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Research Papers

On the Response of a Strongly Diffusing Flow to Propagating Wakes

[+] Author and Article Information
J. P. Gostelow

Department of Engineering, University of Leicester, Leicester LE1 7RH, UKjpg7@le.ac.uk

R. L. Thomas

Whittle Laboratory, University of Cambridge, Cambridge CB3 0DY, UKrt280@cam.ac.uk

D. S. Adebayo

Department of Engineering, University of Leicester, Leicester LE1 7RH, UKdsa5@le.ac.uk

J. Turbomach 131(2), 021006 (Jan 22, 2009) (8 pages) doi:10.1115/1.2950054 History: Received June 08, 2007; Revised June 28, 2007; Published January 22, 2009

Further evidence on the similarities between transition and separation phenomena occurring in turbomachinery and wind tunnel flows is provided by measurements on a large scale flat plate under a strong adverse pressure gradient. The flat plate has a long laminar separation bubble and is subjected to a range of disturbances with triggering caused by injection of a transverse jet and subsequently by wakes generated by rods moving transversely upstream of the leading edge. Wakes were originally presented individually. Each individual wake provoked a vigorous turbulent patch, resulting in the instantaneous collapse of the separation bubble. This was followed by a very strong, and stable, calmed region. Following the lead given by the experiments of Gutmark and Blackwelder (1987, “On the Structure of Turbulent Spot in a Heated Laminar Boundary Layer  ,” Exp. Fluids, 5, pp. 207–229.) on triggered turbulent spots, wakes were then presented in pairs at different wake spacing intervals. In this way wake interaction effects could be investigated in more detail. As in the work on triggered turbulent spots the spacing between impinging wakes was systematically varied; it was found that for close wake spacings the calmed region acted to suppress the turbulence in the following turbulent patch. To investigate whether this phenomenon was a recurring one or whether the flow then reverted back to its unperturbed state, the experiments were repeated with three and four rods instead of two. This has the potential for making available a wide range of variables including direction and speed of rod rotation. It was found that the subsequent wakes were also suppressed by the calming effect. It may be anticipated that this repeating situation is present in a turbomachine, resulting in hidden benefits for blade count and efficiency. There may also conceivably be blade loading advantages while retaining favorable heat transfer conditions in high pressure turbines or stall margin in axial compressors. The inherent and prospective benefits of the calming effect therefore need to be understood thoroughly and new opportunities exploited where this is feasible.a

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Flat plate installation with fairing, hot-wire traverse and upstream wake generator

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Figure 2

Pressure distribution along the flat plate

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Figure 3

rms velocity perturbation, u′, and intermittency, γ, for a turbulent patch induced by a single primary wake at three chordwise locations. Freestream velocities are U∞=8.9m∕s, 9.0m∕s, and 8.8m∕s, respectively.

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Figure 4

Effect of increasing wake proximity on rms velocity. (a) x=0.2m; (b) x=0.4m; (c) x=0.6m; (d); x=0.8m.

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Figure 5

Space-time plots of rms velocity perturbation for four different wake proximity settings. Data from y=2.0mm.

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Figure 6

Variation of peak rms velocity with wake proximity parameter, ψ

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Figure 7

Four pairs of wakes at 40deg spacing

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Figure 10

Timewise variation of rms velocity perturbation at y=1mm, x=0.8m, and a wake spacing of 30deg

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Figure 9

Space-time plots of rms velocity perturbation for four rods at 30deg proximity setting

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Figure 8

Four pairs of wakes at 30deg spacing

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