Experimental study on instantaneous thrust and lift of two plunging wings in tandem

Gong, Wuqi; Jia, Bo Bo; Xi, Guang

doi:10.1007/s00348-015-2096-2

Cited by 12 publications

(8 citation statements)

References 27 publications

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“…These have the form of parallel 'ridges' and 'valleys'. This has previously been observed for single frequencies (Boschitsch et al 2014;Gong et al 2015Gong et al , 2016, but until now this effect has never been observed over a range of frequencies. The fact that such a large thrust augmentation occurs for all these Strouhal numbers shows that the effect is important over a wide range of conditions.…”

Section: Results From Full Simulationsmentioning

confidence: 62%

“…Previous studies have shown that the force production on the hind foil of a tandemfoil configuration is affected by the phase difference in the kinematics between the fore and hind foils (ϕ) and the inter-foil spacing (S) Rival et al 2011;Kumar & Hu 2011;Broering & Lian 2010;Kinsey & Dumas 2012;Lian et al 2014;Boschitsch et al 2014;Gong et al 2015Gong et al , 2016. This effect has been addressed as "wake recapture", "wing/wake interaction", or "thrust augmentation For the purposes of this work, "performance augmentation" is deemed the most suitable description, as this term accounts for the augmentation of both thrust and efficiency.…”

Section: Introductionmentioning

confidence: 99%

“…Many studies have investigated both the phase and spacing, but at only one flapping frequency Kumar & Hu 2011;Boschitsch et al 2014;Gong et al 2015Gong et al , 2016. One study has considered the effects of both spacing and phase at different flapping frequencies, or Strouhal numbers (Broering & Lian 2010), although the parameter space was sparse and the experimental data consisted of combined forces of both foils, with very little information on the flowfield.…”

Section: Introductionmentioning

confidence: 99%

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Performance augmentation mechanism of in-line tandem flapping foils

2017

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The propulsive performance of a pair of tandem flapping foils is sensitively dependent on the spacing and phasing between them. Large increases in thrust and efficiency of the hind foil are possible, but the mechanisms governing these enhancements remain largely unresolved. Two-dimensional numerical simulations of tandem and single foils oscillating in heave and pitch at a Reynolds number of 7,000 are performed over a broad and dense parameter space, allowing the effects of inter-foil spacing (S) and phasing (ϕ) to be investigated over a range of non-dimensional frequencies (or Strouhal number, St). Results indicate that the hind foil can produce from no thrust, to twice the thrust of a single foil depending on its spacing and phasing with respect to the fore foil, which is consistent with previous studies that were carried out over a limited parameter space. Examination of instantaneous flowfields indicate that high thrust occurs when the hind foil weaves in between the vortices that have been shed by the fore foil, and low thrust occurs when the hind foil intercepts these vortices. Contours of high thrust and minimal thrust appear as inclined bands in the S − ϕ parameter space and this behaviour is apparent over the entire range of Strouhal numbers considered (0.2 St 0.5). A novel quasi-steady model that utilises kinematics of a virtual hind foil together with data obtained from simulations of a single flapping foil shows that performance augmentation is primarily determined through modification of the instantaneous angle of attack of the hind foil by the vortex street established by the fore foil. This simple model provides estimates of thrust and efficiency for the hind foil, which is consistent with data obtained through full simulations. The limitations of the virtual hind foil method and its physical significance is also discussed.

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Section: Results From Full Simulationsmentioning

confidence: 62%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Performance augmentation mechanism of in-line tandem flapping foils

2017

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“…These highly three-dimensional spatial configurations found within collectives can be decomposed into canonical in-line, side-by-side, or tip-to-tip arrangements as presented in Figure 1 . To date, these interactions have mostly been studied for propulsors in in-line arrangements [ 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ], although a few efforts have been made to understand interactions in side-by-side arrangements [ 20 , 21 , 22 , 23 ], as well as staggered arrangements [ 24 , 25 , 26 , 27 ]. Here, our focus is on the propulsive performance and flow interactions in in-line , and staggered arrangements.…”

