Abstracts - KTH Mechanics

166
Experiments on the reverse Bénard-von Kármán vortex street
produced by a flapping foil
R. Godoy-Diana ∗ ,J.L. Aider ∗ and J.E. Wesfreid ∗
Control of wake vortices to generate propulsive forces is the everyday task of
swimming fish and other animals. Many studies on actual flapping tails and fins
have been motivated looking forward to gain a better understanding of this form
of propulsion with the ultimate goal of enhancing man-made propulsive mechanisms
(see e.g. Triantafyllou, et al. 1 for a review). A feature that is present in almost
every configuration involving flapping foils is the generation of a wake vortex street
with the sign of vorticity in the core of each vortex reversed with respect to the
Bénard-von Kármán (BvK) street in the wake of non-flapping body. The goal of the
present work is to study experimentally this reverse BvK vortex street in a simple
configuration in order to identify basic dynamical mechanisms. The experimental
setup consists of a high-aspect-ratio pitching foil placed in a hydrodynamic tunnel.
The main control parameters on the experiment are the forcing frequency (f) and
oscillation amplitude (A) of the foil and the flow velocity in the tunnel (U). In figure
1 we present visualizations of the wake of the foil obtained in two different parameter
configurations by injecting two fluorescein dye filaments upstream at mid-height of
the foil. In figure 1.a the flapping motion produces a typical example of a reverse BvK
vortex street whereas in figure 1.b an asymmetric regime is established in the wake
of the foil. Particle image velocimetry (PIV) measurements give access to the spatiotemporal
characteristics of the vorticity field in the wake and allow for a calculation
of the spatial distribution of velocity fluctuations as well as an estimate of the forces
on the foil. We compare these results for reverse BvK vortex streets, with respect to
those of a forced BvK wake produced by a cylinder performing rotary oscillations 2 .
∗ PMMH-ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France
1 Triantafyllou et al., Annu. Rev. Fluid Mech. 32, 33 (2000).
2 Thiria et al., J. Fluid Mech. to be published (2006).
Figure 1: (a) Reverse BvK vortex street (f =0.75s 1 ); (b) Asymmetric wake (f =
1s 1 ). In both cases the Reynolds number based on the width of the foil D is Re =
UD/ν = 95.