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Stability of momentumless wakes

Published online by Cambridge University Press:  28 October 2016

M. Rizqie Arbie
Affiliation:
Aix Marseille Univ, CNRS, Centrale Marseille, IRPHE, 13013 Marseille, France
Uwe Ehrenstein*
Affiliation:
Aix Marseille Univ, CNRS, Centrale Marseille, IRPHE, 13013 Marseille, France
Christophe Eloy
Affiliation:
Aix Marseille Univ, CNRS, Centrale Marseille, IRPHE, 13013 Marseille, France
*
Email address for correspondence: ehrenstein@irphe.univ-mrs.fr

Abstract

The caudal fin of swimming animals can be modelled as a thrust-producing flapping foil. When considered alone, such a foil produces on average a jet wake with a positive net momentum. It has been argued that the instability characteristics of these averaged wakes are linked to the propulsion efficiency of swimming animals. Here, we reconsider this question by taking into account both the thrust and the drag exerted on a self-propelled swimming body. To do so, we study the stability of a family of momentumless wakes, constructed as the Oseen approximation of a force doublet moving at constant velocity. By performing a local stability analysis, we first show that these wakes undergo a transition from absolute to convective instability. Then, using the time stepper approach by integrating the linearised Navier–Stokes system, we investigate the global stability and reveal the influence of a non-parallel base flow as well as the role of the locally absolutely unstable upstream region in the wake. Finally, to complete the global scenario, we address the nonlinear evolution of the wake disturbance. These results are then discussed in the context of aquatic locomotion. According to the present stability results, and assuming the Oseen approximation whose validity has been assessed only for moderate Reynolds number, the momentumless wake of aquatic animals is generally stable, whereas the corresponding thrust part is unstable. It is therefore essential to consider all forces exerted on a self-propelled animal when discussing its wake stability and its propulsion efficiency.

Type
Papers
Copyright
© 2016 Cambridge University Press 

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