Generation and Development of Klebanoff Streaks in Low-Pressure Turbine Cascade Under Upstream Wakes

2020 ◽  
Author(s):  
Shuang Sun ◽  
Xingshuang Wu ◽  
Tianrong Tan ◽  
Canlin Zuo ◽  
Sirui Pan ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Separation Bubble ◽  
Transition Process ◽  
Low Pressure ◽  
Suction Surface ◽  
High Lift ◽  
Low Pressure Turbine ◽  
Wall Shear

Abstract At low Reynolds numbers operating condition, the boundary layer of the high-lift low-pressure turbine (LPT) of aero-engines is prone to separate on the suction surface of the airfoil. The profile losses of the airfoil are largely governed by the size of the separation bubble and the transition process in the boundary layer. However, the wake-induced transition, the natural transition and the instability induced by the Klebanoff streaks complicate the transition process. The boundary layer on the suction surface of a high-lift LPT was investigated at Re = 50,000 with upstream wakes. The numerical simulation was performed with the CFX software using large eddy simulations (LES), and the experiment was performed on a linear cascade. In this study, the wake is divided into the wake center and the wake tail, the unsteady formation process of the streaks and the wall shear stress caused by the wake are analyzed. A new mechanism of generation and development of Klebanoff Streaks was presented to better understand the effect of the wake on the boundary layer. Moreover, it was found that after entering the blade passage, the wake center does not contact the blade but causes the wall shear stress of the front part on the suction surface to increase. However, it is not possible to form strong Klebanoff streaks at the leading edge of the blade by shear sheltering effect. Only the wake tail can form Klebanoff streaks when it contacts the blade.

Effects of periodic wakes on boundary layer development on an ultra-high-lift low pressure turbine airfoil

2016 ◽  
Vol 231 (1) ◽  
pp. 25-38 ◽  
Author(s):  
Xingen Lu ◽  
Yanfeng Zhang ◽  
Wei Li ◽  
Shuzhen Hu ◽  
Junqiang Zhu
Keyword(s):  
Boundary Layer ◽  
Transition Process ◽  
Low Pressure ◽  
Suction Surface ◽  
High Lift ◽  
Low Pressure Turbine ◽  
Turbulent Transition ◽  
Boundary Layer Development ◽  
Unsteady Wake ◽  
Layer Development

The laminar-turbulent transition process in the boundary layer is of significant practical interest because the behavior of this boundary layer largely determines the overall efficiency of a low pressure turbine. This article presents complementary experimental and computational studies of the boundary layer development on an ultra-high-lift low pressure turbine airfoil under periodically unsteady incoming flow conditions. Particular emphasis is placed on the influence of the periodic wake on the laminar-turbulent transition process on the blade suction surface. The measurements were distinctive in that a closely spaced array of hot-film sensors allowed a very detailed examination of the suction surface boundary layer behavior. Measurements were made in a low-speed linear cascade facility at a freestream turbulence intensity level of 1.5%, a reduced frequency of 1.28, a flow coefficient of 0.70, and Reynolds numbers of 50,000 and 100,000, based on the cascade inlet velocity and the airfoil axial chord length. Experimental data were supplemented with numerical predictions from a commercially available Computational Fluid Dynamics code. The wake had a significant influence on the boundary layer of the ultra-high-lift low pressure turbine blade. Both the wake’s high turbulence and the negative jet behavior of the wake dominated the interaction between the unsteady wake and the separated boundary layer on the suction surface of the ultra-high-lift low pressure turbine airfoil. The upstream unsteady wake segments convecting through the blade passage behaved as a negative jet, with the highest turbulence occurring above the suction surface around the wake center. Transition of the unsteady boundary layer on the blade suction surface was initiated by the wake turbulence. The incoming wakes promoted transition onset upstream, which led to a periodic suppression of the separation bubble. The loss reduction was a compromise between the positive effect of the separation reduction and the negative effect of the larger turbulent-wetted area after reattachment due to the earlier boundary layer transition caused by the unsteady wakes. It appeared that the successful application of ultra-high-lift low pressure turbine blades required additional loss reduction mechanisms other than “simple” wake-blade interaction.

