Numerical Investigation of the Wake-Boundary Layer Interaction on a Highly Loaded LP Turbine Cascade Blade

2002 ◽  
Author(s):  
Pasquale Cardamone ◽  
Peter Stadtmu¨ller ◽  
Leonhard Fottner
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Transition Process ◽  
Reynolds Numbers ◽  
Numerical Approach ◽  
Experimental Investigations ◽  
Turbine Cascade ◽  
Suction Surface ◽  
Time Resolved ◽  
Highly Loaded

The effects of wake passing on the development of the profile boundary layer of a highly loaded low-pressure turbine cascade are studied using the RANS code TRACE-U. The numerical results are compared with available experimental data to verify the accuracy of the code in predicting the periodic-unsteady transition and separation mechanisms at low Reynolds number conditions. The experimental investigations have been carried out on a turbine cascade called T106D-EIZ subjected to wakes generated by an up-stream moving bar-type generator. The cascade pitch was increased by about 30% with respect to design conditions without modifying the blade geometry in order to obtain a large separation bubble on the suction surface. The extensive database containing time-averaged as well as time-resolved results was presented in a separate paper by Stadtmu¨ller and Fottner (2001) and is discussed only briefly. The time-accurate multistage Navier-Stokes solver TRACE-U developed by the DLR Cologne used for the numerical simulations employs a modified version of the one-equation Spalart-Allmaras turbulence model coupled with a transition correlation based on the work of Abu-Ghannam and Shaw in the formulation of Drela. The objective of this paper is to provide further insight into the aerodynamics of the wake-induced transition process and to rate the application limits of the numerical approach for exit Reynolds numbers as low as 60.000. The CFD predictions for two different flow conditions are compared with the measurements. Plots of wall-shear stress, blade loading, shape factor and loss behaviour are used to verify the reliability of the code. The periodic-unsteady development of the boundary layer as well as the loss behaviour is well reproduced for higher Reynolds numbers. For the case with massive separation, large discrepancies between numerical and experimental results are observed.

Actuated Transition in an LP Turbine Laminar Separation: An Experimental Approach

2011 ◽  
Author(s):  
Jenny Baumann ◽  
Ulrich Rist ◽  
Martin Rose ◽  
Tobias Ries ◽  
Stephan Staudacher
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Reynolds Numbers ◽  
Experimental Investigations ◽  
Stream Velocity ◽  
Laminar Separation ◽  
Low Reynolds Numbers ◽  
Low Reynolds ◽  
Lp Turbine

The reduction of blade counts in the LP turbine is one possibility to cut down weight and therewith costs. At low Reynolds numbers the suction side laminar boundary layer of high lift LP turbine blades tends to separate and hence cause losses in turbine performance. To limit these losses, the control of laminar separation bubbles has been the subject of many studies in recent years. A project is underway at the University of Stuttgart that aims to suppress laminar separation at low Reynolds numbers (60,000) by means of actuated transition. In an experiment a separating flow is influenced by disturbances, small in amplitude and of a certain frequency, which are introduced upstream of the separation point. Small existing disturbances are therewith amplified, leading to earlier transition and a more stable boundary layer. The separation bubble thus gets smaller without need of a high air mass flow as for steady blowing or pulsed vortex generating jets. Frequency and amplitude are the parameters of actuation. The non-dimensional actuation frequency is varied from 0.2 to 0.5, whereas the normalized amplitude is altered between 5, 10 and 25% of the free stream velocity. Experimental investigations are made by means of PIV and hot wire measurements. Disturbed flow fields will be compared to an undisturbed one. The effectiveness of the presented boundary layer control will be compared to those of conventional ones. Phase-logged data will give an impression of the physical processes in the actuated flow.

Unsteady Transition in an Axial-Flow Turbine: Part 1—Measurements on the Turbine Rotor

1990 ◽  
Vol 112 (2) ◽  
pp. 206-214 ◽  
Author(s):  
J. S. Addison ◽  
H. P. Hodson
Keyword(s):  
Boundary Layer ◽  
Cross Sections ◽  
Axial Flow ◽  
Transition Process ◽  
Turbine Rotor ◽  
Boundary Layer Transition ◽  
Surface Boundary ◽  
Suction Surface ◽  
Layer Transition ◽  
Time Resolved

Previously published measurements in a low-speed, single-stage, axial-flow turbine have been reanalyzed in the light of more recent understanding. The measurements include time-resolved hot-wire traverses and surface hot film gage measurements at the midspan of the rotor suction surface with three different rotor-stator spacings. Part 1 investigates the suction surface boundary layer transition process, using surface-distance time plots and boundary layer cross sections to demonstrate the unsteady and two-dimensional nature of the process. Part 2 of the paper will describe the results of supporting experiments carried out in a linear cascade together with a simple transition model, which explains the features seen in the turbine.

