Passive control of laminar separation bubble with spanwise groove on a low-speed highly loaded low-pressure turbine blade

LES (Large-Eddy Simulation) computations were preformed to investigate the mechanisms of a kind of spanwise groove for the passive control of laminar separation bubble on the suction surface of a low-speed highly loaded low-pressure turbine blade at Re = 50,000 (Reynolds number, based on inlet velocity and axial chord length). Compared with the smooth suction surface, the numerical results indicate that: (1) the groove is effective to shorten and thin the separation bubble, which contributes the flow loss reduction on the groove surface, by thinning the boundary layer behind the groove and promoting earlier transition inception in the separation bubble; (2) upstream movement of the transition inception location on the grooved surface is suggested being the result of the lower frequency at which the highest amplification rate of instability waves occurs, and the larger initial amplitude of the disturbance at the most unstable frequency before transition; and (3) the viscous instability mode is promoted on the grooved surface, due to the thinning of the boundary layer behind the groove.     

Free-stream turbulence effects on the boundary layer of a high-lift low-pressure-turbine blade

The suction side boundary layer evolution of a high-lift low-pressure turbine cascade has been experimentally investigated at low and high free-stream turbulence intensity conditions.Measurements have been carried out in order to analyze the boundary layer transition and separation processes at a low Reynolds number,under both steady and unsteady inflows.Static pressure distributions along the blade surfaces as well as total pressure distributions in a downstream tangential plane have been measured to evaluate the overall aerodynamic efficiency of the blade for the different conditions.Particle Image Velocimetry has been adopted to analyze the time-mean and time-varying velocity fields.The flow field has been surveyed in two orthogonal planes (a blade-to-blade plane and a wall-parallel one).These measurements allow the identification of the Kelvin-Helmholtz large scale coherent structures shed as a consequence of the boundary layer laminar separation under steady inflow,as well as the investigation of the three-dimensional effects induced by the intermittent passage of low and high speed streaks.A close inspection of the time-mean velocity profiles as well as of the boundary layer integral parameters helps to characterize the suction side boundary layer state,thus justifying the influence of free-stream turbulence intensity on the blade aerodynamic losses measured under steady and unsteady inflows.     

Unsteady inflow effects on the wake shed from a high-lift LPT blade subjected to boundary layer laminar separation

An experimental investigation on the near and far wake of a cascade of high-lift low-pressure turbine blades subjected to boundary layer separation over the suction side surface has been carried out, under steady and unsteady inflows. Two Reynolds number conditions, representative of take-off/landing and cruise operating conditions of the real engine, have been tested. The effect of upstream wake-boundary layer interaction on the wake shed from the profile has been investigated in a three-blade large-scale linear turbine cascade. The comparison between the wakes shed under steady and unsteady inflows has been performed through the analysis of mean velocity and Reynolds stress components measured at midspan of the central blade by means of a two-component crossed miniature hot-wire probe. The wake development has been analyzed in the region between 2% and 100% of the blade chord from the central blade trailing edge, aligned with the blade exit direction. Wake integral parameters, half-width and maximum velocity defects have been evaluated from the mean velocity distributions to quantify the modifications induced on the vane wake by the upstream wake. Moreover the thicknesses of the two wake shear layers have been considered separately in order to identify the effects of Reynolds number and incoming flow on the wake shape. The self-preserving state of the wake has been looked at, taking into account the different thicknesses of the two shear layers. The evaluation of the power density spectra of the velocity fluctuations allowed the study of the wake unsteady behavior, and the detection of the effects induced by the different operating conditions on the trailing edge vortex shedding.     

Numerical study of improving aerodynamic performance of low solidity LPT cascade through increasing trailing edge thickness

This paper presents a new idea to reduce the solidity of low-pressure turbine (LPT) blade cascades, while remain the structural integrity of LPT blade. Aerodynamic performance of a low solidity LPT cascade was improved by increasing blade trailing edge thickness (TET). The solidity of the LPT cascade blade can be reduced by about 12.5% through increasing the TET of the blade without a significant drop in energy efficiency. For the low solidity LPT cascade, increasing the TET can decrease energy loss by 23.30% and increase the flow turning angle by 1.86% for Reynolds number (Re) of 25,000 and freestream turbulence intensities (FSTI) of 2.35%. The flow control mechanism governing behavior around the trailing edge of an LPT cascade is also presented. The results show that appropriate TET is important for the optimal design of high-lift load LPT blade cascades.     

