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Research Papers: Fundamental Issues and Canonical Flows

Control of Flow Separation in a Turbulent Boundary Layer Using Time-Periodic Forcing

[+] Author and Article Information
Minjeong Cho

Department of Mechanical and
Aerospace Engineering,
Seoul National University,
1 Gwanak-ro, Gwanak-gu,
Seoul 08826, South Korea
e-mail: minjeongcho1127@gmail.com

Sangho Choi

Department of Mechanical and
Aerospace Engineering,
Seoul National University,
1 Gwanak-ro, Gwanak-gu,
Seoul 08826, South Korea
e-mail: sirius_35@hanmail.net

Haecheon Choi

Department of Mechanical and
Aerospace Engineering;
Institute of Advanced Machines and Design,
Seoul National University,
1 Gwanak-ro, Gwanak-gu,
Seoul 08826, South Korea
e-mail: choi@snu.ac.kr

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received March 7, 2016; final manuscript received June 7, 2016; published online July 29, 2016. Assoc. Editor: D. Keith Walters.

J. Fluids Eng 138(10), 101204 (Jul 29, 2016) (10 pages) Paper No: FE-16-1146; doi: 10.1115/1.4033977 History: Received March 07, 2016; Revised June 07, 2016

A time-periodic blowing/suction is provided to control turbulent separation in a boundary layer using direct numerical simulation. The blowing/suction is given just before the separation point, and its nondimensional forcing frequency ranges from F*= fLb/U = 0.28–8.75, where f is the forcing frequency, Lb is the streamwise length of uncontrolled separation bubble, and U is the freestream velocity. The size of separation bubble is minimum at F*= 0.5. At low forcing frequencies of F*≤ 0.5, vortices generated by the forcing travel downstream at convection velocity of 0.32–0.35 U, bring high momentum toward the wall, and reduce the size of separation bubble. However, at high forcing frequencies of F*≥ 1.56, flow separation disappears and appears in time during the forcing period. This phenomenon occurs due to high wall-pressure gradients alternating favorably and adversely in time. A potential flow theory indicates that this rapid change of the wall pressure in time occurs through an inviscid mechanism. Finally, it is shown that this high-frequency forcing requires a large control input power due to high pressure work.

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Figures

Grahic Jump Location
Fig. 3

Inflow mean streamwise velocity profile at Re = 300: ––– present DNS; ▪ DNS (Spalart) [31]

Grahic Jump Location
Fig. 2

Wall-normal velocity distribution along the top boundary

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Fig. 1

Schematic diagram of the computational domain

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Fig. 4

Mean streamlines: (a) uncontrolled, (b) F*= 8.75, (c) 3.5, (d) 1.56, (e) 0.93, (f) 0.78, (g) 0.5, and (h) 0.28. Here, thick black (blue online) lines and arrows denote the mean separation bubble and the center of forcing slot, respectively.

Grahic Jump Location
Fig. 5

Contours of the spanwise-averaged skin friction coefficient: (a) uncontrolled, (b) F*= 8.75, (c) 3.5, (d) 1.56, (e) 0.93, (f) 0.78, (g) 0.5, and (h) 0.28. Here, black curves and arrows (blue online) correspond to the locations of Cf = 0 and the center of forcing slot, respectively.

Grahic Jump Location
Fig. 6

Flow fields for the uncontrolled case and controlled case of F*= 0.5: (a) contours of the instantaneous spanwise-averaged spanwise vorticity with velocity vectors in an (x, y) plane for no control (upper) and control at F*= 0.5 (lower) and (b) contours of the instantaneous skin friction coefficient on the wall. The arrow in the x-axis indicates the location of the forcing slot center. In (b), thick black-dashed lines denote the mean separation and reattachment lines for the uncontrolled case, and black solid curves indicate Cf = 0 for the case of F*= 0.5, respectively.

Grahic Jump Location
Fig. 7

Contours of phase- and spanwise-averaged spanwise vorticity with streamlines at the phases of blowing start, blowing maximum, suction start, and suction maximum: (a) F*= 0.28 and (b) 0.5. Here, black arrows denote the forcing slot center.

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Fig. 8

Flow fields for the control at F*= 8.75 at the phases of blowing maximum (upper) and suction maximum (lower): (a) contours of the instantaneous spanwise-averaged spanwise vorticity with velocity vectors in an (x, y) plane and (b) contours of the instantaneous skin friction coefficient on the wall. In (b), thick-dashed lines indicate Cf = 0 for the uncontrolled flow.

Grahic Jump Location
Fig. 9

Phase- and spanwise-averaged wall-pressure distributions: (a) F*= 8.75 and (b) 0.5. – – – –, blowing start; –––––, blowing maximum; – · – · –, suction start; and – ·· – ·· –, suction maximum. Here, thin black curves (blue online) and black arrows denote the mean wall-pressure distribution of uncontrolled flow and the location of the forcing slot center, respectively.

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Fig. 10

Wall-pressure distributions predicted by a potential flow theory: (a) F*= 8.75 and (b) 0.5. – – – –, blowing start; –––––, blowing maximum; – · – · –, suction start; and – ·· – ·· –, suction maximum. Here, black arrows denote the forcing slot center.

Grahic Jump Location
Fig. 11

Three-dimensional vortical structures identified by λ2 = −0.005 [33]: (a) F*= 8.75 and (b) 0.5

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Fig. 12

Variation of the mean input power with the forcing frequency: ———, total mean input power; - - - - - -, pressure work; and — — —, kinetic energy convection

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