Studies on Effects of Periodic Wake Passing upon a Blade Leading Edge Separation Bubble (Suppresion of Separation Bubble Due to Wake Passing)

This paper describes an attempt to suppress a blade leading edge separation bubble by utilizing a stationary bar wake. This study aims at exploration of a possibility for reducing the aerodynamic loss due to blade boundary layer that is accompanied with the separation bubble. The test model used in this study consists of semi-circular leading edge and two parallel flat plates. It can be tilted against the inlet flow so as to change the characteristics of the separation bubble. Detailed flow measurements over the test model are conducted using a single hot-wire probe. Emphasis in this study is placed on the effect of bar shifting or bar clocking across the inlet flow in order to see how the bar-wake position with respect to the test model affects the separation bubble as well as aerodynamic loss generated within the boundary layer. The present study reveals a loss reduction through the separation bubble control using a properly clocked bar wake.

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Studies on Effects of Periodic Wake Passing Upon a Blade Leading Edge Separation Bubble: Experimental Investigation Using a Simple Leading Edge Model

Volume 5: Turbo Expo 2003, Parts A and B ◽

10.1115/gt2003-38281 ◽

2003 ◽

Author(s):

K. Funazaki ◽

K. Yamada ◽

Y. Kato

Keyword(s):

Experimental Investigation ◽

Separation Bubble ◽

Leading Edge ◽

Test Cases ◽

Test Model ◽

Flat Plates ◽

Aerodynamic Interaction ◽

Edge Separation ◽

Wake Passing ◽

Probe Measurements

This paper describes experimental investigation on aerodynamic interaction between incoming periodic wakes and leading edge separation bubble on a compressor or turbine blade, using a scaled leading edge model. The studies aims at expanding the range of the test conditions from that of the previous study (Funazaki and Kato [15]) in order to deepen the knowledge on how and to what extent upstream wake passing suppresses the leading edge separation bubble. Special attention is paid to the transitional behaviors of the separated boundary layers, in particular, to the emergence of wake-induced turbulence spots. Hot-wire probe measurements are then executed under five different flow conditions. The test model has a simple structure consisting of a semi-circular leading edge and two flat-plates. Cylindrical bars of the wake generator generate the periodic wakes in front of the test model. Effects of Reynolds number, Strouhal number, direction of the bar movement and incidence of the test model against the incoming flow are examined in this paper. The measurements reveal that the wake moving over the separation bubble does not directly suppress the separation bubble. Instead, wake-induced turbulence spots and the subsequent calmed regions have dominant impacts on the separation bubble suppression for the all test cases. Distinct difference of the bubble suppressing effect by the wakes is also observed when the direction of the bar movement is altered.

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Flow Visualization Experiments on Tethered Flying Green Lacewings Chrysopa Dasyptera

Journal of Experimental Biology ◽

10.1242/jeb.169.1.143 ◽

1992 ◽

Vol 169 (1) ◽

pp. 143-163 ◽

Cited By ~ 1

Author(s):

DMITRY L. GRODNTTSKY ◽

PAHVEL P. MOROZOV

Keyword(s):

Spatial Structure ◽

Functional Morphology ◽

Insect Flight ◽

Separation Bubble ◽

Leading Edge ◽

Vortex Wake ◽

Green Lacewings ◽

Visualization Experiments ◽

Edge Separation ◽

Stroke Cycle

Experiments on dust visualization of the flow around tethered flying green lacewings showed that, contrary to expectations based on the Weis-Fogh clap-andfling mechanism, a leading edge separation bubble does not exist near either fore-or hindwings. At the beginning of the stroke cycle each wing operates as an independent generator of vorticity. The vortex bubbles of all the four wings then unite, producing a single U-shaped bubble. A hypothetical spatial structure for the vortex wake is derived from a series of registrated sections of the wake illuminated with a flat light beam. Some problems of wing functional morphology and insect flight aerodynamics are also discussed.

