Wake structure and thrust generation of a flapping foil in two-dimensional flow

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Abstract

We present a combined numerical (particle vortex method) and experimental (soap film tunnel) study of a symmetric foil undergoing prescribed oscillations in a two-dimensional free stream. We explore pure pitching and pure heaving, and contrast these two generic types of kinematics. We compare measurements and simulations when the foil is forced with pitching oscillations, and we find a close correspondence between flow visualisations using thickness variations in the soap film and the numerically determined vortex structures. Numerically, we determine wake maps spanned by oscillation frequency and amplitude, and we find qualitatively similar maps for pitching and heaving. We determine the drag–thrust transition for both pitching and heaving numerically, and we discuss it in relation to changes in wake structure. For heaving with low oscillation frequency and high amplitude, we find that the drag–thrust transition occurs in a parameter region with wakes in which two vortex pairs are formed per oscillation period, in contrast to the common transition scenario in regions with inverted von Kármán wakes.
Original languageEnglish
Article numberR4
JournalJournal of Fluid Mechanics
Volume812
Number of pages12
ISSN0022-1120
DOIs
Publication statusPublished - 2017

Keywords

  • Propulsion
  • Vortex shedding
  • Wakes/jets

Cite this

@article{302e5ec5c86143c0b5d68e476caf36af,
title = "Wake structure and thrust generation of a flapping foil in two-dimensional flow",
abstract = "We present a combined numerical (particle vortex method) and experimental (soap film tunnel) study of a symmetric foil undergoing prescribed oscillations in a two-dimensional free stream. We explore pure pitching and pure heaving, and contrast these two generic types of kinematics. We compare measurements and simulations when the foil is forced with pitching oscillations, and we find a close correspondence between flow visualisations using thickness variations in the soap film and the numerically determined vortex structures. Numerically, we determine wake maps spanned by oscillation frequency and amplitude, and we find qualitatively similar maps for pitching and heaving. We determine the drag–thrust transition for both pitching and heaving numerically, and we discuss it in relation to changes in wake structure. For heaving with low oscillation frequency and high amplitude, we find that the drag–thrust transition occurs in a parameter region with wakes in which two vortex pairs are formed per oscillation period, in contrast to the common transition scenario in regions with inverted von K{\'a}rm{\'a}n wakes.",
keywords = "Propulsion, Vortex shedding, Wakes/jets",
author = "Andersen, {Anders Peter} and Tomas Bohr and Teis Schnipper and Walther, {Jens Honore}",
year = "2017",
doi = "10.1017/jfm.2016.808",
language = "English",
volume = "812",
journal = "Journal of Fluid Mechanics",
issn = "0022-1120",
publisher = "Cambridge University Press",

}

Wake structure and thrust generation of a flapping foil in two-dimensional flow. / Andersen, Anders Peter; Bohr, Tomas; Schnipper, Teis; Walther, Jens Honore.

In: Journal of Fluid Mechanics, Vol. 812, R4, 2017.

Research output: Contribution to journalJournal articleResearchpeer-review

TY - JOUR

T1 - Wake structure and thrust generation of a flapping foil in two-dimensional flow

AU - Andersen, Anders Peter

AU - Bohr, Tomas

AU - Schnipper, Teis

AU - Walther, Jens Honore

PY - 2017

Y1 - 2017

N2 - We present a combined numerical (particle vortex method) and experimental (soap film tunnel) study of a symmetric foil undergoing prescribed oscillations in a two-dimensional free stream. We explore pure pitching and pure heaving, and contrast these two generic types of kinematics. We compare measurements and simulations when the foil is forced with pitching oscillations, and we find a close correspondence between flow visualisations using thickness variations in the soap film and the numerically determined vortex structures. Numerically, we determine wake maps spanned by oscillation frequency and amplitude, and we find qualitatively similar maps for pitching and heaving. We determine the drag–thrust transition for both pitching and heaving numerically, and we discuss it in relation to changes in wake structure. For heaving with low oscillation frequency and high amplitude, we find that the drag–thrust transition occurs in a parameter region with wakes in which two vortex pairs are formed per oscillation period, in contrast to the common transition scenario in regions with inverted von Kármán wakes.

AB - We present a combined numerical (particle vortex method) and experimental (soap film tunnel) study of a symmetric foil undergoing prescribed oscillations in a two-dimensional free stream. We explore pure pitching and pure heaving, and contrast these two generic types of kinematics. We compare measurements and simulations when the foil is forced with pitching oscillations, and we find a close correspondence between flow visualisations using thickness variations in the soap film and the numerically determined vortex structures. Numerically, we determine wake maps spanned by oscillation frequency and amplitude, and we find qualitatively similar maps for pitching and heaving. We determine the drag–thrust transition for both pitching and heaving numerically, and we discuss it in relation to changes in wake structure. For heaving with low oscillation frequency and high amplitude, we find that the drag–thrust transition occurs in a parameter region with wakes in which two vortex pairs are formed per oscillation period, in contrast to the common transition scenario in regions with inverted von Kármán wakes.

KW - Propulsion

KW - Vortex shedding

KW - Wakes/jets

U2 - 10.1017/jfm.2016.808

DO - 10.1017/jfm.2016.808

M3 - Journal article

VL - 812

JO - Journal of Fluid Mechanics

JF - Journal of Fluid Mechanics

SN - 0022-1120

M1 - R4

ER -