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Jan 2010

Volume 22, Issue 1, Articles (01xxxx)

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Phys. Fluids 22, 016102 (2010); http://dx.doi.org/10.1063/1.3278523 (13 pages)

Andrew D. Johnson and Dimitri Papamoschou
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Magnetic field effect on the cooling of a low-Pr fluid in a vertical cylinder

I. E. Sarris, A. I. Iatridis, C. D. Dritselis, and N. S. Vlachos

Phys. Fluids 22, 017101 (2010); http://dx.doi.org/10.1063/1.3291074 (10 pages) | Cited 1 time

Online Publication Date: 11 January 2010

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Results of direct numerical simulations are presented for the transient and turbulent natural convection cooling of an initially isothermal quiescent liquid metal placed in a vertical cylinder in the presence of a vertical magnetic field. The electrically conductive low-Prandtl number fluid is put to motion when the cylindrical wall is suddenly cooled to a uniform lower temperature. For this particular cooling process, the flow is characterized by three sequential almost discrete stages: (a) development of momentum and thermal boundary layers along the cylindrical cold wall, (b) intrusion of the cooled fluid into the main fluid body, and (c) flow and thermal stratification. The selected Rayleigh numbers in the present study are high enough so that turbulent convection is established. The numerical results show that the magnetic field has no observable effect at the initial stage of the vertical boundary layer development and conduction heat transfer is favored during the intrusion stage. An interesting effect of the magnetic field during the stratification stage is the deceleration of the cooling process for low Rayleigh numbers and its acceleration for high ones. This dependence of the magnetic field effect on the Rayleigh number was found to be related to the cold vortices emanating from the vertical boundary layer. In contrast with the hydrodynamic cooling, the magnetic field was also found to accelerate the cooling near the bottom of the cylinder.
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47.65.-d Magnetohydrodynamics and electrohydrodynamics
47.27.E- Turbulence simulation and modeling
47.55.P- Buoyancy-driven flows; convection
47.27.nb Boundary layer turbulence

Morphological instability of the solid-liquid interface in crystal growth under supercooled liquid film flow and natural convection airflow

Kazuto Ueno and Masoud Farzaneh

Phys. Fluids 22, 017102 (2010); http://dx.doi.org/10.1063/1.3291075 (15 pages) | Cited 1 time

Online Publication Date: 11 January 2010

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Ringlike ripples on the surface of icicles are an example of morphological instability of the ice-water interface during ice growth under supercooled water film flow. The surface of icicles is typically covered with ripples of about 1 cm in wavelength, and the wavelength appears to be almost independent of external temperature, icicle radius, and volumetric water flow rate. One side of the water layer consists of the water-air surface and growing ice is the other. This is one of the more complicated moving phase boundary problems with two interfaces. A recent theoretical work [ K. Ueno, Phys. Rev. E 68, 021603 (2003) ] to address the underlying instability that produces ripples is based on the assumption of the absence of airflow around icicles. In this paper, we extend the previous theoretical framework to include a natural convection airflow ahead of the water-air surface and consider whether the effect of natural convection airflow on the wavelength of ripples produced on an ice surface is essential or not.
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47.55.P- Buoyancy-driven flows; convection
68.15.+e Liquid thin films

On Young’s Paradox, and the attractions of immersed parallel plates

Robert Finn

Phys. Fluids 22, 017103 (2010); http://dx.doi.org/10.1063/1.3276213 (10 pages)

Online Publication Date: 25 January 2010

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A seemingly paradoxical prediction for behavior of objects with nonconstant contact angles when dipped into fluids is clarified in a new way. The method is then applied to the problem of determining the attraction (or repulsion) of parallel vertical plates dipped into an infinite liquid bath. A criterion is given for determining whether the plates attract or repel each other, and estimates for the forces are obtained, which are asymptotically exact for small plate separations. It is shown that the attracting force is asymptotically proportional inversely to the square of the distance between the plates; however the repelling force remains under a fixed bound in magnitude. In all cases the net forces are independent of the contact angles of the exterior fluid with the plates, although that is not the case for the individual forces ascribable to pressure differences and to surface tensions. It is shown that regardless of the data, each of the plates experiences the same net force as does the other. Finally a new and more precise and inclusive clarification is given for a phenomenon described in 1806 by Laplace, who noted conditions under which repelling forces can change abruptly into (much larger) attracting forces.
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68.03.Cd Surface tension and related phenomena
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