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Feb 2013

Volume 25, Issue 2, Articles (02xxxx)

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Phys. Fluids 25, 025102 (2013); http://dx.doi.org/10.1063/1.4790640 (31 pages)

T. A. Casey, J. Sakakibara, and S. T. Thoroddsen
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back to top Biofluid Mechanics

Stability of passive locomotion in inviscid wakes

Babak G. Oskouei and Eva Kanso

Phys. Fluids 25, 021901 (2013); http://dx.doi.org/10.1063/1.4789901 (12 pages)

Online Publication Date: 8 February 2013

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We consider the passive locomotion of rigid bodies in inviscid point-vortex wakes. This work is motivated by a common belief that live and inanimate objects may extract energy from unsteady flows for locomotory advantages. Studies on energy extraction from unsteady flows focus primarily on energy efficiency. Besides efficiency, a fundamental aspect of energy extraction for locomotion purposes is stability of motion. Here, we propose idealized wake models using periodically generated point vortices to emulate shedding of vortices from an un-modeled moving or stationary object. We assess the stability of these point-vortex wakes and find that they are stable for a range of periods, unlike the von Kármán street model which is mainly unstable. We then investigate the dynamics of a rigid body submerged in such wakes. In particular, we calculate periodic trajectories where the rigid body “swims” passively against the flow by extracting energy from the ambient vortices. All the periodic trajectories we find are unstable. The largest instabilities reported are for elliptic bodies where rotational effects play a role in destabilizing their motion. Within the context of this model, we conclude that passive locomotion of rigid bodies in inviscid wakes is unstable. Questions as to whether passive stability can be achieved when accounting for fluid viscosity and body elasticity remain open.
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47.20.Cq Inviscid instability
47.27.wb Turbulent wakes
47.32.cd Vortex stability and breakdown
47.11.-j Computational methods in fluid dynamics
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Aerodynamic forces and vortical structures in flapping butterfly's forward flight

Naoto Yokoyama, Kei Senda, Makoto Iima, and Norio Hirai

Phys. Fluids 25, 021902 (2013); http://dx.doi.org/10.1063/1.4790882 (24 pages)

Online Publication Date: 21 February 2013

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Forward flights of a bilaterally symmetrically flapping butterfly modeled as a four-link rigid-body system consisting of a thorax, an abdomen, and left and right wings are numerically simulated. The joint motions of the butterflies are adopted from experimental observations. Three kinds of the simulations, distinguished by ways to determine the position and attitude of the thorax, are carried out: a tethered simulation, a prescribed simulation, and free-flight simulations. The upward and streamwise forces as well as the wake structures in the tethered simulation, where the thorax of the butterfly is fixed, reasonably agree with those in the corresponding tethered experiment. In the prescribed simulation, where the thoracic trajectories as well as the joint angles are given by those observed in a free-flight experiment, it is confirmed that the butterfly can produce enough forces to achieve the flapping flights. Moreover, coherent vortical structures in the wake and those on the wings are identified. The generation of the aerodynamic forces due to the vortical structures are also clarified. In the free-flight simulation, where only the joint angles are given as periodic functions of time, it is found that the free flight is longitudinally unstable because the butterfly cannot maintain the attitude in a proper range. Focusing on the abdominal mass, which largely varies owing to feeding and metabolizing, we have shown that the abdominal motion plays an important role in periodic flights. The necessity of control of the thoracic attitude for periodic flights and maneuverability is also discussed.
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47.85.Gj Aerodynamics
87.19.rj Contraction
02.60.Cb Numerical simulation; solution of equations
47.32.-y Vortex dynamics; rotating fluids
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