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Three-dimensional coherent structure in a separated and reattaching flow over a backward-facing step

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Abstract

An experimental study was carried out to elucidate the large-scale vortical structure in a separated and reattaching flow over a backward-facing step. The Reynolds number based on the step height (H) was Re H =33,000. The large-scale vortical structure was probed by means of three-dimensional velocity measurements performed at the recirculation zone (x/H=4.0) and the reattachment zone (x/H=7.5). A 32-channel microphone array extending in the streamwise and spanwise directions was used for sensing the wall pressure fluctuations. The relationship between the flow field and the relevant spatial mode of the pressure field was determined by examining the spatial box filtering. From the relevant spatial mode of the wall pressure fluctuations, a conditional averaging technique was employed to characterize the coherent structure. In addition, the cross-correlation between velocity and wall pressure fluctuations was calculated to identify the structure and the length scale of the large-scale vortex. The cross-correlation results revealed that the large-scale hairpin vortices have a three-dimensional structure, in agreement with previous findings. The present results clearly show the growth and downstream elongation of the hairpin vortices.

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Abbreviations

H :

step height, m

k :

turbulent kinetic energy, m2/s2

q :

freestream dynamic pressure, Pa

Re H :

Reynolds number based on U 0 and H,U oH/ν

U 0 :

freestream velocity, m/s

U c :

convection velocity, m/s

X 0 :

streamwise coordinate of the measurement origin, m

x R :

time mean reattachment length, m

γ p :

forward flow time fraction

ρ :

cross-correlation coefficient

τ :

time delay, s

ω :

vorticity, m2/s

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Acknowledgement

This work was supported by a grant from the National Research Laboratory of the Ministry of Science and Technology, Korea.

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Authors and Affiliations

  1. Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong, 305-701, Yusong-ku, Taejon, Korea

    I. Lee, S. K. Ahn & H. J. Sung

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  1. I. Lee
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  2. S. K. Ahn
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  3. H. J. Sung
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Correspondence to H. J. Sung.

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Lee, I., Ahn, S.K. & Sung, H.J. Three-dimensional coherent structure in a separated and reattaching flow over a backward-facing step. Exp Fluids 36, 373–383 (2004). https://doi.org/10.1007/s00348-003-0637-6

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  • DOI: https://doi.org/10.1007/s00348-003-0637-6

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