Effects of three-dimensionality on thrust production by a pitching panel

Green, Melissa A.; Smits, Alexander J.

doi:10.1017/s0022112008003583

Cited by 62 publications

(42 citation statements)

References 16 publications

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“…However, as the panel decelerates, this pair does not continue to develop, and instead interacts with the swept edge. From Green & Smits (2008) we infer that at this point in the phase the pressure on the advancing surface of a rectangular pitching panel is high, with a favourable gradient toward the spanwise edges. This favourable pressure gradient will induce the flow toward the edges, and break up the swept edge structures.…”

Section: Planar Laser-induced Fluorescencementioning

confidence: 87%

“…This vortex pair has a component in the streamwise direction, but it is also expected to have a component in the spanwise direction, since it is aligned with the swept edge. Green & Smits (2008) measured time-resolved pressures on a similarly actuated rectangular pitching panel, and saw a strong pressure differential between the two sides of the panel exists at the extremes of the panel motion. Here, on the trapezoidal panel, we infer that, half a period earlier, a similar pressure difference drew fluid from what was the advancing surface (right) to the retreating surface (left).…”

Section: Planar Laser-induced Fluorescencementioning

confidence: 99%

“…Experiments and computations on pitching and flapping bodies of finite aspect ratio have revealed that the three-dimensional wake structure is considerably more complex than the two-dimensional case, and that its form is a strong function of both the Strouhal number and aspect ratio AR (=S/c, where S is the span and c is the chord) (von Ellenrieder, Parker & Soria 2003;Guglielmini 2004;Dong et al 2005;Ghovardhan & Williamson 2005;Sarkar & Venkatraman 2006;Zhu & Shoele 2008;Green & Smits 2008). Buchholz (2006) performed flow visualization experiments on a series of low-aspect-ratio propulsors and found that with increasing Strouhal number or decreasing aspect ratio, the wake transitions from the 2S configuration to a 2P configuration, where two pairs of opposite sign vortices are shed during each pitching cycle.…”

Section: Introductionmentioning

confidence: 99%

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The unsteady three-dimensional wake produced by a trapezoidal pitching panel

2011

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Particle image velocimetry (PIV) is used to investigate the three-dimensional wakes of rigid pitching panels with a trapezoidal geometry, chosen to model idealized fish caudal fins. Experiments are performed for Strouhal numbers from 0.17 to 0.56 for two different trailing edge pitching amplitudes. A Lagrangian coherent structure (LCS) analysis is employed to investigate the formation and evolution of the panel wake. A classic reverse von Kármán vortex street pattern is observed along the mid-span of the near wake, but the vortices realign and exhibit strong interactions near the spanwise edges of the wake. At higher Strouhal numbers, the complexity of the wake increases downstream of the trailing edge as the spanwise vortices spread transversely and lose coherence as the wake splits. This wake transition is shown to correspond to a qualitative change in the LCS pattern surrounding each vortex core, and can be identified as a quantitative event that is not dependent on arbitrary threshold levels. The location of this transition is observed to depend on both the pitching amplitude and free stream velocity, but is not constant for a fixed Strouhal number. On the panel surface, the trapezoidal planform geometry is observed to create additional vortices along the swept edges that retain coherence for low Strouhal numbers or high sweep angles. These additional swept-edge structures are conjectured to add to the complex three-dimensional flow near the tips of the panel.

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Section: Planar Laser-induced Fluorescencementioning

confidence: 87%

Section: Planar Laser-induced Fluorescencementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The unsteady three-dimensional wake produced by a trapezoidal pitching panel

2011

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“…A further consideration is that a resonant frequency may not necessarily align best with the hydrodynamic demands of swimming. For effective swimming, vortices must be shed appropriately into the wake (Hover et al, 2004; Buchholz and Smits, 2006; Green and Smits, 2008). Undulating at a resonant frequency may not necessarily shed vortices at the right times to develop a good wake structure for swimming, and thus it may not result in efficient propulsion even if it reduces the energy demands for oscillation.…”

Section: Whither Resonance?mentioning

confidence: 99%

The role of mechanical resonance in the neural control of swimming in fishes

Tytell¹,

Hsu²,

Fauci³

2014

Zoology

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The bodies of many fishes are flexible, elastic structures; if you bend them, they spring back. Therefore, they should have a resonant frequency: a bending frequency at which the output amplitude is maximized for a particular input. Previous groups have hypothesized that swimming at this resonant frequency could maximize efficiency, and that a neural circuit called the central pattern generator might be able to entrain to a mechanical resonance. However, fishes swim in water, which may potentially damp out many resonant effects. Additionally, their bodies are elongated, which means that bending can occur in complicated ways along the length of the body. We review previous studies of the mechanical properties of fish bodies, and then present new data that demonstrate complex bending properties of elongated fish bodies. Resonant peaks in amplitude exist, but there may be many of them depending on the body wavelength. Additionally, they may not correspond to the maximum swimming speed. Next, we describe experiments using a closed-loop preparation of the lamprey, in which a preparation of the spinal cord is linked to a real-time simulation of the muscle and body properties, allowing us to examine resonance entrainment as we vary the simulated resonant frequency. We find that resonance entrainment does occur, but is rare. Gain had a significant, though weak, effect, and a nonlinear muscle model produced resonance entrainment more often than a linear filter. We speculate that resonance may not be a critical effect for efficient swimming in elongate, anguilliform swimmers, though it may be more important for stiffer carangiform and thunniform fishes.

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“…Study of the role of vortex formation, shedding, and breaking down in the lift histories of pitching flat plates (as models of fins) has been previously carried out both computationally and experimentally Smits 2006, 2008;Green and Smits 2008). This classic case is a first, fundamental step toward understanding the more complicated unsteady flow (Ringuette et al 2007).…”

mentioning

confidence: 99%