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Research Papers

Transition Mechanisms in Separation Bubbles Under Low- and Elevated-Freestream Turbulence

[+] Author and Article Information
Brian R. McAuliffe1

Department of Mechanical and Aerospace Engineering,  Carleton University, 1125 Colonel By Drive, Ottawa, Ontaria K1S 5B6, Canadabrian.mcauliffe@nrc-cnrc.gc.ca

Metin I. Yaras

Department of Mechanical and Aerospace Engineering,  Carleton University, 1125 Colonel By Drive, Ottawa, Ontaria K1S 5B6, Canadametiṉyaras@carleton.ca

1

Present address: Aerodynamics Laboratory, Institute for Aerospace Research, National Research Council of Canada, 1200 Montreal Road, Building M-2 Ottawa, ON K1A 0R6, Canada.

J. Turbomach 132(1), 011004 (Sep 11, 2009) (10 pages) doi:10.1115/1.2812949 History: Received June 12, 2007; Revised August 03, 2007; Published September 11, 2009

Through numerical simulations, this paper examines the nature of instability mechanisms leading to transition in separation bubbles. The results of two direct numerical simulations are presented in which separation of a laminar boundary layer occurs over a flat surface in the presence of an adverse pressure gradient. The primary difference in the flow conditions between the two simulations is the level of freestream turbulence with intensities of 0.1% and 1.45% at separation. In the first part of the paper, transition under a low-disturbance environment is examined, and the development of the Kelvin–Helmholtz instability in the separated shear layer is compared to the well-established instability characteristics of free shear layers. The study examines the role of the velocity-profile shape on the instability characteristics and the nature of the large-scale vortical structures shed downstream of the bubble. The second part of the paper examines transition in a high-disturbance environment, where the above-mentioned mechanism is bypassed as a result of elevated-freestream turbulence. Filtering of the freestream turbulence into the laminar boundary layer results in streamwise streaks, which provide conditions under which turbulent spots are produced in the separated shear layer, grow, and then merge to form a turbulent boundary layer. The results allow identification of the structure of the instability mechanism and the characteristic structure of the resultant turbulent spots. Recovery of the reattached turbulent boundary layer is then examined for both cases. The large-scale flow structures associated with transition are noted to remain coherent far downstream of reattachment, delaying recovery of the turbulent boundary layer to an equilibrium state.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 2

Boundary-layer-edge velocity distributions for both simulations in the region 0.40m<x<0.78m, and time-averaged locations of separation, transition onset, and reattachment

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Figure 4

Power spectra of velocity in the separation bubble under low-freestream turbulence (at y=0.004m)

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Figure 5

Characteristics of the separation bubble under low-Tu freestream conditions (wall-normal coordinate stretched for better visualization); (a) time-averaged velocity vector profiles with every sixth profile shown; (b) rms-velocity contours

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Figure 6

Transitional structures in the separation bubble under low-Tu freestream conditions; (a) vorticity isosurface Ω=1500s−1, (b) pressure isosurfaces p′=Pa (dark), p′=1Pa (light)

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Figure 8

Power spectra of velocity through the bubble region under elevated-freestream turbulence (at y=0.00125m)

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Figure 12

Turbulent boundary-layer properties downstream of the separation bubble

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Figure 1

Schematic of computational domain

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Figure 3

Vorticity roll-up of the separated shear layer caused by the Kelvin–Helmholtz instability (trajectories of two roll-up vortices shown by white lines)

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Figure 7

Streamwise streaks in the shear layer observed in a plane where U¯≈1∕2Ue¯; (a) streamwise fluctuations; (b) wall-normal fluctuations

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Figure 9

Evolution of a turbulent spot through time sequence of x‐y planes along the centerline of the spot (averaged vectors and spanwise vorticity in uppermost plot)

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Figure 10

Topology of a turbulent spot in a separated shear layer

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Figure 11

Comparison of streamwise intermittency distribution, boundary layer growth characteristics, and velocity-fluctuation correlations for the two cases examined

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