Analysis of leading-edge separation bubbles on airfoils

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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Experiments on the Weis-Fogh mechanism of lift generation by insects in hovering flight. Part 1. Dynamics of the ‘fling’

Journal of Fluid Mechanics ◽

10.1017/s0022112079001774 ◽

1979 ◽

Vol 93 (1) ◽

pp. 47-63 ◽

Cited By ~ 244

Author(s):

T. Maxworthy

Keyword(s):

Initial Phase ◽

Angle Of Attack ◽

Large Angle ◽

Leading Edge ◽

Tip Vortex ◽

Hovering Flight ◽

Separation Bubbles ◽

Mechanical Models ◽

Series Of Experiments ◽

Edge Separation

From a series of experiments using simplified mechanical models we suggest certain minor modifications to the Weis-Fogh (1973)–Lighthill (1973) explanation of the so-called ‘clap and fling’ mechanism for the generation of large lift coefficients by insects in hovering flight. Of particular importance is the production and motion of a leading edge, separation vortex that accounts for virtually all of the circulation generated during the initial phase of the ‘fling’ process. The magnitude of this circulation is substantially larger than that calculated using inviscid theory. During the motion that subsequently separates the wings, the vorticity over each of them is convected and combined to become a tip vortex of uniform circulation spanning the space between them. This combined vortex moves downwards as a part of a ring, of large impulse, that is then continuously fed from quasi-steady separation bubbles that move with the wings as they continue to open at a large angle of attack. Such effects are able to account for the large lift forces generated by the insect.

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Some Measurements of Leading Edge Separation Bubbles on a 10 per cent Thick Symmetrical Aerofoil

Journal of the Royal Aeronautical Society ◽

10.1017/s0368393100131542 ◽

1953 ◽

Vol 57 (516) ◽

pp. 819-823 ◽

Cited By ~ 3

Author(s):

J. Black ◽

R. D. Hunt

Keyword(s):

Thin Film ◽

Boundary Layer ◽

Liquid Film ◽

Pressure Distribution ◽

Constant Pressure ◽

Leading Edge ◽

Pressure Distributions ◽

Separation Bubbles ◽

Edge Separation

Pressure Distribution and liquid-film tests on a 10 per cent, thick aerofoil revealed the presence of separation “bubbles” close to the leading edge. These bubbles are formed beneath the boundary layer which separates near the leading edge and re-attaches farther aft; their existence is usually indicated by localised constant-pressure regions in the pressure distributions. It is also believed that if a thin film of liquid (such as a suspension of lamp-black in paraffin) is spread on the surface, the scrubbing action of the air rotating in the bubble will tend to draw liquid forward into the bubble, and hence the location and extent of the bubble may be indicated approximately by the accumulation of the fluid.Many boundary layer traverses of bubbles on N.A.C.A. aerofoils have been made, but it was felt that similar measurements of the bubbles on this particular aerofoil would provide useful data, since the separation characteristics of this section appeared to differ from those in the N.A.C.A. tests.

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Closure to “Discussion of ‘Leading Edge Separation Bubbles on Turbomachine Blades’” (1995, ASME J. Turbomach., 117, p. 125)

Journal of Turbomachinery ◽

10.1115/1.2835628 ◽

1995 ◽

Vol 117 (1) ◽

pp. 125-125

Author(s):

R. E. Walraevens ◽

N. A. Cumpsty

Keyword(s):

Leading Edge ◽

Separation Bubbles ◽

Edge Separation

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The Effect of Reynolds Number on the Behaviour of a Leading Edge Separation Bubble

Volume 1: Turbomachinery ◽

10.1115/96-gt-410 ◽

1996 ◽

Cited By ~ 1

Author(s):

Birinchi K. Hazarika ◽

Charles Hirsch

Keyword(s):

Reynolds Number ◽

Shear Layer ◽

Production Rate ◽

Free Stream ◽

Separation Bubble ◽

Leading Edge ◽

Reynolds Numbers ◽

Separation Point ◽

Separation Bubbles ◽

Edge Separation

An experimental investigation of a separation bubble on a C4 leading edge plate at an incidence in a low turbulence free stream at six Reynolds numbers, is reported. The long separation bubble, formed at the leading edge, has a short laminar and transitional zone followed by a long turbulent zone. The increase in Reynolds number reduced the laminar and transitional part significantly, but its effect on the length of the separation bubble is marginal till the transition starts at the separation point. The peak intermittency factor, which occurs at the centre of the shear layer, follows the universal intermittency distribution curve. The spot production rate for the separated flows are several orders of magnitude higher than that for the attached boundary layers. The transition process is initiated by the amplification of the instability waves in the shear layer similar to the natural mode of transition. At high Reynolds numbers, the onset of transition is likely to take place at the separation point. At lower chord Reynolds numbers, the separation to onset Reynolds number and the spot production rate parameter are functions of the separation momentum thickness Reynolds number. The free stream turbulence intensity has a strong influence on the spot production rate. New correlations for transition in the leading edge separation bubbles are proposed based on all the available intermittency measurements in the leading edge separation bubbles.

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