Predicting Transitional Separation Bubbles

2004 ◽  
Author(s):  
John A. Redford ◽  
Mark W. Johnson
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Transition Process ◽  
Reynolds Numbers ◽  
Test Cases ◽  
Transition Model ◽  
Flow Transition ◽  
Attached Flow

This paper describes the modifications made to a successful attached flow transition model to produce a model capable of predicting both attached and separated flow transition. This transition model is used in combination with the Fluent CFD software, which is used to compute the flow around the blade assuming that it remains entirely laminar. The transition model then determines the start of transition location and the development of the intermittency. These intermittency values weight the laminar and turbulent boundary layer profiles to obtain the resulting transitional boundary layer parameters. The ERCOFTAC T3L test cases are used to validate the predictions. The T3L blade is a flat plate with a semi-circular leading edge, which results in the formation of a separation bubble the length of which is strongly dependent on the transition process. Predictions were performed for five T3L test cases for differing freestream turbulence levels and Reynolds numbers. For the majority of these test cases the measurements were accurately predicted.

A Low Reynolds Number Experimental Evaluation of Tubercles on a Low-Pressure Turbine Cascade

2019 ◽  
Author(s):  
Stephen A. Pym ◽  
Asad Asghar ◽  
William D. E. Allan ◽  
John P. Clark
Keyword(s):  
Boundary Layer ◽  
Turbine Blade ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Blade Profile ◽  
Suction Surface ◽  
Low Reynolds Numbers ◽  
Low Pressure Turbine ◽  
Low Reynolds ◽  
Highly Loaded

Abstract Aircraft are operating at increasingly high-altitudes, where decreased air density and engine power settings have led to increasingly low Reynolds numbers in the low-pressure turbine portion of modern-day aeroengines. These operating conditions, in parallel with highly-loaded blade profiles, result in non-reattaching laminar boundary layer separation along the blade suction surface, increasing loss and decreasing engine performance. This work presents an experimental investigation into the potential for integrated leading-edge tubercles to improve blade performance in this operating regime. A turn-table cascade test-section was constructed and commissioned to test a purpose-designed, forward-loaded, low-pressure turbine blade profile at various incidences and Reynolds numbers. Baseline and tubercled blades were tested at axial chord Reynolds numbers at and between 15 000 and 60 000, and angles of incidence ranging from −5° to +10°. Experimental data collection included blade surface pressure measurements, total pressure loss in the blade wakes, hot-wire anemometry, surface hot-film measurements, and surface flow visualization using tufts. Test results showed that the implementation of tubercles did not lead to a performance enhancement. However, useful conclusions were drawn regarding the ability of tubercles to generate stream-wise vortices at ultra-low Reynolds numbers. Additional observations helped to characterize the suction surface boundary layer over the highly-loaded, low-pressure turbine blade profile when at off-design conditions. Recommendations were made for future work.

Large Eddy Simulation of Periodic Wake Impact on Boundary Layer Transition Mechanisms on a Highly Loaded Low-Pressure Turbine Blade

2020 ◽  
Author(s):  
Benjamin Winhart ◽  
Martin Sinkwitz ◽  
Andreas Schramm ◽  
Pascal Post ◽  
Francesca di Mare
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Separation Bubble ◽  
Boundary Layer Transition ◽  
Suction Surface ◽  
Layer Transition ◽  
Array Measurements ◽  
Numerical Predictions ◽  
Large Eddy ◽  
Highly Loaded

Abstract In the proposed paper the transient interaction between periodic incoming wakes and the laminar separation bubble located on the rear suction surface of a typical, highly loaded LPT blade is investigated by means of highly resolved large-eddy simulations. An annular, large scale, 1.5-stage LPT test-rig, equipped with a modified T106 turbine blading and an upstream rotating vortex generator is considered and the numerical predictions are compared against hot film array measurements. In order to accurately assess both baseline transition and wake impact, simulations were conducted with unperturbed and periodically perturbed inflow conditions. Main mechanisms of transition and wake-boundary layer interaction are investigated utilizing a frequency-time domain analysis. Finally visualizations of the main flow structures and shear layer instabilities are provided utilizing the q-criterion as well as the finite-time Lyapunov exponent.

