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Research Papers

Numerical Study of Instability Mechanisms Leading to Transition in Separation Bubbles

[+] Author and Article Information
Brian R. McAuliffe

Aerodynamics Laboratory,Institute for Aerospace Research, 1200 Montreal Road, Building M-2, Ottawa, ON K1A 0R6, Canada

Metin I. Yaras

Department of Mechanical and Aerospace Engineering, Carleton University, 1125 Colonel By Drive, Ottawa, ON, K1S 5B6, Canada

J. Turbomach 130(2), 021006 (Feb 12, 2008) (8 pages) doi:10.1115/1.2750680 History: Received July 13, 2006; Revised December 14, 2006; Published February 12, 2008

In this paper, transition in a separation bubble is examined through numerical simulation. The flow Reynolds number and streamwise pressure distribution are typical of the conditions encountered on the suction side of low-pressure turbine blades of gas-turbine engines. The spatial and temporal resolutions utilized in the present computations correspond to a coarse direct numerical simulation, wherein the majority of turbulence scales, including the inertial subrange, are adequately resolved. The accuracy of the simulation results is demonstrated through favorable comparisons to experimental data corresponding to the same flow conditions. The results of the simulation show linear Tollmien-Schlichting (T-S) instability growth downstream of the point of separation, leading to the roll up of spanwise vorticity into discrete vortical structures, characteristic of Kelvin-Helmholtz (K-H) instability growth. The extent of cross-stream momentum exchange associated with packets of amplified T-S waves is examined, along with details of the time-periodic breakdown into turbulence occurring upon the development of the K-H instability. Reynolds-averaged properties of the separation bubble are presented and provide evidence of the strong three-dimensional nature of the reattachment process.

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Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Schematic of test section

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Figure 2

Schematic of computational domain

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Figure 3

Experimentally measured freestream velocity distributions

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Figure 4

Sample hot-wire velocity-fluctuation time traces

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Figure 5

Comparison of measured and computed velocity-fluctuation time traces for test case 1 (y=2mm)

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Figure 6

Disturbance spectra in the separation bubble: (a) experiment and (b) simulation (test case 1)

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Figure 7

Instability growth rates at the dominant frequency (test case 1)

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Figure 8

Effects of instability growth on velocity profiles at x′=85mm for test case 1 (p=peak, t=trough)

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Figure 9

Instantaneous vector field through the transition region of test case 1 (x-y plane at z=30mm and x-z plane at y=4mm)

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Figure 10

Streamwise velocity fluctuation profiles in the separation-bubble region (test case 1)

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Figure 11

Turbulence statistics for test case 1

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