Tip Clearance Cavitation Research Articles

A method for quantitatively depicting the influence of vortices on the pressure forces acting on a hydrofoil in cavitating flow with tip clearance

In this paper, the force decomposition method (FDM) is proposed for decomposing the pressure forces acting on the immersed body. The major improvement in FDM is the applicability in RANS and LES simulations with non-constant density flows, e.g. cavitating flow. In the single-phase flow over a circular cylinder (Re = 3900), the FDM results show excellent agreement with the pressure force given by conventional method. In the cavitating flow over a hydrofoil with tip clearance, FDM can also reproduce the same tendency of the pressure force with an average deviation below 17%. In this case, the vorticity and kinematic effect induced force dominates the pressure force. In order to isolate the effect of attached cavitation near the suction side and tip clearance cavitation, a method combing domain cutting (manually) and cell extracting (by a threshold of vorticity magnitude) is proposed. The lift caused by vorticity force is mainly affected by the vortices shedding from the suction side of the hydrofoil. Also, the tip clearance region should not be ignored, where the lift generation by TLV is the interaction of strong shear and rotating flows.

Numerical study of porous tip treatment in suppressing tip clearance vortices in cavitating flow

Tip clearance cavitation (TCC) is a type of vortex cavitation. It widely exists in axial flow hydraulic machinery and has significant negative influence on the mechanical service life and the operating stability. It is necessary to suppress the tip clearance vortices (TCV) to control the TCC in engineering applications. Based on the analysis of the advantages and disadvantages of the present various suppression strategies, a new coupling method is proposed in this study by combining the damping approach and the diversion approach. Porous medium material is used to realize the coupling effect. A 2 mm span length porous tip is installed on the solid tip surface of a hydrofoil under two gap sizes conditions (representing two types of gap flow pattern), and excellent suppression results of the TCV and TCC are obtained. The characteristics and mechanism of the clearance flow are analyzed by numerical simulation. The numerical accuracy is verified by experimental qualitative observations. The simulation results show that the temporal and spatial stability of the clearance flow field is enhanced, and the leakage velocity and the TCV strength are weakened via the combined action of damping and diversion effects. There is a difference in the damping mechanism between the two gap flow patterns. It is a comprehensive result of viscous dissipation and momentum loss in the jet pattern represented by the small gap size, and primarily, the result of momentum loss in the rolling pattern represented by the large gap size.

The cavitation characteristics of aerospace high-speed centrifugal pumps with different tip clearance

This study investigates the Tip Clearance Cavitation (TCC) characteristics of three different Tip Clearances (TC) (0.4, 0.6, 0.8) and five inlet negative pressure conditions Pj = (− 20–60)kPa to improve the reliability of the aerospace high-speed centrifugal pump during in-orbit operation, based on the premise of good agreement between the TC 0.6 test curve and the simulation performance curve. Under negative pressure and high-speed conditions, the variation gradient of cavitation characteristics with various inlet negative pressures is non-linear and has a sudden change, but the trend becomes stable after the inlet negative pressure drops to a certain stage. The tip clearance cavitation characteristics vary from the blade surface cavitation characteristics due to the difference in forces on both sides. This study is a proper starting point for the design of aerospace power pumps.

Research on blade tip clearance cavitation and turbulent kinetic energy characteristics of axial flow pump based on the partially-averaged Navier-Stokes model

Research on blade tip clearance cavitation and turbulent kinetic energy characteristics of axial flow pump based on the partially-averaged Navier-Stokes model

Numerical study of two types of rough groove in suppressing the tip clearance cavitation