scholarly journals Detailed Studies on Separated Boundary Layers Over Low-Pressure Turbine Airfoils Under Several High Lift Conditions: Effect of Freesteam Turbulence

2009 ◽  
Author(s):  
Ken-ichi Funazaki ◽  
Kazutoyo Yamada ◽  
Nozomi Tanaka ◽  
Yasuhiro Chiba
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Separation Bubble ◽  
Low Pressure ◽  
Scale Model ◽  
Freestream Turbulence ◽  
Suction Surface ◽  
High Lift ◽  
Low Pressure Turbine ◽  
Turbine Airfoils

This paper deals with experimental investigation on the interaction between inlet freestream turbulence and boundary layers with separation bubble on a low-pressure turbine airfoil under several High Lift conditions. Solidity of the cascade can be reduced by increasing the airfoil pitch by 25%, while maintaining the throat in the blade-to-blade passage. Reynolds number examined is 57000, based on chord length and averaged exit velocity. Freestream turbulence intensity at the inlet is varied from 0.80% (no grid condition) to 2.1% by use of turbulence grid. Hot-wire probe measurements of the boundary layer on the suction surface for Low Pressure (LP) turbines rotor are carried out to obtain time-averaged and time-resolved characteristics of the boundary layers under the influence of the freestream turbulence. Frequency analysis extracts some important features of the unsteady behaviors of the boundary layer, including vortex formation and shedding. Numerical analysis based on high resolution Large Eddy Simulation is also executed to enhance the understanding on the flow field around the highly loaded turbine airfoils. Standard Smagorinsky model is employed as subgrid scale model. Emphasis of the simulation is placed on the relationship of inherent instability of the shear layer of the separation bubble and the freestream turbulence.

Separated Flow Measurements on a Highly Loaded Low-Pressure Turbine Airfoil

2008 ◽  
Author(s):  
Ralph J. Volino
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Separation Bubble ◽  
Boundary Layer Separation ◽  
Low Pressure ◽  
Transition To Turbulence ◽  
Suction Surface ◽  
Airfoil Surface ◽  
Flow Measurements ◽  
Low Pressure Turbine

Boundary layer separation, transition and reattachment have been studied on a new, very high lift, low-pressure turbine airfoil. Experiments were done under low freestream turbulence conditions on a linear cascade in a low speed wind tunnel. Pressure surveys on the airfoil surface and downstream total pressure loss surveys were documented. Velocity profiles were acquired in the suction side boundary layer at several streamwise locations using hot-wire anemometry. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) ranging from 25,000 to 330,000. In all cases the boundary layer separated, but at high Reynolds number the separation bubble remained very thin and quickly reattached after transition to turbulence. In the low Reynolds number cases, the boundary layer separated and did not reattach, even when transition occurred. This behavior contrasts with previous research on other airfoils, in which transition, if it occurred, always induced reattachment, regardless of Reynolds number.

Loss reduction on ultra high lift low-pressure turbine blades using selective roughness and wake unsteadiness

2007 ◽  
Vol 111 (1118) ◽  
pp. 257-266 ◽  
Author(s):  
R. J. Howell ◽  
K. M. Roman
Keyword(s):  
Boundary Layer ◽  
Steady Flow ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Low Pressure ◽  
Surface Boundary ◽  
Suction Surface ◽  
High Lift ◽  
Profile Losses ◽  
Lp Turbine

