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Phys Rev E ; 100(5-1): 053107, 2019 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-31869921


A direction adaptive approach for the reduction of drag and the suppression of lift fluctuation in flow passing a circular cylinder is developed. Flexible filaments are attached to the surface of the cylinder, and different configurations, including the number, lengths, and angles of attachment of the filaments, as well as their tension and bending features, are investigated. In this comprehensive numerical study, the configuration with two filaments 180^{o} apart is found to be optimal for drag reduction and lift fluctuation suppression and is adaptive to the direction of the incoming flow. A drag reduction of 10.8% and a lift fluctuation suppression of 34.6% can be achieved as one filament is attached to the rear stagnation point and the other to the front stagnation point. The hairy coating resembled by 12 evenly attached filaments is also considered. Though marked drag reduction has not been found for this configuration, we leave it an open question for future studies to explore various properties of the filaments in turbulent flow, whose interaction with the filaments would be significant.

Bioinspir Biomim ; 14(4): 046009, 2019 06 07.
Artigo em Inglês | MEDLINE | ID: mdl-31117061


Flying fish is a family of unique aerial-aquatic animals, which can both swim in the water and glide over the sea surface. Most previous studies on their aerodynamic characteristics were based on field observations or measurements of their morphometric parameters. In the present study, we consider three different flying fish models, of which the preliminary one mimics the Cypselurus hiraii in the pectoral fin morphology, following a previous wind tunnel experiment (Park and Choi 2010 J. Exp. Biol. 213 3269-79). Their aerodynamic performances are numerically studied by the computational fluid dynamics (CFD) method. The maximum lift force coefficient of 1.03 is reached at the angle of attack [Formula: see text], and the maximum lift-to-drag ratio of 4.7 is achieved at [Formula: see text]. By choosing appropriately the center of gravity, the flying fish model is proved to be longitudinally stable, according to the negative slope of pitching moment profile. Furthermore, we build a three-degrees-of-freedom (3-DOF) dynamic model in the longitudinal plane based on the aerodynamic coefficients obtained in our simulations, to predict its gliding performance. The results show that the flying fish can achieve a distance up to 45.4 m, and reach a height of 13.2 m, indicating an extraordinary gliding performance. Our numerical simulations are consistent with previous experimental results and theoretical prediction, which can be taken as the basis of further research on robotic flying fish.

Phys Rev E ; 94(3-1): 033107, 2016 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-27739714


We propose a natural model to probe in a controlled fashion the instability of interacting vortex rings shed from the edge of an oblate spheroid disk of major diameter c, undergoing oscillations of frequency f_{0} and amplitude A. We perform a Floquet stability analysis to determine the characteristics of the instability modes, which depend strongly on the azimuthal (integer) wave number m. We vary two key control parameters, the Keulegan-Carpenter number K_{C}=2πA/c and the Stokes number ß=f_{0}c^{2}/ν, where ν is the kinematic viscosity of the fluid. We observe two distinct flow regimes. First, for sufficiently small ß, and hence low frequency of oscillation corresponding to relatively weak interaction between sequentially shedding vortex rings, symmetry breaking occurs directly to a single unstable mode with m=1. Second, for sufficiently large yet fixed values of ß, corresponding to a higher oscillation frequency and hence stronger ring-ring interaction, the onset of asymmetry is predicted to occur due to two branches of high m instabilities as the amplitude is increased, with m=1 structures being dominant only for sufficiently large values of K_{C}. These two branches can be distinguished by the phase properties of the vortical structures above and below the disk. The region in (K_{C},ß) parameter space where these two high m instability branches arise can be described accurately in terms of naturally defined Reynolds numbers, using appropriately chosen characteristic length scales. We subsequently carry out direct numerical simulations of the fully three-dimensional flow to verify the principal characteristics of the Floquet analysis, in particular demonstrating that high wave-number symmetry-breaking generically occurs when vortex rings sequentially interact sufficiently strongly.