686 research outputs found
Patterns and bifurcations in low-Prandtl number Rayleigh-Benard convection
We present a detailed bifurcation structure and associated flow patterns for
low-Prandtl number () Rayleigh-B\'{e}nard
convection near its onset. We use both direct numerical simulations and a
30-mode low-dimensional model for this study. We observe that low-Prandtl
number (low-P) convection exhibits similar patterns and chaos as zero-P
convection \cite{pal:2009}, namely squares, asymmetric squares, oscillating
asymmetric squares, relaxation oscillations, and chaos. At the onset of
convection, low-P convective flows have stationary 2D rolls and associated
stationary and oscillatory asymmetric squares in contrast to zero-P convection
where chaos appears at the onset itself. The range of Rayleigh number for which
stationary 2D rolls exist decreases rapidly with decreasing Prandtl number. Our
results are in qualitative agreement with results reported earlier
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Sub-picosecond energy transfer from a highly intense THz pulse to water: a computational study based on the TIP4P/2005 model
The dynamics of ultrafast energy transfer to water clusters and to bulk water
by a highly intense, sub-cycle THz pulse of duration ~150~fs is
investigated in the context of force-field molecular dynamics simulations. We
focus our attention on the mechanisms by which rotational and translational
degrees of freedom of the water monomers gain energy from these sub-cycle
pulses with an electric field amplitude of up to about 0.6~V/{\AA}. It has been
recently shown that pulses with these characteristics can be generated in the
laboratory [PRL 112, 213901 (2014)]. Through their permanent dipole moment,
water molecules are acted upon by the electric field and forced off their
preferred hydrogen-bond network conformation. This immediately sets them in
motion with respect to one another as energy quickly transfers to their
relative center of mass displacements. We find that, in the bulk, the operation
of these mechanisms is strongly dependent on the initial temperature and
density of the system. In low density systems, the equilibration between
rotational and translational modes is slow due to the lack of collisions
between monomers. As the initial density of the system approaches 1~g/cm,
equilibration between rotational and translational modes after the pulse
becomes more efficient. In turn, low temperatures hinder the direct energy
transfer from the pulse to rotational motion owing to the resulting stiffness
of the hydrogen bond network. For small clusters of just a few water molecules
we find that fragmentation due to the interaction with the pulse is faster than
equilibration between rotations and translations, meaning that the latter
remain colder than the former after the pulse
Energy Fluxes during Dynamo Reversals
Using direct numerical simulations of the equations of magnetohydrodynamics,
we study reversals of the magnetic field generated by the flow of an
electrically conducting fluid in a sphere. We show that at low magnetic Prandtl
numbers, Pm=0.5, the decrease of magnetic energy, ohmic dissipation and power
of the Lorentz force during a reversal is followed by an increase of the power
injected by the force driving the flow and an increase of viscous dissipation.
Cross correlations show that the Lorentz energy flux is in advance with respect
to the other energy fluxes. We also observe that during a reversal, the maximum
of the magnetic energy density migrates from one hemisphere to the other and
comes back to its initial position, in agreement with recent experimental
observations. For larger magnetic Prandtl numbers (Pm= 1, 2), the magnetic
field reversals do not display these trends and strongly differ one from
another.Comment: 6 pages, 5 figure
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