Tip clearance cavitation (TCC) which is consisted of tip leakage vortex (TLV) cavitation and gap cavitation (occurs inside the gap region) is a common phenomenon in the gap flow filed in axial turbomachinery, and can trigger a series of negative effects. Based on the suppression mechanism of a type of prototypical horizontal casing groove, two types of rough groove structures are proposed in this paper aiming at enhancing the suppression effect on the TCC by utilizing the Coanda effect of the gap leakage jet. The gap flow field is generated via a NACA0009 hydrofoil and a solid wall. The gap size is fixed as 2 mm. The numerical simulation method is adopted. The results show that combining the detached-eddy simulation (DES) approach, SchnerrSauer cavitation model and a type of varRhoTurbVOF method on the OpenFOAM platform, accurate numerical results of cavitating gap flow fields under the small gap conditions can be obtained. The gap flow field analysis reveals that the control ability of the grooves on the leakage flow is enhanced via divergent–convergent flow passage in the roughness groove cases. The leakage rate is reduced while the local turbulence intensity is increased, which weakens the TLV and the tip separation vortex (TSV) dramatically. The rough casing grooves show a superior suppression effect on the TCC. The present work can inspire new casing treatment method for improving the anti-cavitation performance in the axial-flow hydraulic machinery.

The Analysis of Cavitation Flow and Pressure Pulsation of Bi-Directional Pump

A bi-directional pump is designed by using S-shaped hydrofoil, is the most convenient way to achieve bi-directional operation. In this paper, high-speed photography is used to visualize the flow field characteristics of the bidirectional pump under different cavitation numbers, and the flow field changes caused by cavitation are quantitatively analyzed in combination with the pressure pulsation sensor. The results show that the operation efficiency of the bidirectional pump in reverse operation is lower than that in forward operation. Tip clearance cavitation occurs on both suction and pressure surfaces of the impeller under reverse operation and large flow. In reverse operation, the influence of guide vane on the main frequency of pressure pulsation in the impeller is obvious. The quasi-periodic vertical cavitation flow phenomenon increases the amplitude of pressure pulsation in the impeller and becomes the main component of the internal flow in the bidirectional axial flow pump.

Effects of cavitation on the hydrodynamic loading and wake vortex evolution of a pre-swirl pump-jet propulsor

The purpose of this study is to investigate the effects of cavitation on the hydrodynamic loading and wake vortex evolution in a pre-swirl pump-jet propulsor, and also the cavitation-vortex interaction mechanism. The cavitating flow is simulated by delayed detached eddy simulation coupled with a homogeneous cavitation model. Based on available experimental validation, the cavitation patterns, hydrodynamic loadings, the tip leakage vortex (TLV) evolutions and trailing edge vortex interactions are orderly investigated under two typical cavitation conditions. Results show that the blade sheet cavitation, TLV cavitation and tip clearance cavitation are regard as the main cavitation types of the rotor, where the sheet cavity on adjacent blades is non-uniformly distributed under the perturbations of the stator wakes and phase effects. The interaction between the thickening sheet cavity and stator wakes causes the shift of dominant frequencies of rotor loading from the rotor blade passing frequency fBPF and its harmonics to stator blade passing frequency fs and its harmonics. The relative vorticity transport equation is used to analyze cavitation-vortex interaction of TLV. The TLV cavitation promotes the vorticity production of TLV at the incipient stage and increases its intensity downstream. The instability of TLV is triggered earlier when cavitation is heavy due to the enhanced mutual interaction between consecutive spirals of the TLVs and their interaction with sheet cavity induced shedding vortices. The trailing edge vortices of stator and rotor mutually interacts with blade sheet cavity, which accelerates the breakdown of trailing vortex system downstream.

Research on Cavitation Flow Dynamics and Entropy Generation Analysis in an Axial Flow Pump

The entropy generation theory is introduced to investigate the effects of different NPSH and tip clearance size on the cavitation flow dynamics and mechanical energy dissipation intuitively and quantitatively within an axial flow pump through numerical simulations. The results indicate that main mechanical energy dissipation of the pump gathers in part impeller and diffuser, and most are turbulent dissipation. Meanwhile, the impeller is the largest place of mechanical energy dissipation of the pump under cavitation conditions, accounting for more than 50%. NPSH has significant effects on the cavitation pattern, which reflects on the field that the areas of attached sheet cavitation and tip leakage vortex cavitation around blades increase obviously with NPSH reducing under the tip clearance of 0.1% span. With NPSH decreasing, high regions of turbulent dissipation in the impeller mainly expands along blades and move downstream, with span S0.98 near the shroud having larger turbulent dissipation. Besides, high regions of turbulent dissipation are mainly distributed at the rear part of the cavity for every corresponding span of the impeller, which indicates that the turbulent dissipation has a strong relation with the cavitation pattern. In the impeller, the unstable flows cause cavity shedding at the rear of the cavity and wake flows near the blade trail induce higher turbulent kinetic energy, finally resulting in higher turbulent dissipation there. Under the same NPSH, areas of tip leakage vortex cavitation and areas of tip clearance cavitation around the tip both expand with the tip clearance increasing from 0.1% span to 0.8% span. And high areas of turbulent dissipation also are distributed at the rear of the cavity and moving downstream along the blade suction side, especially at span S0.98. Therefore, the tip clearance width mainly affects the cavitation development and turbulent dissipation distribution near the impeller’s shroud under same NPSH.

