Article

# Oscillating pendulum decay by emission of vortex rings.

Department of Civil Engineering an Geological Sciences, University of Notre Dame, Notre Dame, Indiana 46556, USA.

Physical Review E (Impact Factor: 2.31). 04/2010; 81(4 Pt 2):046317. DOI: 10.1103/PhysRevE.81.046317 Source: PubMed

- [Show abstract] [Hide abstract]

**ABSTRACT:**We consider vortex rings moving in a Bose-Einstein condensate. By numerically solving the Gross-Pitaevskii equation, we show that if the circular shape of the ring is perturbed by helical Kelvin waves of given amplitude and azimuthal wave number, the translational self-induced velocity of the vortex ring is reduced; at large amplitude, the vortex ring halts.Physical Review A 04/2011; 83(4):045601-045601. · 3.04 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**The simple pendulum remains one of the most fundamental systems studied in physics. It is commonly used as a model to illustrate a broad variety of mechanisms in a wide range of areas. However, in spite of this popularity, subtle behaviours still remain to be discovered and to be explored when the pendulum is strongly coupled to fluid mechanics. This is for instance illustrated in recent studies by Neill, Livelybrooks & Donnelly (Am. J. Phys., vol. 75, 2007, pp. 226–229) and Bolster, Hershberger & Donnelly (Phys. Rev. E, vol. 81, 2010, pp. 1–6) which highlight the impact on a simple spherical pendulum of vortex shedding and added mass effects. In the present work we show that the equilibrium of a pendular disk facing a flow exhibits bi-stability and hysteresis. We give a simple interpretation of this behaviour in terms of a two-potential-well description, only requiring to know the angular dependence of the normal drag coefficient of an inclined plate. We investigate the influence of turbulence on the equilibrium of the pendulum in general and on the observed bi-stability in particular. Our results have potentially important fundamental and practical consequences: (i) they extend the attractiveness of the pendulum as a model to investigate generic questions related to bi-stable stochastic processes, (ii) they highlight important fluid dynamic mechanisms, including turbulent drag enhancement and fluid–structure interaction issues.Journal of Fluid Mechanics 08/2013; 728. · 2.29 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**The onset of turbulent flow around an oscillating sphere is known to occur at a critical velocity $v_{c} \sim\sqrt{\kappa\,\omega}$ where κ is the circulation quantum and ω is the oscillation frequency. However, in a small interval of driving force amplitudes F (or corresponding velocity amplitudes of few percent above v c ) the turbulent flow is found to be unstable. The flow pattern switches intermittently between potential flow and turbulence. The lifetimes of the turbulent phases have an exponential distribution and the mean lifetimes τ grow very rapidly with increasing driving force, namely as τ(F)∼exp[(F/F 1)2]. In this work this experimental result is analyzed in more detail than before, in particular the force F 1 is identified. As a result, the turbulent drag force can be ascribed quantitatively to the shedding of vortex rings having the size of the sphere.Journal of Low Temperature Physics 11/2013; · 1.18 Impact Factor

Data provided are for informational purposes only. Although carefully collected, accuracy cannot be guaranteed. The impact factor represents a rough estimation of the journal's impact factor and does not reflect the actual current impact factor. Publisher conditions are provided by RoMEO. Differing provisions from the publisher's actual policy or licence agreement may be applicable.