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The Effect of Wake Passing on a Flow Separation in a Low-Pressure Turbine Cascade

Journal of Fluids Engineering ◽

10.1115/1.1667884 ◽

2004 ◽

Vol 126 (2) ◽

pp. 250-256 ◽

Cited By ~ 2

Author(s):

Michael J. Brear ◽

Howard P. Hodson

Keyword(s):

Separation Bubble ◽

Phase Locking ◽

Experimental Studies ◽

Leading Edge ◽

Low Pressure ◽

Pressure Surface ◽

Low Pressure Turbine ◽

Numerical Predictions ◽

Unsteady Interaction ◽

Wake Passing

This paper describes an investigation into the effect that passing wakes have on a separation bubble that exists on the pressure surface and near the leading edge of a low-pressure turbine blade. Previous experimental studies have shown that the behavior of this separation is strongly incidence dependent and that it responds to its disturbance environment. The results presented in this paper examine the effect of wake passing in greater detail. Two-dimensional, Reynolds averaged, numerical predictions are first used to examine qualitatively the unsteady interaction between the wakes and the separation bubble. The separation is predicted to consist of spanwise vortices whose development is in phase with the wake passing. However, comparison with experiments shows that the numerical predictions exaggerate the coherence of these vortices and also overpredict the time-averaged length of the separation. Nonetheless, experiments strongly suggest that the predicted phase locking of the vortices in the separation onto the wake passing is physical.

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Three-Dimensional Vortex Structure in a Leading-Edge Separation Bubble at Moderate Reynolds Numbers

Journal of Fluids Engineering ◽

10.1115/1.2909510 ◽

1991 ◽

Vol 113 (3) ◽

pp. 405-410 ◽

Cited By ~ 38

Author(s):

Kyuro Sasaki ◽

Masaru Kiya

Keyword(s):

Reynolds Number ◽

Separation Bubble ◽

Plate Thickness ◽

Three Dimensional ◽

Leading Edge ◽

Reynolds Numbers ◽

Vortex Filaments ◽

Separated Shear Layer ◽

Hydrogen Bubbles ◽

Edge Separation

This paper describes the results of a flow visualization study which concerns three-dimensional vortex structures in a leading-edge separation bubble formed along the sides of a blunt flat plate. Dye and hydrogen bubbles were used as tracers. Reynolds number (Re), based on the plate thickness, was varied from 80 to 800. For 80 < Re < 320, the separated shear layer remains laminar up to the reattachment line without significant spanwise distortion of vortex filaments. For 320 < Re < 380, a Λ-shaped deformation of vortex filaments appears shortly downstream of the reattachment and is arranged in-phase in the downstream direction. For Re > 380, hairpin-like structures are formed and arranged in a staggered manner. The longitudinal and spanwise distances of the vortex arrangement are presented as functions of the Reynolds number.

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The Effect of Wake Passing on a Flow Separation in a Low Pressure Turbine Cascade

Volume 1: Fora, Parts A, B, C, and D ◽

10.1115/fedsm2003-45580 ◽

2003 ◽

Author(s):

Michael J. Brear ◽

Howard P. Hodson

Keyword(s):

Separation Bubble ◽

Phase Locking ◽

Experimental Studies ◽

Leading Edge ◽

Low Pressure ◽

Pressure Surface ◽

Low Pressure Turbine ◽

Numerical Predictions ◽

Unsteady Interaction ◽

Wake Passing

This paper describes an investigation into the effect that passing wakes have on a separation bubble that exists on the pressure surface and near the leading edge of a low pressure turbine blade. Previous experimental studies have shown that the behaviour of this separation is strongly incidence dependent and that it responds to its disturbance environment. The results presented in this paper examine the effect of wake passing in greater detail. Two dimensional, Reynolds averaged, numerical predictions are first used to examine qualitatively the unsteady interaction between the wakes and the separation bubble. The separation is predicted to consist of spanwise vortices whose development is in phase with the wake passing. However, comparison with experiments shows that the numerical predictions exaggerate the coherence of these vortices and also overpredict the time-averaged length of the separation. Nonetheless, experiments strongly suggest that the predicted phase locking of the vortices in the separation onto the wake passing is physical.