The Tip Clearance Cavitation Mechanism of a High-Speed Centrifugal Pump with a Splitter-Bladed Inducer

For a high-speed centrifugal pump, cavitation occurs easily. To equip a high-performance splitter-bladed inducer upstream of the pump is an effective method to suppress cavitation. In this paper, an external characteristics experiment of the high-speed centrifugal pump with a splitter-bladed inducer is carried out, and the corresponding numerical calculations are completed. The research shows that the results of the numerical calculation are credible. Numerical cavitation calculations under eight different tip clearance conditions are carried out. First, it is found that the tip clearance (TC) has a certain impact on the head of the centrifugal pump. When TC is in a small range, the clearance leakage is small, and the impact on the head of the pump is not so obvious, which can give the pump a higher performance. Second, it is found that TC has a certain influence on the static pressure distribution in the cascade passage of the splitter-bladed inducer. When TC is in a certain range, the increasement in TC will aggravate the cavitation at the suction surface of the long blades near the inlet. When it exceeds the certain range, it will cause cavitation at the outlet of the inducer. At last, it is found that the cavitation’s severity and position of the inducer are closely related to TC. TC affects the magnitude and position of vorticity in the inducer’s passage. In this paper the flow mechanism of TC is revealed, and its research results can provide theoretical basis and technical support for the design of the tip clearance of the inducers.

Numerical Prediction of the Pumpjet Propulsor Tip Clearance Vortex Cavitation in Uniform Flow

Previous studies show that the tip clearance loss limits the improvement of pumpjet propulsor (PJP) performance, and the tip clearance flow field is the most complicated part of PJP flow. In this work, the non-cavitation and cavitation hydrodynamic performances of PJP with a tip clearance size of 1 mm are obtained by using the detached-eddy simulation (DES). At constant oncoming velocity, cavitation first occurs on the duct and then disappears with the decrease of the advance ratio. The rotor blade cavitation occurs at the low advance ratio and comprises tip clearance cavitation, tip leakage cavitation, and blade sheet cavitation. In the rotor region, the typical vortices include tip separation vortex, tip leakage vortex, trailing edge shedding vortex, and blade root horseshoe vortex. Combined with the pressure distribution, both the Q and λ2 criteria give reliable results of vortex identification. The cavitation causes an expansion of tip leakage vortex in the circumferential direction and decreases the intensities of tip separation vortex in the whole tip clearance area and tip leakage vortex in the cavitation area, and enhances the strength of tip leakage vortex in the downstream non-cavitation area.

Experimental verification of the effect of gap geometry on the tip-leakage flow pattern and tip-clearance cavitation

Tip-clearance cavitation is one of the most aggressive and widespread forms of cavitation in hydraulic machinery that occurs due to liquid leaking through narrow gaps between tips or end faces of blades/vanes and a stator wall. The research is aimed at the study of a passive control of tip-leakage flow and tip-clearance cavitation by modifying the gap geometry. The test object was a NACA0022-34 hydrofoil with a 100 mm chord that was equipped with a double-sided axis of rotation. The gap geometry was changed by mounting side plates with different end surfaces (flat and grooved) to the hydrofoil end face. We used high-speed imaging to analyze the temporal and spatial cavity evolution simultaneously on the foil suction side and inside the clearance using three cameras. The implemented control method was shown to allow an effective management of the tip-leakage flow and tip-clearance cavitation, especially at higher angles of attack. The modified end plate makes the tip-leakage flow less prone to cavitation as compared to the original one, i.e. the tip-clearance cavitation inception and development appear to be hampered.