This paper describes how it is possible to reduce the profile losses on ultra high lift low pressure (LP) turbine blade profiles with the application of selected surface roughness and wake unsteadiness. Over the past several years, an understanding of wake interactions with the suction surface boundary layer on LP turbines has allowed the design of blades with ever increasing levels of lift. Under steady flow conditions, ultra high lift profiles would have large (and possibly open) separation bubbles present on the suction side which result from the very high diffusion levels. The separation bubble losses produced by it are reduced when unsteady wake flows are present. However, LP turbine blades have now reached a level of loading and diffusion where profile losses can no longer be controlled by wake unsteadiness alone. The ultra high lift profiles investigated here were created by attaching a flap to the trailing edge of another blade in a linear cascade — the so called flap-test technique. The experimental set-up used in this investigation allows for the simulation of upstream wakes by using a moving bar system. Hotwire and hotfilm measurements were used to obtain information about the boundary-layer state on the suction surface of the blade as it evolved in time. Measurements were taken at a Reynolds numbers ranging between 100,000 and 210,000. Two types of ultra high lift profile were investigated; ultra high lift and extended ultra high lift, where the latter has 25% greater back surface diffusion as well as a 12% increase in lift compared to the former. Results revealed that distributed roughness reduced the size of the separation bubble with steady flow. When wakes were present, the distributed roughness amplified disturbances in the boundary layer allowing for more rapid wake induced transition to take place, which tended to eliminate the separation bubble under the wake. The extended ultra high lift profile generated only slightly higher losses than the original ultra high lift profile, but more importantly it generated 12% greater lift.

Effect of Unsteady Wakes on Boundary Layer Separation on a Very High Lift Low Pressure Turbine Airfoil

2010 ◽  
Author(s):  
Ralph J. Volino
Keyword(s):  
Boundary Layer ◽  
Boundary Layer Separation ◽  
Low Pressure ◽  
Freestream Turbulence ◽  
Suction Surface ◽  
High Lift ◽  
Layer Separation ◽  
Low Pressure Turbine ◽  
Wake Passing ◽  
Very High

Boundary layer separation has been studied on a very high lift, low-pressure turbine airfoil in the presence of unsteady wakes. Experiments were done under low (0.6%) and high (4%) freestream turbulence conditions on a linear cascade in a low speed wind tunnel. Wakes were produced from moving rods upstream of the cascade. Flow coefficients were varied from 0.35 to 1.4 and wake spacing was varied from 1 to 2 blade spacings, resulting in dimensionless wake passing frequencies F = fLj-te/Uave (f is the frequency, Lj-te is the length of the adverse pressure gradient region on the suction surface of the airfoils, and Uave is the average freestream velocity) ranging from 0.14 to 0.56. Pressure surveys on the airfoil surface and downstream total pressure loss surveys were documented. Instantaneous velocity profile measurements were acquired in the suction surface boundary layer and downstream of the cascade. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) of 25,000 and 50,000. In cases without wakes, the boundary layer separated and did not reattach. With wakes, separation was largely suppressed, particularly if the wake passing frequency was sufficiently high. At lower frequencies the boundary layer separated between wakes. Background freestream turbulence had some effect on separation, but its role was secondary to the wake effect.

Effects of Surface Groove on Separated Flow Transition on a High-Lift Low-Pressure Turbine Profile Under Steady Inflow Conditions

2009 ◽  
Author(s):  
Hualing Luo ◽  
Weiyang Qiao ◽  
Kaifu Xu
Keyword(s):  
Boundary Layer ◽  
Separated Flow ◽  
Low Pressure ◽  
Separation Point ◽  
Flow Transition ◽  
Suction Surface ◽  
High Lift ◽  
Low Pressure Turbine ◽  
Surface Groove ◽  
Inflow Conditions

LES (Large-Eddy Simulation) computations for a high-lift low-pressure turbine profile equipped with the span-wise groove on the suction surface are done to investigate the mechanism of the surface groove for separated flow transition control under steady inflow conditions, employing the dynamic Smagorinsky model. In addition to the baseline case (no groove), three groove positions which depend on the relative position of the groove trailing edge and the separation point on the suction surface are considered at two Reynolds numbers (Re, based on the inlet velocity and axial chord length). The results show that all grooves can reduce the calculated loss for Re = 50000, due to the further upstream transition inception in the separated shear layer. The analyses indicate two kinds of control mechanism such as the thinning of boundary layer behind the groove and the introduction of disturbances within the groove, depending on the groove position and Reynolds number. At Re = 50000, for the groove located upstream of the separation point, the reason for the further upstream transition inception location is the thinning of boundary layer behind the groove, and for the groove located downstream of the separation point, the reason is the introduction of disturbances within the groove. At Re = 100000, disturbances can also be generated within the groove located upstream of the separation point, promoting earlier transition inception.

