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Universal scaling law for drag-to-thrust wake transition in flapping foils

Universal scaling law for drag-to-thrust wake transition in flapping foils
Universal scaling law for drag-to-thrust wake transition in flapping foils
Reversed von Kármán streets are responsible for a velocity surplus in the wake of flapping foils, indicating the onset of thrust generation. However, the wake pattern cannot be predicted based solely on the flapping peak-to-peak amplitude and frequency because the transition also depends sensitively on other details of the kinematics. In this work we replace with the cycle-averaged swept trajectory of the foil chordline. Two-dimensional simulations are performed for pure heave, pure pitch and a variety of heave-to-pitch coupling. In a phase space of dimensionless we show that the drag-to-thrust wake transition of all tested modes occurs for a modified Strouhal . Physically, the product expresses the induced velocity of the foil and indicates that propulsive jets occur when this velocity exceeds . The new metric offers a unique insight into the thrust-producing strategies of biological swimmers and flyers alike, as it directly connects the wake development to the chosen kinematics, enabling a self-similar characterisation of flapping foil propulsion.
0022-1120
Lagopoulos, Nikolaos
8a38fce1-8fa9-4dec-a2b0-db0e9372c575
Weymouth, Gabriel
b0c85fda-dfed-44da-8cc4-9e0cc88e2ca0
Ganapathisubramani, Bharathram
5e69099f-2f39-4fdd-8a85-3ac906827052
Lagopoulos, Nikolaos
8a38fce1-8fa9-4dec-a2b0-db0e9372c575
Weymouth, Gabriel
b0c85fda-dfed-44da-8cc4-9e0cc88e2ca0
Ganapathisubramani, Bharathram
5e69099f-2f39-4fdd-8a85-3ac906827052

Lagopoulos, Nikolaos, Weymouth, Gabriel and Ganapathisubramani, Bharathram (2019) Universal scaling law for drag-to-thrust wake transition in flapping foils. Journal of Fluid Mechanics, 872. (doi:10.1017/jfm.2019.361).

Record type: Article

Abstract

Reversed von Kármán streets are responsible for a velocity surplus in the wake of flapping foils, indicating the onset of thrust generation. However, the wake pattern cannot be predicted based solely on the flapping peak-to-peak amplitude and frequency because the transition also depends sensitively on other details of the kinematics. In this work we replace with the cycle-averaged swept trajectory of the foil chordline. Two-dimensional simulations are performed for pure heave, pure pitch and a variety of heave-to-pitch coupling. In a phase space of dimensionless we show that the drag-to-thrust wake transition of all tested modes occurs for a modified Strouhal . Physically, the product expresses the induced velocity of the foil and indicates that propulsive jets occur when this velocity exceeds . The new metric offers a unique insight into the thrust-producing strategies of biological swimmers and flyers alike, as it directly connects the wake development to the chosen kinematics, enabling a self-similar characterisation of flapping foil propulsion.

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Accepted/In Press date: 28 April 2019
e-pub ahead of print date: 7 June 2019
Published date: August 2019

Identifiers

Local EPrints ID: 431773
URI: http://eprints.soton.ac.uk/id/eprint/431773
ISSN: 0022-1120
PURE UUID: 08bb371e-39f8-4387-8c0a-8cbbdecd6ee3
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: 14 Jun 2019 16:30
Last modified: 07 Oct 2020 04:51

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