Vortex dynamic characteristics of unsteady tip clearance cavitation in a waterjet pump determined with different vortex identification methods

The cavitating flow field in waterjet pumps is complex, especially tip leakage cavitation (TLC), which has been a puzzle to researchers for decades. In this study, high-speed video (HSV) of the instantaneous inner structures of tip cavitation flow is used to show the evolution of TLC. Numerical simulation is conducted with the scale adaptive simulation (SAS) turbulence model and Zwart-Gerber-Belamri (ZGB) cavitation model to understand the cavitation-vortex interaction in the blade tip region. The predicted cavitation performance curve exhibits reasonable agreement with the experimental results, and the time-dependent vapor iso-surfaces (αv = 0.1) are consistent with HSV at different times in a typical cycle. Numerical simulation results show that cavitation can enhance the turbulent kinetic energy in the unstable vortex cavitation region and downstream tip leakage vortex region along the blade. Different vortex identification methods, including vorticity, Q criterion, λci, λ2 criterion, Ω, and Liutex/Rortex, are investigated. Analysis and comparison of the iso-surfaces of the different vortex identification methods indicate that the Ω and Liutex iso-surfaces can effectively predict the tip leakage vortex core and vortex pair in the unstable vortex cavitation region. Analysis of the fifth clip contours shows that no obvious difference exists between Q and λ2 criteria in terms of predicting the vortex core. All vortex identification methods can accurately predict the tip separation vortex in the blade tip region, but only the Ω and Liutex iso-surfaces can predict weak vortices in the cavitation region. The influence of the small parameter e of the Ω method on tip vortex identification is also discussed.

Numerical investigation of blade tip cavitation on large discharge condition of diagonal turbine

A series of numerical simulations was carried out to estimate the cavitation characteristics for a diagonal flow turbine. The interaction between the tip leakage flow and the cavitation bubbles was investigated using the obtained numerical data. The evaporation and condensation process for cavitation phenomenon was described by using a simplified Rayleigh-Plesset equation. A two-phase homogeneous model was adopted to calculate the mixture of gas and liquid phases. Three types of cavitation with different origins were obtained around the blade tip region. The incipient cavitation was well-known tip clearance cavitation. In the present study, the occurrence of tip clearance cavitation was limited in the position where the clearance between blade tip and discharge ring spread, irrespective of Thoma number. Tip clearance cavitation was found out not to contribute to the decrease of the turbine efficiency. On the contrary, the turbine efficiency increased with the growth of tip clearance cavitation. When Thoma number decreased, the other type cavitation arose on the discharge ring surface. The development of the discharge ring cavitation led to the decrease of the turbine efficiency. In addition, so-called tip vortex cavitation obtained in the lower Thoma number condition than the appearance of discharge ring cavitation. These three types of cavitation were also observed in the experimental approach. The correlation between the decrease of the turbine efficiency and the development of discharge ring cavitation was also verified in the model test.

Tip-leakage cavitation in the clearance of a 2D hydrofoil with fillets: high-speed visualization and PIV/PTV measurements

Tip-clearance cavitation is one of the most aggressive forms of cavitation as it can cause surface erosion of hydraulic machinery elements and, as a result, their fatigue damage and disturb designed operating conditions. At present, the literature lacks for detailed experimental data on the inception and development of this type of cavitation at various flow conditions. In the paper, a tip-leakage cavitation occurring in the clearance between an end face of a 2D hydrofoil (a scaled-down model of guide vanes (GV) of a Francis turbine) and a transparent wall of the test section was studied. The experiments were carried out for different cavitating regimes on the cavitation number and two attack angles of 3° and 9°, with the gap size (tip clearance width) varied in the range from 0.4 to 0.8 mm. In order to determine the cavitation inception conditions and investigate the dynamics of the tip-leakage cavitation, a high-speed visualization was applied. A modified PIV/PTV technique with a diverging laser beam instead of a laser light sheet was used to measure the mean velocity distributions within the gap. It was shown that the cavitation pattern on the suction side of the GV model impacts the dynamics of the leakage flow in the gap but does not affect the sheet cavity formed close to the foil leading edge in the clearance as well as its size and dynamics. When the gap size is increased, the tip-leakage cavitation initiates at higher cavitation numbers or, in other words, conditions for the cavitation occurrence become more favorable.

