Pinch-off of non-axisymmetric vortex rings

O’Farrell, Clara; Dabiri, John O.

doi:10.1017/jfm.2013.639

Cited by 53 publications

(23 citation statements)

References 86 publications

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“…These features were also observed in previous studies of flow around cylinders 42 and convecting vortex rings, 43 and represent the boundary between regions which are quickly swept downstream and fluid which interacts with the viscous boundary layer surrounding the cylinder. These upstream ridges were observed in the σ + f fields computed for each mode.…”

Section: B A-ii Modesupporting

confidence: 75%

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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Section: B A-ii Modesupporting

confidence: 75%

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

Cagney

Balabani

2016

Physics of Fluids

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“…A study by O'Farrell & Dabiri (2014) demonstrates that for non-axisymmetric orifices the formation number still holds when expressed as L/D eq , where D eq (m) is the equivalent diameter of the orifice; the diameter of an axisymmetric orifice with the same cross-sectional area. For a rectangular orifice, the equivalent diameter is given by…”

Section: Dependency Of Vectoring Behaviour On Strouhal Numbermentioning

confidence: 99%

“…Smith & Glezer (1998); Glezer & Amitay (2002) ;Cattafesta & Sheplak (2011);O'Farrell & Dabiri (2014)). Both experimental and numerical studies have focussed on the development of actuators, the formation and evolution of synthetic jets and on flow control applications.…”

Section: Introductionmentioning

confidence: 99%

Vectoring of parallel synthetic jets: a parametric study

2016

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The vectoring of a pair of parallel synthetic jets can be described using five dimensionless parameters: the aspect ratio of the slots, the Strouhal number, the Reynolds number, the phase difference between the jets and the spacing between the slots. In the present study, the influence of the latter four on the vectoring behaviour of the jets is examined experimentally, using particle image velocimetry. Time-averaged velocity maps are used to give a qualitative description of the variations in vectoring for a parametric sweep of each of the four parameters independently. A diverse set of vectoring behaviour is observed in which the resulting jet can be merged or bifurcated and either vectored towards the actuator leading in phase or the actuator lagging in phase. Three performance metrics are defined to give a quantitative description of the vectoring behaviour: the included angle between bifurcated branches, the vectoring angle of the total flow and the normalized momentum flux of the flow. Using these metrics, the influence of changes in the Strouhal number, Reynolds number, phase difference and spacing are quantified. Phase-locked maps of the swirling strength are used to track vortex pairs. Vortex trajectories are used to define three Strouhal number regimes for the vectoring behaviour. In the first regime, vectoring behaviour is dominated by the pinch-off time, which is written as function of Strouhal number only. In the second regime, the pinch-off time is invariant and the vectoring behaviour slightly changes with Strouhal number. In the third regime, given by the formation criterion, no synthetic jet is formed. Vortex positions at a single phase, shortly after creation of the lagging vortex pair, are used to propose a vectoring mechanism. This vectoring mechanism explains the observed qualitative and quantitative variations for all four parameters.

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“…It is chosen to satisfy u 0 T/D = 4, where T is the period of the entire stroke. In the recent literature on vortex rings (Dabiri 2009;O'Farrell & Dabiri 2014), the formation time (t * ) is often used to describe unsteady vortex formation, defined as t * = ut/D = L(t)/D. When the stroke is over, the PoVR has a final stroke ratio T * given by T * = L/D = uT/D.…”

Section: Computational Set-upmentioning

confidence: 99%

Transient force augmentation due to counter-rotating vortex ring pairs

2015

J. Fluid Mech.

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Of particular significance to biological locomotion is vortex ring interaction. In the wakes of animals, this unsteady process determines the changes in the impulse of counter-rotating vortex ring pairs (VRPs), which consist of two vortex rings with opposite sense of rotation. In this paper, these VRPs are proposed to be of particular importance to unsteady force generation. We carry out numerical computations, simulating the piston-cylinder apparatus, to study the transient changes in the impulse of counter-rotating VRPs composed of a positive and a negative vortex ring. We model the negative vortex ring (NeVR) of a VRP by making reasonable assumptions about their vorticity distributions and spatial locations, which are initially prescribed. The result of modelling is superimposed on the flow, which has a pre-existing positive vortex ring (PoVR), leading to a VRP. The simulation quantitatively demonstrates that the unsteady force resulting from a VRP is significantly larger compared with an isolated PoVR (without an NeVR). The force enhancement is also correlated to vortex configurations. A normalised force coefficient characterising force augmentation over the entire stroke is given. The force augmentation coefficient grows significantly and then reaches a plateau as the momentum input increases. The results are consistent with those in fully unsteady vortex interaction, which involves the generation of an NeVR. It is suggested that counter-rotating VRPs could offer a new perspective to explain unconventional force generation for biological swimming and flying.

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Pinch-off of non-axisymmetric vortex rings

Cited by 53 publications

References 86 publications

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

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

Vectoring of parallel synthetic jets: a parametric study

Transient force augmentation due to counter-rotating vortex ring pairs

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