Measurements of the Effects of Freestream Turbulence on Separation-Bubble Transition

2002 ◽  
Author(s):  
M. I. Yaras
Keyword(s):  
Pressure Distribution ◽  
Separation Bubble ◽  
Reynolds Numbers ◽  
Pressure Gradients ◽  
Freestream Turbulence ◽  
Test Plate ◽  
Laminar To Turbulent Transition ◽  
Turbulent Transition ◽  
Boundary Layer Development ◽  
Layer Development

In this paper, measurements are presented on the effects of freestream turbulence on laminar-to-turbulent transition in separation bubbles, and correlations are proposed for the locations of transition and reattachment on the basis of this data. The boundary layer development is measured on a smooth, flat plate upon which streamwise pressure gradients are imposed by a flexible, contoured wall opposite to the test plate. Two variations in the streamwise pressure distribution are investigated, and two Reynolds numbers are considered for each pressure-gradient setting. For each combination of pressure distribution and Reynolds number, the freestream turbulence intensity and length scale are adjusted systematically by varying the open-area-ratio and cell size of the grid installed at the test-section inlet. Measured quantities consist of velocity obtained with a single-hot wire probe and surface pressures measured through pressure taps.

Effects of Reynolds Number and Freestream Turbulence Intensity on the Unsteady Boundary Layer Development on an Ultra-High-Lift Low Pressure Turbine Airfoil

2009 ◽  
Vol 132 (1) ◽  
Author(s):  
Xue Feng Zhang ◽  
Howard Hodson
Keyword(s):  
Boundary Layer ◽  
Reynolds Numbers ◽  
Freestream Turbulence ◽  
Unsteady Boundary Layer ◽  
Suction Surface ◽  
High Lift ◽  
Low Pressure Turbine ◽  
Boundary Layer Development ◽  
Unsteady Wake ◽  
Layer Development

The effects of Reynolds numbers and the freestream turbulence intensities (FSTIs) on the unsteady boundary layer development on an ultra-high-lift low-pressure turbine airfoil, so-called T106C, are investigated. The measurements were carried out at both Tu=0.5% and 4.0% within a range of Reynolds numbers, based on the blade chord and the isentropic exit velocity, between 100,000 and 260,000. The interaction between the unsteady wake and the boundary layer depends on both the strength of the wake and the status of the boundary layer. At Tu=0.5%, both the wake’s high turbulence and the negative jet behavior of the wake dominate the interaction between the unsteady wake and the separated boundary layer on the suction surface of the airfoil. Since the wake turbulence cannot induce transition before separation on this ultra-high-lift blade, the negative jet of the wake has the opportunity to induce a rollup vortex. At Tu=4.0%, the time-mean separation on the suction surface is much smaller. With elevated FSTI, the turbulence in the wake just above the boundary layer is no longer distinguishable from the background turbulence level. The unsteady boundary layer transition is dominated by the wake’s negative jet induced boundary layer variation.

Effects of Reynolds Number and Freestream Turbulence Intensity on the Unsteady Boundary Layer Development on an Ultra-High-Lift LPT Airfoil

2007 ◽  
Author(s):  
Xue Feng Zhang ◽  
Howard Hodson
Keyword(s):  
Boundary Layer ◽  
Reynolds Numbers ◽  
Boundary Layer Transition ◽  
Freestream Turbulence ◽  
Unsteady Boundary Layer ◽  
Suction Surface ◽  
High Lift ◽  
Boundary Layer Development ◽  
Unsteady Wake ◽  
Layer Development

The effects of Reynolds numbers and the freestream turbulence intensities (FSTI) on the unsteady boundary layer development on an ultra-high-lift low-pressure (LP) turbine airfoil, so-called T106C, are investigated. The measurements were carried out at both Tu = 0.5% and 4.0% within a range of Reynolds numbers, based on the blade chord and the isentropic exit velocity, between 100,000 and 260,000. The interaction between the unsteady wake and the boundary layer depends on both the strength of the wake and the status of the boundary layer. At Tu = 0.5%, both the wake’s high turbulence and the negative jet behaviour of the wake dominate the interaction between the unsteady wake and the separated boundary layer on the suction surface of the airfoil. Since the wake turbulence cannot induce transition before separation on this ultra-high-lift blade, the negative jet of the wake has the opportunity to induce a rollup vortex. At Tu = 4.0%, the time-mean separation on the suction surface is much smaller. With elevated FSTI, the turbulence in the wake just above the boundary layer is no longer distinguishable from the background turbulence level. The unsteady boundary layer transition is dominated by the wake’s negative jet induced boundary layer variation.

