Bimodal vortex shedding in a perturbed cylinder wake

Konstantinidis, Efstathios; Balabani, Stavroula; Yianneskis, M.

doi:10.1063/1.2432152

Cited by 91 publications

(41 citation statements)

References 19 publications

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“…3 that the first mode of the vorticity-based POD exhibits similar spatial features than those of the second mode. This is in good agreement with the results of Ma et al (2000), Dipankar et al (2007) and Konstantinidis et al (2007). Similar observations can be made for velocity-based POD.…”

Section: Podsupporting

confidence: 93%

“…The average vortex wavelength c = U c f s (U c is the convection velocity defined as 0.86U ∞ and f s is the vortex shedding frequency at x/d = 10 (Zhou et al 2002;Zhou and Antonia 1992) for both PODs is equal to 4.2d. Konstantinidis et al (2007) showed that a symmetric distribution of the POD modes with respect to the centerline reflects the antisymmetric characteristic of the flow field (with respect to the flow centerline). Consequently, the first two modes of both PODs, illustrated in Fig.…”

Section: Podmentioning

confidence: 98%

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Comparison between velocity- and vorticity-based POD methods in a turbulent wake

et al. 2015

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Section: Podsupporting

confidence: 93%

Section: Podmentioning

confidence: 98%

Comparison between velocity- and vorticity-based POD methods in a turbulent wake

et al. 2015

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“…In the current work, the distinction between the A-II and SA modes is maintained to aid discussion. Forced oscillation studies have shown that the wake can exhibit the A-IV mode (in which two pairs of counter-rotating vortices are shed per cycle; figure 1) at similar ranges of reduced velocity and vibration amplitude to that which are experienced by a cylinder in this region of the response regime (Ongoren & Rockwell 1988;Nishihara, Kaneko & Watanabe 2005;Konstantinidis, Balabani & Yianneskis 2007). Cagney & Balabani (2013b) showed that this mode can also occur in the wake of a freely vibrating cylinder; the mode was stable, and caused the second branch to occur in a slightly different form, with a lower-amplitude response compared with when the SA mode was present.…”

Section: Streamwise Response Regimementioning

confidence: 99%

Streamwise vortex-induced vibrations of cylinders with one and two degrees of freedom

Cagney

Balabani

2014

J. Fluid Mech.

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Measurements are presented of the structural response and wake of a two-degree-offreedom (2-DOF) pivoted cylinder undergoing streamwise vortex-induced vibrations (VIV), which were carried out using particle-image velocimetry (PIV). The results are compared with those of previous studies performed in the same experimental facility examining a cylinder free to move only in the streamwise direction (1-DOF). The aim of this study is to examine to what extent the results of previous work on streamwise-only VIV can be extrapolated to the more practical, multi-DOF case. The response regimes measured for the 1-and 2-DOF cases are similar, containing two response branches separated by a low-amplitude region. The first branch is characterised by negligible transverse motion and the appearance of both alternate and symmetric vortex shedding. The two wake modes compete in an unsteady manner; however, the competition does not appear to have a significant effect on either the streamwise or transverse motion. Comparison of the phase-averaged vorticity fields acquired in the second response branch also indicates that the additional DOF does not alter the vortex-shedding process. However, the additional DOF affects the cylinder-wake system in other ways; for the 1-DOF case the second branch can appear in three different forms (each associated with a different wake mode), while for the 2-DOF case the second branch only exists in one form, and does not exhibit hysteresis. The cylinder follows a figure-of-eight trajectory throughout the lock-in range. The phase angle between the streamwise and transverse motion decreases linearly with reduced velocity. This work highlights the similarities and differences between the fluid-structure interaction and wake dynamics associated with 1-and 2-DOF cylinders throughout the streamwise response regime, which has not received attention to date.

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“…The A-IV mode has been observed in studies in which a cylinder is forced to oscillate 17,25 or is placed in pulsating flow. 32 This mode is characterised by the shedding of four vortices per wake cycle, with a pair of counter-rotating vortices shed in each cylinder oscillation cycle (Figure 1), and has recently been shown to also occur in the wake of freely oscillating cylinders in the second response branch, 33 using the same experimental data presented in this paper. Wake modes are typically identified in flow-visualisation studies using dye 1,25,34 or laserinduced fluorescence.…”

Section: Introductionmentioning

confidence: 95%

Lagrangian structures and mixing in the wake of a streamwise oscillating cylinder

Cagney

Balabani

2016

Physics of Fluids

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Lagrangian analysis is capable of revealing the underlying structure and complex phenomena in unsteady flows. We present particle-image velocimetry measurements of the wake of a cylinder undergoing streamwise vortex-induced vibrations and calculate the Finite-Time Lyapunov Exponents (FTLE) in backward- and forward-time. The FTLE fields are compared to the phase-averaged vorticity fields for the four different wake modes observed while the cylinder experiences streamwise vortex-induced vibrations. The backward-time FTLE fields characterise the formation of vortices, with the roll up of spiral-shaped ridges coinciding with the roll up of the shear layers to form the vortices. Ridges in the forward-time fields tend to lie perpendicular to the flow direction and separate nearby vortices. The shedding of vortices coincides with a “peel off” process in the forward-time FTLE fields, in which a ridge connected to the cylinder splits into two strips, one of which moves downstream. Particular attention is given to the “wake breathing” process, in which the streamwise motion of the cylinder causes both shear layers to roll up simultaneously and two vortices of opposite sign to be shed into the wake. In this case, the ridges in forward-time FTLE fields are shown to define “vortex cells,” in which the new vortices form, and the FTLE fields allow the wake to be decomposed into three distinct regions. Finally, the mixing associated with each wake mode is examined, and it is shown that cross-wake mixing is significantly enhanced when the vibration amplitude is large and the vortices are shed alternately. However, while the symmetric shedding induces large amplitude vibrations, no increase in mixing is observed relative to the von Kármán vortex street observed behind near-stationary bodies.

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Bimodal vortex shedding in a perturbed cylinder wake

Cited by 91 publications

References 19 publications

Comparison between velocity- and vorticity-based POD methods in a turbulent wake

Comparison between velocity- and vorticity-based POD methods in a turbulent wake

Streamwise vortex-induced vibrations of cylinders with one and two degrees of freedom

Lagrangian structures and mixing in the wake of a streamwise oscillating cylinder

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