Tip Flow Research Articles

Flow and heat transfer characteristics of a squealer tip with bent rail in turbine blades

Squealer tip is an effective method for reducing heat transfer and leakage losses of blade tips in gas turbines and is currently being extensively researched and developed. In this study, the squealer tip has been innovatively developed by introducing bent rails, and three types of squealer structures with bent rails were proposed: the bent pressure-side rail scheme (Case_P), the bent suction-side rail scheme (Case_S), and the bent double-side rail scheme (Case_PS). This study first compared the performance of the proposed novel schemes with the traditional squealer tip scheme with vertical rails (Baseline) in terms of tip heat transfer and flow control by analyzing parameters such as the tip heat transfer coefficient (h), leakage flow rate (LFR), and total pressure loss (Cpt) under fixed bending values (S). Subsequently, the effects of different bending values (S) on the three novel schemes were investigated. The results show that, in terms of heat transfer, both Case_P and Case_S can effectively reduce the tip h, but their mechanisms are entirely different. Case_P primarily relies on the increasing separation bubble size at pressure-side rail edge and gradually expanded low-energy flow region in the pressure-side rail corner. In contrast, in Case_S, it is mainly because the cavity flow enters the suction-side gap more smoothly due to the bent suction-side rail. In terms of aerodynamics, the bent pressure-side rail of Case_P significantly inhibits the entering velocity of leakage flow, thereby reducing the LFR and Cpt; however, Case_S causes the LFR to increase. In Case_PS, the heat transfer and leakage phenomena resemble a combination of Case_P and Case_S. With the increase in S, the tip h in the three schemes all decreases, and the tip heat transfer in Case_PS is more comprehensively reduced because it incorporates all the flow factors that reduce h from the other two schemes. In Case_PS, the rail types that both hinder and increase LFR coexist, resulting in the LFR remaining relatively stable with varied S.

Effect of Leading Edge Erosion on the Tip Leakage Flow in a Compressor Cascade

Abstract The interaction between tip leakage flow and passage flow in axial compressor blade rows creates complex flow mechanisms that impair flow stability. Caused by erosion of the compres- sor blades, these flow mechanisms become time dependent. Es- pecially the compressor tip regions experience erosion-related surface loss owing to the particle-laden flow inside the engine. The corresponding reduction in chord length varies in height and leads to varying shapes of eroded compressor blades. It is likely that these impact the engine's operability. The optical accessi- bility of flow structures in water was exploited by analyzing four blade tip geometries at different angles of incidence in a linear compressor cascade. The impact of these geometries on the tip flow structures is analyzed by observing the movement of inked fluid elements in space and time. Particle Tracking Velocimetry (PTV) is used to obtain 3D trajectories of both stable and un- stable flows. Whilst the non-eroded blade geometry shows stable vortex formation, a notable change in the flow characteristics is observed for particular erosion patterns. In these cases, the tip leakage flow exhibits unsteadiness, featuring fluctuations and unstable flow phenomena. A large blockage of the flow passage occurs due to a vortex breakdown.

Catheter Exchange With Elongation of Tunnel (CEET) Procedure-A Novel Technique for Cuff Extrusion of Tunneled Dialysis Catheter: Surgical Experience and Early Outcomes.

Cuff extrusion of tunneled dialysis catheter (TDC) leads to catheter dysfunction, leading to loss of vascular access and the need for new catheter. Definitive management is to remove TDC and reinsert new catheter by new venous puncture and tunnel, which may not be possible in all cases. The study evaluated the surgical experience and early outcomes of a novel "Catheter Exchange with Elongation of Tunnel (CEET)" procedure for cuff extrusion. The retrospective study included all cases of hemodialysis with TDC with partial or complete cuff extrusion and excluded complete catheter dislodgement, tunnel infection, or any catheter related infection. All patients also underwent the CEET procedure under fluoroscopy guidance, and the clinical details and outcomes were analyzed. Eleven cases of TDC cuff extrusion underwent the CEET procedure of which three (27.2%) had previous and four (36.4%) had partial cuff extrusion, and seven cases (63.6%) had short tunnel length, which likely predisposed to cuff extrusion. CEET procedure was successful in 10 cases (success rate 90.1%) with desired position of catheter tip and good blood flow. Study population was divided into early and late cuff extrusion (≥1 month). Short tunnel length was associated with late extrusion (p = 0.05), whereas premature removal of TDC anchor sutures was associated with early cuff extrusion (p = 0.04). CEET procedure is a successful alternative technique for correction of cuff extrusion of TDC with good success rate. Premature removal of anchor sutures was associated with early cuff extrusion, whereas short tunnel length was associated with late cuff extrusion.

