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Performance augmentation mechanism of in-line tandem flapping foils

Performance augmentation mechanism of in-line tandem flapping foils
Performance augmentation mechanism of in-line tandem flapping foils
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 ($\varphi$) 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 $S-\varphi$ parameter space and this behaviour is apparent over the entire range of Strouhal numbers considered (0.2 $\leq St \leq$ 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.
Flapping, foil, swimming, wakes, jets, fluid structure interaction, tandem
0022-1120
484-505
Muscutt, Luke
c10a4f10-38d0-4c9e-bfa5-9605856838ad
Weymouth, Gabriel
b0c85fda-dfed-44da-8cc4-9e0cc88e2ca0
Ganapathisubramani, Bharathram
5e69099f-2f39-4fdd-8a85-3ac906827052
Muscutt, Luke
c10a4f10-38d0-4c9e-bfa5-9605856838ad
Weymouth, Gabriel
b0c85fda-dfed-44da-8cc4-9e0cc88e2ca0
Ganapathisubramani, Bharathram
5e69099f-2f39-4fdd-8a85-3ac906827052

Muscutt, Luke, Weymouth, Gabriel and Ganapathisubramani, Bharathram (2017) Performance augmentation mechanism of in-line tandem flapping foils. Journal of Fluid Mechanics, 484-505. (doi:10.1017/jfm.2017.457).

Record type: Article

Abstract

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 ($\varphi$) 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 $S-\varphi$ parameter space and this behaviour is apparent over the entire range of Strouhal numbers considered (0.2 $\leq St \leq$ 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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Muscutt2017_PerformanceAugmentationOfTandemFlappingFoils - Accepted Manuscript
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More information

Accepted/In Press date: 11 June 2017
e-pub ahead of print date: 24 August 2017
Published date: 25 September 2017
Keywords: Flapping, foil, swimming, wakes, jets, fluid structure interaction, tandem

Identifiers

Local EPrints ID: 414374
URI: http://eprints.soton.ac.uk/id/eprint/414374
ISSN: 0022-1120
PURE UUID: eac057fe-c1f8-4194-a401-7f4d1274acc9
ORCID for Gabriel Weymouth: ORCID iD orcid.org/0000-0001-5080-5016
ORCID for Bharathram Ganapathisubramani: ORCID iD orcid.org/0000-0001-9817-0486

Catalogue record

Date deposited: 27 Sep 2017 16:30
Last modified: 07 Oct 2020 06:07

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