Asymmetric vortex pair induces secondary traveling wave vibration of a flexible cylinder from still water to incoming flow

Abstract. We investigate the elliptical instability of a strongly asymmetric vortex pair in a stratified fluid, generated by the acceleration and deceleration of the rotation of a single flap. The dominant parameter is the Froude number, Fr=U/(NR), based on the maximum azimuthal velocity, U, and corresponding radius, R, of the strongest vortex, i.e. the principal vortex, and buoyancy frequency N. For Fr>1, both vortices are elliptically unstable while the instability is suppressed for Fr<1. In an asymmetric vortex pair, the principal vortex is less – and the secondary vortex more – elliptical than the vortices in an equivalent symmetric dipolar vortex. The far more unstable secondary vortex interacts with the principal vortex and increases the strain on the latter, thus increasing its ellipticity and its instability growth rate. The nonlinear interactions render the elliptical instability more relevant. An asymmetric dipole can be more unstable than an equivalent symmetric dipole. Further, the wavelength of the instability is shown to be a function of the Froude number for strong stratifications corresponding to small Froude numbers, whereas it remains constant in the limit of a homogenous fluid.

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PLASMA FLOW CONTROL OVER FOREBODY AT HIGH ANGLES OF ATTACK

Modern Physics Letters B ◽

10.1142/s0217984910023736 ◽

2010 ◽

Vol 24 (13) ◽

pp. 1405-1408 ◽

Cited By ~ 2

Author(s):

ZIJIE ZHAO ◽

CHAO GAO ◽

FENG LIU ◽

SHIJUN LUO

Keyword(s):

Wind Tunnel ◽

Flow Control ◽

Plasma Flow ◽

Vortex Pair ◽

Apex Angle ◽

Plasma Actuators ◽

Pressure Measurements ◽

Asymmetric Vortex ◽

Plasma Flow Control ◽

High Angles Of Attack

Forward blowing from a pair of plasma actuators on the leeward surface and near the apex is used to switch the asymmetric vortex pair over a cone of semi-apex angle 10° at high angles of attack. Wind tunnel pressure measurements show that by appropriate design of the actuators and appropriate choice of the AC voltage and frequency, side forces and yawing moments of opposite signs can be obtained at a given angle of attack by activating one of the plasma actuators. Further work is suggested.

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Hydrodynamic Performance of Two-Dimensional Undulating Foils in Triangular Formation

Journal of Mechanics ◽

10.1017/jmech.2011.21 ◽

2011 ◽

Vol 27 (2) ◽

pp. 177-190 ◽

Cited By ~ 1

Author(s):

M.-H. Chung

Keyword(s):

Traveling Wave ◽

Wave Speed ◽

Vortex Pair ◽

Leading Edge ◽

Viscous Flows ◽

Chord Length ◽

Hydrodynamic Performance ◽

Two Dimensional ◽

Efficiency Enhancement ◽

Trailing Edges

ABSTRACTAs inspired by studies of fish schooling in literature, this work investigates hydrodynamic performance of a two-dimensional undulating-foil triad in viscous flows via numerical simulation. The chord of foil oscillates in the form of a streamwise traveling wave. The triad is in triangular formation, i.e., two foils followed by one. A series of triad configuration are computed assuming the same wave speed, amplitude, and frequency of chord traveling wave for each foil. The results show that, to achieve highest thrust efficiency, the two leading foils should separate from each other by 0.4 chord length, perform antiphase undulating motion, and the leading edge of the trailing foil stay 0.2 chord length in front of the trailing edges of the leading foils. An underlining mechanism, vortex pair shedding from the leading foil interacting with boundary-layer vorticity field of the trailing foil, has been identified to explain the efficiency enhancement. This optimal triad configuration is different from that obtained in a previous potential flow analysiss.

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