Numerical investigation of the influence of suction surface synthetic jet on the performance of transonic axial flow compressor

In order to investigate the effect of suction surface synthetic jet on the aerodynamic performance of transonic axial compressor, Rotor35 was used for numerical simulation. The results show that at four different positions, the stability margin of the compressor is slightly reduced by suction surface excitation, but there is an optimal position that can improve the compressor total pressure ratio and efficiency. At this position, there is little change in the total pressure ratio at the peak efficiency, and the efficiency is improved by 0.58%. The total pressure ratio and efficiency of the design point are increased by 0.18% and 0.55%, respectively. The periodic excitation of synthetic jet can effectively separate the suction surface and reduce the separation loss, which is the reason for the improvement of compressor efficiency. However, for the transonic compressor, synthetic jet on the suction surface obstructs the radial migration vortex that reaches the tip to the exit, but cause it to gather in the tip channel, resulting in the blockage and reduction of the stability margin of the compressor. In order to take into account the stability margin of the compressor, the coupled flow control is carried out by combining the casing treatment with synthetic jet on the suction surface. The results show that the flow margin of the compressor is increased by 8.76%, the total pressure ratio is increased by 1.76%, and the efficiency is only decreased by 0.13%. In coupled flow control, the suction and injection effects of the casing treatment can make up for the negative influence of synthetic jet on the tip flow, effectively improve the tip flow and expand the stable working range of the compressor. Compared with flow control that only carries out casing treatment, coupled flow control can exert the advantages of synthetic jet suppressing suction surface separation, reduce shock loss, outlet loss and casing treatment loss, and make up for the shortage of excessive efficiency reduction after casing treatment, which is the main reason for the improvement of compressor performance.

Surface roughness effects in a transonic axial flow compressor operating at near-stall conditions

Surface roughness is a major contributor to performance degradation in gas turbine engines. The fan and the compressor, as the first components in the engine's air path, are especially vulnerable to the effects of surface roughness. Debris ingestion, accumulation of grime, dust, or insect remnants, typically at low atmospheric conditions, over several cycles of operation are some major causes of surface roughness over the blade surfaces. The flow in compressor rotors is inherently highly complex. From the perspective of the component designers, it is, thus, important to study the effect of surface roughness on the performance and flow physics, especially at near-stall conditions. In this study, we examine the effect of surface roughness on flow physics such as shock-boundary layer interactions, tip and hub flow separations, the formation and changes in the critical points, and tip leakage vortices among other phenomena. Steady and unsteady Reynolds Averaged Navier Stokes calculations are conducted at near-stall conditions for smooth and rough National Aeronautics and Space Administration rotor 67 blades. Surface streamlines, Q-criterion, and entropy contours aid in analyzing the flow physics qualitatively and quantitatively. It is observed that from the onset of stall, to fully stalled conditions, the blockage varies from 21.7% to 59.6% from 90% span to the tip in the smooth case, and from 40.5% to 75.2% in the rough case. This significant blockage, caused by vortex breakdown and chaotic flow structures, leads to the onset of full rotor stall.

Shock wave structures and vortex unsteadiness in the tip region of a transonic turbine cascade under different conditions

