Tip Clearance Vortex Research Articles

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.

Numerical and experimental analysis of the cavitation and flow characteristics in liquid nitrogen submersible pump

In this paper, the cavitation and flow characteristics of the unsteady liquid nitrogen (LN2) cavitating flow in a submersible pump are investigated through both experimental and numerical approaches. The performance curve of the LN2 submersible pump is obtained via experimental measurement. Numerical simulations are performed using a modified shear stress transport k–ω turbulence model, incorporating corrections for rotation and thermal effects as per the Schnerr–Sauer cavitation model. The numerical framework is verified by comparing the cavitation morphology features with previously reported visual data of the LN2 inducer and aligning pump performance data with those obtained from experimental tests of the LN2 submersible pump. The results indicate that cavitation at the designed flow rate predominantly manifests as tip clearance vortex cavitation in the inducer. Increased flow rates exacerbate cavitation, potentially obstructing the flow passage of the impeller. The vortex identification method and the vorticity transport equation are employed to identify the vortex structures and analyze the interaction between cavitation and vortices in the unsteady LN2 cavitating flow. The vortex structures primarily concentrate at the outlet of the impeller flow passage, largely attributed to the vortex dilation term and baroclinic torque. The influence of thermal effects on the cavitation flow of submersible pumps is analyzed. An entropy production analysis model, comprehensively involving various contributing factors, is proposed and utilized to accurately predict the entropy production rate within the pump. This study not only offers an effective numerical approach but also provides valuable insight into the cavitation flow characteristics of the LN2 submersible pump.

Numerical Analysis of Unsteady Phenomena in a Contra-Rotating Stage Based on the Reduction of Local Entropy Production

Abstract The primary purpose of this study is the reduction of local entropy production in a contra-rotating stage. As such, the unsteady flow phenomena and the impact of radial load distribution on these phenomena and local entropy production need to be clarified. In this study, the stress-blended eddy simulation (SBES) turbulence model is utilized to capture the vortices in the flow separation zone, and the γ-Reθ transition model is employed to predict the transition phenomenon within the boundary layer. Entropy production rate models suitable for different turbulence models are constructed separately to calculate local entropy production. Vortex visualization is achieved according to the λci criterion, and the relative vorticity change rate is used to analyze the components of the tip clearance vortices. The transition phenomenon is analyzed from the perspectives of both the Euler and the Lagrange descriptions. The primary findings can be summarized as follows: the transition begins earlier and progresses more rapidly in the rear rotor. Wake propagation, occurring at double the frequency, entropy production rate within the boundary layer changes in synchrony with the wall shear stress at the same frequency. Additionally, an investigation of the tip clearance vortices concludes that the main structure of the tip clearance vortices coincides with the flow pattern of the high entropy production rate region, and the flow structure related to the high divergence area is essential for considering subsequent optimization with the aim of reducing the entropy production rate.

Dynamic mode characteristics of flow instabilities in a single-stage compressor under different throttling processes

To investigate the influence of throttling processes on dynamic characteristics of compressor stability, the rotating stall development of National Aeronautics and Space Administration Stage 35 was simulated with full-annulus Unsteady Reynolds-Averaged Navier–Stokes under different throttling processes. The numerical methods were verified. By combining Dynamic Mode Decomposition and flow field evolution research, the flow structures and dynamic characteristics of “critical mass flow” under different throttling processes were deeply studied; the flow mechanism of flow instabilities under different throttling processes was explored. It is found that the “critical mass flow” corresponds to the beginning of a rapid decrease in mass flow, mainly characterized by shock forward movement and a larger range of spillage flow. Around “critical mass flow,” if the throttle is still tightening, it presents stall pattern 2; otherwise, it presents stall pattern 1. During the pre-stall, both patterns are dominated by tip clearance vortex (TCV)-shock interference. Stall inception disturbance is generated from TCV-shock interference; pattern 1 presents a single disturbance, while pattern 2 presents multiple disturbances. Subsequently, the TCV-shock interference gradually weakens. The single stall disturbance of pattern 1 gradually develops and stabilizes. The multiple stall disturbances in pattern 2 undergo processes including fusion and disappearance, ultimately developing into a single stall cell. During the stable stall, the throttling processes have no significant impact on the speed of the stall cell, and the flow in the un-stalled region is basically consistent with the speedline. However, the tighter the throttle is, the larger the stalled region, and the weaker the flow capacity of the un-stalled region.