雷诺数改变对S825翼型的影响

文章采用CFD软件对风力机翼型NREL S825在雷诺数分别为1×10^6、2×10^6、3×10^6的情况进行气动性能的数值研究。通过对比数值计算结果与实验数据,确认计算结果的可靠性。在此基础上,详细地分析边界层参数,包括边界层内速度分布、边界层位移厚度和动量损失厚度。结果表明：相同攻角下,随着雷诺数的增加,边界层厚度变薄,分离点后移．进而使升力系数增加、阻力系数减小。     

Turbine blade boundary layer separation suppression via synthetic jet: An experimental and numerical study

The present paper focuses on the analysis of a synthetic jet device (with a zero net massflow rate) on a separated boundary layer. Separation has been obtained on a flat plate installed within a converging-diverging test section specifically designed to attain a local velocity distribution typical of a high-lift LPT blade. Both experimental and numerical investigations have been carried out. Unsteady RANS results have been compared with experiments in terms of time-averaged velocity and turbulence intensity distributions. Two different Reynolds number cases have been investigated, namely Re = 200,000 and Re = 70,000, which characterize low-pressure turbine operating conditions during take-off/landing and cruise. A range of synthetic jet aerodynamic parameters (Strouhal number and blowing ratio) has been tested in order to analyze the features of control-separated boundary layer interaction for the aforementioned Reynolds numbers.     

Numerical simulation of separated flow transition and heat transfer around a two‐dimensional rib

Numerical simulations of separated flow transition and heat transfer around a two‐dimensional rib mounted in a laminar boundary layer were performed. The separated shear layer becomes unstable due to the Kelvin–Helmholtz instability and generates a two‐dimensional vortex. This vortex becomes three‐dimensional and collapses in the downstream part of the separation bubble. As a result, transition from laminar to turbulent flow occurs in the separated shear layer. Streamwise vortices exist downstream of the reattachment flow region. The low‐frequency flapping motion and transition of the separated shear layer are influenced by three‐dimensional dynamics upstream of the separation bubble. Large‐scale vortices around the reattachment flow region have substantial effects on heat transfer. Downstream of the reattachment point, the surface friction coefficient and Nusselt number are different from their profiles in the laminar boundary layer and approach the distributions seen in the turbulent boundary layer. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(8): 513–528, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20177     

Experimental investigation of separation and transition processes on a high-lift low-pressure turbine profile under steady and unsteady inflow at low Reynolds number

The effects induced by the presence of incoming wakes on the boundary layer developing over a high-lift low-pressure turbine profile have been investigated in a linear cascade at mid-span. The tested Reynolds number is 70000, typical of the cruise operating condition. The results of the investigations performed under steady and unsteady inflow conditions are analyzed. The unsteady investigations have been performed at the reduced fre- quency off=0.62, representative of the real engine operating condition. Profile aerodynamic loadings as well as boundary layer velocity profiles have been measured to survey the separation and transition processes. Spectral analysis has been also performed to better understand the phenomena associated with the transition process under steady inflow. For the unsteady case, a phase-locked ensemble averaging technique has been employed to reconstruct the time-resolved boundary layer velocity distributions from the hot-wire instantaneous signal output. The ensemble-averaging technique allowed a detailed analysis of the effects induced by incoming wakesboundary layer interaction in separation suppression. Time-resolved results are presented in terms of mean velocity and unresolved unsteadiness time-space plots.     

Reynolds number dependence of turbulence statistics in the wake of wind turbines

A wind tunnel experiment has been performed to quantify the Reynolds number dependence of turbulence statistics in the wake of a model wind turbine. A wind turbine was placed in a boundary layer flow developed over a smooth surface under thermally neutral conditions. Experiments considered Reynolds numbers on the basis of the turbine rotor diameter and the velocity at hub height, ranging from Re = 1.66 × 10⁴ to 1.73 × 10⁵. Results suggest that main flow statistics (mean velocity, turbulence intensity, kinematic shear stress and velocity skewness) become independent of Reynolds number starting from Re ≈ 9.3 × 10⁴. In general, stronger Reynolds number dependence was observed in the near wake region where the flow is strongly affected by the aerodynamics of the wind turbine blades. In contrast, in the far wake region, where the boundary layer flow starts to modulate the dynamics of the wake, main statistics showed weak Reynolds dependence. These results will allow us to extrapolate wind tunnel and computational fluid dynamic simulations, which often are conducted at lower Reynolds numbers, to full‐scale conditions. In particular, these findings motivates us to improve existing parameterizations for wind turbine wakes (e.g. velocity deficit, wake expansion, turbulence intensity) under neutral conditions and the predictive capabilities of atmospheric large eddy simulation models. Copyright © 2011 John Wiley & Sons, Ltd.     