Separation Control on a Very High Lift Low Pressure Turbine Airfoil Using Pulsed Vortex Generator Jets

2011 ◽  
Vol 133 (4) ◽  
Author(s):  
Ralph J. Volino ◽  
Olga Kartuzova ◽  
Mounir B. Ibrahim
Keyword(s):  
Boundary Layer ◽  
Total Pressure ◽  
Vortex Generator ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Separation Control ◽  
Suction Surface ◽  
High Lift ◽  
Low Pressure Turbine ◽  
Very High

Boundary layer separation control has been studied using vortex generator jets (VGJs) on a very high lift, low-pressure turbine airfoil. Experiments were done under high (4%) freestream turbulence conditions on a linear cascade in a low speed wind tunnel. Pressure surveys on the airfoil surface and downstream total pressure loss surveys were documented. Instantaneous velocity profile measurements were acquired in the suction surface boundary layer. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) of 25,000 and 50,000. Jet pulsing frequency, duty cycle, and blowing ratio were all varied. Computational results from a large eddy simulation of one case showed reattachment in agreement with the experiment. In cases without flow control, the boundary layer separated and did not reattach. With the VGJs, separation control was possible even at the lowest Reynolds number. Pulsed VGJs were more effective than steady jets. At sufficiently high pulsing frequencies, separation control was possible even with low jet velocities and low duty cycles. At lower frequencies, higher jet velocity was required, particularly at low Reynolds numbers. Effective separation control resulted in an increase in lift and a reduction in total pressure losses. Phase averaged velocity profiles and wavelet spectra of the velocity show the VGJ disturbance causes the boundary layer to reattach, but that it can reseparate between disturbances. When the disturbances occur at high enough frequency, the time available for separation is reduced, and the separation bubble remains closed at all times.

The Effect of FSTI to the Combined Separation Control Strategy of Surface Roughness With Upstream Wakes

2016 ◽  
Author(s):  
Sun Shuang ◽  
Lei Zhi-jun ◽  
Lu Xin-gen ◽  
Zhang Yan-feng ◽  
Zhu Jun-qiang
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Reynolds Number ◽  
Transition Region ◽  
Boundary Layer Separation ◽  
Low Pressure ◽  
Suction Surface ◽  
High Lift ◽  
Number Range ◽  
Low Pressure Turbine

Boundary layer separation can lead to partial loss of lift and higher aerodynamic losses on low-pressure turbine airfoils at low Reynolds number in high bypass ratio engines. The combined effects of upstream wakes and surface roughness on boundary layer development have been investigated experimentally to improve the performance of ultra-high-lift low-pressure turbine (LPT) blades. The measurement was performed on a linear cascade with an ultra-high-lift aft-loaded LP turbine profile named IET-LPTA with Zweifel loading coefficient of about 1.37. The wakes were simulated by the moving cylindrical bars upstream of the cascade. The time-mean aerodynamic performance and the boundary layer behavior on suction surface had been measured with two 3-hole probes and a hot-wire probe. Three roughness heights ranging from 8.8–20.9μm combined with three roughness deposit positions ranging from 5.2%–39.5% suction surface length formed a large measurement matrix. The roughness with height of 8.8μm (1.05×10−4 chord length) covering 5.2% suction surface reduced the profile loss across the whole Reynolds number range. Under the effect of roughness associated with upstream wakes, the freestream turbulence intensity (FSTI) is responsible in part for the development of the wake-induced transition region, calmed region and natural transition region of the boundary layer. The transition length and the transition onset of the boundary layer were also affected by the FSTI.

