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Research Papers

Numerical Study of Turbulent-Spot Development in a Separated Shear Layer

[+] Author and Article Information
Brian R. McAuliffe1

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

Metin I. Yaras

Department of Mechanical and Aerospace Engineering, Carleton University, 1125 Colonel By Drive, Ottawa, ON, 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 130(4), 041018 (Aug 04, 2008) (9 pages) doi:10.1115/1.2812948 History: Received June 12, 2007; Revised August 03, 2007; Published August 04, 2008

The development of turbulent spots in a separation bubble under elevated freestream turbulence levels is examined through direct numerical simulation. The flow Reynolds number, freestream turbulence level, and streamwise pressure distribution are typical of the conditions encountered on the suction side of low-pressure turbine blades of gas-turbine engines. Based on the simulation results, the spreading and propagation rates of the turbulent spots and their internal structure are documented, and comparisons are made to empirical correlations that are used for predicting the transverse growth and streamwise propagation characteristics of turbulent spots. The internal structure of the spots is identified as a series of vortex loops that develop as a result of low-velocity streaks generated in the shear layer. A frequency that is approximately 50% higher than that of the Kelvin–Helmholtz instability is identified in the separated shear layer, which is shown to be associated with the convection of these vortex loops through the separated shear layer. While freestream turbulence is noted to promote breakdown of the laminar separated shear layer into turbulence through the generation of turbulent spots, evidence is found to suggest coexistence of the Kelvin–Helmholtz instability, including the possibility of breakdown to turbulence through this mechanism.

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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 computational domain

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Figure 2

Streamwise distributions of freestream velocity and turbulence intensity

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Figure 3

Comparison of simulated and measured velocity time traces with an intermittency of 0.5 (experimental data from a test case of Roberts and Yaras(2))

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Figure 4

Velocity profiles, intermittency distribution, and spot origin histogram through the bubble region

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Figure 5

Unsteadiness of separation location: (a) z-x near-wall plane and (b) z-y plane at the averaged separation location

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Figure 6

Vortex loops within a turbulent spot

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Figure 7

Turbulent-spot growth as observed in the y-t and z-t planes

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Figure 8

Schematic of vortex loop structure during the early development of a turbulent spot

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Figure 9

Power spectra of (a) velocity and (b) static pressure

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Figure 10

Spot spreading half angles

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Figure 11

Identification of spot celerities: (a) leading edge and (b) trailing edge

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