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Effects of a geometrical surface disturbance on flow past a circular cylinder: a large-scale spanwise wire

  • A. EKMEKCI (a1) and D. ROCKWELL (a2)

Abstract

Flow control induced by a single wire that is attached on the outer surface and parallel to the span of a stationary circular cylinder is investigated experimentally. The Reynolds number has a value of 10 000 and the wire diameter is nearly two orders of magnitude smaller than the cylinder diameter, while being larger than the thickness of the unperturbed boundary layer forming around the cylinder. A technique of high-image-density particle image velocimetry is used to characterize mean and unsteady structures of the separating shear layer and the near wake. Only one of the shear layers is directly perturbed by the surface wire. This disturbance, however, has important global consequences over the entire near wake, provided that the wire is located within a certain range of angular positions with respect to the approach flow. Over this range, there are two angles that can be defined as critical on the basis of the streamwise extent of the near-wake structure. In a simplified sense, these critical angles are associated with significant extension and contraction of the near wake, relative to the wake in the absence of the effect of a surface disturbance. The critical angle of the wire that yields the most significant extension of the near wake is also found to lead to bistable oscillations of the separating shear layer at irregular time intervals, much longer than the time scale associated with the classical Kármán vortex shedding. The foregoing two critical states of extension and contraction of the near wake are, respectively, linked to attenuation or enhancement of the Kármán instability. Moreover, the onset of the shear-layer instability, Reynolds stress, Strouhal number and the transverse extent of shear-layer flapping are all shown to depend on the angular position of the wire within the defined range of angles.

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Corresponding author

Email address for correspondence: ekmekci@utias.utoronto.ca

References

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Type Description Title
VIDEO
Movies

Ekmekci supplementary movie
Movie 1. Vorticity contours when the surface wire is located at the first critical angular position qc1 = 55°. Absolute values of normalized vorticity are plotted for purposes of comparison of the opposite shear layers. Minimum and incremental values are as follows: [|w|D/Uo]min = 2 and D[|w|D/Uo] = 1. Herein, a rhythmic formation of large-scale vorticity clusters does not exist. Hence, this movie confirms that the Karman instability is significantly attenuated in this case.

 Video (3.8 MB)
3.8 MB
VIDEO
Movies

Ekmekci supplementary movie
Movie 1. Vorticity contours when the surface wire is located at the first critical angular position qc1 = 55°. Absolute values of normalized vorticity are plotted for purposes of comparison of the opposite shear layers. Minimum and incremental values are as follows: [|w|D/Uo]min = 2 and D[|w|D/Uo] = 1. Herein, a rhythmic formation of large-scale vorticity clusters does not exist. Hence, this movie confirms that the Karman instability is significantly attenuated in this case.

 Video (6.8 MB)
6.8 MB
VIDEO
Movies

Ekmekci supplementary movie
Movie 2. Vorticity contours when the surface wire is located at the second critical angular position qc2 = 65°. Absolute values of normalized vorticity are plotted for purposes of comparison of the opposite shear layers. Minimum and incremental values are as follows: [|w|D/Uo]min = 2 and D[|w|D/Uo] = 1. Large-scale vortical structures are formed in an alternate fashion in the near wake. Hence, this movie clearly depicts the periodic formation of Karman vortices.

 Video (3.8 MB)
3.8 MB
VIDEO
Movies

Ekmekci supplementary movie
Movie 2. Vorticity contours when the surface wire is located at the second critical angular position qc2 = 65°. Absolute values of normalized vorticity are plotted for purposes of comparison of the opposite shear layers. Minimum and incremental values are as follows: [|w|D/Uo]min = 2 and D[|w|D/Uo] = 1. Large-scale vortical structures are formed in an alternate fashion in the near wake. Hence, this movie clearly depicts the periodic formation of Karman vortices.

 Video (7.0 MB)
7.0 MB
VIDEO
Movies

Ekmekci supplementary movie
Movie 3. Streamwise velocity u/U­o contours at the wire angular position q = 50°, which is smaller than the critical angle qc1. Positive values of u/U­o are indicated by solid lines and the negative values are indicated by broken lines. It is clear from the movie that the shear layer, distinguished by the dense region of solid lines, reattaches to the cylinder surface after separation at the wire, and a final separation always takes place from the cylinder surface.

 Video (2.3 MB)
2.3 MB
VIDEO
Movies

Ekmekci supplementary movie
Movie 3. Streamwise velocity u/U­o contours at the wire angular position q = 50°, which is smaller than the critical angle qc1. Positive values of u/U­o are indicated by solid lines and the negative values are indicated by broken lines. It is clear from the movie that the shear layer, distinguished by the dense region of solid lines, reattaches to the cylinder surface after separation at the wire, and a final separation always takes place from the cylinder surface.

 Video (9.4 MB)
9.4 MB
VIDEO
Movies

Ekmekci supplementary movie
Movie 4. Streamwise velocity u/U­o contours at the critical angular position qc1 = 55° of the wire. Positive values of u/U­o are indicated by solid lines and the negative values are indicated by broken lines. This movie clearly shows bistable oscillations of the shear layer between two different modes of separation. In one mode, the separating shear layer at the wire reattaches onto the cylinder surface, and then separates from the cylinder surface. In the other mode, the shear layer separates at the wire and shows no reattachment.

 Video (4.8 MB)
4.8 MB
VIDEO
Movies

Ekmekci supplementary movie
Movie 4. Streamwise velocity u/U­o contours at the critical angular position qc1 = 55° of the wire. Positive values of u/U­o are indicated by solid lines and the negative values are indicated by broken lines. This movie clearly shows bistable oscillations of the shear layer between two different modes of separation. In one mode, the separating shear layer at the wire reattaches onto the cylinder surface, and then separates from the cylinder surface. In the other mode, the shear layer separates at the wire and shows no reattachment.

 Video (21.2 MB)
21.2 MB
VIDEO
Movies

Ekmekci supplementary movie
Movie 5. Streamwise velocity u/U­o contours at the wire angular position q = 60°, which is larger than the critical angle qc1. Positive values of u/U­o are indicated by solid lines and the negative values are indicated by broken lines. The movie depicts the separation of the shear layer at the wire with no reattachment afterwards.

 Video (2.5 MB)
2.5 MB
VIDEO
Movies

Ekmekci supplementary movie
Movie 5. Streamwise velocity u/U­o contours at the wire angular position q = 60°, which is larger than the critical angle qc1. Positive values of u/U­o are indicated by solid lines and the negative values are indicated by broken lines. The movie depicts the separation of the shear layer at the wire with no reattachment afterwards.

 Video (11.7 MB)
11.7 MB

Effects of a geometrical surface disturbance on flow past a circular cylinder: a large-scale spanwise wire

  • A. EKMEKCI (a1) and D. ROCKWELL (a2)

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