flow separation at the middle of SS caused by wake flow increased energy dissipation to nearly ten times that of PS. Jet-wake flow near volute inlet was the dominant factor for energy dissipation at part-load, while separation flows in volute discharge was more significant at overload. The induced high local shear strength was responsible for energy dissipation. Therefore, reducing local shear is a potential energy-saving approach in pumps.

CFD Simulation of Centrifugal Pump with Different Impeller Blade Trailing Edges

The centrifugal pump is one of the most widely used types of power machinery in the field of ship and ocean engineering, and the shape of the impeller blade trailing edge has an important influence on their performance. To reveal the mechanism of the effect of different trailing edges on external performance, the internal flow of 16 types of impeller blade trailing edges of a centrifugal pump, consisting of Bezier trailing edges, rounding on the pressure side, cutting on the suction side, and the original trailing edge is studied by numerical simulation. The reverse flow, shaft power, and energy loss distribution in the impeller and diffuser along the streamwise direction are analyzed by calculating them on each micro control body sliced from the fluid domain. The entropy production theory and Ω-vortex identification method are used to display the magnitude and location of energy loss and the vortex structure. Finally, a static structural analysis of the impeller with different trailing edges is performed. The results show that different impeller trailing edges can clearly affect the efficiency of the pump, i.e., the thinner the trailing edge, the higher the efficiency, with the thickest model reducing efficiency by 5.71% and the thinnest model increasing efficiency by 0.59% compared to the original one. Changing the shape of the impeller trailing edge has a great influence on the reverse flow, shaft power, and energy loss near the impeller trailing edge and diffuser inlet but has little influence on the leading part of the impeller. The distribution of local entropy production rate, energy loss, and reverse flow along the streamwise direction shows similar rules, with a local maximum near the leading edge of the impeller due to the impact effect, and a global maximum near the impeller trailing edge resulting from strong flow separation and high vortex strength due to the jet-wake flow. Thinning the impeller trailing edge and smoothing its connection with the blade can reduce the vortex strength and entropy production near the impeller trailing edge and diffuser inlet, improve the flow pattern, and reduce energy loss, thus improving the pump efficiency. In all models, the maximum equivalent stress is less than 6.5 MPa and the maximum total deformation is less than 0.065mm. The results are helpful for a deeper understanding of the complex flow mechanism of the centrifugal pump with different blade trailing edges.

Flow behaviour in vented brake discs with straight and airfoil-shaped radial vanes

This paper presents experimentally obtained flow characteristics inside the vane passage of a vented brake disc and at the brake disc exit. The flow conditions of three different brake discs were compared: a basic design with straight vanes and two prototypes with different vane heights, shapes (airfoil shape) and numbers. The possible effect on brake disc cooling performance was studies. Hot-wire anemometry was applied to measure the pumping performance of different discs, and particle imaging velocimetry was used to map the velocity field. All three discs were tested on a brake disc dynamometer according to the SAE J2522 standard, and the maximum temperature was measured during the thermal capacity test. No prototype outperformed the original brake disc. Moreover, the brake disc with the increased vane thickness and reduced vane height (prototype C) performed significantly worse, whereas prototype B, designed with an increased vane height and increased number of vanes, performed similarly to the original brake disc. A comparison of the flow characteristics of the three discs indicated that the significant decrease in the cooling airflow rate deteriorated the cooling performance of prototype C the most. On the other hand, the improved pumping performance did not improve the cooling performance of prototype B. Only the comparison of the internal cooling air flow pattern of the original and prototype B brake discs revealed the reason. While the characteristic jet-wake flow structure formed within the vane passage of the original brake disc, the inter-vane channel flow within brake disc B was distributed much more uniformly in a circumferential direction, with significantly lower flow velocities near the pressure vane side. This reduced the convective heat transfer on the brake disc vanes, which number, as well as the heat transfer surface area increased without the desired cooling effect.

