Uncovering the physics of flapping flat plates with artificial evolution

Milano, Michele; Gharib, Morteza

doi:10.1017/s0022112005004842

Cited by 108 publications

(60 citation statements)

References 7 publications

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“…In this Re range, the dependence of T on Re is very significant. Previous studies in the vortex formation time typically employed higher Reynolds numbers than in this study, [14][15][16] so Reynolds number effects there may not be apparent.…”

Section: U͑t͒dt ͑22͒mentioning

confidence: 87%

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The leading-edge vortex and quasisteady vortex shedding on an accelerating plate

2010

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A computational inquiry focuses on leading-edge vortex ͑LEV͒ growth and shedding during acceleration of a two-dimensional flat plate at a fixed 10°-60°angle of attack and low Reynolds number. The plate accelerates from rest with a velocity given by a power of time ranging from 0 to 5. During the initial LEV growth, subtraction of the added mass lift from the computed lift reveals an LEV-induced lift augmentation evident across all powers and angles of attack. For the range of Reynolds numbers considered, a universal time scale exists for the peak when ␣ Ն 30°, with augmentation lasting about four to five chord lengths of translation. This time scale matches well with the half-stroke of a flying insect. An oscillating pattern of leading-and trailing-edge vortex shedding follows the shedding of the initial LEV. The nondimensional frequency of shedding and lift coefficient minima and maxima closely match their values in the absence of acceleration. These observations support a quasisteady theory of vortex shedding, where dynamics are determined primarily by velocity and not acceleration. Finally, the nondimensional vortex formation time is found to be a function of the Reynolds number, but only weakly when the Reynolds number is high.

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Section: U͑t͒dt ͑22͒mentioning

confidence: 87%

“…13 For example, such a vortex formation time was first observed in piston-cylinder vortex ring generators, 14 but the same T Ϸ 4 was also observed in flow over a cylinder 15 and over flapping wings. 16 Under the current setup, the vortex formation time is defined as…”

Section: -9mentioning

confidence: 99%

The leading-edge vortex and quasisteady vortex shedding on an accelerating plate

2010

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“…The appearance of optimal vortex formation as a successful strategy in response to selective pressures at the relatively low Reynolds numbers observed here (Re<200) supports that the notion that optimal vortex formation may exist even more broadly in aquatic locomotion and biological propulsion. This suggestion of a broader role for the concept of optimal vortex formation in swimming and flying has been buoyed by recent comparative biomechanics studies at higher Reynolds numbers (Linden and Turner, 2004;Dabiri and Gharib, 2005a;Milano and Gharib, 2005).…”

Section: Discussionmentioning

confidence: 99%

Fast-swimming hydromedusae exploit velar kinematics to form an optimal vortex wake

Dabiri

Colin

Costello

2006

Journal of Experimental Biology

114

123

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SUMMARY Fast-swimming hydromedusan jellyfish possess a characteristic funnel-shaped velum at the exit of their oral cavity that interacts with the pulsed jets of water ejected during swimming motions. It has been previously assumed that the velum primarily serves to augment swimming thrust by constricting the ejected flow in order to produce higher jet velocities. This paper presents high-speed video and dye-flow visualizations of free-swimming Nemopsis bachei hydromedusae, which instead indicate that the time-dependent velar kinematics observed during the swimming cycle primarily serve to optimize vortices formed by the ejected water rather than to affect the speed of the ejected flow. Optimal vortex formation is favorable in fast-swimming jellyfish because,unlike the jet funnelling mechanism, it allows for the minimization of energy costs while maximizing thrust forces. However, the vortex `formation number'corresponding to optimality in N. bachei is substantially greater than the value of 4 found in previous engineering studies of pulsed jets from rigid tubes. The increased optimal vortex formation number is attributable to the transient velar kinematics exhibited by the animals. A recently developed model for instantaneous forces generated during swimming motions is implemented to demonstrate that transient velar kinematics are required in order to achieve the measured swimming trajectories. The presence of velar structures in fast-swimming jellyfish and the occurrence of similar jet-regulating mechanisms in other jet-propelled swimmers (e.g. the funnel of squid) appear to be a primary factor contributing to success of fast-swimming jetters, despite their primitive body plans.

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“…Further advancement of the piston resulted in the formation of a jet trailing the vortex ring. The formation number has been generalized to predict the generation of vortex rings through temporally varying orifices (Dabiri & Gharib 2005) and the optimal oscillatory motion of a finite-aspect-ratio plate in a quiescent fluid (Milano & Gharib 2005).…”

Section: Introductionmentioning

confidence: 99%

The wake structure and thrust performance of a rigid low-aspect-ratio pitching panel

2008

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Thrust performance and wake structure were investigated for a rigid rectangular panel pitching about its leading edge in a free stream. For Re C = O(10 4 ), thrust coefficient was found to depend primarily on Strouhal number St and the aspect ratio of the panel AR. Propulsive efficiency was sensitive to aspect ratio only for AR less than 0.83; however, the magnitude of the peak efficiency of a given panel with variation in Strouhal number varied inversely with the amplitude to span ratio A/S, while the Strouhal number of optimum efficiency increased with increasing A/S. Peak efficiencies between 9 % and 21 % were measured. Wake structures corresponding to a subset of the thrust measurements were investigated using dye visualization and digital particle image velocimetry. In general, the wakes divided into two oblique jets; however, when operating at or near peak efficiency, the near wake in many cases represented a Kármán vortex street with the signs of the vortices reversed. The three-dimensional structure of the wakes was investigated in detail for AR = 0.54, A/S = 0.31 and Re C = 640. Three distinct wake structures were observed with variation in Strouhal number. For approximately 0.20 < St < 0.25, the main constituent of the wake was a horseshoe vortex shed by the tips and trailing edge of the panel. Streamwise variation in the circulation of the streamwise horseshoe legs was consistent with a spanwise shear layer bridging them. For St > 0.25, a reorganization of some of the spanwise vorticity yielded a bifurcating wake formed by trains of vortex rings connected to the tips of the horseshoes. For St > 0.5, an additional structure formed from a perturbation of the streamwise leg which caused a spanwise expansion. The wake model paradigm established here is robust with variation in Reynolds number and is consistent with structures observed for a wide variety of unsteady flows. Movies are available with the online version of the paper.

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Uncovering the physics of flapping flat plates with artificial evolution

Cited by 108 publications

References 7 publications

The leading-edge vortex and quasisteady vortex shedding on an accelerating plate

The leading-edge vortex and quasisteady vortex shedding on an accelerating plate

Fast-swimming hydromedusae exploit velar kinematics to form an optimal vortex wake

The wake structure and thrust performance of a rigid low-aspect-ratio pitching panel

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