Abstract
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 ⩽ A 0 ⩽ 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.
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Abbreviations
- a 0 :
-
forcing amplitude=(Q forced−Q unforced)/U 0
- AR :
-
aspect ratio=W/H
- C p :
-
wall-pressure coefficient=(P-P 0)/(l/2) ρU 20
- ER :
-
expansion ratio=(2H+H)/2H
- f f :
-
forcing frequency, Hz
- f s :
-
shedding frequency, Hz
- g :
-
slit width = 1.0 ± 0.1 mm
- H :
-
step height = 50 mm
- P :
-
wall-static pressure, Pa
- P 0 :
-
wall-static pressure at x/H= -2.0, Pa
- Q forced :
-
total velocity measured at reference position for forced flow, m/s
- Q unforced :
-
total velocity measured at reference position for unforced flow, m/s
- Re H :
-
Reynolds number based on H and U 0,= U 0 H/v
- St H :
-
Reduced forcing frequency, Strouhal number = f f H/U 0
- St θ :
-
Reduced forcing frequency based on the momentum thickness = f f θ/U 0
- U, V :
-
streamwise and vertical time-mean velocity, m/s
- u :
-
streamwise fluctuation velocity, m/s
- U 0 :
-
free-stream velocity, m/s
- \(\sqrt {\bar u^2 } \) :
-
r.m.s. intensity of streamwise velocity fluctuation, m/s
- x r :
-
reattachment length, m
- X r 0 :
-
reattachment length for A 0 = 0, m
- x, y, z :
-
distance of streamwise, vertical and spanwise respectively, m
- W :
-
width of test section = 625 mm
- δ:
-
boundary-layer thickness, cm
- δ* :
-
displacement thickness, cm
- γp :
-
forward-flow time fraction
- ρ:
-
density of air for measurement, kg/m3
- v :
-
kinematic viscosity of air for measurement, m2/s
- θ:
-
momentum thickness, cm
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Chun, K.B., Sung, H.J. Control of turbulent separated flow over a backward-facing step by local forcing. Experiments in Fluids 21, 417–426 (1996). https://doi.org/10.1007/BF00189044
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DOI: https://doi.org/10.1007/BF00189044