scholarly journals The scales of the leading-edge separation bubble

2021 ◽  
Vol 33 (4) ◽  
pp. 045101
Author(s):  
J. A. Smith ◽  
G. Pisetta ◽  
I. M. Viola
Keyword(s):  
Separation Bubble ◽  
Leading Edge ◽  
Edge Separation

Flow Visualization Experiments on Tethered Flying Green Lacewings Chrysopa Dasyptera

1992 ◽  
Vol 169 (1) ◽  
pp. 143-163 ◽  
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.

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

1991 ◽  
Vol 113 (3) ◽  
pp. 405-410 ◽  
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.

Control of Separation Bubble on a Blade Leading Edge by a Stationary Bar Wake

2000 ◽  
Author(s):  
K. Funazaki ◽  
Y. Harada ◽  
E. Takahashi
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Leading Edge ◽  
Test Model ◽  
Flat Plates ◽  
Flow Measurements ◽  
Inlet Flow ◽  
Aerodynamic Loss ◽  
Edge Separation ◽  
Blade Leading Edge

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.

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

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.

scholarly journals An Inviscid-Viscous Method to Model Leading Edge Separation Bubbles

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.

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

2000 ◽  
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.

On the steady separated flow around an inclined flat plate

1997 ◽  
Vol 333 ◽  
pp. 403-413 ◽  
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.

scholarly journals Control of a Leading-Edge Separation Bubble by Bimodal Disturbances.

1994 ◽  
Vol 60 (570) ◽  
pp. 432-437
Author(s):  
Masayuki Shimizu ◽  
Masaru Kiya ◽  
Osamu Mochizuki ◽  
Yasushi Ido
Keyword(s):  
Separation Bubble ◽  
Leading Edge ◽  
Edge Separation

The aerodynamics of hovering insect flight. VI. Lift and power requirements

1984 ◽  
Vol 305 (1122) ◽  
pp. 145-181 ◽  
Keyword(s):  
Vortex Shedding ◽  
Power Output ◽  
Flight Muscle ◽  
Insect Flight ◽  
Separation Bubble ◽  
Leading Edge ◽  
Mechanical Power ◽  
Mechanical Power Output ◽  
Edge Vortex ◽  
Edge Separation

The lift and power requirements for hovering insect flight are estimated by combining the morphological and kinematic data from papers II and III with the aerodynamic analyses of papers IV and V. The lift calculations are used to evaluate the importance in hovering of two distinct types of aerodynamic mechanisms: (i) the usual quasi-steady mechanism, where the circulation for lift is primarily determined by translation of the wing, and (ii) rotational mechanisms, where the circulation is largely governed by wing rotation at either end of the wingbeat. Power estimates are compared with the available measurements of metabolic rate during hovering to investigate the role of elastic energy storage, the maximum mechanical power output of the flight muscles, and the muscle efficiency. The quasi-steady mechanism proves inadequate for the lift requirements of hover-flies using an inclined stroke plane, and for a ladybird beetle and a crane-fly hovering with a horizontal stroke plane. Observed angles of attack rule out lift enhancement by unsteady modifications to the quasi-steady mechanism, such as delayed stall, but the rotational lift mechanisms proposed in paper IV seem consistent with the kinematics. The rotational mechanisms rely on concentrated vortex shedding from the leading edge during rotation, with attachment of that vorticity as a leading edge separation bubble during the subsequent half-stroke. Strong leading edge vortex shedding should result from delayed pronation for the hover-fly, a near fling and partial fling for the ladybird, and profile flexion for the crane-fly (the flex mechanism). The kinematics for the other insects hovering with a horizontal stroke plane are basically the same as for the anomalous crane-fly, and the quasi-steady mechanism cannot be accepted for them while rejecting it for the crane-fly. All of these insects flex their wings in a similar manner during rotation, and could use the flex mechanism for lift generation. The implication is that most, if not all, hovering animals do not rely on quasi-steady aerodynamics, but use rotational lift mechanisms instead. It is not possible to reconcile the power estimates with the commonly accepted values of both the mechanochemical efficiency of insect flight muscle (about 25%) and its maximum mechanical power output (about 20 W N -1 of muscle). Maximum efficiencies of 12-29% could be obtained only if there is no elastic storage of the kinetic energy of the flapping wings, but this would require more than twice the accepted value for maximum mechanical power output. The available evidence suggests that substantial elastic storage does occur, and that the maximum mechanical power output is close to the accepted value. If so, then the efficiency of both fibrillar and non-fibrillar flight muscle is likely to be only 5-9%.

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