98 research outputs found
Smoothed particle hydrodynamics modelling in continuum mechanics: fluid-structure interaction
Within this study, the implementation of the smoothed particle hydrodynamics (SPH) method solving the complex problem of interaction between a quasi-incompressible fluid involving a free surface and an elastic structure is outlined. A brief description of the SPH model for both the quasi-incompressible fluid and the isotropic elastic solid is presented. The interaction between the fluid and the elastic structure is realised through the contact algorithm. The results of numerical computations are confronted with the experimental as well as computational data published in the literature
Direct measurement of the radiative lifetime of vibrationally excited OH radicals
Neutral molecules, isolated in the gas-phase, can be prepared in a long-lived
excited state and stored in a trap. The long observation time afforded by the
trap can then be exploited to measure the radiative lifetime of this state by
monitoring the temporal decay of the population in the trap. This method is
demonstrated here and used to benchmark the Einstein -coefficients in the
Meinel system of OH. A pulsed beam of vibrationally excited OH radicals is
Stark decelerated and loaded into an electrostatic quadrupole trap. The
radiative lifetime of the upper -doublet component of the level is determined as ms, in good
agreement with the calculated value of ms.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Quantum Calculation of Inelastic CO Collisions with H. II. Pure Rotational Quenching of High Rotational Levels
Carbon monoxide is a simple molecule present in many astrophysical
environments, and collisional excitation rate coefficients due to the dominant
collision partners are necessary to accurately predict spectral line
intensities and extract astrophysical parameters. We report new quantum
scattering calculations for rotational deexcitation transitions of CO induced
by H using the three-dimensional potential energy surface~(PES) of Song et al.
(2015). State-to-state cross sections for collision energies from 10 to
15,000~cm and rate coefficients for temperatures ranging from 1 to
3000~K are obtained for CO(, ) deexcitation from to all lower
levels, where is the rotational quantum number. Close-coupling and
coupled-states calculations were performed in full-dimension for =1-5, 10,
15, 20, 25, 30, 35, 40, and 45 while scaling approaches were used to estimate
rate coefficients for all other intermediate rotational states. The current
rate coefficients are compared with previous scattering results using earlier
PESs. Astrophysical applications of the current results are briefly discussed.Comment: 8 figures, 1 tabl
RNA silencing can explain chlorotic infection patterns on plant leaves
<p>Abstract</p> <p>Background</p> <p>RNA silencing has been implicated in virus symptom development in plants. One common infection symptom in plants is the formation of chlorotic tissue in leaves. Chlorotic and healthy tissue co-occur on a single leaf and form patterns. It has been shown that virus levels in chlorotic tissue are high, while they are low in healthy tissue. Additionally, the presence of siRNAs is confined to the chlorotic spots and the boundaries between healthy and infected tissue. These results strongly indicate that the interaction between virus growth and RNA silencing plays a role in the formation of infection patterns on leaves. However, how RNA silencing leads to the intricate patterns is not known.</p> <p>Results</p> <p>Here we elucidate the mechanisms leading to infection patterns and the conditions which lead to the various patterns observed. We present a modeling approach in which we combine intra- and inter-cellular dynamics of RNA silencing and viral growth. We observe that, due to the spread of viruses and the RNA silencing response, parts of the tissue become infected while other parts remain healthy. As is observed in experiments high virus levels coincide with high levels of siRNAs, and siRNAs are also present in the boundaries between infected and healthy tissue. We study how single- and double-stranded cleavage by Dicer and amplification by RNA-dependent RNA polymerase can affect the patterns formed.</p> <p>Conclusion</p> <p>This work shows that RNA silencing and virus growth within a cell, and the local spread of virions and siRNAs between cells can explain the heterogeneous spread of virus in leaf tissue, and therewith the observed infection patterns in plants.</p
On the role of the magnetic dipolar interaction in cold and ultracold collisions: Numerical and analytical results for NH() + NH()
We present a detailed analysis of the role of the magnetic dipole-dipole
interaction in cold and ultracold collisions. We focus on collisions between
magnetically trapped NH molecules, but the theory is general for any two
paramagnetic species for which the electronic spin and its space-fixed
projection are (approximately) good quantum numbers. It is shown that dipolar
spin relaxation is directly associated with magnetic-dipole induced avoided
crossings that occur between different adiabatic potential curves. For a given
collision energy and magnetic field strength, the cross-section contributions
from different scattering channels depend strongly on whether or not the
corresponding avoided crossings are energetically accessible. We find that the
crossings become lower in energy as the magnetic field decreases, so that
higher partial-wave scattering becomes increasingly important \textit{below} a
certain magnetic field strength. In addition, we derive analytical
cross-section expressions for dipolar spin relaxation based on the Born
approximation and distorted-wave Born approximation. The validity regions of
these analytical expressions are determined by comparison with the NH + NH
cross sections obtained from full coupled-channel calculations. We find that
the Born approximation is accurate over a wide range of energies and field
strengths, but breaks down at high energies and high magnetic fields. The
analytical distorted-wave Born approximation gives more accurate results in the
case of s-wave scattering, but shows some significant discrepancies for the
higher partial-wave channels. We thus conclude that the Born approximation
gives generally more meaningful results than the distorted-wave Born
approximation at the collision energies and fields considered in this work.Comment: Accepted by Eur. Phys. J. D for publication in Special Issue on Cold
Quantum Matter - Achievements and Prospects (2011
Size consistent multireference single and double excitation configuration interaction calculations. The multireference coupled electron‐pair approximation
The dynamics and efficacy of antiviral RNA silencing: A model study
<p>Abstract</p> <p>Background</p> <p>Mathematical modeling is important to provide insight in the complicated pathway of RNA silencing. RNA silencing is an RNA based mechanism that is widely used by eukaryotes to fight viruses, and to control gene expression.</p> <p>Results</p> <p>We here present the first mathematical model that combines viral growth with RNA silencing. The model involves a plus-strand RNA virus that replicates through a double-strand RNA intermediate. The model of the RNA silencing pathway consists of cleavage of viral RNA into siRNA by Dicer, target cleavage of viral RNA via the RISC complex, and a secondary response. We found that, depending on the strength of the silencing response, different viral growth patterns can occur. Silencing can decrease viral growth, cause oscillations, or clear the virus completely. Our model can explain various observed phenomena, even when they seem contradictory at first: the diverse responses to the removal of RNA dependent RNA polymerase; different viral growth curves; and the great diversity in observed siRNA ratios.</p> <p>Conclusion</p> <p>The model presented here is an important step in the understanding of the natural functioning of RNA silencing in viral infections.</p
Magnetic anisotropy of individually addressed spin states
Controlling magnetic anisotropy is a key requirement for the fundamental understanding of molecular magnetism and is a prerequisite for numerous applications in magnetic storage, spintronics, and all-spin logic devices. In order to address the question of molecular magnetic anisotropy experimentally, we have synthesized single crystals of a molecular spin system containing four antiferromagnetically coupled s=5/2 manganese(II) ions. Using low-temperature cantilever magnetometry, we demonstrate the selective population of the
S=0,1,...,10 spin states upon application of magnetic fields up to 33 T and map the magnetic anisotropy of each of these states. We observe a strong dependence of the shape and size of the magnetic anisotropy on the populated spin states, and, in particular, reveal an anisotropy reversal upon going from the lowest to the highest spin state
- …