Computational Investigation of Blade slotting on a High-Load Low-Pressure Turbine Profile at various Reynolds Numbers：Part Ⅰ——Slotting Scheme＇s Verification

Boundary layer separation and reattachment is often an unavoidable feature of low pressure (LP) turbine,one of the main causes of this phenomenon is the high altitude low Reynolds number experienced by the modern LP turbine stage in aero-engine.Although an excellent turbine airfoil design can avoid flow separation on certain extent,but within flight envelope,LP turbine’s characteristic Reynolds number may varied greatly,so it will be still under the risk of the presence of separation bubble.In this two parts paper a new concept of slotted-blade was raised to testify the gain of the blade slotting.A high aerodynamic loading LP turbine blade IET-LPTA was under investigated with different Reynolds number.Computational results reveal that the blade slotting could be a way of choice to suppress separation bubble and reduce profile loss under the condition of low Reynolds number,although its position and geometry need to be further investigated.     

Numerical study of laminar-turbulent transition on a plate in a low-speed tunnel with contoured wall

INTRODUCTIONItiswellknownthattheboundarylayerphenomena,especiallyitslaminar-turbulenttransitionintheinternalandexternalflows,areveryimportanttothetheoreticalunderstandingandalsototheengineeringapplications,duetothefactthattrsnsitioncontfolstheevoluhonofimportantaerodynamicquantitiessuchasdragorheattransfer.Forexample,separationandstallonlowReynolds-numberairfoilsandturbinebladesstronglydependsonwhethertheboundarylayerislaminar,tfallsitional,ortllrbulent(Saric,1993);andtheheatingratesgener…     

Theoretical and Experimental Investigations of Laminar—Turbulent Transition in the Non—Isothermal Boundary Layer

Theoretical investigation of LaminarBoundary Layer StabilityThere is well known Schlichting's theoreticalsolution of the laminar boundary layer stability atsimple flow conditions: isothermal boundary layer,very small external opulence intensity and verysmall rouglmess of streamlined scarce. Schlichtingsolved the Orra-Sommerfield's equation I I ]:where u, u M - velocity and its second derivativein boundary layer aZu/fy'; Re-Reynolds number ofmain flow; a-wave number; 9, 9", MamplitUdeof s…     

Turbulence characteristics and vortical structures in combined-convection boundary layers along a heated vertical flat plate

Time-developing direct numerical simulations are performed for the combined-convection boundary layers created by imposing aiding and opposing freestreams to the pure natural-convection boundary layer in air along a heated vertical flat plate to clarify their structural characteristics. The numerical results reveal that with a slight increase in freestream velocity, the transition region moves downstream for aiding flow and upstream for opposing flow. This fact correlates well with the existing experimental results showing that the transition delays for aiding flow and quickens for opposing flow in the practical space-developing boundary layer. Thereby, for aiding flow, turbulence characteristics indicate the behavior proceeding to the laminarization of the boundary layer. On the other hand, for opposing flow, the large scale fluid motions are apparent and become larger than those for the pure natural-convection boundary layer with increasing freestream velocity. For the occurrence of such fluid motions, the budgets of turbulent energy and two-point spatial correlations in the turbulent combined-convection boundary layers are also examined. Consequently, it is found in the spatial correlations that the turbulence structures are mainly controlled by fluid motions in the outer region of the boundary layer.     

Computational Investigation of Blade Slotting on a High-Load Low-Pressure Turbine Profile at Various Reynolds Numbers：Part Ⅱ——Optimization of Slotting Position

Boundary layer separation and reattachment is often an unavoidable feature of low pressure (LP) turbine, one of the main causes of this phenomenon is the high altitude low Reynolds number experienced by the modern LP turbine stage in aero-engine. Although an excellent turbine airfoil design can avoid flow separation on certain extent, but within flight envelope, LP turbine’s characteristic Reynolds number may varied greatly, so it will be still under the risk of the presence of separation bubble. In this two parts paper a new concept of slotted-blade was raised to testify the gain of the blade slotting. A high aerodynamic loading LP turbine blade IET-LPTA was under investigated with different Reynolds number. Computational results reveal that the blade slotting could be a way of choice to suppress separation bubble and reduce profile loss under the condition of low Reynolds number, although its position and geometry need to be further investigated.     

An Experimental Study on 3—D Flow in an Annular Cascade of High Turning Angle Turbine Blades

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A numerical study of the effect of turbulence on mass transfer from a single fuel droplet evaporating in a hot convective flow

A three-dimensional numerical model is developed to investigate the effect of turbulence on mass transfer from a single droplet exposed to a freestream of air. The freestream temperature, turbulence intensity and Reynolds number are varied to provide a wide range of test conditions, whereas the ambient pressure is kept atmospheric. The turbulence terms in the conservation equations of the gas-phase are modelled by using the shear-stress transport (SST) model. A Cartesian grid based blocked-off technique is used in conjunction with the finite-volume method to solve numerically the governing equations of the gas and liquid-phases. This study showed that the vaporization Damköhler number proposed in the literature to correlate the effect of turbulence on the droplet's vaporization rate is invalid at air temperatures higher than room temperature. Additionally, an attempt is made to correlate the effect of the freestream turbulence on the droplet's mass transfer rate by using Sherwood number over a wide range of freestream temperatures.     