Turbulent Spots in Strong Adverse Pressure Gradients: Part 2 — Spot Propagation and Spreading Rates

2001 ◽  
Author(s):  
A. D’Ovidio ◽  
J. A. Harkins ◽  
J. P. Gostelow
Keyword(s):  
Separation Bubble ◽  
Reynolds Numbers ◽  
Boundary Layer Transition ◽  
Pressure Gradients ◽  
Flat Plates ◽  
Layer Transition ◽  
Turbulent Spots ◽  
Laminar Separation ◽  
Transition Length ◽  
Spreading Rates

The study of turbulent spots in strong adverse pressure gradients is of current interest in turbomachinery research. The aim of this investigation is to use information gathered from boundary layer transition and laminar separation, in wind tunnel tests on flat plates, to predict the equivalent phenomena occurring on turbomachinery blade surfaces. In Part 1 turbulent spot behavior was documented for two Reynolds numbers, corresponding to a laminar separation bubble (LSB) and an incipient separation condition (IS). In Part 2 further results are reported characterizing typical spot propagation and spreading rates and serving to validate or modify existing correlations for predicting transition length.

Experimental Study of the Effect of Periodic Unsteady Wake Flow on Boundary Layer Development, Separation, and Reattachment Along the Surface of a Low Pressure Turbine Blade

2004 ◽  
Vol 126 (4) ◽  
pp. 663-676 ◽  
Author(s):  
M. T. Schobeiri ◽  
B. O¨ztu¨rk
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Wake Flow ◽  
Suction Surface ◽  
Low Pressure Turbine ◽  
Reduced Frequency ◽  
Boundary Layer Development ◽  
Unsteady Wake ◽  
Unsteady Wake Flow ◽  
Layer Development

The paper experimentally studies the effects of periodic unsteady wake flow on boundary layer development, separation and reattachment along the suction surface of a low pressure turbine blade. The experimental investigations were performed on a large scale, subsonic unsteady turbine cascade research facility at the Turbomachinery Performance and Flow Research Laboratory (TPFL), Texas A&M University. The experiments were carried out at a Reynolds number of 110,000 (based on suction surface length and exit velocity) with a free-stream turbulence intensity of 1.9%. One steady and two different unsteady inlet flow conditions with the corresponding passing frequencies, wake velocities, and turbulence intensities were investigated. The reduced frequencies cover the entire operating range of LP turbines. In addition to the unsteady boundary layer measurements, blade surface measurements were performed at the same Reynolds number. The surface pressure measurements were also carried out at one steady and two periodic unsteady inlet flow conditions. The results presented in ensemble-averaged, and the contour plot forms help to understand the physics of the separation phenomenon under periodic unsteady wake flow. It was found that the suction surface displayed a strong separation bubble for these three different reduced frequencies. For each condition, the locations and the heights defining the separation bubble were determined by carefully analyzing and examining the pressure and the mean velocity profile data. The location of boundary layer separation was independent of the reduced frequency level. However, the extent of the separation was strongly dependent on the reduced frequency level. Once the unsteady wake started to penetrate into the separation bubble, the turbulent spot produced in the wake paths caused a reduction of the separation bubble height.