The Effect of Acoustic Excitation on Boundary Layer Separation of a Highly Loaded LPT Blade

2012 ◽  
Author(s):  
Chiara Bernardini ◽  
Stuart Benton ◽  
Jeffrey P. Bons
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Separation Bubble ◽  
Active Flow Control ◽  
Excitation Frequency ◽  
Acoustic Excitation ◽  
Intensity Level ◽  
Inlet Section ◽  
Suction Surface ◽  
Highly Loaded

An experimental investigation of the effect of acoustic excitation on the boundary layer development of a highly loaded low-pressure turbine blade at low-Reynolds number is investigated. The aim of this work is to study the effect of excitation at select frequencies on separation which could give indications about active flow control exploitation. The front-loaded L2F blade is tested in a low-speed linear cascade. The uncontrolled flow presents a separation bubble on the suction surface at Reynolds numbers below 40,000. For these conditions, the instability of the shear layer is documented using hot-wire anemometry. A loudspeaker upstream of the cascade is directed towards the passage inlet section. A parametric study on the effect of amplitude and frequency is carried out. The effect of the excitation frequency is observed to delay separation for a range of frequencies. However, the control authority of sound is found to be most effective at the fundamental frequency of the shear layer. The amplitude of perturbation is significant in the outcome of control until a threshold value is reached. PIV measurements allow a deeper understanding of the mechanisms leading to the reduction of separation. Data has been acquired with a low inlet turbulence level (<1%) in order to provide a cleaner environment which magnifies the effects of the excitation frequency, and with an increased turbulence intensity level of 3% which is representative of more typical engine values. Integrated wake loss values are also presented to evaluate the effect on blade performance.

The Effect of Acoustic Excitation on Boundary Layer Separation of a Highly Loaded LPT Blade

2013 ◽  
Vol 135 (5) ◽  
Author(s):  
Chiara Bernardini ◽  
Stuart I. Benton ◽  
Jeffrey P. Bons
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Separation Bubble ◽  
Active Flow Control ◽  
Excitation Frequency ◽  
Acoustic Excitation ◽  
Intensity Level ◽  
Inlet Section ◽  
Suction Surface ◽  
Highly Loaded

An experimental investigation of the effect of acoustic excitation on the boundary layer development of a highly loaded low-pressure turbine blade at low-Reynolds number is investigated. The aim of this work is to study the effect of excitation at select frequencies on separation which could give indications about active flow control exploitation. The front-loaded L2F blade is tested in a low-speed linear cascade. The uncontrolled flow presents a separation bubble on the suction surface at Reynolds numbers below 40,000. For these conditions, the instability of the shear layer is documented using hot-wire anemometry. A loudspeaker upstream of the cascade is directed towards the passage inlet section. A parametric study on the effect of amplitude and frequency is carried out. The effect of the excitation frequency is observed to delay separation for a range of frequencies. However, the control authority of sound is found to be most effective at the fundamental frequency of the shear layer. The amplitude of perturbation is significant in the outcome of control until a threshold value is reached. PIV measurements allow a deeper understanding of the mechanisms leading to the reduction of separation. Data has been acquired with a low inlet turbulence level (<1%) in order to provide a cleaner environment which magnifies the effects of the excitation frequency, and with an increased turbulence intensity level of 3% which is representative of more typical engine values. Integrated wake loss values are also presented to evaluate the effect on blade performance.

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.

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.

Aerodynamics of a Low-Pressure Turbine Airfoil at Low Reynolds Numbers—Part I: Steady Flow Measurements

2012 ◽  
Vol 135 (1) ◽  
Author(s):  
Ali Mahallati ◽  
Brian R. McAuliffe ◽  
Steen A. Sjolander ◽  
Thomas J. Praisner
Keyword(s):  
Turbulence Intensity ◽  
Separation Bubble ◽  
Transition Process ◽  
Reynolds Numbers ◽  
Freestream Turbulence ◽  
Suction Surface ◽  
Low Reynolds Numbers ◽  
Low Pressure Turbine ◽  
Turbulence Length ◽  
Hot Film

This two-part paper presents a detailed experimental investigation of the laminar separation and transition phenomena on the suction surface of a high-lift low-pressure turbine airfoil, PakB. The first part describes the influence of Reynolds number, freestream turbulence intensity and turbulence length scale on the PakB airfoil under steady inflow conditions. The present measurements are distinctive in that a closely-spaced array of hot-film sensors has allowed a very detailed examination of the suction surface boundary layer behavior. In addition, this paper presents a technique for interpreting the transition process in steady, and periodically unsteady, separated flows based on dynamic and statistical properties of the hot-film measurements. Measurements were made in a low-speed linear cascade facility at Reynolds numbers between 25,000 and 150,000 at three freestream turbulence intensity levels of 0.4%, 2%, and 4%. Two separate grids were used to generate turbulence intensity of 4% with integral length scales of about 10% and 40% of the airfoil axial chord length. While the higher levels of turbulence intensity promoted earlier transition and a shorter separation bubble, turbulence length scale did not have a noticeable effect on the transition process. The size of the suction side separation bubble increased with decreasing Reynolds number, and under low freestream turbulence levels the bubble failed to reattach at low Reynolds numbers. As expected, the losses increased with the length of the separation bubble, and increased significantly when the bubble failed to reattach.

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