Wake mechanics for thrust generation in oscillating foils

Triantafyllou, Michael S.; Triantafyllou, George S.; Gopalkrishnan, R.

doi:10.1063/1.858173

Cited by 410 publications

(292 citation statements)

References 8 publications

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“…The experimental investigation of Anderson et al [11] showed that optimum propulsive efficiencies were obtained within an approximate range of Strouhal numbers 0.2-0.4. This range coincides with that presented by Triantafyllou et al [12] for observed fish and cetaceans swimming at or near their maximum observed speed. However, Young and Lai [13] concluded through a numerical study that Strouhal number alone was insufficient to characterize the efficiency of flapping foil propulsion.…”

Section: Introductionsupporting

confidence: 77%

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A study on forces acting on a flapping wing

Vuruskan

Fenercioglu

Çeti̇ner

2013

EPJ Web of Conferences

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Abstract. In order to study the forces acting on a flapping wing, an experimental investigation is performed in steady water flow. In this study, a SD7003 airfoil undergoes combined pitching and plunging motion which simulates the forward flight of small birds. The frequency of pitching motion is equal to the frequency of plunging motion and pitch leads the plunge by a phase angle of 90 degrees. The experiments are conducted at Reynolds numbers of 2500 ≤ Re ≤ 13700 and the vortex formation is recorded using the digital particle image velocimetry (DPIV) technique. A prediction of thrust force and efficiency is calculated from the average wake deficit of DPIV data, the near-wake vorticity patterns and time dependent velocity vectors are determined to comment on the thrust and drag indication. Direct force measurements are attempted using a Force/Torque sensor which is capable of measuring forces and moments in three axial directions.

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Section: Introductionsupporting

confidence: 77%

“…Therefore, a lift production along with thrust is expected. In accordance with Equation (12), high efficiencies are observed in general for low Strouhal numbers, just before a C d ≥ 0 value is observed. These cases are in general designated as Category D.…”

Section: Velocity Deficit Profiles From Dpiv Imagessupporting

confidence: 61%

A study on forces acting on a flapping wing

Vuruskan

Fenercioglu

Çeti̇ner

2013

EPJ Web of Conferences

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“…This Strouhal number is also near the range in which aquatic animals are known to swim (Triantafyllou et al 1991(Triantafyllou et al , 1993, forming a reverse Kármán pattern in the symmetry plane (Triantafyllou & Triantafyllou 1995). The significant change in performance between the low-and moderate-Reynoldsnumber cases is attributed to the relative magnitude of surface pressures from which thrust is generated, to viscous shear stresses which primarily produce drag (note that separated flows may produce small shear stresses that make a positive contribution to the thrust).…”

Section: Sensitivity Of Wake Structure To Reynolds Numbermentioning

confidence: 87%

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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“…Other factors, such as stiffness of the tail, swimming frequency and Strouhal number may be responsible for differences in wake structure [32,33].…”

Section: Discussionmentioning

confidence: 99%

Volumetric imaging of shark tail hydrodynamics reveals a three-dimensional dual-ring vortex wake structure

et al. 2011

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Understanding how moving organisms generate locomotor forces is fundamental to the analysis of aerodynamic and hydrodynamic flow patterns that are generated during body and appendage oscillation. In the past, this has been accomplished using two-dimensional planar techniques that require reconstruction of three-dimensional flow patterns. We have applied a new, fully three-dimensional, volumetric imaging technique that allows instantaneous capture of wake flow patterns, to a classic problem in functional vertebrate biology: the function of the asymmetrical (heterocercal) tail of swimming sharks to capture the vorticity field within the volume swept by the tail. These data were used to test a previous three-dimensional reconstruction of the shark vortex wake estimated from two-dimensional flow analyses, and show that the volumetric approach reveals a different vortex wake not previously reconstructed from two-dimensional slices. The hydrodynamic wake consists of one set of dual-linked vortex rings produced per half tail beat. In addition, we use a simple passive shark-tail model under robotic control to show that the three-dimensional wake flows of the robotic tail differ from the active tail motion of a live shark, suggesting that active control of kinematics and tail stiffness plays a substantial role in the production of wake vortical patterns.

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Wake mechanics for thrust generation in oscillating foils

Cited by 410 publications

References 8 publications

A study on forces acting on a flapping wing

A study on forces acting on a flapping wing

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

Volumetric imaging of shark tail hydrodynamics reveals a three-dimensional dual-ring vortex wake structure

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