Numerical Investigation of Gas-Liquid Flow in a Multiphase Pump with Special Emphasis on the Effect of Tip Leakage Vortex on the Gas Flow Pattern

In this paper, the gas-liquid flow is comprehensively analyzed under different inlet gas void fractions, and the effect of tip leakage vortex (TLV) on the gas flow pattern in multiphase pumps is revealed. The results show that the gas flow pattern in an impeller is closely related to the centrifugal force, low-pressure region, and vortex motion. Most gas is present near the hub and suction surface of the blade as well as in the TLV. The two- and three-dimensional spatiotemporal evolution of the gas is presented, and the gas motion during the inception, development, and dissipation of TLV is revealed. It is reflected that the gas volume fraction is the highest at the TLV core and gradually weakens along the radial direction with the vortex core at the center. Additionally, the TLV energy dissipation is closely related to the gas and pressure difference, and strong energy dissipation occurs in the jet-wake flow.

Study of rotor–stator interaction phenomenon in a double-suction centrifugal pump with impeller vane trailing edge as a design parameter

Various geometrical parameters, such as cut-water clearance, volute tongue location, tongue radius, vane trailing edge profile, and flow parameters like speed and operating point, affect the rotor–stator interaction in a centrifugal pump. In the present investigation, vane trailing edge is selected as a design parameter for profile modifications to study and reduce the rotor–stator interaction intensity and hence the pressure pulsations. A double-suction centrifugal pump with tangential discharge volute (M1) and specific speed (ns) 19 has been selected for the numerical experiments. Transient analysis using detached eddy simulation is used for predicting flow parameter behavior at impeller periphery (primary source) and volute tongue locations (secondary source). The impeller periphery probes in the rotor–stator interaction zone are used to identify the intensity of the jet-wake flow phenomenon and its interaction with the volute tongue. Similar strategy has been applied by modifying the trailing-edge profile of the original geometry with vane underfiling (M2) and the M2 vane geometry with novel trailing-edge profile (M3). The pressure pulsation behavior along the impeller periphery (primary source) shows the strong reduction of pressure amplitude near volute tongue by 42% in M3 compared to M1 aligning with the reduction of jet-wake flow intensity. The pressure fluctuation (rms) near volute tongue (secondary source) shows an average reduction of 11% in modified trailing edge (M3) relative to M1. The intensified rotor–stator interaction due to vortex shedding in M2 causes relatively higher pressure fluctuations (rms) near volute tongue compared to M1 and M3.

Study on fluid dynamic characteristics of a low specific speed centrifugal pump with emphasis on trimming operations

In the present investigation, the fluid dynamic characteristics of a narrow channel centrifugal pump are studied using numerical simulations with special emphasis on pump trimming operations. The trimming operations at optimized rate, reduce the pressure fluctuations and hydraulic noise in pumps, but at the cost of pump performance characteristics. This has been extensively studied by the researchers in the past; however, the efficacy of trimming operations on industrial pumps with narrow channel passage impellers is to be explored. A low specific speed pump with narrow channel impeller passage and radial discharge volute (M1) with specific speed of 14.7 is considered for the present investigation. Detached Eddy Simulation (DES) is used in transient simulations and Fast Fourier Transform (FFT) is used to understand the pressure fluctuations and hydraulic noise at the Blade Pass Frequency (BPF) and its harmonics. The pressure fluctuation behavior along the impeller periphery is monitored at 30 equidistant probes and 7 probes covering the volute tongue. These results are compared with the similar study for impeller with oblique trimming of 30° (M2) and impeller oblique trimming along with volute trimming (M3). The flow behavior in M1 is dominated by a Jet-wake flow in the impeller passage which acts as a primary source of flow non-uniformity. The resulting non-uniform flow on the impeller periphery interacting with the volute tongue causes vortex shedding behind the volute tongue, subsequently leading to discharge recirculation in the impeller. The coefficient of pressure fluctuations near volute tongue at BPF is reduced by 10.3% in M2, whereas it is enhanced by 24% in M3 relative to M1. The enhanced vortex shedding behind the volute tongue in addition to the vortex train along the impeller periphery causes rise of the pressure fluctuations in M3.