The tip region of transonic turbine blades (exit Mach number of 0.95) exhibits complex flow characteristics with the coexistence of multiple shock wave systems and multiscale vortices. Based on a validated detached Eddy simulation method, unsteady flow features such as the spatial-temporal dynamic evolution of tip leakage vortices (TLV) and the periodic buffet/oscillation of shock trains inside the tip gap are revealed and discussed systematically under different incidence angles (i) and heights of tip clearance. Moreover, the distinction of tip flow structures between the subsonic and transonic conditions is also manifested. Results indicate that the wandering behavior of the TLV is influenced by both the swirling strength of the vortex itself and the interaction of adjacent secondary vortices. The TLV under a tip gap of 5% blade height (h) exhibits “binary and bimodal” wandering characteristics both along the pitchwise and spanwise direction, whereas under the 1%h case, only the pitchwise wandering is prominent. The dominant characteristic frequency of the TLV wandering under different conditions falls within the spectrum range of 2.2–2.4 kHz. As for the shock trains inside the tip clearance (τ), coherently movement back and forth along the pitchwise direction with varying amplitudes can be observed, where the magnitude within the τ=1%h exceeds that observed in the τ=5%h, depending on the intensity of the shock waves. Notably, significant shock wave oscillations are present throughout the range of the chord length (c) within the τ=1%h, whereas within the tip gap of 5%h, shock wave systems exhibit more pronounced oscillations predominantly near the trailing edge.

Effect of tip clearance on non-synchronous propagating flow disturbances of compressor rotors under high aerodynamic loading conditions

The complex tip flow instability and its induced non-synchronous vibration have become significant challenges, especially as aerodynamic loading continues to increase. This study investigates the effects of tip clearance on non-synchronous propagating flow disturbances of compressor rotors under high aerodynamic loading conditions by conducting full-annulus unsteady numerical simulations with three typical tip clearance values for a 1-1/2 stage transonic compressor. The non-synchronous aerodynamic excitation frequency, circumferential mode characteristics, and annular unstable flow structures are analyzed under near stall conditions. The results show that the total pressure ratio and normalized mass flow parameters first increase and then decrease as the tip clearance increases from 0.5%C (where C represents the tip chord length) to 2%C under high aerodynamic loading conditions, instead of constantly decreasing. For the 0.5%C tip clearance case, the traveling large-scale tornado-like separation vortices cause a low non-synchronous aerodynamic excitation frequency and severe pressure fluctuations. The periodic shedding and reattachment processes of the rotor blades separated by 2 – 3 pitches result in 19 dominant mode orders in the circumferential direction. As the tip clearance increases from 1%C to 2%C, the difference of tip flow structures in each blade passage is significantly weakened, and the dominant mode order of the disturbance is equal to the rotor blade-passing number. The pressure fluctuation is mainly caused by cross-channel tip leakage flow, and the aerodynamic excitation frequency exhibits evident broadband hump characteristics, which has been reported as a rotating instability phenomenon.

The relationship between intraosseous catheter tip placement, flow rates, and infusion pressures in a high bone density cadaveric swine (Sus scrofa) model.

Intraosseous (IO) infusion is a life-preserving technique when intravenous access is unobtainable. Successful IO infusion requires sufficiently high flow rates to preserve life but at low enough pressures to avoid complications. However, IO catheter tips are often misplaced, and the relative flow rates and pressures between IO catheter tips placed in medullary, trabecular, and cortical bone are not well described, which has important implications for clinical practice. We developed the Zone Theory of IO Catheter Tip Placement based on bone density and proximity to the venous central sinus and then tested the influence of catheter tip placement locations on flow rates and pressures in a cadaveric swine model. Three cross-trained participants infused 500mL of crystalloid fluid into cadaveric swine humerus and sternum (N=210 trials total) using a push‒pull method with a 60 cm3 syringe. Computed tomography scans were scored by radiologists and categorized as zone 1 (medullary space), zone 2 (trabecular bone), or zone 3 (cortical bone) catheter tip placements. Differences between zones in flow rates, mean pressures, and peak pressures were assessed using analysis of variance and analysis of covariance to account for participant and site differences at the p<0.05 threshold. Zone 1 and zone 2 placements were essentially identical in flow rates, mean pressures, and peak pressures (each p>0.05). Zone 1 and zone 2 placements were significantly higher in flow rates and lower in pressures than zone 3 placements (each p<0.05 or less). Within the limitations of an unpressurized cadaveric swine model, the present findings suggest that IO catheter tip placements need not be perfect to acquire high flow rates at low pressures, only accurate enough to avoid the dense cortical bone of zone 3. Future research using in vivo animal and human models is needed to better define the clinical impact of IO catheter placement on infusion flow rates and pressures.

