Formation mechanism of a secondary vortex street in a cylinder wake

The strength (initial circulation) and spacing of vortices in the wake of a circular cylinder have been obtained for conditions under which the body undergoes lateral vibrations. The vibrations of the cylinder were at all times synchronized with those in the wake, thereby suppressing the natural Strouhal frequency in favour of a common synchronized or ‘locked-in’ frequency for the body-wake system. All experiments were performed at a Reynolds number of 144 or 190. An inverse relation between the initial circulation K and the length lF of the vortex formation region was obtained for cylinder oscillations of up to 50% of a diameter, at vibration frequencies both above and below the Strouhal shedding frequency. The initial circulation K of the vortices was increased by as much as 65%, at lF = 1·6 diameters, from the stationary-cylinder value of K corresponding to lF = 3·2d. An increase in the rate A of vorticity generation of 80% from the stationary-cylinder wake value was obtained with the cylinder vibrating at 30% of a diameter and 110% of the Strouhal frequency. Both flow-visualization and hot-wire results show that the lateral spacing of the vortex street decreases as the vibration amplitude of the cylinder is increased, but that the longitudinal vortex spacing is independent of changes in amplitude. The longitudinal spacing, however, varies inversely with the vibration frequency. The street approaches a single line of vortices of alternating sign as the amplitude of vibration approaches values near a full cylinder diameter, and secondary vortex formation at these large amplitudes is associated with the vanishing lateral spacing of the street. Observation of the wake has elucidated the mechanism of vortex formation; the entrainment processes in the formation region have been observed at small intervals over a cycle of the cylinder's motion.

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Secondary vortex street in the wake of a rectangular cylinder: Effects of Reynolds number, aspect ratio and free-stream perturbation

International Journal of Heat and Fluid Flow ◽

10.1016/j.ijheatfluidflow.2021.108893 ◽

2022 ◽

Vol 93 ◽

pp. 108893

Author(s):

Xiaoying Ju ◽

Hongyi Jiang

Keyword(s):

Reynolds Number ◽

Aspect Ratio ◽

Free Stream ◽

Vortex Street ◽

Secondary Vortex ◽

Rectangular Cylinder

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Mechanism underlying Kármán vortex street breakdown preceding secondary vortex street formation

Physics of Fluids ◽

10.1063/1.4947449 ◽

2016 ◽

Vol 28 (5) ◽

pp. 054101 ◽

Cited By ~ 12

Author(s):

G. Ya. Dynnikova ◽

Ya. A. Dynnikov ◽

S. V. Guvernyuk

Keyword(s):

Vortex Street ◽

Secondary Vortex ◽

Karman Vortex Street ◽

Kármán Vortex Street ◽

Karman Vortex

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Secondary vortex dynamics in the cylinder wake during laminar-to-turbulent transition

Physical Review Fluids ◽

10.1103/physrevfluids.4.124702 ◽

2019 ◽

Vol 4 (12) ◽

Author(s):

Jeffrey McClure ◽

Colin Pavan ◽

Serhiy Yarusevych

Keyword(s):

Vortex Dynamics ◽

Secondary Vortex ◽

Cylinder Wake ◽

Laminar To Turbulent Transition ◽

Turbulent Transition

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On the origin of the secondary vortex street

Journal of Fluid Mechanics ◽

10.1017/jfm.2012.421 ◽

2012 ◽

Vol 711 ◽

pp. 641-666 ◽

Cited By ~ 20

Author(s):

Bhaskar Kumar ◽

Sanjay Mittal

Keyword(s):

Stability Analysis ◽

Linear Stability ◽

Vortex Shedding ◽

Linear Stability Analysis ◽

Broad Band ◽

Vortex Street ◽

Secondary Vortex ◽

Primary Vortex ◽

Near Wake ◽

Far Wake

AbstractThe origin of the secondary vortex street, observed in the far wake in the flow past a circular cylinder, is investigated. The Reynolds number, based on the diameter of the cylinder, is 150. The von Kármán vortex street, which originates in the near wake, decays exponentially downstream of the cylinder. Beyond the region of decay, a broad band of frequencies are selectively amplified, leading to the formation of a secondary vortex street consisting of packets of large-scale vortex structures. The streamwise location of the onset of the instability, frequency of the generation of packets and their convection speed are estimated via direct numerical simulation (DNS). Global linear stability analysis of the time-averaged flow reveals the presence of unstable convective modes that travel at almost the same speed and have a structure similar to the packet-like disturbances as observed in the DNS. Sensitivity analysis of the global convective modes to structural perturbations is carried out to locate the region of the wake that is most significant in generating the modes responsible for the appearance of the secondary vortex street. This information is utilized to control the flow. By placing a ‘slip’ splitter plate along the wake centre line, in the overlap region of the direct and the adjoint modes, the oscillations in the far wake are significantly reduced, though the oscillations related to the primary vortex shedding in the near wake are not. It is also found that suppression of the primary vortex shedding leads to annihilation of the secondary vortex street as well. Linear stability analysis of the steady-state flow does not yield any modes that can explain the appearance of the secondary vortex street. The steady and time-averaged wake profiles, for the $\mathit{Re}= 150$ flow, are compared to bring out the differences in the two. The effect of free-stream oscillations on the evolution of the secondary vortex street is investigated. By reducing the amplitude of inlet excitation, a gradual transition from ordered shedding in the far wake to the appearance of a broad-band spectrum of frequencies, as in the unforced wake, is observed. All the computations have been carried out using a stabilized finite element method.

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