Effect of blade tip geometry on tip leakage vortex dynamics and cavitation pattern in axial-flow pump

A series of blade tip geometries, including original plain tip, rounded tip on the pressure side and diverging tip towards the suction side, were adopted to investigate the effect of blade geometry on tip leakage vortex dynamics and cavitation pattern in an axial-flow pump. On the basis of the computation, it clearly shows the flow structure in the clearance for different tip configurations by the detailed data of axial velocity and turbulent kinetic energy. The in-plain trajectory, in aspects of the angle between the blade suction side and vortex core and the initial point of tip leakage vortex, was presented using the maximum swirling strength method. The most striking feature is that the inception location of tip leakage vortex is delayed for chamfered tip due to the change of blade loading on suction side. Some significant non-dimensional parameters, such as pressure, swirling strength and turbulent kinetic energy, were used to depict the characteristics of tip vortex core. By the distribution of circumferential vorticity which dominates the vortical flows near the tip region, it is observed that the endwall detachment as the leakage flow meets the mainstream varies considerably for tested cases. The present study also indicates that the shear layer feeds the turbulence into tip leakage vortex core, but the way is different. For the chamfered tip, high turbulence level in vortex core is mainly from the tip clearance where large turbulent kinetic energy emerges, while it is almost from a layer extending from the suction side corner for rounded tip. At last, the visualized observations show that tip clearance cavitation is eliminated dramatically for rounded tip but more intensive for chamfered tip, which can be associated with the vortex structure in the clearance.

Numerical investigation of tip clearance cavitation in Kaplan runners

There is a gap between the Kaplan runner blade and the shroud that makes for a special kind of cavitation: cavitation in the tip leakage flow. Two types of cavitation caused by the presence of clearance gap are known: tip vortex cavitation that appears at the core of the rolled up vortex on the blade suction side and tip clearance cavitation that appears precisely in the gap between the blade tip edge and the shroud. In the context of this work numerical investigation of the model Kaplan runner has been performed taking into account variable tip clearance for several cavitation regimes. The focus is put on investigation of structure and origination of mechanism of cavitation in the tip leakage flow. Calculations have been performed with the help of 3-D unsteady numerical model for two-phase medium. Modeling of turbulent flow in this work has been carried out using full equations of Navier-Stokes averaged by Reynolds with correction for streamline curvature and system rotation. For description of this medium (liquid-vapor) simplification of Euler approach is used; it is based on the model of interpenetrating continuums, within the bounds of this two- phase medium considered as a quasi-homogeneous mixture with the common velocity field and continuous distribution of density for both phases. As a result, engineering techniques for calculation of cavitation conditioned by existence of tip clearance in model turbine runner have been developed. The detailed visualization of the flow was carried out and vortex structure on the suction side of the blade was reproduced. The range of frequency with maximum value of pulsation was assigned and maximum energy frequency was defined; it is based on spectral analysis of the obtained data. Comparison between numerical computation results and experimental data has been also performed. The location of cavitation zone has a good agreement with experiment for all analyzed regimes.

Numerical simulation of tip clearance impact on a pumpjet propulsor

Numerical simulation based on the Reynolds Averaged Navier–Stokes (RANS) Computational Fluid Dynamics (CFD) method had been carried out with the commercial code ANSYS CFX. The structured grid and SST k–ω turbulence model had been adopted. The impact of non-condensable gas (NCG) on cavitation performance had been introduced into the Schnerr and Sauer cavitation model. The numerical investigation of cavitating flow of marine propeller E779A was carried out with different advance ratios and cavitation numbers to verify the numerical simulation method. Tip clearance effects on the performance of pumpjet propulsor had been investigated. Results showed that the structure and characteristics of the tip leakage vortex and the efficiency of the propulsor dropped more sharply with the increase of the tip clearance size. Furthermore, the numerical simulation of tip clearance cavitation of pumpjet propulsor had been presented with different rotational speed and tip clearance size. The mechanism of tip clearance cavitation causing a further loss of the efficiency had been studied. The influence of rotational speed and tip clearance size on tip clearance cavitation had been investigated.