Experimental study of flow control on tip leakage flow in variable geometry linear turbine cascades with different pivot layouts

Variable geometry turbines are essential for adjusting operational conditions in industrial gas turbines and variable cycle engines. These adjustments necessitate partial gaps at both ends of the variable guide vanes to alter the turning angle, consequently introducing an aerodynamic performance penalty. Moreover, the pivot layout profoundly influences aerodynamic losses. Research on turbine cascades that considers various partial gap layouts is limited, particularly in terms of experimental studies, which are rarely conducted. This study aims to diminish aerodynamic losses and augment the efficiency of gas turbines by examining the impact of pivot layouts on partial gap clearance and secondary flow. It further investigates the effectiveness of flow control strategies at the blade tip across different pivot configurations within a variable geometry turbine cascade, utilizing pneumatic probe scanning and surface oil flow visualization techniques. The results reveal that employing a cavity at the tip can significantly reduce aerodynamic losses in schemes both with and without a pivot, achieving maximum loss reductions of 15.8% and 3.7%, respectively. Additionally, a narrower squealer width can further decrease these losses. However, with a pivot located at the tip, the resulting separation flow and wake vortex become predominant sources of losses. The presence of the pivot weakens the tip leakage flow rate and the intensity of the tip leakage vortex (TLV), thus diminishing the effectiveness of cavity tip flow control. The cavity moderates TLV and enhances the interaction between TLV and the wake vortex, leading to increased aerodynamic losses.

An investigation on film cooling and aerodynamic performance of squealer blade tip with inclined cavity bottom

The gas turbine blade tip poses as one of the most challenging areas for blade cooling, primarily due to its intricate three-dimensional flow characteristics, which make it one of the primary contributors to aerodynamic loss within the turbine stage. In this paper, the adiabatic film cooling effectiveness (η) distributions at five squealer tip structures (Original, PS7.5, PS15, SS7.5, SS15) with varying inclined angles of cavity bottom surface were obtained using pressure sensitive paint technique (PSP) under five coolant blowing ratios (BR = 0.4, 0.8, 1.2, 1.6, 2.0). Combined with the numerical simulations verified by the experimental data, the tip flow field details were described to reveal the cooling characteristics of the tip region. The results indicate that, at low BRs, the film hole downstream exhibits apparent cooling traces, which are most pronounced at the SS15 tip. With the increase of BR, the cooling traces gradually fade, and the area-averaged film cooling effectiveness (ηΑ-avg) rises. At low BRs (BR = 0.4, 0.8), the ηΑ-avg of the SS15 tip is 12.8 %, and 8.4 % higher than that of the Original tip, respectively. At BR = 1.6, the ηΑ-avg of the PS15 tip is the highest, 3.9 % higher than that of the Original tip. The ηΑ-avg of SS7.5 is the lowest at all BRs. In addition, the pitch-averaged η (ηavg) of PS7.5 at some BRs is higher in the front part of the tip, compared to the Original tip. At BR = 1.2, the area-averaged total pressure loss coefficient (Cpt¯)of the PS7.5 and PS15 is 1.46 % and 1.58 % higher than that of the Original tip, respectively. However, the Cpt¯ of the SS7.5 and SS15 is 1.77 % and 10.28 % lower than that of the Original tip, respectively.

Effect of Tip Gap Size on the Tip Flow Structure and Turbulence Generation in a Low Reynolds Number Compressor Cascade

Abstract Efficient and compact axial compressors are currently undergoing rapid development for use in microcooling systems and small-scale vehicles. Limited experimental work concentrates on the inner flow field of the compressors working at such low Reynolds numbers (Re∼104). This study examines the vortical structures and the resulting turbulence production in the transitional flow over a C4 compressor blade at a Reynolds number Re of 24,000, with a specific focus on the impact of tip clearance. The particle image velocimetry measurements reveal the tip flow structures in detail, including the tip leakage vortex (TLV) and its induced complex vortical structures. The tip secondary flow at the low Reynolds number can be divided as the tip leakage flow (TLF)/vortex and transitional boundary layer both at the end walls and the blade surfaces. The TLV propagates at the highest spanwise positions and farthest pitchwise positions at the middle tip gap size (τ/C = 3%) for the three tip gap sizes investigated. The tip flow fluctuations decrease from τ/C = 5% to τ/C = 3% and then increase from τ/C = 3% to τ/C = 1%. The spatial distribution, streamwise evolution, and individual Reynolds normal stress components contributing to the turbulent kinetic energy (TKE) are discussed. The primary contributors to the turbulence generation are examined to elucidate the flow mechanism leading to the distinct anisotropic turbulence structure in the tip region with various tip gap sizes.

