7,667 research outputs found
Helicity of convective flows from localized heat source in a rotating layer
Experimental and numerical study of the steady-state cyclonic vortex from
isolated heat source in a rotating fluid layer is described. The structure of
laboratory cyclonic vortex is similar to the typical structure of tropical
cyclones from observational data and numerical modelling including secondary
flows in the boundary layer. Differential characteristics of the flow were
studied by numerical simulation using CFD software FlowVision. Helicity
distribution in rotating fluid layer with localized heat source was analysed.
Two mechanisms which play role in helicity generation are found. The first one
is the strong correlation of cyclonic vortex and intensive upward motion in the
central part of the vessel. The second one is due to large gradients of
velocity on the periphery. The integral helicity in the considered case is
substantial and its relative level is high
Theory of x-ray scattering from laser-driven electronic systems
We describe, within the framework of quantum electrodynamics, an interaction
between a non-resonant hard x-ray pulse and an electronic system in the
presence of a temporally periodic laser field driving electron dynamics in this
system. We apply Floquet theory to describe the laser-driven electronic system,
and then obtain the scattering probability of an arbitrary nonresonant x-ray
pulse from such a system employing the density-matrix formalism. We show that
the scattering probability can be connected to the time-dependent electron
density of the driven electronic system only under certain conditions, in
particular, if the bandwidth of the probe x-ray pulse is sufficiently narrow to
spectroscopically resolve transitions to different final states. A special
focus is laid on application of the theory to laser-driven crystals in a
strongly nonperturbative regime. We show how the time-dependent electron
density of a crystal can be reconstructed from energy-resolved scattering
patterns. This is illustrated by a calculation of a diffraction signal from a
driven MgO crystal.Comment: accepted to Phys Rev
Thermal decomposition of a honeycomb-network sheet - A Molecular Dynamics simulation study
The thermal degradation of a graphene-like two-dimensional triangular
membrane with bonds undergoing temperature-induced scission is studied by means
of Molecular Dynamics simulation using Langevin thermostat. We demonstrate that
the probability distribution of breaking bonds is highly peaked at the rim of
the membrane sheet at lower temperature whereas at higher temperature bonds
break at random anywhere in the hexagonal flake. The mean breakage time
is found to decrease with the total number of network nodes by a power law
and reveals an Arrhenian dependence on temperature .
Scission times are themselves exponentially distributed. The fragmentation
kinetics of the average number of clusters can be described by first-order
chemical reactions between network nodes of different coordination. The
distribution of fragments sizes evolves with time elapsed from a
-function through a bimodal one into a single-peaked again at late
times. Our simulation results are complemented by a set of -order
kinetic differential equations for which can be solved exactly and
compared to data derived from the computer experiment, providing deeper insight
into the thermolysis mechanism.Comment: 21pages, 9 figures, LaTeX, revised versio
Inversionless light amplification and optical switching controlled by state-dependent alignment of molecules
We propose a method to achieve amplification without population inversion by
anisotropic molecules whose orientation by an external electric field is
state-dependent. It is based on decoupling of the lower-state molecules from
the resonant light while the excited ones remain emitting. The suitable class
of molecules is discussed, the equation for the gain factor is derived, and the
magnitude of the inversionless amplification is estimated for the typical
experimental conditions. Such switching of the sample from absorbing to
amplifying via transparent state is shown to be possible both with the aid of
dc and ac control electric fields.Comment: AMS-LaTeX v1.2, 4 pages with 4 figure
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