A Spectral Study of a Moderately Loaded Low-Pressure Turbine Airfoil—Part I: Identifying Frequencies Affecting Bypass Transition

2013 ◽  
Vol 135 (4) ◽  
Author(s):  
J. R. S. Graveline ◽  
S. A. Sjolander
Keyword(s):  
Shear Layer ◽  
Low Pressure ◽  
Bypass Transition ◽  
Free Shear Layer ◽  
Airflow Velocity ◽  
Single Wire ◽  
Low Pressure Turbine ◽  
Wire Probe ◽  
Empirical Relations ◽  
Tollmien Schlichting

A single wire, hot-wire, probe is used to examine the airflow in, and in close vicinity to, the shear layer of a low-pressure turbine (LPT) airfoil. A spectral analysis of the data identifies two sets of wide peaks of turbulent kinetic energy, one near 200 Hz and a second near 1 kHz. A method is developed to identify these peaks based on a combination of empirical relations between the airflow velocity and boundary layer thickness and on the location of the frequency peaks relative to the state of the free shear layer as it transitioned from laminar to turbulent. The method suggests the presence of Tollmien–Schlichting waves and Kelvin–Helmholtz instabilities. The Kelvin–Helmholtz instabilities are shown to pair.

A Spectral Study of a Moderately Loaded LPT Airfoil: Part 1—Identifying Frequencies Affecting By-Pass Transition

2012 ◽  
Author(s):  
J. R. S. Graveline ◽  
S. A. Sjolander
Keyword(s):  
Boundary Layer ◽  
Spectral Analysis ◽  
Boundary Layer Thickness ◽  
Hot Wire ◽  
Airflow Velocity ◽  
Single Wire ◽  
Low Pressure Turbine ◽  
Wire Probe ◽  
Empirical Relations ◽  
Tollmien Schlichting

A single wire, hot-wire, probe is used to examine the airflow in, and in close vicinity to, the shearlayer of a Low-Pressure Turbine (LPT) airfoil. A spectral analysis of the data identifies two sets of wide peaks of turbulent kinetic energy, one near 200 Hz and a second near 1 kHz. A method is developed to identify these peaks based on a combination of empirical relations between the airflow velocity and boundary layer thickness and on the location of the frequency peaks relative to the state of the free shearlayer as it transitioned from laminar to turbulent. The method suggests the presence of Tollmien-Schlichting waves and Kelvin-Helmholtz instabilities. The Kelvin-Helmholtz instabilities are shown to pair.

A Spectral Study of a Moderately Loaded LPT Airfoil: Part 2—Effects of Turbulence Intensity and Reynolds Number on Frequencies Affecting By-Pass Transition

2012 ◽  
Author(s):  
J. R. S. Graveline ◽  
S. A. Sjolander
Keyword(s):  
Reynolds Number ◽  
Turbulence Intensity ◽  
Spectral Study ◽  
Reynolds Numbers ◽  
Hot Wire ◽  
Spectral Measures ◽  
Single Wire ◽  
Low Pressure Turbine ◽  
Wire Probe ◽  
Tollmien Schlichting

A single wire, hot-wire, probe is used to examine the airflow in, and in close vicinity to, the shearlayer of a Low-Pressure Turbine (LPT) airfoil. The experiment was performed with varying turbulence intensities (Tu) and Reynolds numbers (ReBx); in this work, Re is based on the cascade inflow velocity and axial chord length. In part 1 of the present study [1], the methodology used to identify the key frequencies in the free shearlayer using a combination of statistical and spectral measures of the airflow was first discussed. Here, the focus is on the effects of ReBx and Tu on the spectral results. The frequencies and location in the shearlayer of the Tollmien-Schlichting (TS) waves and Kelvin-Helmholtz (KH) instabilities are shown to be affected by both Tu and ReBx. Additionally, the KH instabilities are shown to undergo pairing.