Influence mechanism of tip clearance on flutter stability with different aerodynamic coupling effects for transonic compressor

Flutter is a highly destructive aeroelastic problem in modern compressors, which hinders the improvements of aero-engine performance and reliability. Because the aerodynamic work is mainly concentrated in the blade tip region, the tip clearance which is associated to the complex tip clearance flow has a considerable effect on the flutter stability. In this paper, in order to investigate the influence mechanism of the tip clearance on the flutter stability at different nodal diameters (ND), a series of compressor models were established with different tip clearances based on a transonic compressor rotor. The aerodynamic damping at each ND is obtained by influence coefficient method (ICM), while phase-shifted boundary method (PSB) is adopted to analyze the influence of the tip clearance on the flutter stability at different NDs. The results indicate the worst flutter stability for each tip clearance always appears at ND=1, where the aerodynamic damping exhibits a nonmonotonic trend of increasing first and decreasing thereafter along with the rising tip clearance. Two kinds of action are exerted by the tip clearance flow on the flow structures to alter the flutter stability: one is the tip clearance vortices which is generated on the suction side and impinge on the pressure side; the other is the interference of tip clearance vortices to the shock wave and to the flow separation. Besides, the influence of the changing aerodynamic coupling effect makes the above action bring about more drastic fluctuations to the unsteady pressure. At larger NDs, the unsteady pressure amplitude and phase fluctuate more sharply for larger tip clearances. Therefore, diverse change trends of the aerodynamic damping with the increasing tip clearance appear at different NDs, while the impingement of tip clearance vortices on the adjacent pressure side has a constantly stabilizing effect at different NDs.

Numerical analysis of the influence of hull-modulated inflow on unsteady force fluctuations and vortex dynamics of pump-jet propulsor

The influence of the hull-modulated inflow on the propulsion performance of the propeller is related to the matching design of the propeller–hull system. In the present study, considering the working conditions of the pump-jet propulsor in uniform inflow and two types of hull-modulated inflow, based on improved delay detached eddy simulation, the influence of hull-modulated inflow on unsteady force fluctuations and vortex dynamics of pump-jet propulsor under design conditions is carried out. The results show that the hull-modulated inflow increases the propulsion efficiency of the pump-jet propulsor to varying degrees within the range of the calculated advance coefficient and has a significant influence on the frequency characteristics of the unsteady force spectra characteristics of each component of the pump-jet propulsor. It also shows changes in the magnitude characteristics, that is, the energy transfer process of an individual rotor blade from the stator blade passing frequency to other harmonics of the shaft rotation frequency, and the thrust spectrum of an individual stator blade presents broad-spectrum characteristics in the high-frequency range. Furthermore, the application of hull-modulated inflow directly affects the shape of the stator shedding vortex, causing some of the stator blade shedding vortices to separate early and aggravating its short-wave instability. More secondary vortices are induced to accelerate the instability of the rotor blade tip clearance vortex. The energy transfer mechanism from the rotor blade passing frequency and its harmonics to the broadband spectra appears in the wake field of the pump-jet propulsor.

Numerical Investigation of Rotating Instability Development in a Wide Tip Gap Centrifugal Compressor

In the current study, full-stage unsteady simulations were performed to investigate rotating instability inception mechanisms in a particularly large tip clearance centrifugal compressor with a vaneless diffuser and a volute. Four operating points along a speed line were analysed to understand the influence of the mass flow reduction on flow structures. Close to the peak efficiency, an unsteady interaction between the tip clearance vortices and splitter blades was observed. Considering other studies, the influence of the tip gap size was analysed. Then, a large-scale vortex shedding from the leading edges of the main blades was detected when the stage operated near the maximum pressure ratio. It was demonstrated that shed vortices were caused by the combination of the radial gradient of the tangential velocity under the tip vortex and the reverse backflow near the casing. Previous studies on axial compressors refer to these vortical structures as backflow vortices. These vortices cause a significant increase in the incidence angle in the tip region.

