Biomedical subjects

Carlo F Barenghi

Publications and source records attributed to Carlo F Barenghi.

6 recordsLinked to original sources

PubMed2005

Reversing and nonreversing modulated Taylor-Couette flow at finite aspect ratio.

Recent work on the Taylor-Couette problem in the infinite-cylinder approximation has revealed that, if the driving velocity of the inner cylinder is not steady but modulates in time, two classes of time-dependent flow exist: reversing flows and nonreversing flows. The latter are at first surprising, since the direction of rotation of the Taylor vortices is decoupled from the (driving) azimuthal flow. Since experiments are performed in cylinders of finite height, the natural question which we address in this paper is whether the Ekman circulation induced by the end walls suppresses the reversing-nonreversing effect. We find that the answer is negative--nonreversing flows are actually favoured--and we reveal a variety of new flow patterns including "side-by-side" vortices.

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PubMed2005

Measurement of the normal-fluid velocity in superfluids.

Numerical calculations of (finite temperature) superfluid vortex ring propagation against a particulate sheet show that the solid particle trajectories collapse to a very good approximation to the normal-fluid path lines. We propose an experiment in which, by measuring the solid particles' velocities, direct information about the instantaneous normal-fluid velocity values could be obtained.

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PubMed2004

Vortex nucleation by collapsing bubbles in Bose-Einstein condensates.

Nucleation of vortex rings accompanies the collapse of ultrasound bubbles in superfluids. Using the Gross-Pitaevskii equation for a uniform condensate we elucidate the various stages of the collapse of a stationary spherically symmetric bubble and establish conditions necessary for vortex nucleation. The minimum radius of the stationary bubble, whose collapse leads to vortex nucleation, was found to be 28+/-1 healing lengths. The time after which the nucleation becomes possible is determined as a function of the bubble's radius. We show that vortex nucleation takes place in moving bubbles of even smaller radius if the motion makes them sufficiently oblate.

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PubMed2003

Rotating superfluid turbulence.

Almost all studies of vortex states in helium II have been concerned with either ordered vortex arrays or disordered vortex tangles. This work numerically studies what happens in the presence of both rotation (which induces order) and thermal counterflow (which induces disorder). We find a new statistically steady state in which the vortex tangle is polarized along the rotational axis. Our results are used to interpret an instability that was discovered experimentally by Swanson et al. [Phys. Rev. Lett. 50, 190 (1983)]] and the vortex state beyond the instability that has been unexplained until now.

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PubMed2002

Polarization of superfluid turbulence.

We show that normal-fluid eddies in turbulent helium II polarize the tangle of quantized vortex lines present in the flow, thus inducing superfluid vorticity patterns similar to the driving normal-fluid eddies. We also show that the polarization is effective over the entire inertial range. The results help explain the surprising analogies between classical and superfluid turbulence which have been observed recently.

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PubMed2002

Evaporation of a packet of quantized vorticity.

We study the diffusion of a packet of quantized vorticity initially confined inside a small region. We find that reconnections fragment the packet into a gas of small vortex loops which fly away. The time scale of the process is in order-of-magnitude agreement with recent experiments performed in 3He-B.

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