Section: Introductionmentioning

confidence: 99%

Flow Interactions Between Low Aspect Ratio Hydrofoils in In-line and Staggered Arrangements

2020

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Many species of fish gather in dense collectives or schools where there are significant flow interactions from their shed wakes. Commonly, these swimmers shed a classic reverse von Kármán wake, however, schooling eels produce a bifurcated wake topology with two vortex rings shed per oscillation cycle. To examine the schooling interactions of a hydrofoil with a bifurcated wake topology, we present tomographic particle image velocimetry (tomo PIV) measurements of the flow interactions and direct force measurements of the performance of two low-aspect-ratio hydrofoils ( A R = 0.5 ) in an in-line and a staggered arrangement. Surprisingly, when the leader and follower are interacting in either arrangement there are only minor alterations to the flowfields beyond the superposition of the flowfields produced by the isolated leader and follower. Motivated by this finding, Garrick’s linear theory, a linear unsteady hydrofoil theory based on a potential flow assumption, was adapted to predict the lift and thrust performance of the follower. Here, the follower hydrofoil interacting with the leader’s wake is considered as the superposition of an isolated pitching foil with a time-varying cross-stream velocity derived from the wake flow measurements of the isolated leader. Linear theory predictions accurately capture the time-averaged lift force and some of the major peaks in thrust derived from the follower interacting with the leader’s wake in a staggered arrangement. The thrust peaks that are not predicted by linear theory are likely driven by spatial variations in the flowfield acting on the follower or nonlinear flow interactions; neither of which are accounted for in the simple theory. This suggests that unsteady potential flow theory that does account for spatial variations in the flowfield acting on a hydrofoil can provide a relatively simple framework to understand and model the flow interactions that occur in schooling fish. Additionally, schooling eels can derive thrust and efficiency increases of 63-80% in either a in-line or a staggered arrangement where the follower is between two branched momentum jets or with one momentum jet branch directly impinging on it, respectively.

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“…Supporting this argument, there have been extensive research efforts explaining energetic benefits of animals in collectives; ranging from a 15% thrust boost for fish schools [2], to 11 − 14% of metabolic energy savings for white pelicans [3]. So far this problem has been studied mostly for propulsors in in-line arrangements [5][6][7][8][9][10][11][12][13][14][15], as well as mixtures of side-by-side and in-line arrangements [15][16][17].…”

Section: Introductionmentioning

confidence: 99%

Unsteady Performance of Finite-Span Pitching Propulsors in Side-by-Side Arrangements

Kurt

Moored

2018

2018 Fluid Dynamics Conference

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Experimental measurements are presented on the performance of two finite-span pitching wings with an aspect ratio of AR = 2 interacting in a side-by-side arrangement. Experiments are conducted for various synchronies and cross-stream spacings. The thrust, power and lift coefficients as well as the efficiency are reported for both the individual wings and the collective. The collective thrust and power consumption are found to increase while the efficiency decreases during out-of-phase oscillations (φ = π) for close spacings of Y * ≤ 0.75. Similarly, the collective thrust and power consumption are found to decrease while the efficiency increases during inphase oscillations (φ = 0) for spacings of Y * ≤ 1.125. In contrast to this trade-off between improved thrust or efficiency, there are conditions such as at Y * = 1 and φ ≈ π/2 and 3π/2 that give a 20% increase in collective thrust and 17% increase in collective efficiency at the same time. It is further demonstrated that the direction of the collective lift shows a dependence on both the synchrony and the spacing.

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Experimental study on instantaneous thrust and lift of two plunging wings in tandem

Cited by 12 publications

References 27 publications

Performance augmentation mechanism of in-line tandem flapping foils

Performance augmentation mechanism of in-line tandem flapping foils

Flow Interactions Between Low Aspect Ratio Hydrofoils in In-line and Staggered Arrangements

Unsteady Performance of Finite-Span Pitching Propulsors in Side-by-Side Arrangements

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