Flow Characteristics and Energy Loss of a Multistage Centrifugal Pump with Blade-Type Guide Vanes

Multistage pumps with blade-type guide vanes are widely used in offshore oil production, the petrochemical and coal-chemical industries, and nuclear power fields for its advantages of large flow rate, high pressure, and excellent operation stability. However, the internal flow of this kind of pump is complex; in particular, the hydraulic, flow, and pressure pulsation characteristics of the different stages are quite different, which has a great impact on the design and performance predictions of this kind of pump. Thus, in this paper, the hydraulic performance, unsteady flow characteristics, evolution of vortex structures and pressure pulsation characteristics in a 10 stage centrifugal pump are investigated numerically. The results show that inverse flow, jet-wake flow, and rotor-stator interaction flow are the key factors causing energy loss and efficiency decline at every stage and in the whole pump. The vortex evolution at the rotor–stator interaction regions is actually the process that the vortex structures fall off and impact on the pressure surface at the leading edge of the guide vane blade at a frequency that equals to the impeller blade passing frequency. Furthermore, under the actions of the guide vane with confluence cavity, the pressure pulsation within the final-stage guide vane contains low-frequency components with large bandwidths, which mainly results from the confluence flow disturbance at the outlet of the cylindrical guide passage.

Aerodynamic instabilities modeling under asymmetric boundary in a centrifugal compressor for turbocharger

The increasing demand for compression systems with high pressure ratio and wide safety margin has set new prerequisites for designers to meet the industrial needs without increasing the manufacturing costs excessively. In this work, the turbulent stability of the vaneless diffuser of the centrifugal compressor was analyzed. Unsteady Reynolds-averaged numerical simulations of the isolated diffuser and full annular diffuser with or without circumferential asymmetric boundary conditions downstream were performed. And a continuous adjoint approach was adopted, which is rarely applied in the stability analysis of compressor flow. Then, the origin of instability under different inflow and outflow conditions was sought with a sensitivity analysis. The prediction of the growth rate reveals that the flow near the shroud dominates the global stability of the diffuser. When connected with an impeller in the upstream direction, the most unstable region is localized at the backflow regions near the outlet. The wave number, however, is altered under the impact of the jet-wake flow. When connected to a circumferential asymmetric condition, the structural sensitivity of the vaneless diffuser with a radius ratio of 1.53 indicates that the interaction between the inlet reverse flow and outlet backflow is responsible for the occurrence of stall. The most unstable regions are localized at the region 90°–135° away from the volute tongue. The present work mainly contributes to the instabilities identification with novel sensitivity methods under asymmetric boundary conditions.

Improve of Unsteady Pressure Pulsation Based on Jet–Wake Suppression for a Low Specific Centrifugal Pump

Abstract In this study, three impellers with different blade pressure side (PS) profiles are designed and the influence on the hydraulic and dynamic performance of a low specific speed centrifugal pump is investigated by numerical simulation and experimental research. The results show that blade PS modification introduced in this study can efficiently alleviate the unsteady pressure pulsation of model pump. In order to study the effects of blade modification on the internal flow filed, the volute domain is replaced by an even outlet region for computational fluid dynamic (CFD) analysis. Relative velocity distribution is extracted to visualize the three-dimensional (3D) flow characteristics at the impeller outlet. Results show that the flow at impeller outlet presents a typical jet–wake structure, which is significantly suppressed after the blade modification. The suppression of jet–wake structure, which is attributed to the redistribution of pressure and velocity in the impeller caused by the change of blade work capacity, can directly reduce the intensity of pressure pulsation in the volute by increasing the velocity uniformity at impeller outlet. Given that the existence of jet–wake flow results in large mixing loss and velocity deviation at the impeller outlet, entropy generation rate and slip velocity calculation are introduced here to measure the extent of jet–wake configuration. Results show that both indicators introduced here can be used to quantify the extent of the jet–wake structure at impeller outlet, and thus, indirectly predict the strength of unsteady pressure pulsation in pump volute.