Experimental and Computational Investigations of Separation and Transition on a Highly Loaded Low-Pressure Turbine Airfoil: Part 2 — High Freestream Turbulence Intensity

2008 ◽  
Author(s):  
Ralph J. Volino ◽  
Olga Kartuzova ◽  
Mounir B. Ibrahim
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Shear Layer ◽  
Separation Bubble ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Transition Model ◽  
Freestream Turbulence ◽  
Low Pressure Turbine ◽  
Separated Shear Layer

Boundary layer separation, transition and reattachment have been studied on a very high lift, low-pressure turbine airfoil. Experiments were done under high (4%) freestream turbulence conditions on a linear cascade in a low speed wind tunnel. Pressure surveys on the airfoil surface and downstream total pressure loss surveys were documented. Velocity profiles were acquired in the suction side boundary layer at several streamwise locations using hot-wire anemometry. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) ranging from 25,000 to 300,000. At the lowest Reynolds number the boundary layer separated and did not reattach, in spite of transition in the separated shear layer. At higher Reynolds numbers the boundary layer did reattach, and the separation bubble became smaller as Re increased. High freestream turbulence increased the thickness of the separated shear layer, resulting in a thinner separation bubble. This effect resulted in reattachment at intermediate Reynolds numbers, which was not observed at the same Re under low freestream turbulence conditions. Numerical simulations were performed using an unsteady Reynolds averaged Navier-Stokes (URANS) code with both a shear stress transport k-ω model and a 4 equation shear stress transport Transition model. Both models correctly predicted separation and reattachment (if it occurred) at all Reynolds numbers. The Transition model generally provided better quantitative results, correctly predicting velocities, pressure, and separation and transition locations. The model also correctly predicted the difference between high and low freestream turbulence cases.

Forcing of Separation Bubbles by Main Flow Unsteadiness or Pulsed Vortex Generating Jets—A Comparison

2013 ◽  
Vol 136 (5) ◽  
Author(s):  
Christoph Lyko ◽  
Jerrit Dähnert ◽  
Dieter Peitsch
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Separation Bubble ◽  
Active Flow Control ◽  
Transition Process ◽  
Low Pressure ◽  
Main Flow ◽  
Suction Surface ◽  
Forcing Mechanisms ◽  
The One

Low pressure turbines typically operate in the low Reynolds number regime. Depending on the loading of the blade, they may exhibit detached flow with associated reattachment in the rear part of the suction surface. Additionally, the flow is highly time-dependent due to the sequence of rotating and stationary blade rows. The work presented in this paper covers experimental efforts taken to investigate this type of flow in detail. Typical low pressure turbine flow conditions have been chosen as baseline for the experimental work. A pressure distribution has been created on a flat plate by means of a contoured upper wall in a low speed wind tunnel. The distribution matches the one of the Pak-B airfoil. Unsteadiness is then superimposed in two ways: A specific unsteadiness was created by using a rotating flap (RF) downstream of the test section. This results in almost sinusoidal periodic unsteady flow across the plate, simulating the interaction between stator and rotor of a turbine stage. Furthermore, pulsed blowing by vortex generating jets (VGJ) upstream of the suction peak was used to influence the transition process and development of the separation bubble. Measurements have been performed with hot-wire anemometry. Experimental results are presented to compare both forcing mechanisms. In sinusoidal unsteady main flow, the transition occurs naturally by the breakdown of the shear layer instability, which is affected by periodic changes in the overall Reynolds number and thus pressure gradient. In opposition, active flow control (AFC) by VGJ triggers the transition process by impulse and vorticity injection into the boundary layer, while maintaining a constant Reynolds number. The flow fields are compared using phase averaged data of velocity und turbulence intensity as well as boundary layer parameters, namely shape factor and momentum thickness Reynolds number. Finally, a model to describe the time mean intermittency distribution is refined to fit the data.

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