Jet meandering by a foil pitching in quiescent fluid

Shinde, Sachin Y.; Arakeri, Jaywant H.

doi:10.1063/1.4800321

Cited by 23 publications

(26 citation statements)

References 16 publications

(17 reference statements)

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“…The flow from the rigid foil is discussed in detail by Shinde & Arakeri (2013). In dramatic contrast, for the same pitching conditions, attaching a chordwise flexible flap to the foil creates a coherent, undulating jet composed of vortices in a 'reverse Bénard-Kármán vortex street' aligned along the centreline (figure 2b).…”

Section: Flow Structurementioning

confidence: 97%

“…increase in U ∞ ). For the limiting condition of St tending to infinity (U ∞ = 0), non-symmetric jets are observed for heaving and pitching rigid foils (Freymuth 1990;Gustafson et al 1992;Lai & Platzer 2001); the jets meander across the centreline either quasiperiodically (Heathcote & Gursul 2007, for purely heaving motion), or continually and randomly (Shinde & Arakeri 2013, for purely pitching motion). Marais et al (2012) recently showed that chordwise flexibility increases separation between successively shed vortices and prevents jet inclination up to St = 1.2 compared with the value of St 0.4 for rigid foils.…”

Section: Introductionmentioning

confidence: 94%

“…While most studies are with forward velocity/free-stream flow, very few explore the limit of zero free-stream velocity (e.g . Freymuth 1990;Gustafson, Leben & McArthur 1992;Wang 2000;Lai & Platzer 2001;Shinde & Arakeri 2013), and even fewer explore flexible foils with zero free-stream velocity (e.g. Heathcote, Martin & Gursul 2004;Heathcote & Gursul 2007;Eldredge, Toomey & Medina 2010).…”

Section: Introductionmentioning

confidence: 98%

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Flexibility in flapping foil suppresses meandering of induced jet in absence of free stream

Shinde

Arakeri

2014

J. Fluid Mech.

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Thrust-generating flapping foils are known to produce jets inclined to the free stream at high Strouhal numbers St = fA/U ∞ , where f is the frequency and A is the amplitude of flapping and U ∞ is the free-stream velocity. Our experiments, in the limiting case of St → ∞ (zero free-stream speed), show that a purely oscillatory pitching motion of a chordwise flexible foil produces a coherent jet composed of a reverse Bénard-Kármán vortex street along the centreline, albeit over a specific range of effective flap stiffnesses. We obtain flexibility by attaching a thin flap to the trailing edge of a rigid NACA0015 foil; length of flap is 0.79 c where c is rigid foil chord length. It is the time-varying deflections of the flexible flap that suppress the meandering found in the jets produced by a pitching rigid foil for zero free-stream condition. Recent experiments (Marais et al., J. Fluid Mech., vol. 710, 2012, p. 659) have also shown that the flexibility increases the St at which non-deflected jets are obtained. Analysing the near-wake vortex dynamics from flow visualization and particle image velocimetry (PIV) measurements, we identify the mechanisms by which flexibility suppresses jet deflection and meandering. A convenient characterization of flap deformation, caused by fluid-flap interaction, is through a non-dimensional 'effective stiffness', EI * = 8 EI/(ρ V 2 TE max s f c 3 f /2), representing the inverse of the flap deflection due to the fluid-dynamic loading; here, EI is the bending stiffness of flap, ρ is fluid density, V TE max is the maximum velocity of rigid foil trailing edge, s f is span and c f is chord length of the flexible flap. By varying the amplitude and frequency of pitching, we obtain a variation in EI * over nearly two orders of magnitude and show that only moderate EI * (0.1 EI * 1) generates a sustained, coherent, orderly jet. Relatively 'stiff' flaps (EI * 1), including the extreme case of no flap, produce meandering jets, whereas highly 'flexible' flaps (EI * 0.1) produce spread-out jets. Obtained from the measured mean velocity fields, we present values of thrust coefficients for the cases for which orderly jets are observed.