Vortex Structure Topology Analysis of the Transonic Rotor 37 Based on Large Eddy Simulation

Highly three–dimensional and complex flow structures are closely related to the aerodynamic losses occurring in the transonic axial–flow compressor. The large eddy simulation (LES) approach was adopted to study the aerodynamic performance of the NASA rotor 37 for the cases at the design, the near stall (NS), and the near choke (NC) flow rate. The internal flow vortex topology was analyzed by the Q–criterion method, the omega (Ω) vortex identification method, and the Liutex identification method. It was observed that the Q–criterion method was vulnerable to being influenced by the flow with high–shear deformation rate, especially near the end–wall regions. The Ω method was adopted to recognize the three–dimensional vortex structure with a higher precision than that of the Q–criterion method. Meanwhile, the Liutex vortex identification method showed a good performance in vortex identification, and the corresponding contribution of Liutex components in the vortex topology was analyzed. The results show that the high–vortex fields around the separation line and reattachment line had high vortex components in the x–axis, the tip clearance vortices presented a high–vortex component in the y–axis, and the suction side corner vortex possessed high–vortex components in the y– and z–axes.

Study on the thrust fluctuation and vortices of a pump-jet propulsor under different duct parameters

Improving underwater propulsor performance is endlessly pursued, not only the propulsion but the vibration and radiated noise. The latter is highly related to the unsteady characteristics of forces and flow. This paper investigates the effects of duct parameters on the thrust fluctuation and flow features of a pre-swirl stator pump-jet propulsor (PJP) via embedded large eddy simulation (ELES). The duct parameters include inclination angle, tip clearance, axial length, maximum thickness, and diameters of duct inlet and duct outlet. The numerical approach is verified with experiments, showing good precision. The performance is first discussed, followed by the thrust fluctuation analysis to reveal the varying law of the thrust fluctuation level with duct parameters. Then, the velocity and pressure around the duct are presented to inspect the causes of differences in performance and thrust fluctuation. Finally, the vortices in the flow field are identified and considered the vortex evolution of the tip clearance vortex for inspecting the instability mechanism under different ducts. Results show that the duct parameters significantly affect the performance and thrust fluctuation. The instability mechanism of the tip clearance vortex also presents notable differences. Overall, the study should be helpful in the duct design and reduce thrust fluctuation, and will eventually benefit low-noise pump-jet design.

Analysis on spike-type rotating stall in transonic axial compressor by dynamic mode decomposition

The spike-type rotating stall in axial compressor is one of the major causes for the aeroengine failure. The evolution of the flow structures, especially in the rotor tip region, is very important to understand the mechanism and predict the onset of the rotating stall. However, with the limitation of the experimental technologies, the detailed flowfield during the stall was hard to be obtained. Not only that, the flow structures were not easy to be extracted from the results by the traditional numerical analysis method. In this paper, the detailed unsteady flowfields are obtained under the conditions from the near stall to the stable stall by the validated numerical simulation with a transonic axial compressor model. Then, the dynamic mode decomposition, as a novel method for the model reduction and decomposition, is verified and implemented to extract the flow structures from the flowfield of the spike-type rotating stall. It is found that the flow structures are well decomposed with their characteristic frequencies. By reconstructing the unsteady flowfields based on the selected modes, the effect and the evolution of the corresponding flow structure are clearly observed. From the near-stall to the stable stall, there are three types of flow structure that play important role in tip region. First, the structure of the tip clearance vortex shedding with blade passing frequency dominates the unsteadiness in the near-stall flowfield. Next, the structure of the vortex breakdown with the self-induced frequency provides the initial non-uniform disturbance for the spike generation, but it is soon eliminated with the strengthening of the stall cell. The third one is the stall-related structure, which is triggered by the spike disturbance and evolved to the stable stall cell. This kind of the developing process plays the dominant role on the unsteadiness of the stall.

Unsteady Effects of Wake on Downstream Rotor at Low Reynolds Numbers

In a compressor, the periodic wake is an inherently unsteady phenomenon that affects the downstream flow conditions and loading distribution. Thus, understanding the physical mechanisms of these unsteady effects is important for eliminating flow losses and improving compressor performance, particularly at low Reynolds numbers. To understand the influence of the upstream wake on the downstream flow field structure, this paper describes numerical simulations of a one-stage high-pressure compressor at altitudes of 10–20 km. The influence of the wake on rotor flow blockage at different Reynolds numbers is analyzed, and the unsteady interaction between the upstream wake and boundary layer or tip leakage flow is discussed. The results indicate that the wake has a beneficial effect on the efficiency of the rotor at high Reynolds numbers, but this weakens and becomes negative as the Reynolds number decreases. The wake can reduce the flow blockage in the mainflow region. Due to the wake, the length of the laminar separation bubble at high Reynolds numbers decreases and that at low Reynolds numbers increases. In addition, the unsteadiness of the wake causes separation bubbles to appear periodically at high Reynolds numbers and induces an open separation bubble at low Reynolds numbers. The Kelvin–Helmholtz instability can dominate the transition process of the boundary layer, which is also affected by the disturbance vortex induced by the wake. Regarding the tip leakage flow, the wake can reduce the flow blockage at high Reynolds numbers but increase the flow blockage at low Reynolds numbers. The interaction at low Reynolds numbers causes a double-leakage flow, which finally leads to the large-scale separation of the suction surface boundary layer. The large-scale separation causes flow blockage in the tip region and prevents the rotor wake from propagating downstream. On the contrary, the unsteady wake can pass through the tip clearance vortex and inhibit the separation of the suction boundary layer at high Reynolds numbers, which is reflected in a larger amplitude of one blade passage frequency. Therefore, the flow loss in the downstream flow field at high Reynolds numbers is significantly reduced at high Reynolds numbers.

