5,023 research outputs found
The Dynamical Dipole Mode in Fusion Reactions with Exotic Nuclear Beams
We report the properties of the prompt dipole radiation, produced via a
collective bremsstrahlung mechanism, in fusion reactions with exotic beams. We
show that the gamma yield is sensitive to the density dependence of the
symmetry energy below/around saturation. Moreover we find that the angular
distribution of the emitted photons from such fast collective mode can
represent a sensitive probe of its excitation mechanism and of fusion dynamics
in the entrance channel.Comment: 5 pages, 3 figures, to appear in Phys.Rev.
Multiscale lattice Boltzmann approach to modeling gas flows
For multiscale gas flows, kinetic-continuum hybrid method is usually used to
balance the computational accuracy and efficiency. However, the
kinetic-continuum coupling is not straightforward since the coupled methods are
based on different theoretical frameworks. In particular, it is not easy to
recover the non-equilibrium information required by the kinetic method which is
lost by the continuum model at the coupling interface. Therefore, we present a
multiscale lattice Boltzmann (LB) method which deploys high-order LB models in
highly rarefied flow regions and low-order ones in less rarefied regions. Since
this multiscale approach is based on the same theoretical framework, the
coupling precess becomes simple. The non-equilibrium information will not be
lost at the interface as low-order LB models can also retain this information.
The simulation results confirm that the present method can achieve model
accuracy with reduced computational cost
Pulse-like and crack-like ruptures in experiments mimicking crustal earthquakes
Theoretical studies have shown that the issue of rupture modes has important implications for fault constitutive laws, stress conditions on faults, energy partition and heat generation during earthquakes, scaling laws, and spatiotemporal complexity of fault slip. Early theoretical models treated earthquakes as crack-like ruptures, but seismic inversions indicate that earthquake ruptures may propagate in a self-healing pulse-like mode. A number of explanations for the existence of slip pulses have been proposed and continue to be vigorously debated. This study presents experimental observations of spontaneous pulse-like ruptures in a homogeneous linear-elastic setting that mimics crustal earthquakes; reveals how different rupture modes are selected based on the level of fault prestress; demonstrates that both rupture modes can transition to supershear speeds; and advocates, based on comparison with theoretical studies, the importance of velocity-weakening friction for earthquake dynamics
Quiet Sun magnetic fields from space-borne observations: simulating Hinode's case
We examine whether or not it is possible to derive the field strength
distribution of quiet Sun internetwork regions from very high spatial
resolution polarimetric observations in the visible. In particular, we consider
the case of the spectropolarimeter attached to the Solar Optical Telescope
aboard Hinode. Radiative magneto-convection simulations are used to synthesize
the four Stokes profiles of the \ion{Fe}{1} 630.2 nm lines. Once the profiles
are degraded to a spatial resolution of 0\farcs32 and added noise, we infer the
atmospheric parameters by means of Milne-Eddington inversions. The comparison
of the derived values with the real ones indicates that the visible lines yield
correct internetwork field strengths and magnetic fluxes, with uncertainties
smaller than 150 G, when a stray light contamination factor is included
in the inversion. Contrary to the results of ground-based observations at
1\arcsec, weak fields are retrieved wherever the field is weak in the
simulation.Comment: Accepted for publication in ApJ Letter
The Dynamical Dipole Mode in Dissipative Heavy Ion Collisions
We study the effect of a direct Giant Dipole Resonance () excitation in
intermediate dinuclear systems with exotic shape and charge distributions
formed in charge asymmetric fusion entrance channels. A related enhancement of
the gamma yield in the evaporation cascade of the fused nucleus is
expected. The dynamical origin of such extra strength will show up in a
characteristic anisotropy of the dipole gamma-emission. A fully microscopic
analysis of the fusion dynamics is performed with quantitative predictions of
the photon yield based on a dynamics- statistics coupling model. In
particular we focus our attention on the energy and mass dependence of the
effect.
We suggest a series of new experiments, in particular some optimal entrance
channel conditions. We stress the importance of using the new available
radioactive beams.Comment: 20 pages (Latex), 14 Postscript figure
Helicopter tail rotor thrust and main rotor wake coupling in crosswind flight
The tail rotor of a helicopter with a single main rotor configuration can experience a significant reduction in thrust when the aircraft operates in crosswind flight. Brownâs vorticity transport model has been used to simulate a main rotor and tail rotor system translating at a sideslip angle that causes the tail rotor to interact with the main rotor tip vortices as they propagate downstream at the lateral extremities of the wake. The tail rotor is shown to exhibit a distinct directionally dependent mode during which tail rotors that are configured so that the blades travel forward at the top of the disk develop less thrust than tail rotors with the reverse sense of rotation. The range of flight speeds over which this mode exists is shown to vary considerably with the vertical location of the tail rotor. At low flight speeds, the directionally dependent mode occurs because the tail rotor is immersed within not only the downwash from the main rotor but also the rotational flow associated with clusters of largely disorganized vorticity within the main rotor wake. At higher flight speeds, however, the tail rotor is immersed within a coherent supervortex that strongly influences the velocity field surrounding the tail rotor
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