Effects of Lean Mode of Blade Trailing Edge on Pressure Fluctuation Characteristics of a Vertical Centrifugal Pump With Vaned Diffuser

Abstract A vertical centrifugal pump with a vaned diffuser is very attractive in the field of long-distance water supply. Excessive pressure fluctuations in the vaneless region due to rotor–stator interaction (RSI) need careful evaluation. In the present investigation, the hydraulic performance and pressure fluctuation characteristics of a vertical centrifugal pump with three different lean modes of the blade trailing edge were quantitatively analyzed by comparison experiments, using the same test rig. Results showed that the pressure fluctuation level was the highest in the vaneless region, closest to the volute tongue, and increased as the flow rate deviated from the design flow rate. The lean mode of the blade trailing edge was found to have a slight influence on hydraulic performance, and the relative deviation of experimental specific speeds with three different lean modes was within 6%. The influence of the lean mode of the blade trailing edge on the pressure fluctuation level was experimentally verified for the first time. In particular, the flow rate-averaged peak-to-peak value of pressure fluctuation with the positive lean mode (PLM) was 62% of the corresponding value with the zero lean mode (ZLM), while no significant improvement was observed for the negative lean mode (NLM). The flow mechanism behind this may be explained as a weakening of the jet-wake flow pattern with PLM.

Experimental studies of unsteady cavitation at the tongue of a pump-turbine in pump mode

The paper deals with the cavitation at tongue of a pump-turbine in pump mode, including the typical patterns, unsteady behaviors, and the influence on pump performance and fluid-borne noise. Visualization experiments were conducted under different flow rates and rotational speeds for variation cavity regimes. The results show the patterns can be divided into three regimes with cavitation development: bubble cavitation, cloud cavitation, and lock cavitation for the gas-lock phenomenon and performance plummet. The rotor-stator interaction and jet-wake flow could affect the cavitation generation. The blade passing effect is the leading cause of bubble clusters periodic collapsing or shedding at the early stages of cavitation, while playing a small role as cavities become thick and evaporation rate rises. The combined action of the attached vortex (re-entrant jet) and the depression by pressure gradient outside cavity surface account for the shedding at the developed cloud cavitation stage. Based on the noise measurement by hydrophone at the outlet, the broadband sound in the range of 1000 HZ-2000 Hz can be used for cavitation detection since it can be rarely influenced by operating conditions under non-cavitation.

Analysis of Unsteady Flow Characteristics of Centrifugal Pump under Part Load Based on DDES Turbulence Model

To better reveal the mechanism of the rotor-stator interference between the impeller and the guide vane and the evolution process of the stall vortex under the part-load conditions, numerical simulation is carried out based on the DDES turbulence model, which can better capture vortex structure. And the pressure pulsation and the radial velocity distribution of the centrifugal pump are studied. The vortex structure and pressure fluctuation of pump internal flow field under part-load condition of Q = 0.4 Qdes are mainly analyzed. The analysis results show that the stall vortex is formed at the inlet of the impeller and evolves to the outlet of the impeller, the front cover to the rear cover according to the fluid flow direction, and then disappears. Besides, under the part-load condition, the vorticity of the impeller outlet is always obviously less than that of the impeller inlet as the flow rate increases. Due to the asymmetric action of the volute, the radial velocity distribution law of flow channel C1 is different from other flow channels at different blade heights. By analyzing the radial velocity, the phenomenon that the jet-wake flow impacts the guide vane with the rotation of the impeller is the main reason for the rotor-stator interference. And large radial velocity gradients appear at the front and rear cover plates, which will cause high energy loss and reduce pump efficiency. Besides, the conclusion can be drawn that the region with the strongest rotor-stator interference is the inlet region of the guide vane suction surface. It also occurs near the volute tongue but is lower due to the effect of the guide vane. This research may serve as a reference for the safe operation of centrifugal pumps under part-load conditions.

