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TECHNICAL PAPERS

Sensitivity of a Square Cylinder Wake to Forced Oscillations

[+] Author and Article Information
Sushanta Dutta, P. K. Panigrahi, K. Muralidhar

Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India

J. Fluids Eng 129(7), 852-870 (Jan 19, 2007) (19 pages) doi:10.1115/1.2742736 History: Received July 18, 2006; Revised January 19, 2007

The wake of a square cylinder at zero angle of incidence oscillating inline with the incoming stream has been experimentally studied. Measurement data are reported for Reynolds numbers of 170 and 355. The cylinder aspect ratio is set equal to 28 and a limited study at an aspect ratio of 16 has been carried out. The frequency of oscillation is varied around the Strouhal frequency of a stationary cylinder, and the amplitude of oscillation is 10–30% of the cylinder size. Spatial and temporal flow fields in the cylinder wake have been studied using particle image velocimetry and hot-wire anemometry, the former providing flow visualization images as well. A strong effect of forcing frequency is clearly seen in the near wake. With an increase in frequency, the recirculation length substantially reduces and diminishes the time-averaged drag coefficient. The time-averaged vorticity contours show that the large-scale vortices move closer to the cylinder. The rms values of velocity fluctuations increase in magnitude and cluster around the cylinder as well. The production of turbulent kinetic energy shows a similar trend as that of spanwise vorticity with the former showing greater asymmetry at both sides of the cylinder centerline. The instantaneous vorticity contours show that the length of the shear layer at separation decreases with increasing frequency. The effect of amplitude of oscillation on the flow details has been studied when the forcing frequency is kept equal to the vortex-shedding frequency of the stationary cylinder. An increase in amplitude diminishes the time-averaged drag coefficient. The peak value of rms velocity increases, and its location moves upstream. The length of the recirculation bubble decreases with amplitude. The reduction in drag coefficient with frequency and amplitude is broadly reproduced in experiments with the cylinder of lower aspect ratio.

Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic drawing of the experimental apparatus

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Figure 2

Validation in terms of Strouhal number as a function of Reynolds number for flow past a square cylinder at zero angle of incidence

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Figure 3

Flow visualization images for inline oscillations of a square cylinder (f∕f0=2, below) with a circular cylinder (above) as reported by Griffin and Ramberg (1)

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Figure 4

Time-averaged nondimensional velocity vectors above an oscillating cylinder: Effect of frequency ratio, Re=170, A∕B=0.1; flooded contours represent the absolute velocity magnitude

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Figure 5

Time-average u-velocity profiles above an oscillating cylinder at various x-locations: Effect of frequency ratio, Re=170, A∕B=0.1

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Figure 6

Time-average v-velocity profiles above an oscillating cylinder at various x-locations: Effect of frequency ratio, Re=170, A∕B=0.1

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Figure 7

Time-averaged spanwise vorticity field (top) and streamlines (below) in the wake of an oscillating square cylinder as a function of the forcing frequency, Re=170, A∕B=0.1

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Figure 8

Nondimensional contours of turbulent intensity in the wake of an oscillating square cylinder as a function of the forcing frequency, Re=170, A∕B=0.1

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Figure 9

Dimensionless production of turbulent kinetic energy, Re=170, A∕B=0.1

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Figure 10

Comparison of centerline recovery of streamwise velocity of the present study (left) at various oscillation frequencies with Konstantinides (15) (right). The reference study is for a circular cylinder.

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Figure 11

Instantaneous spanwise vorticity contours above an oscillating cylinder. First row: f∕f0=0.5; second row: f∕f0=1; third row: f∕f0=2. A∕B=0.1; maximum, minimum, and increments in ωz are 3,−3,0.25.

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Figure 12

Power spectra of the transverse velocity component in the wake of an oscillating cylinder; Re=170

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Figure 13

Instantaneous flow visualization images on the x-z plane for various forcing frequencies (f∕f0=0.5, bottom; 1, middle; 2, top); 80% of the cylinder length is included in each frame. Main flow direction is from the right to the left.

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Figure 14

Instantaneous flow visualization images in the x-y plane for various forcing frequencies (frequency ratios=0.5, 1, 2)

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Figure 15

Time-averaged spanwise vorticity contours (ωz) and streamlines in the wake of a square cylinder at various frequency ratios (0.5,1,2) with a perturbation amplitude at A∕B=0.1 and Re=355

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Figure 17

Rms contours for various amplitudes of oscillation, Re=170

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Figure 19

Time-averaged spanwise vorticity contours (ωz) for various forcing frequencies (f∕f0=0.5,1,2) (top) and amplitude (A∕B=0.1,0.14,0.21) (bottom) at Re=170; aspect ratio=16

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Figure 18

Power spectra for various amplitudes of oscillation, Re=170

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Figure 16

Time-averaged vorticity and stream traces for various amplitudes of oscillation (A∕B=0.1,0.17,0.26) at a forcing frequency f∕f0=1, Re=170

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