Exploitation of Subharmonics for Separated Shear Layer Control on a High-Lift Low-Pressure Turbine Using Acoustic Forcing

2013 ◽  
Vol 136 (5) ◽  
Author(s):  
Chiara Bernardini ◽  
Stuart I. Benton ◽  
Jen-Ping Chen ◽  
Jeffrey P. Bons
Keyword(s):  
Shear Layer ◽  
Separation Bubble ◽  
Boundary Layer Separation ◽  
Low Pressure ◽  
Linear Instability ◽  
Laminar Separation ◽  
Significant Control ◽  
Low Pressure Turbine ◽  
Separated Shear Layer ◽  
Sound Control

The mechanism of separation control by sound excitation is investigated on the aft-loaded low-pressure turbine (LPT) blade profile, the L1A, which experiences a large boundary layer separation at low Reynolds numbers. Previous work by the authors has shown that on a laminar separation bubble such as that experienced by the front-loaded L2F profile, sound excitation control has its best performance at the most unstable frequency of the shear layer due to the exploitation of the linear instability mechanism. The different loading distribution on the L1A increases the distance of the separated shear layer from the wall and the exploitation of the same linear mechanism is no longer effective in these conditions. However, significant control authority is found in the range of the first subharmonic of the natural unstable frequency. The amplitude of forced excitation required for significant wake loss reduction is higher than that needed when exploiting linear instability, but unlike the latter case, no threshold amplitude is found. The fluid-dynamics mechanisms under these conditions are investigated by particle image velocimetry (PIV) measurements. Phase-locked PIV data gives insight into the growth and development of structures as they are shed from the shear layer and merge to lock into the excited frequency. Unlike near-wall laminar separation sound control, it is found that when such large separated shear layers occur, sound excitation at subharmonics of the fundamental frequency is still effective with high-Tu levels.

Separated Flow Transition Under Simulated Low-Pressure Turbine Airfoil Conditions—Part 1: Mean Flow and Turbulence Statistics

2002 ◽  
Vol 124 (4) ◽  
pp. 645-655 ◽  
Author(s):  
Ralph J. Volino
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Shear Layer ◽  
Free Stream ◽  
Low Pressure ◽  
Turbulent Shear ◽  
Trailing Edge ◽  
Free Stream Turbulence ◽  
Low Pressure Turbine ◽  
Turbulent Shear Stress

Boundary layer separation, transition and reattachment have been studied experimentally under low-pressure turbine airfoil conditions. Cases with Reynolds numbers (Re) ranging from 25,000 to 300,000 (based on suction surface length and exit velocity) have been considered at low (0.5%) and high (9% inlet) free-stream turbulence levels. Mean and fluctuating velocity and intermittency profiles are presented for streamwise locations all along the airfoil, and turbulent shear stress profiles are provided for the downstream region where separation and transition occur. Higher Re or free-stream turbulence level moves transition upstream. Transition is initiated in the shear layer over the separation bubble and leads to rapid boundary layer reattachment. At the lowest Re, transition did not occur before the trailing edge, and the boundary layer did not reattach. Turbulent shear stress levels can remain low in spite of high free-stream turbulence and high fluctuating streamwise velocity in the shear layer. The beginning of a significant rise in the turbulent shear stress signals the beginning of transition. A slight rise in the turbulent shear stress near the trailing edge was noted even in those cases which did not undergo transition or reattachment. The present results provide detailed documentation of the boundary layer and extend the existing database to lower Re. The present results also serve as a baseline for an investigation of turbulence spectra in Part 2 of the present paper, and for ongoing work involving transition and separation control.

303 Bypass Transition of Separated Boundary Layer on Low Pressure Turbine Blade

2008 ◽  
Vol 2008.44 (0) ◽  
pp. 79-80
Author(s):  
Atsushi CHIBA ◽  
Ken-ichi FUNAZAKI ◽  
Hideo TANIGUCHI ◽  
Kazutoyo YAMADA
Keyword(s):  
Boundary Layer ◽  
Turbine Blade ◽  
Low Pressure ◽  
Bypass Transition ◽  
Low Pressure Turbine

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.

