Three-dimensional transition after wake deflection behind a flapping foil

Deng, Jian; Caulfield, C. P.

doi:10.1103/physreve.91.043017

Cited by 37 publications

(46 citation statements)

References 36 publications

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“…Beyond the inverted vKm transition, for StA > 0.28, the wake is deflected from the horizontal centre line and C D becomes negative. The results shown that the distance between the tail of the foil and the point of deflection seems to decrease as StA increases, in agreement with the numerical simulations made by Deng and Caulfield (2015) (see figures 5(b), (c), (f) and (g)). For Re = 255, the performed simulations shown that the wake deflects when StA > 0.23, in good agreement with the 3D experimental observations of Godoy-Diana et al (2008) and the numerical simulations of He et al (2012) and Deng and Caulfield (2015).…”

Section: A Fixed Flapping Foilsupporting

confidence: 89%

On the forced flow around a rigid flapping foil

Mandujano

Málaga

2018

Physics of Fluids

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The two dimensional incompressible viscous flow past a flapping foil immersed in a uniform stream is studied numerically. Numerical simulations were performed using a Lattice-Boltzmann model for moderate Reynolds numbers. The computation of the hydrodynamic force on the foil is related to the the wake structure. In particular, when the foil's centre of mass is fixed in space, numerical results suggest a relation between drag coefficient behaviour and the flapping frequency which determines the transition from the von Kármán (vKm) to the inverted von Kármán wake. Beyond the inverted vKm transition the foil was released. Upstream swimming was observed at high enough flapping frequencies. Computed hydrodynamic forces suggest the propulsion mechanism for the swimming foil.arXiv:1608.03618v1 [physics.flu-dyn]

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Section: A Fixed Flapping Foilsupporting

confidence: 89%

On the forced flow around a rigid flapping foil

Mandujano

Málaga

2018

Physics of Fluids

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“…To investigate the nonlinear development of the identified unstable Floquet modes, we use a finite volume numerical code to solve the fully nonlinear threedimensional Navier-Stokes equations. This code has been applied to both a flapping airfoil [32][33][34], and an oscillating elliptical foil [35]. We ensure that the Courant number of all cells is less than one, and that each oscillation period T is decomposed into at least 2000 time steps so that the unsteadiness caused by the oscillation is well-resolved.…”

Section: B Numerical Methodologymentioning

confidence: 99%

Instabilities of interacting vortex rings generated by an oscillating disk

et al. 2016

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We propose a natural model to probe in a controlled fashion the instability of interacting vortex rings shed from the edge of an oblate spheroid disk of major diameter c, undergoing oscillations of frequency f_{0} and amplitude A. We perform a Floquet stability analysis to determine the characteristics of the instability modes, which depend strongly on the azimuthal (integer) wave number m. We vary two key control parameters, the Keulegan-Carpenter number K_{C}=2πA/c and the Stokes number β=f_{0}c^{2}/ν, where ν is the kinematic viscosity of the fluid. We observe two distinct flow regimes. First, for sufficiently small β, and hence low frequency of oscillation corresponding to relatively weak interaction between sequentially shedding vortex rings, symmetry breaking occurs directly to a single unstable mode with m=1. Second, for sufficiently large yet fixed values of β, corresponding to a higher oscillation frequency and hence stronger ring-ring interaction, the onset of asymmetry is predicted to occur due to two branches of high m instabilities as the amplitude is increased, with m=1 structures being dominant only for sufficiently large values of K_{C}. These two branches can be distinguished by the phase properties of the vortical structures above and below the disk. The region in (K_{C},β) parameter space where these two high m instability branches arise can be described accurately in terms of naturally defined Reynolds numbers, using appropriately chosen characteristic length scales. We subsequently carry out direct numerical simulations of the fully three-dimensional flow to verify the principal characteristics of the Floquet analysis, in particular demonstrating that high wave-number symmetry-breaking generically occurs when vortex rings sequentially interact sufficiently strongly.

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“…Unlike traditional Bénard-von Kármán (BvK) vortex streets viewed in a two-dimensional plane (Deng and Caulfield 2015), here the flow wakes produced by the forked tail (caudal fin) show strongly three-dimensionality. Nevertheless, the reversed BvK streets signaling propulsive wakes can still by observed.…”

Section: Validationmentioning

confidence: 76%

Why does a flying fish taxi on sea surface before take-off? A hydrodynamic interpretation

Deng

Wang

Zhang

et al. 2019

Preprint

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Flying fish have been observed jumping out of warm ocean waters worldwide. Before take-off, the flying fish are seen to taxi on the water surface by rapidly beating their semi-submerged tail fins, which process may help them airborne with enough speed to glide over a long distance. To understand the underlying physical mechanisms, here, we study a flying fish, 0.25 m in length and 0.191 kg in weight, considering both its underwater swimming and surface taxiing locomotion.Its hydrodynamic characteristics are numerically studied by computational fluid dynamics (CFD).Underwater, the fish is assumed to swim at a constant speed of 10 m s −1 . Different critical frequencies are identified for various maximum deflected angles, ranging from θ 0 = 10 o to 30 o , at which the fish reaches cruising states, when the horizontal forces are balanced. The corresponding minimum power required for cruising swimming is 350 W, obtained at a deflected angle of 10 o and a critical frequency of 145 Hz. In contrast, in the taxiing stage, the minimum power required for a stead-state locomotion at 10 m s −1 is 36 W, occurring at a deflected angle of 15 o and a frequency of 50 Hz. We note that the power is significantly smaller than the swimming locomotion. Further, by increasing the flapping power, we find that larger speeds can be achieved. In specific, when the power is brought up to 350 W, it can reach a speed of 16.5 m s −1 . Clearly, from the direct comparison between the two locomotive modes, it is apparently evidenced that the flying fish can be further accelerated by taxiing along the water surface.

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Three-dimensional transition after wake deflection behind a flapping foil

Cited by 37 publications

References 36 publications

On the forced flow around a rigid flapping foil

On the forced flow around a rigid flapping foil

Instabilities of interacting vortex rings generated by an oscillating disk

Why does a flying fish taxi on sea surface before take-off? A hydrodynamic interpretation

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