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

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.

Critical effects of a spanwise surface wire on flow past a circular cylinder and the significance of the wire size and Reynolds number

“…6(a) depicts the autospectral density S u of streamwise velocity signals obtained from the present CTA measurements at selected wire locations (θ). Note that this case is analogous to the cylinder-wire configuration previously studied in the PIV investigation of Ekmekci and Rockwell (2010). As evident from Fig.…”

“…To establish a baseline for this study, first, in Section 3.1, the spectral nature of CTA signals is evaluated and compared with the earlier PIV findings for a cylinder fitted with a spanwise wire identical in size to the one used in the previous investigations of Ekmekci (2006) and Ekmekci and Rockwell (2010) under identical Reynolds number condition. After this comparison, the subsequent two sections (Sections 3.2 and 3.3) will aim for an understanding of the effects of the wire size and Reynolds number.…”

Some aspects of aerodynamic flow control using synthetic-jet actuation

“…As shown by Honohan [53], the reduction in the width of the wake (and in pressure drag) by the synthetic jet and the obstruction is accompanied by suppression of vortex shedding in the near wake and a significant reduction in Reynolds stresses within the boundary layer upstream of separation. In a recent investigation of the flow past a circular cylinder (Re D = 10 4 ) with a surface-mounted wire (diameter larger than the thickness of the base-flow boundary layer), Ekmekci & Rockwell [68] reported that the near wake can significantly broaden or contract depending on the azimuthal position of the wire. Broadening was associated with bistable oscillations of the separating shear layer (on a time scale that is longer than that of the Kármán vortex shedding), and contraction occurs when the Kármán vortices are suppressed.…”

Influence of obstacle aspect ratio on tripped cylinder wakes

“…4. The results for the 2D symmetrically (Alam et al, 2010) and asymmetrically (Ekmekci and Rockwell, 2010) tripped cases, in contrast to the case AR = 6, show that the length of the vortex formation region decreases from a = 55°(<a c ) to a = 60°(>a c ). This difference highlights the importance of the mass flux contribution due to the tip flow on the organisation of the wake structure in the three dimensional geometry.…”

“…They also reported bi-stable regimes when the boundary layer was transitional. Ekmekci and Rockwell (2010), for a nominally two-dimensional cylinder and wire of 0.02D at Re % 10 4 show that a change in shedding structure occurs about a critical angle of 50°< a c < 60°. They report that the transition is non-monotonic (expansion and contraction of the wake) at the extremes of the critical angle range.…”