Experimental Investigation of Flow Instabilities in a Laminar Separation Bubble

The present paper reports the results of a detailed experimental study aimed at investigating the dynamics of a laminar separation bubble, from the origin of separation up to the breakdown to turbulence of the large scale co- herent structures generated as a consequence of the Kelvin-Helmholtz instability process. Measurements have been performed along a fiat plate installed within a double contoured test section, designed to produce an adverse pressure gradient typical of Ultra-High-Lift turbine blade profiles, which induces the formation of a laminar separation bubble at low Reynolds number condition. Measurements have been carried out by means of comple- mentary techniques： hot-wire （HW） anemometry, Laser Doppler Velocirnetry （LDV） and Particle Image Veloci- metry （PIV）. The high accuracy 2-dimensional LDV results allow investigating reverse flow magnitude and both Reynolds normal and shear stress distributions along the separated flow region, while the high frequency response of the HW anemometer allows analyzing the amplification process of flow oscillations induced by instability mechanisms. PIV results complement the flow field analysis providing information on the generation and evolu- tion of the large scale coherent structures shed as a consequence of the separated shear layer roll-up, through in- stantaneous velocity vector maps. The simultaneous analysis of the data obtained by means of the different meas- uring techniques allows an in depth view of the instability mechanisms involved in the transition/reattachrnent processes of the separated shear layer.     

Boundary layer transition modeling on leading edge inflatable kite airfoils

We present a computational fluid dynamic analysis of boundary layer transition on leading edge inflatable kite airfoils used for airborne wind energy generation. Because of the operation in pumping cycles, the airfoil is generally subject to a wide range of Reynolds numbers. The analysis is based on the combination of the shear stress transport turbulence model with the transition model, which can handle the laminar boundary layer and its transition to turbulence. The implementation of both models in OpenFOAM is described. We show a validation of the method for a sailwing (ie, a wing with a membrane) airfoil and an application to a leading edge inflatable kite airfoil. For the sailwing airfoil, the results computed with transition model agree well with the existing low Reynolds number experiment over the whole range of angles of attack. For the leading edge inflatable kite airfoil, the transition modeling has both favorable and unfavorable effects on the aerodynamics. On the one hand, the aerodynamics suffer from the laminar separation. But, on the other hand, the laminar boundary layer thickens slower than the turbulent counterpart, which, in combination with transition, delays the separation. The results also indicate that the aerodynamics of the kite airfoil could be improved by delaying the boundary layer transition during the traction phase and tripping the transition in the retraction phase.     

Laminar-to-turbulent transition in supersonic boundary layer: Effects of initial perturbation and wall heat transfer

Direct numerical simulations of supersonic boundary layers (SBLs) over a flat plate for M_∞=2.2 are performed for adiabatic and isothermal (cooled and heated) walls. Receptivity analysis based on five criteria, namely skin-friction coefficient, Stanton number, Reynolds shear stress, wall-normal Reynolds heat flux, and modal decomposition are performed. Effect of perturbation intensity and wall heat transfer on the receptivity and the transitional growth of SBLs are presented. It is found out that increasing perturbation intensity moves the transition onset location upstream and increases the transition length. Additionally, below 1% perturbation intensity, wall cooling stabilizes the flow while beyond this value it has the opposite effect.     

Transition prediction for a two‐dimensional reynolds‐averaged navier–stokes method applied to wind turbine airfoils

Boundary layer transition is significant to many flow fields that include both laminar and turbulent regions. Accurate prediction of transition onset is fundamental to the modelling of these flows. In most flow solvers based on the Reynolds‐averaged Navier–Stokes equations, transition onset must be specified manually. To overcome this weakness and to more accurately predict aerodynamic flow fields, a boundary layer transition prediction methodology is presented. This methodology, which has been applied to a Navier–Stokes solver, dynamically locates transition onset as the flow solution is converging. The prediction methodology identifies several boundary layer transition mechanisms, including Tollmien–Schlichting instability, laminar separation and turbulence contamination. Where possible, the implementation utilizes the calculated boundary layer velocity profiles to strongly couple the predicted transition locations and the flow solution. The transition prediction methodology was used to predicted transition onset for the NLF(1)‐0416 and S809 single‐element wind turbine airfoils. Results obtained with numerical calculations are found to agree well with experimental observations. Copyright © 2001 John Wiley & Sons, Ltd.