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An Inviscid-Viscous Method to Model Leading Edge Separation Bubbles

Volume 1: Turbomachinery ◽

10.1115/94-gt-247 ◽

1994 ◽

Author(s):

W. John Calvert

Keyword(s):

Boundary Layer ◽

Separation Bubble ◽

Leading Edge ◽

Initial Boundary ◽

Axial Compressors ◽

Recent Experimental Study ◽

Separation Bubbles ◽

Blade Section ◽

Set Up ◽

Edge Separation

Separation bubbles are likely to occur near the leading edges of sharp-edged blade sections in axial compressors and turbines, particularly when the sections are operated at positive incidence. Typically the flow reattaches a short distance from the leading edge as a turbulent boundary layer, the thickness of which depends on the details of the separation bubble. The overall performance of the blade section can be significantly affected by the thickness of this initial boundary layer — in some cases blade stall is mainly associated with the change in thickness of the layer as blade incidence is increased. A recent experimental study at the Whittle Laboratory, Cambridge demonstrated the importance of the blade leading edge shape on the separation bubble. In the present work, an inviscid-viscous method has been set up to model the experimental data and to provide a way of predicting the performance of this critical region for different leading edge shapes.

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Prediction of a Laminar Separation Bubble Over a Controlled-Diffusion Compressor Blade

Volume 3: Heat Transfer; Electric Power; Industrial and Cogeneration ◽

10.1115/2000-gt-0277 ◽

2000 ◽

Cited By ~ 3

Author(s):

G. V. Hobson ◽

S. Weber

Keyword(s):

Turbulence Model ◽

Separation Bubble ◽

Turbulence Models ◽

Leading Edge ◽

Equation Model ◽

Implicit Time ◽

Controlled Diffusion ◽

Time Marching ◽

The One ◽

Edge Separation

The paper describes the comparison of the prediction of the flow through a cascade of controlled-diffusion compressor blades with two Navier-Stokes solvers. Both codes solved the thin-layer N-S equations, however; one code performed implicit time marching whereas the other performed explicit time marching. Flow predictions were accomplished with the implicit code using the algebraic turbulence model of Baldwin and Lomax and the one-equation model of Spalart and Allmaras, while predictions were made with the explicit code using the two-equation model by Wilcox. Predictions were made of the detailed laser-anemometry measurements of the flow field taken previously in a low-speed cascade wind tunnel. Comparisons were also made with the experimentally measured blade surface pressures and flow visualization of the extent of the laminar leading edge separation bubble. The one-equation turbulence model was combined with an intermittency based transition-length model for comparisons with fully turbulent calculations. Both codes predicted the leading-edge separation bubble satisfactorily when using higher order turbulence models.

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On the steady separated flow around an inclined flat plate

Journal of Fluid Mechanics ◽

10.1017/s0022112096004247 ◽

1997 ◽

Vol 333 ◽

pp. 403-413 ◽

Cited By ~ 7

Author(s):

W. W. H. YEUNG ◽

G. V. PARKINSON

Keyword(s):

Flat Plate ◽

Bluff Body ◽

Separation Bubble ◽

Separated Flow ◽

Leading Edge ◽

Source Model ◽

Pressure Distributions ◽

Wide Range ◽

Trailing Edges ◽

Edge Separation

An inviscid analytic model is proposed for the steady separated flow around an inclined flat plate. With the plate normal to the stream, the model reduces to the wake-source model of Parkinson & Jandali originally developed for flow external to a symmetrical two-dimensional bluff body and its wake. At any other inclination, the Kutta condition is satisfied at both leading and trailing edges of the plate, and, in the limit that the angle of attack approaches zero, classical airfoil theory is recovered. A boundary condition is formulated based on some experimental results of Abernathy, but no additional empirical information is required. The predicted pressure distributions on the wetted surface for a wide range of angle attack are found to be in good agreement with experimental data, especially at smaller angles of attack. An extension to include a leading-edge separation bubble is explored and results are satisfactory.

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