Numerical Investigation of the Transient Flow and Frequency Characteristic in a Centrifugal Pump with Splitter Blades

To analyze the internal flow characteristics of each subchannel in a low-specific-speed centrifugal pump with splitter blades, the time histories and frequency spectra of pressure fluctuations and the distributions of the corresponding flow states in one impeller channel were investigated through the numerical analysis. Performance experiments and particle image velocimetry (PIV) tests were carried out to verify the results of the numerical calculations. The results suggested that the simulation analysis agreed well with the test results. The time histories and frequency spectra of pressure fluctuations depending on its location (close to or away from the volute tongue) present different changes. The predominant frequency of each monitor point equals to five or ten times shaft frequency. The jet-wake flow pattern at each subchannel separated by splitter blade in one impeller channel is not circumferentially uniform. For the channel away from volute tongue, the magnitude of turbulence kinetic energy in pressure side subchannel is well larger than that in suction side subchannel. With the increase in flow rate, the region close to the elbow of the volute outlet emerges a large-scale vortex.

DDES analysis of unsteady flow evolution and pressure pulsation at off-design condition of a centrifugal pump

In the present paper, to investigate unsteady flow structures and pressure pulsations at off-design conditions, a low specific speed centrifugal pump is investigated by the DDES (Delayed Detached Eddy Simulation) method. Emphasis is laid on the unsteady evolution of flow structure and pressure pulsation at the stalled condition. Results show that the jet-wake flow pattern could be captured by the current numerical simulation method, and it shows a high calculation precision for the jet flow. Relative velocity decreases rapidly in the wake region towards the blade suction side. Significant inflection point exists between the jet and wake flow regions, and it almost keeps unchanged at the identical impeller-volute position under various flow rates. At the stalled status, the inflection point moves towards the blade pressure side with the blade passing the volute tongue, and evident non-uniform flow distributions are formed in the blade channels characterized by the multiple vortices. The blade channel is gradually blocked by the corresponding stall cell when the blade approaches the volute tongue, and it indicates that the volute tongue has a significant effect on the evolution process of the stall structure. By combination analysis of the unsteady evolution of the stall structure and pressure spectrum, the stall frequency at 0.25fn (being fn the impeller rotating frequency) is captured for the current model pump.

Investigation of rotating stall in radial vaneless diffusers with asymmetric inflow

Despite its simple geometry, the turbulent flow in vaneless diffusers is asymmetric and highly skewed. In the present work, a frozen eddy viscosity approach was employed to investigate the instability in a vaneless diffuser with different axial widths. A turbulent stability analysis was performed around the numerically computed mean flows with a non-uniform inflow for both isolated and full-annular vaneless diffusers. The predictions of the flow instability frequency and coherent structure were validated against experimental data. By performing a structural-sensitivity analysis corresponding to the leading eigenvalue, the instability mechanisms for the isolated and full-annular vaneless diffusers were revealed. The sensitivity analysis indicated that the interaction between the boundary layer and the main flow may have been the primary cause of the self-excited instabilities in a narrow diffuser under both axisymmetric and asymmetric inflow. The contribution of the reverse flow near the walls was relatively small. However, the influence of the separation flow near the wall on the instabilities of a wide diffuser was significant, particularly under high-skew inflow conditions. The wavemaker regions were located on the shroud side near the inlet and the hub side near the outlet. When connected to the impeller in the upstream direction, the diffuser outlet backflow was responsible for instability in the diffuser with a radius ratio of 1.53. The jet-wake flow in the diffuser inlet had little impact on the flow instability.