Determination of transient heat transfer by cooling channel in high-pressure die casting using inverse method

Complex shapes of aluminum castings are typically manufactured during the short cycle process known as the high-pressure die casting (HPDC). High productivity is ensured by introducing die cooling through a system of channels, die inserts or jet coolers. Die cooling can also effectively help in reducing internal porosity in cast components. Accurate simulations based on sophisticated numerical models require accurate input data such as material properties, initial and boundary conditions. Although the heat is dominantly dissipated through die cooling, indicating the importance of knowing precise thermal boundary conditions, open literature lacks a detailed information about the spatial distribution of heat transfer coefficient. This study presents an inverse method to determine accurate heat transfer coefficients of a die insert based on temperature measurements in multiple points by 0.5 mm K-type thermocouples and a subsequent solution of the two-dimensional inverse heat conduction problem. The solver was built in the open-source CFD code OpenFOAM and the free library for nonlinear optimization NLopt. The results are presented for the commonly used 10 mm die insert with a hemispherical tip and coolant flow rates ranging from 100 l/h to 200 l/h. Heat transfer coefficients reach values well above 50 kW/m2K in the hemispherical tip, which is followed by a secondary peak and then a gradual drop to values around 1 kW/m2K further downstream.

Tip-leakage-flow excited unsteadiness and associated control

Tip leakage flow in turbomachinery inherently generates intense unsteady features, named self-excited unsteadiness, which significantly affects the operating stability, aerodynamic efficiency, and noise but has not been well understood. A zonalized large eddy simulation is employed for a linear cascade, with wall-modeled large eddy simulation active only in the tip region. The simulation is well validated with advantages demonstrated for effectively reducing the computational cost while maintaining an equivalent prediction accuracy in the region of interest. The time-averaged and spatial-spectral characteristics of tip leakage vortex (TLV) structures are systematically discussed. The self-excited unsteady processes of TLV include the tip gap separation, the tip leakage and jet-mainstream interaction, the primary tip leakage vortex (PTLV) wandering motion, and the induced separation near end wall. The Spectral Proper Orthogonal Decomposition (SPOD) is used to examine the dominant frequencies and their coherent structures. It is found that these unsteady features change from a single high frequency to multiple lower frequencies due to the PTLV breakdown. The SPOD and correlation analyses reveal that the self-excited unsteadiness originates initially from unsteady vortex separation in the tip gap and is then fed by the interactions between the tip leakage jet and mainstream. The associated unsteady fluctuations are convected along the tip leakage jet trajectory, causing the wandering motion of PTLV core. Based on the revealed unsteadiness sources, a micro-offset tip design is proposed and shown to be an effective solution to reducing the tip flow unsteadiness. This work improves the understanding of tip-leakage-flow dynamics and informs the control of the associated unsteady fluid oscillation and noise.

A study of angle parametric optimization of the composite honeycomb on turbine blade tip

To reduce the secondary flow loss of the high-pressure turbine caused by the tip leakage flow, the design of the composite honeycomb tip has been experimentally and numerically examined in this study. The arrangement angle of the composite honeycomb is defined and optimized using the radial basis function and genetic algorithm. The tip flow field of the high-pressure turbine blade with a 2.128 mm clearance is simulated by CFX 18.0, and the arrangement angle of the composite honeycomb is optimized to obtain a lower loss. Moreover, the flat tip, basic honeycomb tip, and optimized honeycomb tip are tested in a low-speed cascade test facility. The results show that the vortices and radial velocity induced by the honeycomb structure cavity increase the kinetic energy loss of the leakage flow and reduce the leakage flow rate. Compared with the basic honeycomb, the optimized honeycomb has a supplemental blocking effect on the tip leakage flow by reducing the crosswise pressure gradient in the blade tip. The optimized honeycomb also changes the outlet angle of the leakage flow in the tip clearance, reduces the leakage vortex, and the total pressure loss is further reduced by 2.5%.

