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Vortex-wake interactions of a flapping foil that models animal swimming and flight.

Lentink, David; Muijres, Florian LU ; Donker-Duyvis, Frits J and Leeuwen, J L Van (2008) In Journal of Experimental Biology 211(Pt 2). p.267-273
Abstract
The fluid dynamics of many swimming and flying animals involves the generation and shedding of vortices into the wake. Here we studied the dynamics of similar vortices shed by a simple two-dimensional flapping foil in a soap-film tunnel. The flapping foil models an animal wing, fin or tail in forward locomotion. The vortical flow induced by the foil is correlated to (the resulting) thickness variations in the soap film. We visualized these thickness variations through light diffraction and recorded it with a digital high speed camera. This set-up enabled us to study the influence of foil kinematics on vortex-wake interactions. We varied the dimensionless wavelength of the foil (lambda=4-24) at a constant dimensionless flapping amplitude... (More)
The fluid dynamics of many swimming and flying animals involves the generation and shedding of vortices into the wake. Here we studied the dynamics of similar vortices shed by a simple two-dimensional flapping foil in a soap-film tunnel. The flapping foil models an animal wing, fin or tail in forward locomotion. The vortical flow induced by the foil is correlated to (the resulting) thickness variations in the soap film. We visualized these thickness variations through light diffraction and recorded it with a digital high speed camera. This set-up enabled us to study the influence of foil kinematics on vortex-wake interactions. We varied the dimensionless wavelength of the foil (lambda=4-24) at a constant dimensionless flapping amplitude (A=1.5) and geometric angle of attack amplitude (A(alpha,geo)=15 degrees ). The corresponding Reynolds number was of the order of 1000. Such values are relevant for animal swimming and flight. We found that a significant leading edge vortex (LEV) was generated by the foil at low dimensionless wavelengths (lambda<10). The LEV separated from the foil for all dimensionless wavelengths. The relative time (compared with the flapping period) that the unstable LEV stayed above the flapping foil increased for decreasing dimensionless wavelengths. As the dimensionless wavelength decreased, the wake dynamics evolved from a wavy von K rm n-like vortex wake shed along the sinusoidal path of the foil into a wake densely packed with large interacting vortices. We found that strongly interacting vortices could change the wake topology abruptly. This occurred when vortices were close enough to merge or tear each other apart. Our experiments show that relatively small changes in the kinematics of a flapping foil can alter the topology of the vortex wake drastically. (Less)
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author
publishing date
type
Contribution to journal
publication status
published
subject
keywords
flapping foil, vortex dynamics, vortex-wake interaction, swimming, flight, soap tunnel.
in
Journal of Experimental Biology
volume
211
issue
Pt 2
pages
267 - 273
publisher
The Company of Biologists Ltd
external identifiers
  • scopus:39049128060
ISSN
1477-9145
DOI
10.1242/jeb.006155
language
English
LU publication?
no
id
b04860a8-2463-4c36-8736-c703a6de2785 (old id 1886262)
date added to LUP
2011-04-06 12:50:09
date last changed
2017-09-24 03:37:36
@article{b04860a8-2463-4c36-8736-c703a6de2785,
  abstract     = {The fluid dynamics of many swimming and flying animals involves the generation and shedding of vortices into the wake. Here we studied the dynamics of similar vortices shed by a simple two-dimensional flapping foil in a soap-film tunnel. The flapping foil models an animal wing, fin or tail in forward locomotion. The vortical flow induced by the foil is correlated to (the resulting) thickness variations in the soap film. We visualized these thickness variations through light diffraction and recorded it with a digital high speed camera. This set-up enabled us to study the influence of foil kinematics on vortex-wake interactions. We varied the dimensionless wavelength of the foil (lambda=4-24) at a constant dimensionless flapping amplitude (A=1.5) and geometric angle of attack amplitude (A(alpha,geo)=15 degrees ). The corresponding Reynolds number was of the order of 1000. Such values are relevant for animal swimming and flight. We found that a significant leading edge vortex (LEV) was generated by the foil at low dimensionless wavelengths (lambda&lt;10). The LEV separated from the foil for all dimensionless wavelengths. The relative time (compared with the flapping period) that the unstable LEV stayed above the flapping foil increased for decreasing dimensionless wavelengths. As the dimensionless wavelength decreased, the wake dynamics evolved from a wavy von K rm n-like vortex wake shed along the sinusoidal path of the foil into a wake densely packed with large interacting vortices. We found that strongly interacting vortices could change the wake topology abruptly. This occurred when vortices were close enough to merge or tear each other apart. Our experiments show that relatively small changes in the kinematics of a flapping foil can alter the topology of the vortex wake drastically.},
  author       = {Lentink, David and Muijres, Florian and Donker-Duyvis, Frits J and Leeuwen, J L Van},
  issn         = {1477-9145},
  keyword      = {flapping foil,vortex dynamics,vortex-wake interaction,swimming,flight,soap tunnel.},
  language     = {eng},
  number       = {Pt 2},
  pages        = {267--273},
  publisher    = {The Company of Biologists Ltd},
  series       = {Journal of Experimental Biology},
  title        = {Vortex-wake interactions of a flapping foil that models animal swimming and flight.},
  url          = {http://dx.doi.org/10.1242/jeb.006155},
  volume       = {211},
  year         = {2008},
}