Mode C flow transition behind a circular cylinder with a near-wake wire disturbance

Abstract

The three-dimensional transition of the flow behind a circular cylinder with a near-wake wire disturbance has been investigated experimentally. The asymmetric placement of a wire in the near-wake region of the cylinder causes an unnatural mode of shedding to occur, namely mode C. We performed flow visualization and particle image velocimetry (PIV) experiments to investigate the influence of the wire on various properties of the flow, such as the dynamics of the streamwise secondary vortices. Experiments were performed at the Reynolds number range of Re = 165-300. From these experiments, it can be concluded that mode C structures are formed as secondary streamwise vortices around the primary von Kármán vortices. The spanwise wavelength of those mode C structures is determined to be approximately two cylinder diameters. The presence of the wire also triggered the occurrence of period doubling in the wake. Each new set of mode C structures is out of phase with the previous set, i.e. doubling the shedding period. This period-doubling phenomenon is due to a feedback mechanism between the consecutively shed upper vortices.

Experimental evidence of new three-dimensional modes in the wake of a rotating cylinder

“…Its nominal spanwise wavelength was λ z /d ≈ 1.1 at α = 1.7, but increased significantly with decreasing rotation rate. This mode has similar characteristics to the subharmonic mode C observed in the wakes of a torus (Sheard et al 2005a), and of a cylinder disturbed by a thin wire (Zhang et al 1995;Yildirim et al 2013). Furthermore, a complex interaction with mode B took place for α 1.0.…”

“…The wavelength was λ z /d = 1.7 in simulations, and λ z /d ≈ 2 in experiments. Yildirim, Rindt & van Steenhoven (2013) extended these experiments and demonstrated the period doubling of the main shedding cycle caused by mode C. Sheard, Thompson & Hourigan (2003b, 2005a observed a subharmonic mode (λ z /d = 1.9) in simulations, and experiments (Sheard et al 2005b), in the wake of a torus aligned normal to the direction of flow. Depending on the aspect ratio of the torus, the mode became unstable at a Reynolds number lower than the well-known modes A and B.…”

The influence of a small upstream wire on transition in a rotating cylinder wake

“…For the non-rotating cylinder, the presence of a wire of diameter D/100 upstream does not trigger the mode C instability, and does not suppress mode A and mode B instabilities, at least at Re = 275, well above the critical Reynolds numbers for the mode A and B transition. This is similar to the situation for the wire placed downstream (Zhang et al 1995;Yildirim et al 2013b).…”

“…Further details of the particular spectral-element implementation can be found in Ryan, and Thompson et al (2006). A secondary validation check was undertaken to confirm that the subharmonic mode observed in the studies of Zhang et al (1995), Yildirim et al (2013b) can indeed be observed when a wire of diameter d = D/100 is placed downstream of a non-rotating cylinder for Re 300. Shown in figure 4 are perturbation contours of spanwise vorticity at Re = 200, where the maximum growth rate occurs at λ/D 1.4.…”

Flow past a rotating cylinder translating at different gap heights along a wall

“…Subharmonic modes have also been observed when the wake symmetry is altered, as in the flow past rings (Sheard et al, 2005b(Sheard et al, , 2004a, and inclined square cylinders (Sheard et al, 2009;Sheard, 2011). In addition, such modes have also been observed in a non-rotating cylinder wake when a wire was placed downstream of the cylinder (Zhang et al, 2005;Yildirim et al, 2013) or upstream (Rao et al, 2015b).…”