Unsteady Wake-Induced Boundary Layer Transition in High Lift LP Turbines

1998 ◽  
Vol 120 (1) ◽  
pp. 28-35 ◽  
Author(s):  
V. Schulte ◽  
H. P. Hodson
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Turbine Blade ◽  
Reynolds Numbers ◽  
Boundary Layer Development ◽  
Unsteady Wake ◽  
Lp Turbine ◽  
Wake Passing ◽  
Layer Development ◽  
Highly Loaded

The development of the unsteady suction side boundary layer of a highly loaded LP turbine blade has been investigated in a rectilinear cascade experiment. Upstream rotor wakes were simulated with a moving-bar wake generator. A variety of cases with different wake-passing frequencies, different wake strength, and different Reynolds numbers were tested. Boundary layer surveys have been obtained with a single hotwire probe. Wall shear stress has been investigated with surface-mounted hot-film gages. Losses have been measured. The suction surface boundary layer development of a modern highly loaded LP turbine blade is shown to be dominated by effects associated with unsteady wake-passing. Whereas without wakes the boundary layer features a large separation bubble at a typical cruise Reynolds number, the bubble was largely suppressed if subjected to unsteady wake-passing at a typical frequency and wake strength. Transitional patches and becalmed regions, induced by the wake, dominated the boundary layer development. The becalmed regions inhibited transition and separation and are shown to reduce the loss of the wake-affected boundary layer. An optimum wake-passing frequency exists at cruise Reynolds numbers. For a selected wake-passing frequency and wake strength, the profile loss is almost independent of Reynolds number. This demonstrates a potential to design highly loaded LP turbine profiles without suffering large losses at low Reynolds numbers.

Experimental Study of the Effect of Periodic Unsteady Wake Flow on Boundary Layer Development, Separation, and Re-Attachment Along the Surface of a Low Pressure Turbine Blade

2004 ◽  
Author(s):  
M. T. Schobeiri ◽  
B. O¨ztu¨rk
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Wake Flow ◽  
Suction Surface ◽  
Low Pressure Turbine ◽  
Reduced Frequency ◽  
Boundary Layer Development ◽  
Unsteady Wake ◽  
Unsteady Wake Flow ◽  
Layer Development

The paper experimentally studies the effects of periodic unsteady wake flow on boundary layer development, separation and re-attachment along the suction surface of a low pressure turbine blade. The experimental investigations were performed on a large scale, subsonic unsteady turbine cascade research facility at Turbomachinery Performance and Flow Research Laboratory (TPFL), Texas A&M University. The experiments were carried out at a Reynolds number of 110,000 (based on suction surface length and exit velocity) with a free-stream turbulence intensity of 1.9%. One steady and two different unsteady inlet flow conditions with the corresponding passing frequencies, wake velocities, and turbulence intensities were investigated. The reduced frequencies cover the entire operating range of LP turbines. In addition to the unsteady boundary layer measurements, blade surface measurements were performed at the same Reynolds number. The surface pressure measurements were also carried out at one steady and two periodic unsteady inlet flow conditions. The results presented in ensemble-averaged, and the contour plot forms help to understand the physics of the separation phenomenon under periodic unsteady wake flow. It was found that the suction surface displayed a strong separation bubble for these three different reduced frequencies. For each condition, the locations and the heights defining the separation bubble were determined by carefully analyzing and examining the pressure and the mean velocity profile data. The location of boundary layer separation was independent of the reduced frequency level. However, the extent of the separation was strongly dependent on the reduced frequency level. Once the unsteady wake started to penetrate into the separation bubble, the turbulent spot produced in the wake paths caused a reduction of the separation bubble height.

scholarly journals Heat Transfer Measurements and Predictions on a Power Generation Gas Turbine Blade

2000 ◽  
Author(s):  
Paul W. Giel ◽  
Ronald S. Bunker ◽  
G. James Van Fossen ◽  
Robert J. Boyle
Keyword(s):  
Heat Transfer ◽  
Power Generation ◽  
Three Dimensional ◽  
Thin Foil ◽  
Reynolds Numbers ◽  
Turbine Rotor ◽  
Stagnation Region ◽  
Design Point ◽  
Laminar To Turbulent Transition ◽  
Turbulent Transition

