Séminaire de Mécanique d'Orsay

Le Jeudi 16 mai à 14h00 - Salle de conférences du FAST

The effect of the wake symmetry in biomimetic propulsion

Verónica Raspa
PMMH, CNRS-ESPCI-UPMC, Université Paris Diderot

The way hydrodynamics forces are linked to wake patterns observed behind biological locomotion systems, (such as birds or fishes in fluids) have long been a central question for physicists and engineers; and nowadays is a subject in which the interdisciplinary community experiences a growing interest in. Locomotive animal–fluid interactions in the macroscopic scale, such as those birds and fish have with their natural environments, typically occur in the Reynolds number regime of Re >102. Basically, swimmers and flyers achieve propulsion by periodically pushing fluid backwards resulting in a complex flow field characterized by the formation and periodical shedding of vortices. This is the case for wakes observed behind the flapping wings of a bird, a fish tail or any other swimming appendages such as for squids. Depending on the species, propulsive wakes are distinguished by the spatial ordering of their vortex pattern. A first order classification can be established by observing a cross-section of the wake, where the series of counter-rotating vortices can be arranged either symmetrically (vortex ring configuration: medusa-like wake) or asymmetrically (staggered vortex street configuration: fish-like wake). Two different experiments were conducted to analyze how thrust production can be affected by the wake characteristics (i.e. spatial ordering, mixing rate) while the momentum available for thrust production is kept constant. The first experiment consists in manipulating the wake of a free jet using harmonic acoustic forcing and/or vortex trapping, to give it one of the two considered typical propulsive-like patterns. Direct thrust measurements allow the performance comparison. The strength of this setup is that the available mass flux and the vorticity injection rate for propulsion are controlled independently of the wake pattern, which is chosen by external perturbation. As a second step on this direction, we analyze the displacement curve of a self-propelled swimmer. The experiment was performed in a free–surface water tank where an artificial swimmer is submerged. The synchronized pitching motion of two rigid foils achieves self-propulsion. The spatial ordering of the vortices in the wake is controlled by a phase lag introduced between the pitching motions of the two flaps. Force estimations were made from the swimmer's displacement. Velocity fields in the near wake were recovered from fast 2D-PIV measurements, to access the mean flow and fluctuations. Both experiments show that the vortices spatial ordering in the wake affects the achieved propulsion, and it's due to a pressure effect related to the ability of each wake to produce, or not, significant mixing in the near wake region. Additionally, our measurements confirm that some wake configurations have the ability to minimize mixing (here, for the symmetric mode) and save useful momentum for propulsion. This result suggests that controlling mixing in the near wake (by setting properly the time dependent momentum inputs to save energy) can be valuable strategy for design and optimization of artificial biomimetic propellers.