Transonic compressor Darmstadt Open Test Case – unsteady aerodynamics and stall inception

Predicting unsteady aerodynamics and related phenomena in the vicinity of the stability limit of transonic compressors has been a challenging topic within the turbomachinery community since decades. To improve numerical tools and prediction capabilities the Transonic Compressor Darmstadt Open Test Case has been introduced to provide a reliable validation test data set for steady and unsteady compressor aerodynamics. This work investigates the unsteady aerodynamics before the occurrence of rotating stall within the TU Darmstadt OpenStage compressor rotor and introduces unsteady wall pressure data to the Darmstadt Open Test Case. Exemplary pre-stall aerodynamics and stall inception are investigated for nominal (transonic) and part speed (subsonic) operating conditions, comprising the analysis of the unsteady static pressure field at the rotor tip as well as spectral analysis of the corresponding pressure signals during transient throttling maneuvers. To capture the rotor tip flow field, the rotor casing is instrumented with axially arranged flush mounted time-resolving wall pressure transducers. To determine propagation speeds of occurring phenomena, additional circumferentially distributed sensors are considered. Vibration monitoring shows no significant flutter or non-synchronous blade vibration, prior to the occurrence of rotating stall. Thus, this work focuses on aerodynamics, such as the interaction of secondary flow and shocks.

Complicated flow in tip flow field of a compressor tandem cascade using delayed detached eddy simulation

The complex flow characteristics in the tip region of a tandem cascade with tip clearance have been calculated and analyzed using Delayed Detached Eddy Simulation (DDES). The coherent mechanism of the vortex structures near the blade tip was discussed, and the unsteady behaviors and features in the tip flow field were analyzed. Additionally, the interaction between the tip leakage flow and the gap jet was revealed. The results show that, compared to the datum cascade, the blade tip load of the rear blade increases while that of the front blade decreases. Unsteady fluctuations of the tandem cascade are mainly caused by the interaction between the tip leakage flow and gap jet, and by the mixing of the vortex structures, but there is no essential change in the spectrum feature of the tip leakage flow. Finally, a detailed analysis of the development of vortices in the tip region is conducted by the topological structures of the flow field. Combined with the three-dimensional vortex structures, the schematic diagram of the vortex system of the datum single-row cascade and tandem cascade is summarized.

Study on the Tip Leakage Loss Mechanism of a Compressor Cascade Using the Enhanced Delay Detached Eddy Simulation Method.

The leakage flow has a significant impact on the aerodynamic losses and efficiency of the compressor. This paper investigates the loss mechanism in the tip region based on a high-load cantilevered stator cascade. Firstly, a high-fidelity flow field structure was obtained based on the Enhanced Delay Detached Eddy Simulation (EDDES) method. Subsequently, the Liutex method was employed to study the vortex structures in the tip region. The results indicate the presence of a tip leakage vortex (TLV), passage vortex (PV), and induced vortex (IV) in the tip region. At i=4°,8°, the induced vortex interacts with the PV and low-energy fluid, forming a "three-shape" mixed vortex. Finally, a qualitative and quantitative analysis of the loss sources in the tip flow field was conducted based on the entropy generation rate, and the impact of the incidence on the losses was explored. The loss sources in the tip flow field included endwall loss, blade profile loss, wake loss, and secondary flow loss. At i=0°, the loss primarily originated from the endwall and blade profile, accounting for 40% and 39%, respectively. As the incidence increased, the absolute value of losses increased, and the proportion of loss caused by secondary flow significantly increased. At i=8°, the proportion of secondary flow loss reached 47%, indicating the most significant impact.