Detailed heat transfer measurements and predictions are given for a power generation turbine rotor with 129 deg of nominal turning and an axial chord of 137 mm. Data were obtained for a set of four exit Reynolds numbers comprised of the design point of 628,000, −20%, +20%, and +40%. Three ideal exit pressure ratios were examined including the design point of 1:378, −10%, and +10%. Inlet incidence angles of 0 deg and ±2 deg were also examined. Measurements were made in a linear cascade with highly three-dimensional blade passage flows that resulted from the high flow turning and thick inlet boundary layers. Inlet turbulence was generated with a blown square bar grid. The purpose of the work is the extension of three-dimensional predictive modeling capability for airfoil external heat transfer to engine specific conditions including blade shape, Reynolds numbers, and Mach numbers. Data were obtained by a steady-state technique using a thin-foil heater wrapped around a low thermal conductivity blade. Surface temperatures were measured using calibrated liquid crystals. The results show the effects of strong secondary vortical flows, laminar-to-turbulent transition, and also show good detail in the stagnation region.

Time Scales for Unsteady Mass Transfer From a Sphere at Low-Finite Reynolds Numbers

2003 ◽  
Vol 125 (4) ◽  
pp. 716-723 ◽  
Author(s):  
Stanley J. Kleis ◽  
Ivan Rivera-Solorio
Keyword(s):  
Boundary Layer ◽  
Mass Transfer ◽  
Flow Field ◽  
Time Scales ◽  
Free Stream ◽  
Reynolds Numbers ◽  
Concentration Boundary Layer ◽  
Concentration Boundary ◽  
Boundary Layer Development ◽  
Layer Development

The problem of unsteady mass transfer from a sphere that impulsively moves from rest to a finite velocity in a non-uniform concentration distribution is studied. A range of low Reynolds numbers (Re<1) and moderate Peclet numbers (Pe ranges from 5.6 to 300) is investigated (typical of the parameters encountered in anchorage dependent cell cultures in micro gravity). Using time scales, the effects of flow field development, concentration boundary layer development and free stream concentration variation are investigated. For the range of parameters considered, the development of the flow field has a negligible effect on the time variation of the Sherwood number (Sh). The Sh time dependence is dominated by concentration boundary layer development for early times and free stream concentration variations at later times.

scholarly journals Laminar-to-turbulent transition of pipe flows through puffs and slugs

2008 ◽  
Vol 614 ◽  
pp. 425-446 ◽  
Author(s):  
MINA NISHI ◽  
BÜLENT ÜNSAL ◽  
FRANZ DURST ◽  
GAUTAM BISWAS
Keyword(s):  
Reynolds Number ◽  
Pipe Flow ◽  
Axial Direction ◽  
Reynolds Numbers ◽  
Turbulent Pipe Flow ◽  
Test Facility ◽  
Pipe Flows ◽  
Laminar To Turbulent Transition ◽  
Turbulent Transition ◽  
Slug Flows

Laminar-to-turbulent transition of pipe flows occurs, for sufficiently high Reynolds numbers, in the form of slugs. These are initiated by disturbances in the entrance region of a pipe flow, and grow in length in the axial direction as they move downstream. Sequences of slugs merge at some distance from the pipe inlet to finally form the state of fully developed turbulent pipe flow. This formation process is generally known, but the randomness in time of naturally occurring slug formation does not permit detailed study of slug flows. For this reason, a special test facility was developed and built for detailed investigation of deterministically generated slugs in pipe flows. It is also employed to generate the puff flows at lower Reynolds numbers. The results reveal a high degree of reproducibility with which the triggering device is able to produce puffs. With increasing Reynolds number, ‘puff splitting’ is observed and the split puffs develop into slugs. Thereafter, the laminar-to-turbulent transition occurs in the same way as found for slug flows. The ring-type obstacle height, h, required to trigger fully developed laminar flows to form first slugs or puffs is determined to show its dependence on the Reynolds number, Re = DU/ν (where D is the pipe diameter, U is the mean velocity in the axial direction and ν is the kinematic viscosity of the fluid). When correctly normalized, h+ turns out to be independent of Reτ (where h+ = hUτ/ν, Reτ = DUτ/ν and $U_{\tau}\,{=}\,\sqrt{\tau_{w}/ \rho}$; τw is the wall shear stress and ρ is the density of the fluid).

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