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Section: Flow Structurementioning

confidence: 97%

Section: Introductionmentioning

confidence: 94%

Section: Introductionmentioning

confidence: 98%

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Flexibility in flapping foil suppresses meandering of induced jet in absence of free stream

Shinde

Arakeri

2014

J. Fluid Mech.

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“…As there is no free stream in the present experiments, it needs to be emphasized that the jet flow is entirely due to the flapping motion of the flexible foil. In the absence of free-stream flow, the jet generated by the flapping foil has the tendency to switch direction [20,22]. Shinde & Arakeri [29] have identified the two underlying mechanisms for suppressing the jet meandering and generating the coherent reverse Bénard-Kármán vortex street and the unidirectional thrust along the centreline: first, appropriate spatial and temporal shedding of the vortices from the flap tip, and, second, convection of the vortices away from the place of shedding.…”

Section: The Flowmentioning

confidence: 99%

Physics of unsteady thrust and flow generation by a flexible surface flapping in the absence of a free stream

Shinde¹,

Arakeri²

2018

Proc. R. Soc. A.

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Inspired by the flexible wings and fins of flying and swimming animals, we investigate the flow induced by the interaction between a flapping flexible surface and the surrounding fluid for the limiting case of Strouhal number S t → ∞ (zero free-stream speed). The model selected for this purpose is a two-dimensional sinusoidally pitching rigid symmetric foil to which is attached at the trailing edge a thin chordwise flexible surface (along the chord line). The flow so generated is a coherent jet aligned along the foil centreline, containing a reverse Bénard–Kármán vortex street and delivering a corresponding unidirectional thrust. We analyse the flow and thrust generation process. The measured velocity field suggests that the flow and thrust generation mainly occurs during the phases when the trailing edge is near the centreline. Flexibility of the surface is important in accelerating the near-wake flow and in transferring momentum and energy to the fluid. We present a detailed account of when and where the momentum and energy are added to the fluid. This study shows that the deformations of the flexible surface are responsible for generating a favourable pressure gradient along the jet direction, and for the observed unsteady actuator disc-type action.

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“…They also observed that the switching period increased with increased foil stiffness. In a related study, Shinde & Arakeri (2013) observed the long-time behaviour of the meandering jet to be random in time and not dependent on initial conditions. They opined that the leading-edge vortices observed in Heathcote & Gursul's experiments could account for the differing results.…”

Section: Similarities To Aerofoil Propulsionmentioning

confidence: 94%

Flows produced by the combined oscillatory rotation and translation of a circular cylinder in a quiescent fluid

et al. 2014

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Flows produced by a circular cylinder undergoing oscillatory rotation and translation in a quiescent fluid have been studied via direct numerical simulations. The incompressible Navier-Stokes equations were solved for large dimensionless time windows using an immersed boundary method with adaptive Cartesian grid refinement. Parametric studies were conducted in two dimensions on the Reynolds number, Keulegan-Carpenter number and phase shift. In addition to the previously reported net thrust case (Blackburn et al., Phys. Fluids, vol. 11, 1999, pp. 4-6), the study catalogued the appearance of several streaming jet regimes with varying deflection angles, deflected and horizontal vortex shedding regimes, and a double mirrored jet regime with varying inter-jet angles, as well as several chaotic cases. Visualizations are presented to clarify each observed flow regime and to illustrate the parameter space. Connections are drawn between these canonical bluff-body deflected wakes and a similar phenomenon observed in aerofoils oscillating at high reduced frequencies in a cross-flow. Also, the discovery of the streaming jet regimes with varying deflection angles opens the door for using these flows as a low-Reynolds-number propulsive mechanism requiring only a two-degree-of-freedom actuator. Simulation results suggest that the flow phenomena observed in two dimensions persist in three dimensions, despite spanwise fluctuations.

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Jet meandering by a foil pitching in quiescent fluid

Cited by 23 publications

References 16 publications

Flexibility in flapping foil suppresses meandering of induced jet in absence of free stream

Flexibility in flapping foil suppresses meandering of induced jet in absence of free stream

Physics of unsteady thrust and flow generation by a flexible surface flapping in the absence of a free stream

Flows produced by the combined oscillatory rotation and translation of a circular cylinder in a quiescent fluid

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