Prediction of Cavitation Performance over the Pump-Jet Propulsor Using Computational Fluid Dynamics and Hybrid Deep Learning Method

The cavitation performance of an oblique flow field is different from that under a pure axial flow field. This study analyzed the hydrodynamic performance, bearing force, and tip clearance flow field under different rotating speeds and different cavitation numbers in an oblique flow field. Furthermore, this study proposed a hybrid deep learning model CNN-Bi-LSTM to quickly and accurately predict the bearing force of a pump-jet propulsor (PJP), which will solve the problem of time-consuming calculation and consumption of considerable computing resources in traditional computational fluid dynamics. The Shear–Stress–Transport model and Reynolds-averaged Navier–Stokes equations were utilized to procure the training and testing datasets. The training and testing datasets were reasonably divided in the ratio of 7:3. The results show that the propulsion efficiency decreased more obviously under higher rotating speed conditions, with a maximum decrease of up to 13.59%. The small cavitation numbers 1.4721 and high oblique angle significantly impacted the efficiency reduction; the maximum efficiency loss exceeded 20%. Thus, a small cavitation number 1.4721 is extremely detrimental to the propulsion efficiency of the PJP due to the large cavitation area. Moreover, the intensity of the tip clearance vortex continuously increased with the rotating speed. The CNN-Bi-LSTM deep model successfully predicted the phase difference and trend change of the propulsor bearing force under different conditions. The prediction difference was large at the crest and trough of the bearing force, but it is within the acceptable error range.

Non-synchronous blade vibration analysis of a transonic fan

This study numerically investigates the aeromechanic behavior of a transonic fan model with a flat tip-leading-edge on the NASA rotor 67 test case. Single-passage unsteady calculations at a near stall operating point of 82% design speed show that the dominant frequencies of mass flow were not the harmonics of the rotor rotational frequency. A full-annulus fluid–structure interaction analysis was subsequently carried out to examine the unsteady flows and their interactions with blade vibrations. The results show that the modal displacement of the backward traveling seventh nodal diameter of the second torsion mode grew exponentially, which reveals that the blade vibration was non-synchronous. The vibration pattern indicates that the aerodynamic mode was resonant with the structural vibration mode. Around the rotor tip, the circumferential vortical propagation induced by interactions among the main flow, tip leakage flow, and tip clearance vortex was the source of aerodynamic excitation. To clarify the mechanism of the non-synchronous vibration, the coupling between aerodynamic disturbance and structural response, i.e., aliasing, was summarized. The frequency spectra of the fluctuating pressure show that an aerodynamic Backward Traveling Wave (BTW) was co-aliased to a structural BTW due to the propagation of the circumferential vortex. The correlation between the frequency and free convective speed of the aerodynamic disturbance determined the directions of aliasing.

An Analysis of the Secondary Flow Around a Tandem Blade Under the Presence of a Tip Gap in a High-Speed Linear Compressor Cascade

Abstract Tandem blades have often been under investigation, experimentally as well as numerically, but most of the studies have been about tandem blade stators without tip gap. This work analyzes the influence of a tip gap on the flow field of a tandem blade for engine core compressors. Experiments have been conducted in a high-speed linear compressor cascade on a tandem and a reference geometry. The flow is analyzed using five-hole probe measurements in the wake of the blades and oil flow visualization to show the near surface stream lines. First, the results for design conditions (tandem and conventional blades) are compared to measurements on corresponding blades without a tip gap. Similarities and differences in the flow topology due to the tip clearance are analyzed, showing that the introduction of the tip clearance has a similar influence on the loss and turning development for the tandem and the conventional blades. The tandem blade features two tip clearance vortices with a complex flow interaction and the possible formation of a third counter-rotating vortex between them. An incidence variation from 0 deg to 5 deg for both blades indicates at first a similar behavior. After a separation of the flow field into gap and non-gap half, it becomes apparent that the tandem blade shows higher losses on the gap side, while featuring a close-to-constant behavior on the non-gap side. Further investigation of the flow on the gap side shows indicators of the front blade exhibiting tip clearance vortex break down, while the rear blade seems unaffected.

