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Abstract
We present, in 8 chapters, experiments on and numerical simulations of bodies flapping
in a fluid. Focus is predominantly on a rigid foil, a model fish, that performs
prescribed pitching oscillations where the foil rotates around its leading edge.
In a flowing soap film is measured, with unprecedented accuracy, the vortex wake
structure behind the flapping foil in the space spanned by dimensionless flapping
frequency 0 <StD <0.3 and amplitude 0 <AD <2. We measure not only the
ubiquitous von K´arm´an wake, but also wakes where up to sixteen vortices are shed
each oscillation period. The wake measurements are supplemented with numerical
simulations of the flow and fluid forces, in settings relevant for the experiments. It
is shown that wake transitions and average fluid forces are described by a single
parameter, the Strouhal number, which is a measure of both the dimensionless foil
tip-speed and the strength ratio of the vortices formed at the foil’s leading and
trailing edge. The simulated vortex particles and measured thickness variations in
the soap film show similar behaviour which indicates that the soap film provides a
good approximation the flow of a two-dimensional incompressible and Newtonian
fluid. Also, measurements of the swimming speed of a pitching foil in a water tank
are presented.
Finally, an experimental study of the surprisingly strong fluid-mediated interaction
of two tandem flappers is presented. It is shown that a passively flapping flag
in general is affected by its downstream neighbour. When this neighbour is a second
flag close by, they synchronise in frequency and the leader experiences a reduced
drag compared to that on the lone flag. In case the follower is replaced by a flapping
plate, upstream synchronisation and drag reduction is again found over a wide range
of frequencies. Drag reductions up to a factor 3 are measured.
Many results presented are obtained through flow visualisations. A great effort
is made to produce visualisations of primarily high scientific quality, but often also
with a certain aesthetic appeal.
Original language | English |
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Place of Publication | Kgs. Lyngby, Denmark |
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Publisher | Technical University of Denmark |
Publication status | Published - Feb 2011 |
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Dive into the research topics of 'Exotic wakes of flapping fins'. Together they form a unique fingerprint.Projects
- 1 Finished
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Fluid Dynamics of Animal Locomotion
Schnipper, T. (PhD Student), Andersen, A. P. (Main Supervisor), Aref, H. (Supervisor), Bohr, T. (Supervisor), Sørensen, J. N. (Supervisor), Brøns, M. (Examiner), Water, W. V. D. (Examiner) & Williamson, C. H. K. (Examiner)
15/08/2007 → 02/02/2011
Project: PhD