Biomedical subjects

C F Barenghi

Publications and source records attributed to C F Barenghi.

10 recordsLinked to original sources

PubMed2006

Anomalous translational velocity of vortex ring with finite-amplitude Kelvin waves.

We consider finite-amplitude Kelvin waves on an inviscid vortex assuming that the vortex core has infinitesimal thickness. By numerically solving the governing Biot-Savart equation of motion, we study how the frequency of the Kelvin waves and the velocity of the perturbed ring depend on the Kelvin wave amplitude. In particular, we show that, if the amplitude of the Kelvin waves is sufficiently large, the perturbed vortex ring moves backwards.

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PubMed2006

Depolarization of decaying counterflow turbulence in He II.

We present experimental evidence backed up by numerical simulations that the steady-state vortex tangle created in He II by heat-transfer counterflow is strongly polarized. When the heater that generates the counterflow turbulence is switched off, the vortex tangle decays, the vortex lines randomize their spatial orientation and the tangle's polarization decreases. The process of depolarization slows down the recovery of the transverse second sound signal which measures the vortex line density; at some values of parameters it even leads to a net decrease of the amplitude of the transverse second sound prior to reaching the universal -32 power temporal law decay typical of classical homogeneous isotropic turbulence in a finite-sized channel.

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PubMed2004

Parametric driving of dark solitons in atomic Bose-Einstein condensates.

A dark soliton oscillating in an elongated harmonically confined atomic Bose-Einstein condensate continuously exchanges energy with the sound field. Periodic optical paddles are employed to controllably enhance the sound density and transfer energy to the soliton, analogous to parametric driving. In the absence of damping, the amplitude of the soliton oscillations can be dramatically reduced, whereas with damping, a driven soliton equilibrates as a stable soliton with lower energy, thereby extending the soliton lifetime up to the lifetime of the condensate.

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PubMed2004

Controlled vortex-sound interactions in atomic Bose-Einstein condensates.

The low temperature dynamics of a vortex in a trapped quasi-two-dimensional Bose-Einstein condensate are studied quantitatively. Precession of an off-centered vortex in a dimple trap, embedded in a weaker harmonic trap, leads to the emission of sound in a dipolar radiation pattern. Sound emission and reabsorption can be controlled by varying the depth of the dimple. In a shallow dimple, the power emitted is proportional to the vortex acceleration-squared over the precession frequency, whereas for a deep dimple, periodic sound reabsorption stabilizes the vortex against radiation-induced decay.

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PubMed2001

Quantum signature of superfluid turbulence.

Using a numerical simulation backed up by physical arguments, we predict that the pressure spectrum of superfluid turbulence has a k(-2) dependence on the wave number k, which represents a macroscopic quantum signature not to be found in the classical Kolmogorov theory of turbulence.

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PubMed2001

Fractal dimension of superfluid turbulence.

Superfluid turbulence consists of a disordered tangle of quantized vortex filaments which interact with each other and with the normal fluid. We develop a kinematic model of normal-fluid turbulence to study superfluid vortex tangles at finite temperatures and show by numerical simulation that the system of filaments has a fractal dimension larger than one. We find that the fractal dimension is directly related to the vortex-line density and is independent of temperature over a wide range.

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PubMed2001

Kelvin waves cascade in superfluid turbulence.

We study numerically the interaction of four initial superfluid vortex rings in the absence of any dissipation or friction. We find evidence for a cascade of Kelvin waves generated by individual vortex reconnection events which transfers energy to higher and higher wave numbers k. After the vortex reconnections occur, the energy spectrum scales as k(-1) and the curvature spectrum becomes flat. These effects highlight the importance of Kelvin waves and reconnections in the transfer of energy within a turbulent vortex tangle.

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PubMed2001

Sound emission due to superfluid vortex reconnections.

By performing numerical simulations based on the Gross-Pitaevskii equation, we make direct quantitative measurements of the sound energy released due to superfluid vortex reconnections. We show that the energy radiated expressed in terms of the loss of vortex line length is a simple function of the reconnection angle. In addition, we study the temporal and spatial distribution of the radiation and show that energy is emitted in the form of a sound pulse with a wavelength of a few healing lengths.

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PubMed2000

Triple vortex ring structure in superfluid helium II.

Superfluids such as helium II consist of two interpenetrating fluids: the normal fluid and the superfluid. The helium II vortex ring has generally been considered merely as a superfluid object, neglecting any associated motion of the normal fluid. We report a three-dimensional calculation of the coupled motion of the normal-fluid and superfluid components, which shows that the helium II vortex ring consists of a superfluid vortex ring accompanied by two coaxial normal-fluid vortex rings of opposite polarity. The three vortex rings form a coherent, dissipative structure.

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PubMed1992

Spontaneous symmetry breaking and the onset of chaos in a muscle model.

The Hill length-tension curve of a muscle fibre is not linear. This had led us to investigate the behaviour of a model muscle sarcomere. The muscle is modelled as an oscillator of mass m subject to a nonlinear restoring force, a periodic external driving force and friction. We have found that this simple, idealized model exhibits spontaneous symmetry breaking and transition to chaos via a period doubling sequence. This result suggests that the nonlinearity in the length-tension curve of a muscle fibre could be responsible for irregular temporal behaviour and muscle tremor.

Animals↗