Experiments on Tuned UHBR Open-Test-Case Fan ECL5/CATANA: Performance and Aerodynamics

Abstract With the advancement of modern fan architectures, the lack of experimental benchmarks for the research community became apparent in the recent years. Enormous effort in the method development could not be validated on representative geometries and motivated the development of the open-test-case ECL5/CATANA. A carbon fiber fan stage has been designed by Ecole Centrale de Lyon and shared with the community in 2021. The fan is representative of a modern ultra high bypass ratio (UHBR) architecture with sonic design speed. The reference configuration has been investigated experimentally on a novel test facility with multiphysical instrumentation. In this publication, the results of the first experimental campaign are presented with a detailed analysis of the aerodynamic performance and system symmetry. The presented results comprise full stage mappings across the whole operating range for three main speedlines (55%, 80%, and 100%) using performance rakes, unsteady wall pressure arrays, tip clearance, and stagger angle measurements. Radial profiles of intake boundary layer and rotor exit conditions at selected conditions complete the full validation dataset for steady and unsteady aerodynamic simulations. Results are discussed in comparison to blind-test Reynolds-averaged Navier–Stokes (RANS) simulations of different international institutes. Significant prediction inaccuracies of the tip flow and corner separations of the stator are observed and reveal the requirement of real blade geometry measurements at running conditions. This article is accompanied by a publication with a focus on aeroelastic instabilities observed near the stability limit. The results represent a comprehensive dimensional benchmark dataset and allow method validation on multiple levels of fidelity for the aerodynamic and aeroelastic research community.

A comprehensive CFD investigation of tip vortex trajectory in shrouded wind turbines using compressible RANS solver

It is well known that a shroud placed around a wind turbine can increase its power coefficient, but it brings complex mechanisms by which the shroud alters the flow passing through the rotor. Such mechanisms impose numerical challenges, as the shrouded turbines present nonlinear behavior in the wake. This paper deals with a comprehensive analysis of tip vortex trajectory in shrouded wind turbines using Reynolds Averaged Navier–Stokes numerical solutions. The analysis includes aerodynamic performance and vortex characteristics of the whole wind turbine. The Multiple Reference Frame is used on a high-order unstructured compressible solver to study both, isolated and shrouded rotor. The NREL Phase VI Unsteady Aerodynamic Experiment rotor is used as a test case. The accuracy of results for wind speeds between 7 and 25 ms−1 is discussed. Overall, good agreement is achieved between the computed pressure distributions and the experimental reference values. At stalled blade, more efforts are needed to improve numerical solutions, especially for integrated load quantities. The vortex structure is examined, showing that shroud impacts tip vortex trajectory by the increase of the axial induced velocity at the rotor plane. This result, demonstrates that the classical Prandtl tip loss is not accurate for shrouded turbine analysis, and modern finite blade functions are needed. The influence of the flow conditions on the tip vortex trajectory, flow separation and shroud interaction are also discussed.

Experimental investigation of secondary flow losses in a variable geometry linear turbine cascade with winglet tip

Variable geometry turbines (VGT) hold a crucial role in gas turbines and variable cycle engines (VCE) by allowing adjustments to different operational conditions. In order to realize the adjustable function of the VGT stator blade, there will be partial gap structure at both ends of the stator blade, which will lead to partial leakage flow and serious secondary flow loss at the blade end that cannot be ignored, especially in a large angle adjustment range. At present, there are few researches on turbine cascades considering the real partial gap structure and winglet tip, and the numerical simulation calculation is mainly used, while the in-depth study using wind tunnel test is rarely seen. Therefore, this study attempts to analyze the internal flow characteristics of variable geometry turbine cascade by using five-hole pneumatic probe scanning and surface oil flow visualization, and discusses the mechanism of controlling partial clearance leakage flow and reducing secondary flow loss in the cascade by using winglet tip flow control technology. The winglet offset distance and the change of the relationship between the pivot and the clearance position on the flow field and flow loss is studied experimentally for the first time. The results show that the influence mechanism of winglet tip applied in VGT partial gap is quite different from that of typical blade tip clearance application, and more consideration should be given to the influence of pivot structure. Reasonable parameter selection can achieve a maximum loss reduction of 13.4 % and 20.2 % with and without pivot, respectively, while the unreasonable flow control scheme may lead to the loss or even increase with pivot. The interaction between the new secondary flow structure induced by pivot in the gap and other gap leakage flows is analyzed to explore the new mechanism of using winglet tip to reduce the loss of gap leakage flow.