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Mar 2013

Volume 25, Issue 3, Articles (03xxxx)

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Phys. Fluids 25, 031302 (2013); http://dx.doi.org/10.1063/1.4793543 (13 pages)

Gretar Tryggvason, Sadegh Dabiri, Bahman Aboulhasanzadeh, and Jiacai Lu
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Ferrofluid droplet heating and vaporization under very large magnetic power: A thermal boundary layer model

C. F. C. Cristaldo and F. F. Fachini

Phys. Fluids 25, 037101 (2013); http://dx.doi.org/10.1063/1.4793611 (19 pages) | Cited 1 time

Online Publication Date: 4 March 2013

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In this work, heating and vaporization of a liquid droplet with dispersed magnetic nanoparticles (ferrofluid) are analyzed. The ferrofluid droplet is in a quiescent inert gas phase with a temperature which is set down equal to, higher and lower than the liquid boiling temperature. Under these conditions, an alternating magnetic field is applied and, as a result, the magnetic nanoparticles generate heat by the Brownian relaxation mechanism. In this mechanism, the magnetic dipoles present a random orientation due to collisions between the fluid molecules and nanoparticles. The magnetic dipoles tend to align to the magnetic field causing rotation of the nanoparticles. Consequently the temperature increases due to the energy dissipated by the friction between the resting fluid and the rotating nanoparticles. Assuming a very large magnetic power and a uniform distribution of nanoparticles, the droplet core is uniformly heated. A thermal boundary layer is established in the liquid-phase adjacent to the droplet surface due to heat flux from the ambient atmosphere. The temperature profile inside the thermal boundary layer is obtained in appropriate time and length scales. In the present model, the ferrofluid droplet is heated up to its boiling temperature in a very short time. In addition, the combination of the heat generated by magnetic nanoparticles and heat conduction from gas phase results in a higher vaporization rate. Under specific conditions, the boiling temperature is achieved not at the surface but inside the thermal boundary layer. Moreover, the results point out that the thermal boundary layer depends directly on the vapor Lewis number but the vaporization rate reciprocally on it.
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47.55.dp Cavitation and boiling
05.40.Jc Brownian motion
47.32.Ef Rotating and swirling flows
75.50.Mm Magnetic liquids
82.70.-y Disperse systems; complex fluids

Topology and dynamics of the A-pillar vortex

Benjamin Levy and Pierre Brancher

Phys. Fluids 25, 037102 (2013); http://dx.doi.org/10.1063/1.4792710 (15 pages)

Online Publication Date: 6 March 2013

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The topology and dynamics of the flow bypassing an automobile A-pillar modeled by a 30° dihedron are investigated experimentally. The various components of the A-pillar flow are identified by means of low and high frequency particle image velocimetry. For each component, the time evolution of the position, displacement, vorticity magnitude, circulation, and fluctuating kinetic energy are analyzed along the A-pillar. More precisely, the flow bypassing the A-pillar is composed of two vortex structures with different behaviors. The major structure grows in size, circulation magnitude, and total amount of fluctuating kinetic energy along the A-pillar, whereas the minor structure does not vary significantly. Finally, the displacement of the major structure is identified as a movement of precession and the influence of the A-pillar geometry is emphasized.
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47.32.C- Vortex dynamics
47.80.Jk Flow visualization and imaging

Revisiting the ABC flow dynamo

Ismaël Bouya and Emmanuel Dormy

Phys. Fluids 25, 037103 (2013); http://dx.doi.org/10.1063/1.4795546 (10 pages)

Online Publication Date: 25 March 2013

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The Arnol'd–Beltrami–Childress flow is a prototype for fast dynamo action, essential to the origin of magnetic field in large astrophysical objects. Probably the most studied configuration is the classical 1 : 1 : 1 flow. We investigate its dynamo properties varying the magnetic Reynolds number Rm. We identify two kinks in the growth rate, which correspond, respectively, to an eigenvalue crossing and to an eigenvalue coalescence. The dominant eigenvalue becomes purely real for a finite value of the control parameter. Finally, we show that even for Rm = 25 000, the dominant eigenvalue has not yet reached an asymptotic behaviour. It still varies very significantly with the controlling parameter. Even at these very large values of Rm the fast dynamo property of this flow cannot yet be established.
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47.65.-d Magnetohydrodynamics and electrohydrodynamics
95.30.Lz Hydrodynamics
02.10.Ud Linear algebra
47.27.-i Turbulent flows
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