Experiments in Fluids

, Volume 21, Issue 6, pp 417–426 | Cite as

Control of turbulent separated flow over a backward-facing step by local forcing

  • K. B. Chun
  • H. J. Sung


An experimental study was made of the flow over a backward-facing step. Excitations were given to separated flow by means of a sinusoidally oscillating jet issuing from a thin slit near the separation line. The Reynolds number based on the step height (H) varied 13000 ⩽ Re H ⩽ 33000. Effect of local forcing on the flow structure was scrutinized by altering the forcing amplitude (0 ⩽ A0 ⩽ 0.07) and forcing frequency (0 ⩽ St H ⩽ 5.0). Small localized forcing near the separation edge enhanced the shear-layer growth rate and produced a large roll-up vortex at the separation edge. A large vortex in the shear layer gave rise to a higher rate of entrainment, which lead to a reduction in reattachment length as compared to the unforced flow. The normalized minimum reattachment length (x r )min/x x0 was obtained at St θ ≅ 0.01. The most effective forcing frequency was found to be comparable to the shedding frequency of the separated shear layer.


Vortex Reynolds Number Shear Layer Flow Structure Separate Flow 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

List of symbols


forcing amplitude=(QforcedQunforced)/U0


aspect ratio=W/H


wall-pressure coefficient=(P-P0)/(l/2) ρU 0 2


expansion ratio=(2H+H)/2H


forcing frequency, Hz


shedding frequency, Hz


slit width = 1.0 ± 0.1 mm


step height = 50 mm


wall-static pressure, Pa


wall-static pressure at x/H= -2.0, Pa


total velocity measured at reference position for forced flow, m/s


total velocity measured at reference position for unforced flow, m/s


Reynolds number based on H and U0,= U0H/v


Reduced forcing frequency, Strouhal number = f f H/U0


Reduced forcing frequency based on the momentum thickness = f f θ/U0

U, V

streamwise and vertical time-mean velocity, m/s


streamwise fluctuation velocity, m/s


free-stream velocity, m/s

\(\sqrt {\bar u^2 } \)

r.m.s. intensity of streamwise velocity fluctuation, m/s


reattachment length, m


reattachment length for A0 = 0, m

x, y, z

distance of streamwise, vertical and spanwise respectively, m


width of test section = 625 mm

Greek symbols


boundary-layer thickness, cm


displacement thickness, cm


forward-flow time fraction


density of air for measurement, kg/m3


kinematic viscosity of air for measurement, m2/s


momentum thickness, cm


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  1. Bhattacharjee S; Sheelke B; Troutt TR (1986) Modifications of vortex interactions in a reattaching separated flow. AIAA J 24: 623–629Google Scholar
  2. Bradshaw P; Wong FYF (1972) The reattachment and relaxation of a turbulent shear layer. J Fluid Mech 52: 113–135Google Scholar
  3. de Brederode V; Bradshaw P (1978) Influence of the side walls on the turbulent center-plane boundary-layer in a squareduct. J Fluids Eng 100: 91–96Google Scholar
  4. Cooper PI; Sheridan JC; Flood GJ (1986) The effects of sound on forced convection over a flat plate. Int J Heat Fluid Flow 7: 61–68Google Scholar
  5. Eaton JK; Johnston JP (1980) Turbulent flow reattachment: an experimental study of the flow and structure behind a backwardfacing step. Report MD-39 Thermoscience Division, Dept. of Mechanical Eng., Stanford UniversityGoogle Scholar
  6. Eaton JK; Johnston JP (1981) A review of research on subsonic turbulent flow reattachment. AIAA J 19: 1093–1100Google Scholar
  7. Gai SL; Sharma SD (1987) Pressure distribution behind a rearward facing step. Exp Fluids 14: 154–158Google Scholar
  8. Hasan MAZ (1992) The flow over a backward-facing step under controlled perturbation: laminar separation J Fluid Mech 238- 73–96Google Scholar
  9. Kim J; Kline SJ; Johnston JP (1980) Investigation of a reattaching turbulent shear layer: Flow over a backward-facing step. J Fluids Eng 102: 302–308Google Scholar
  10. Kiya M; Shimizu M; Mochizuki O; Ido Y; Tezuka H (1993) Active forcing of an axisymmetric leading-edge turbulent separation bubble. AIAA paper 93-3245Google Scholar
  11. Nagib HM; Reisenthel PH; Koga DJ (1985) On the dynamical scaling of forced unsteady separated flows. AIAA Shear Flow Control Conference: AIAA-85-0553Google Scholar
  12. Nishioka M; Asai M; Yoshida S (1990) Control of flow separation by acoustic excitation. AIAA J 28: 1909–1915Google Scholar
  13. Rose FW; Kegelman JT (1986) Control of coherent structures in reattaching laminar and turbulent shear layers. AIAA J 24: 1956–1963Google Scholar
  14. Sigurdson LW (1995) The structure and control of a turbulent reattaching flow. J Fluid Mech 298: 139–165Google Scholar
  15. Zaman KBMO; Bar-Serve A; Mangalam SM (1987) Effect of acoustic excitation on the flow over a low-Re airfoil. J Fluid Mech 182: 127–148Google Scholar
  16. Zaman KBMQ; Hussain AKMF (1980) Vortex pairing on a circular jet under controlled excitation. Part 1. General jet response. J Fluid Mech 101: 449–491Google Scholar

Copyright information

© Springer-Verlag 1996

Authors and Affiliations

  • K. B. Chun
    • 1
  • H. J. Sung
    • 1
  1. 1.Department of Mechanical EngineeringKorea Advanced Institute of Science and TechnologyTaejonKorea

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