The Application of Flow Control to an Aft-Loaded Low Pressure Turbine Cascade With Unsteady Wakes

2011 ◽  
Vol 134 (3) ◽  
Author(s):  
Jeffrey P. Bons ◽  
Jon Pluim ◽  
Kyle Gompertz ◽  
Matthew Bloxham ◽  
John P. Clark
Keyword(s):  
Flow Control ◽  
Shear Layer ◽  
Pressure Loss ◽  
Total Pressure ◽  
Separation Zone ◽  
Vortex Generator ◽  
Low Pressure ◽  
Separation Control ◽  
Low Pressure Turbine ◽  
Separated Shear Layer

The synchronous application of flow control in the presence of unsteady wakes was studied on a highly loaded low pressure turbine blade. At low Reynolds numbers, the blade exhibits a nonreattaching separation bubble under steady flow conditions without upstream wakes. Unsteady wakes from an upstream vane row are simulated with a moving row of bars. The separation zone is modified substantially by the presence of unsteady wakes, producing a smaller separation zone and reducing the area-averaged wake total pressure loss by more than 50%. The wake disturbance accelerates transition in the separated shear layer but stops short of reattaching the flow. Rather, a new time-averaged equilibrium location is established for the separated shear layer. The focus of this study was the application of pulsed flow control using two spanwise rows of discrete vortex generator jets. The jets were located at 59% Cx, approximately the peak cp location, and at 72% Cx. The most effective separation control was achieved at the upstream location. The wake total pressure loss decreased 60% from the wake-only level and the cp distribution fully recovered its high Reynolds number shape. The jet disturbance dominates the dynamics of the separated shear layer, with the wake disturbance assuming a secondary role only. When the pulsed jet actuation was initiated at the downstream location, synchronizing the jet to actuate between wake events was key to producing the most effective separation control. Evidence suggests that flow control using vortex generator jets (VGJs) will be effective in the highly unsteady low pressure turbine environment of an operating gas turbine, provided the VGJ location and amplitude are adapted for the specific blade profile.

Influence of Wake Structure on Unsteady Flow in a Low Pressure Turbine Blade Passage

2009 ◽  
Vol 131 (4) ◽  
Author(s):  
S. Sarkar
Keyword(s):  
Shear Layer ◽  
Turbine Blade ◽  
Suction Side ◽  
Low Pressure ◽  
Small Scale ◽  
Suction Surface ◽  
Low Pressure Turbine ◽  
Eddy Simulation ◽  
Flow Past A Cylinder ◽  
Scale Motion

The effect of wake structures on the evolution of the boundary layer over the suction side of a high-lift low-pressure turbine blade is studied using large-eddy simulation (LES) for a Reynolds number Re=7.8×104 (based on the axial chord and the inlet velocity). The wake data of different characteristics (defined by the wake deficit and the small-scale motion) are extracted from a precursor LES of flow past a cylinder. This replaces a moving bar that generates wakes in front of a cascade. LES results illustrate that apart from the wake kinematics, the large pressure oscillations and rollup of the separated shear layer along the rear half of the suction surface depend on the length scale of the convective wake. The transition of this rolled-up shear layer is influenced by the wake turbulence and the small-scale motion.

scholarly journals Experimental Investigation of Boundary Layer Behavior in a Simulated Low Pressure Turbine

1998 ◽  
Author(s):  
Ki H. Sohn ◽  
Rickey J. Shyne ◽  
Kenneth J. DeWitt
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Flow Region ◽  
Suction Side ◽  
Low Pressure ◽  
Careful Examination ◽  
Freestream Turbulence ◽  
Free Shear Layer ◽  
Mean Velocity ◽  
Low Pressure Turbine

A detailed investigation of the flow physics occurring on the suction side of a simulated Low Pressure Turbine (LPT) blade was performed. A contoured upper wall was designed to simulate the pressure distribution of an actual LPT blade onto a flat plate. The experiments were carried out at Reynolds numbers of 100,000 and 250,000 with three levels of freestream turbulence. The main emphasis in this paper is placed on flow field surveys performed at a Reynolds number of 100,000 with levels of freestream turbulence ranging from 0.8% to 3%. Smoke-wire flow visualization data was used to confirm that the boundary layer was separated and formed a bubble. The transition process over the separated flow region is observed to be similar to a laminar free shear layer flow with the formation of a large coherent eddy structure. For each condition, the locations defining the separation bubble were determined by careful examination of pressure and mean velocity profile data. Transition onset location and length determined from intermittency profiles decrease as freestream turbulence levels increase. Additionally, the length and height of the laminar separation bubbles were observed to be inversely proportional to the levels of freestream turbulence.

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