Case study: Evaluation of discrete frequency noise generated from object in wake of centrifugal impeller: Case study on tonal noise of a centrifugal fan Centrifugal Fan

In order to clarify the physical mechanism of unpleasant fan noise, the discrete frequency noise (DFN) generated from a centrifugal fan is evaluated quantitatively based on the wake characteristics. The volute tongue of the scroll casing is replaced by a cylinder as an object in order to simplify the mechanical model. Moreover, an analytical evaluation method for the DFN generated from the volute tongue of the scroll casing based on the wake characteristics of the centrifugal impeller is proposed. From the comparison on the prediction and measurement of the noise level, the influence of the clearance of the volute tongue on the DFN of a centrifugal fan was evaluated from the viewpoint of fluid dynamics. The aerodynamic noise source of the DFN was formed at the rear side of the impeller, which had large momentum in the wake. The momentum was rapidly lost by the diffusion in the near-wake field. Therefore, when the scroll casing of the fan is designed with suitable clearance for the volute tongue, the DFN will be decreased because of the diffusion of the wake. The evaluated noise indicated that the noise level of the DFN was lower in the far-wake field than the broadband noise level. These results imply that the sinusoidal pressure fluctuation caused by the impingement of the jetwake flow became weaker than the pressure fluctuation induced by the turbulence on the blade surface. © 2019 Institute of Noise Control Engineering.

DDES analysis of the unsteady wake flow and its evolution of a centrifugal pump

In the present paper, to investigate the unsteady wake flow and its evolution at different moments when the blade sweeps the volute tongue, numerical simulation method is applied to discuss the flow field in detail based on the DDES (Delayed Detached Eddy Simulation) model when the model pump works at the nominal flow rate. Emphasis is attracted on the relative velocity distribution, besides the typical vorticity and TKE distributions at various moments are also analyzed to obtain the turbulent characteristics of the model pump. Results show that the current method has the ability to capture the main flow structure of the model pump, especially for the jet flow. From the blade pressure side to the suction side, the relative velocity distribution shows typical jet-wake flow pattern. With the impeller rotating, the reflection point of the jet-wake flow pattern changes at the mid span of the impeller. From flow distributions on different spans of the impeller, it is noted that the relative velocity decreases from the front chamber to the back chamber at major region of the blade channel. In the model pump, it is observed that several typical high vorticity regions are generated, where the high TKE values are also expected, especially at the blade trailing edge and in the volute zone caused by the wake flow. So it is inferred that for the unsteady flow in the pump and the corresponding induced pressure pulsation, controlling the wake flow is an effective approach to lower the unsteady flow pulsation in the centrifugal pump.

Unsteady flow structure and its evolution in a low specific speed centrifugal pump measured by PIV

The intense fluid-dynamic interaction associated with the complex flow structure in the centrifugal pump is detrimental to the stable operation of the pump. Based on the particle image velocimetry (PIV) measuring technique, flow structures in a low specific speed centrifugal pump are measured and presented in detail. Emphasis is laid upon the complex flow structures around the volute tongue region, where intense rotor–stator interaction appears. Results show that the typical jet-wake flow pattern at the blade outlet is observed for flow rates lower than the nominal working condition, and the inflection point of the jet-wake region is found to be at the center of the blade channel. At high flow rate, the jet-wake flow pattern is not apparent due to the high momentum fluid concentrating on the blade suction side. Within the pump, several typical vorticity sheets (both positive and negative) are captured on the blade pressure and suction sides at high flow rates, while at low flow rates, the negative vorticity sheets on the blade suction side disappear due to the separated and reverse flow structures occurring. When the vorticity sheet sheds from the blade trailing edge, it would impinge on the volute tongue with subsequent cutting and distortion. Three typical interacting modes are observed at different flow rates, and the cutting effect of the volute tongue on the vorticity sheet is dependent strongly on the local absolute velocity distribution. At high flow rates, the cutting effect is more significant, and the entire vorticity sheets (positive and negative) would be cut and torn by the volute tongue.