A Detailed Loss Analysis Methodology for Centrifugal Compressors

Abstract A deep understanding of loss mechanisms inside a turbomachine is crucial for the design and analysis work. By quantifying the various losses generated from different flow mechanisms, a targeted optimization can be carried out on the blading design. In this paper, an evaluation method for computational fluid dynamics (CFD) simulations has been developed to quantify the loss generation based on entropy production in the flow field. A breakdown of losses caused by different mechanisms (such as skin friction, secondary flow, tip clearance vortex, and shock waves) is achieved by separating the flow field into different zones. Each zone is defined by the flow physics rather than by geometrical locations or empirical correlations, which makes the method a more general approach and applicable to different machine types. The method has been applied to both subsonic and transonic centrifugal compressors, where internal flow is complex due to the Coriolis acceleration and the curvature effect. An evaluation of loss decomposition is obtained at various operational conditions. The impact of design modification is also assessed by applying the same analysis to an optimized design.

Multi-Objective Optimization of an Axial Flow Turbine Design Using Surrogate Modeling and Genetic Algorithm

Abstract A surrogate model-based multi-objective design optimization methodology using a nondominated sorting genetic algorithm is used to maximize the power output and efficiency of an axial flow turbine. A set of high-fidelity data obtained from ANSYS-CFD simulations on space-filling samples is used for developing a surrogate model. The methodology uses (i) a Latin hypercube experimental design for selecting space-filling samples, (ii) a genetic algorithm for determining parameters of a Kriging model, and (iii) axial gap, tip clearance, and rotation angle of nozzle profile of turbine as predictor variables. Flow in the turbine is characterized by the presence of jet and wake flow, shock in the rotor blade flow passage, and tip clearance vortex. Study shows that (i) the axial gap controls the mixing of the jet and the wake flows and provides proper blade incidence which reduces the shock strength in the rotor blade flow passages and (ii) an optimum sized tip clearance vortex controls tip clearance leakage. The optimization provided a set of Pareto-optimal solutions that are nondominated in terms of power and efficiency. Verification of a selected design configuration from the Pareto-optimal solution using computational fluid dynamics (CFD) analysis showed that the process of optimization has been able to fine-tune the axial gap and the tip clearance.

Verification and Validation of Large Eddy Simulation for Tip Clearance Vortex Cavitating Flow in a Waterjet Pump

In this paper, large eddy simulation (LES) was adopted to simulate the cavitating flow in a waterjet pump with emphasis on the tip clearance flow. The numerical results agree well with the experimental observations, which indicates that the LES method can make good predictions of the unsteady cavitating flows around a rotor blade. The LES verification and validation (LES V&V) analysis was used to reveal the influence of cavitation on the flow structures. It can be found that the LES errors in cavitating region are larger than those in the non-cavitating area, which is mainly caused by more complicated cavitating and tip clearance flow structures. Further analysis of the interaction between the cavitating and vortex flow by the relative vorticity transport equation shows that the stretching, dilatation and baroclinic torque terms have major effects on the generation and transport of vortex structure. Meanwhile the Coriolis force term and viscosity term also exacerbate the vorticity transport in the cavitating region. In addition, the flow loss characteristics of this pump are also revealed by the entropy production theory. It is indicated that the tip clearance flow and trailing edge wake flow cause the viscous dissipation and turbulent dissipation, and the cavitation can further enhance the instability of the flow field in the tip clearance.

