Instantaneous Behavior of Streamwise Vortices for Turbulent Boundary Layer Separation Control

[+] Author and Article Information
K. P. Angele

 KTH, Mekanik, S-100 44 Stockholm, Sweden

F. Grewe

Hermann-Föttinger-Institut, Strömungsmechanik Technische Universität, 10623 Berlin, Germany

J. Fluids Eng 129(2), 226-235 (Jun 06, 2006) (10 pages) doi:10.1115/1.2409327 History: Received May 23, 2005; Revised June 06, 2006

The present study investigates turbulent boundary layer separation control by means of streamwise vortices with focus on the instantaneous vortex behavior. A turbulent boundary layer is exposed to a pressure gradient that generates a separation bubble with substantial backflow. The separation bubble is controlled by conventional passive vortex generators creating pairs of counterrotating vortices. Quantitative information is achieved by applying Particle Image Velocimetry (PIV) to the cross-stream plane of the vortices. The characteristics of a pair of counter-rotating vortices shed from a vortex generator is investigated in the near-field downstream of the vortex generator. The vortices were found to grow with the boundary layer in the downstream direction, and the maximum vorticity decreases as the circulation is conserved. The vortices are nonstationary, and the movements in the spanwise direction are larger than those in the wall-normal direction, due to the presence of the wall. The vortices fluctuate substantially and move over a spanwise distance, which is approximately equal to their size. The most probable instantaneous separation between the two vortices shed from one vortex generator equals the difference between their mean positions. The unsteadiness of the vortices contributes to the observed maxima in the Reynolds stresses around the mean vortex centers. The instantaneous vortex size and the instantaneous maximum vorticity are also fluctuating properties, and the instantaneous vortex is generally smaller and stronger than the mean vortex. A correlation was found between a large instantaneous vortex size and a low instantaneous maximum vorticity (and vice versa), suggesting that the vortices are subjected to vortex stretching.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

Vortex generator creating counterrotating streamwise vortices in the downstream direction

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Figure 2

Sketch of the test section

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Figure 3

Uncontrolled separated turbulent boundary layer

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Figure 4

VGs seen from above. The yz planes were captured centered around z∕D=0, covering the region ±z∕D=0.35 and y∕h=4. Spanwise positions of the xy-plane measurements are z∕D=0, 0.165, and 0.5, covering y∕h=1.9 and Δx∕h=2.6.

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Figure 5

Streamwise development of cf downstream of the VGs

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Figure 6

Mean velocity profiles in the xy plane at (a,c) x∕h=5.5 and at (b,d) x∕h=9, compared to the uncontrolled case (LDV dashed line). Circle: z∕D=0; triangle: z∕D=0.165; star: z∕D=0.5. The boundary layer thickness is ∼2.5h and ∼3.5h respectively, in the uncontrolled case.

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Figure 7

(a) Vortex strength Q∕(Uinl∕h)2, (b) vorticity ωx∕(Uinl∕h) and secondary flow components, (c) W∕Uinl, and (d) V∕Uinl at x∕h=5.5

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Figure 8

(a) Vortex strength Q∕(Uinl∕h)2, (b) vorticity ωx∕(Uinl∕h) and secondary flow components, (c) W∕Uinl, and (d) V∕Uinl at x∕h=9

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Figure 9

Downstream development of: triangle: ωx,max∕(Uinl∕h); plus: Qmax∕(Uinlh)2; circle: A∕h2, and star: Γ∕Uinlh

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Figure 10

Downstream development of the mean vortex centers in the (top) spanwise direction and (bottom) wall-normal direction

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Figure 11

(a) Spanwise and (b) wall-normal PDFs of the vortex center positions z′∕D and y′∕h (lines) and instantaneous distance between the vortices Δz′∕D and Δy′∕h (dashed lines) at x∕h=5.5. Note that D∕h=10.

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Figure 12

(a) PDF of the maximum vorticity in the instantaneous vortex center and (b) maximum instantaneous vorticity versus instantaneous vortex size at x∕h=5.5. The line is only included for visual aid.

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Figure 13

The fluctuating secondary flow components (left) v′2¯∕Uinl2 and (right) w′2¯∕Uinl2 in the yz plane at (a,b)x∕h=5.5




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