Numerical analysis of unsteady tip leakage vortex cavitation cloud and unstable suction-side-perpendicular cavitating vortices in an axial flow pump

The objective of this work is to simulate and analyze the formations of three-dimensional tip leakage vortex (TLV) cavitation cloud and the periodic collapse of TLV-induced suction-side-perpendicular cavitating vortice (SSPCV). Firstly, the improved SST k–ω turbulence model and the homogeneous cavitation model were validated by comparing the simulation result with the experiment of unsteady cavitation shedding flow around the NACA66-mod hydrofoil, and then the unsteady TLV cloud cavitation and unstable SSPCV in an axial flow pump were predicted using the improved numerical method. The predicted three-dimensional cavitation structures of TLV and SSPCV as well as the collapsing features show a good qualitative agreement with the high speed photography results. Numerical results show that the TLV cavitation cloud in the axial flow pump mainly includes tip clearance cavitation, shear layer cavitation, and TLV cavitation. The unsteady TLV cavitation cloud occurs near the blade trailing edge (TE) where the shapes of sheet cavitation and TLV cavitation fluctuate. The inception of SSPCV is attributed to the tail of the shedding cavitation cloud originally attached on the suction side (SS) surface of blade, and the entrainment affect of the TLV and the influence of the tip leakage flow at the tailing edge contribute to the orientation and development of the SSPCV. The existence of SSPCV was evidently approved to be a universal phenomenon in axial flow pumps. At the part-load flow rate condition, the SSPCV may trigger cavitation instability and suppress the tip cavitation in the neighboring blade. The cavitation cloud on the SS surface of the neighboring blade grows massively, accompanying with a new SSPCV in the neighboring flow passage, and this SSPCV collapses in a relatively short time.

Numerical simulation of unsteady cavitating flows of pumpjet propulsor

In this study, an attempt has been made to study the cavitating flows of a pumpjet propulsor. A numerical study based on the unsteady Reynolds-averaged Navier–Stokes computational fluid dynamics method of homogeneous multiphase using structured grids and the shear stress transport (SST) k–ω turbulence model has been carried out. Considering the effects of non-condensable gas (NCG) on the cavitation performance, the Schnerr–Sauer cavitation model has been applied and improved by introducing the NCG. The numerical calculations of non-cavitating and cavitating flows for the marine propeller E779A are carried out with different advance ratios at several cavitation numbers to verify the numerical simulation method. Results show that the thrust, the torque, and the cavitation performance, such as the cavitation location and shape, are in good agreement with experimental data. Furthermore, numerical predictions for a pumpjet propulsor on an unmanned underwater vehicle are made with different non-cavitating and cavitating flow conditions. The numerical simulations accurately predict the propulsor efficiency changes and the cavitation inception and extension on the suction side of rotor blades. The circumferential distributions of velocity components in different regions along the axial direction have different characteristics. The pressure distributions around rotor and stator blades are reasonable and consistent with the cavitation phenomenon. Additionally, the existence of a tip vortex and tip clearance cavitation leads to a further loss in efficiency of cavitating flows.

119104 軸流ポンプにおけるキャビテーションの発生状況とウェーブレット解析(一般05 流体力学・流体工学1)

The relationship between pump performance and vibration generated by cavitation was investigated using an axial-flow impeller. By experiments on cavitation of an axial-flow pump, the growth of cavitation due to the change of the Net Positive Suction Head (NPSH) and the pump performance were clarified. Moreover, the frequencies of cavitaion were disclosed by conducting the wavelet analysis on pressure fluctuation and vibration fluctuation at the casing wall. It was made clear in this paper that the tip clearance cavitation is proven to have a specific frequency.