Structure and modal analysis of the last stage blade in steam turbine under low volume flow conditions

To explore the prestressed modal characteristics of the last stage blade in steam turbine under low volume flow conditions, the fluid–structure interaction (FSI) method of ANSYS mechanical + CFX is adopted to consider the effect of flow field on the strength performance of the rotor blade. The cyclic symmetry analysis is adopted to calculate the strength performance and modal characteristics of the rotor blade. The results show that the flow field of the last stage is composed of backflow vortex, separation vortex and tip clearance vortex under low volume flow conditions. The tip clearance vortex has high circumferential velocity and temperature. The maximum deformation of the rotor blade is on the leading edge of the blade at 80% span, and the maximum equivalent stress is on the suction surface at 90% span. With the decrease of the steam flow, the temperature at the blade tip increases significantly, which increases the maximum deformation and the equivalent stress of the rotor blade. The plastic deformation occurs when the steam flow is reduced to 15% turbine heat acceptance (THA). In addition, the elastic modulus of blade material decreases with the increase of temperature, which leads to the decrease of natural frequencies. From 35 %THA to 5% THA condition, the natural frequencies of the blade decrease by 34-136 Hz. This study provides a theoretical reference for the identification, diagnosis and prediction of the vibration of the last stage blade under low volume flow conditions.

Effect of tip clearance on cavitating flow of a hydraulic axial turbine applied in turbopump

The requirements of Liquid Propellant Rocket Engine (LPRE) are high for thrust, specific impulse, and flow rate; thus, its components also have strict requirements. For the turbopumps (TPs), this means high flow rate, high rotational speed, and high pressure ratio, which makes their operations susceptible to the cavitation phenomenon, as observed in two previous works. In the first, cavitation regions were observed in the first stage of the Space Shuttle Main Engine (SSME) Liquid Oxygen (LOX) booster turbine, for 3.0, 5.5, and 8.0% tip clearances (relative to rotor blade height), using monophase flow (Lindquist Whitacker et al., 2017). In the second, the simulations were performed with multiphase flow, producing results more physically coherent for the 3.0% gap configuration (Whitacker et al., 2018). The characteristics of both types of simulations in space propulsion applications still require better understanding. Therefore, to compare monophase and multiphase results at various operating points and turbine configurations, steady-state turbulent 3-D Computational Fluid Dynamics (CFD) simulations were performed, based on Reynolds-Averaged Navier-Stokes (RANS) formulation. The same three tip configurations for the turbine first stage were simulated, and the calculations were validated using experimental results from the National Aeronautics and Space Administration (NASA) (Boynton and Rohlik, 1976). This made it possible to verify the effect of the tip clearance on the machine performance and internal flowfield. When the gap increased, the pressure loading decreased in a large region of the blade tip, the interaction was greater between the Tip Clearance Vortex (TCV) and a vortex generated around the shroud cavitation region (Cavitation Vortex - CV), and this interaction moved towards the middle of the blade-to-blade passage. Thus, the losses increased and the efficiency decreased. Various comparative aspects between the simulations using both mono and multiphase numerical schemes are also discussed.

Lock-in phenomenon of tip clearance flow and its influence on aerodynamic damping under specified vibration on an axial transonic compressor rotor

In this study, the lock-in phenomenon of Tip Clearance Flow (TCF) instabilities and their relationship to blade vibration are investigated numerically on an axial transonic rotor with a large tip clearance. The capabilities of simulating instability flow and lock-in phenomenon are verified on a transonic rotor and a NACA0012 airfoil by comparing with the test data, respectively. The lock-in phenomenon is first numerically confirmed that may occur to TCF instabilities when its frequency is close to the blade vibration frequency. The lock-in region becomes wider with the vibration amplitude increasing, and it is also affected by modal shapes. For the rotor at the simulation conditions in this study, the bending mode results in a wider lock-in region than the torsional mode. In the lock-in region, the phase difference between the Tip Clearance Vortex (TCV) and the blade vibration changes with the flow condition and the frequency ratio of the blade vibration and the TCV instabilities. The frequency of the TCV instabilities reduces with the mass flow decreasing. Therefore, reducing mass flow and increasing frequency ratio have similar effects on the TCV phase, which causes a significant variation on the unsteady pressure amplitude in the blade tip area. Thus, the aerodynamic damping changes significantly with the TCV phase. The aerodynamic damping displays a nonlinear relationship with the vibration amplitude, and it changes from negative to positive with the vibration amplitude increasing at the same frequency ratio. The negative damping is mainly provided by the tip area of the blade. For unlocked conditions, the period of the TCF instabilities fluctuates over time, and it cannot be directly separated by their frequency features. Inter Blade Phase Angle (IBPA) also has an important influence on the feature of the TCV instabilities. The occurrence of frequency lock-in also requires “appropriate” IBPA. For the examined working conditions, the frequency lock-in occurs under 0 ND (Nodal Diameter), but not under 8 ND. However, no matter 0 ND or 8 ND, the phase of TCV always locks onto the IBPA at the examined conditions.