49 research outputs found
Singularities in mass-loaded MHD flow: The cometary bow shock
We present a one-dimensional model of the mass-loading of the solar wind by cometary ions which predicts a singularity in the mass-loaded flow at M = 2. Further, a subshock occurs when the flow speed reaches M almost-equal-to 1.15. The shape of the cometary bow shock in two dimensions is predicted, by requiring that the flow Mach number of the shock is 2 taking the velocity component normal to the shock surface. The Mach number results compare favourably with observations at comet Halley
Universal Pairwise Interatomic van der Waals Potentials Based on Quantum Drude Oscillators
Repulsive short-range and attractive long-range van der Waals (vdW) forces
have an appreciable role in the behavior of extended molecular systems. When
using empirical force fields - the most popular computational methods applied
to such systems - vdW forces are typically described by Lennard-Jones-like
potentials, which unfortunately have a limited predictive power. Here, we
present a universal parameterization of a quantum-mechanical vdW potential,
which requires only two free-atom properties - the static dipole polarizability
and the dipole-dipole dispersion coefficient. This is achieved
by deriving the functional form of the potential from the quantum Drude
oscillator (QDO) model, employing scaling laws for the equilibrium distance and
the binding energy as well as applying the microscopic law of corresponding
states. The vdW-QDO potential is shown to be accurate for vdW binding energy
curves, as demonstrated by comparing to ab initio binding curves of 21
noble-gas dimers. The functional form of the vdW-QDO potential has the correct
asymptotic behavior both at zero and infinite distances. In addition, it is
shown that the damped vdW-QDO potential can accurately describe vdW
interactions in dimers consisting of group II elements. Finally, we demonstrate
the applicability of the atom-in-molecule vdW-QDO model for predicting accurate
dispersion energies for molecular systems. The present work makes an important
step towards constructing universal vdW potentials, which could benefit
(bio)molecular computational studies
Kinetic Description of Ionospheric Outflows Based on the Exact Form of Fokker-Planck Collision Operator: Electrons
We present the results of a finite difference implementation of the kinetic Fokker-Planck model with an exact form of the nonlinear collisional operator, The model is time dependent and three-dimensional; one spatial dimension and two in velocity space. The spatial dimension is aligned with the local magnetic field, and the velocity space is defined by the magnitude of the velocity and the cosine of pitch angle. An important new feature of model, the concept of integration along the particle trajectories, is discussed in detail. Integration along the trajectories combined with the operator time splitting technique results in a solution scheme which accurately accounts for both the fast convection of the particles along the magnetic field lines and relatively slow collisional process. We present several tests of the model's performance and also discuss simulation results of the evolution of the plasma distribution for realistic conditions in Earth's plasmasphere under different scenarios
Peculiar Aspects in Influence of α1-Adrenoceptor Stimulation on Isolated Rat Heart
The study examined the effect of α1-adrenoceptor stimulation with methoxamine on chronotropic function of isolated heart perfused ex vivo according to Langendorff and cardiac chronotropy in vivo. Stimulation of α1-adrenoceptors in isolated heart induced gradually developing bradycardia, which progressed during several minutes. Similar stimulation in vivo produced a short-term bradycardia probably terminated by the compensatory influences in the whole organism. Comparison of the data obtained in both experimental paradigms during α1-adrenoceptor stimulation revealed unidirectional changes in cardiac chronotropy characterized with time-related peculiarities
Fast elliptic solvers in cylindrical coordinates and the Coulomb collision operator
In this paper, we describe a new class of fast solvers for separable elliptic
partial differential equations in cylindrical coordinates with
free-space radiation conditions. By combining integral equation methods in the
radial variable with Fourier methods in and , we show that
high-order accuracy can be achieved in both the governing potential and its
derivatives. A weak singularity arises in the Fourier transform with respect to
that is handled with special purpose quadratures. We show how these solvers
can be applied to the evaluation of the Coulomb collision operator in kinetic
models of ionized gases.Comment: 20 pages, 5 figure
The effects of a kappa-distribution in the heliosheath on the global heliosphere and ENA flux at 1 AU
We investigate heliosheath energetic neutral atom (ENA) fluxes at keV
energies, by assuming that the heliosheath proton distribution can be
approximated by a kappa-distribution. The choice of the kappa parameter derives
from observational data of the solar wind (SW). This has direct applications to
the upcoming IBEX mission. We will look at all-sky ENA maps within the IBEX
energy range (10 eV to 6 keV), as well as ENA energy spectra in several
directions. We find that the use of kappa, as opposed to a Maxwellian, gives
rise to greatly increased ENA fluxes above 1 keV, while medium energy fluxes
are somewhat reduced. We show how IBEX data can be used to estimate the
spectral slope in the heliosheath, and that the use of kappa reduces the
differences between ENA maps at different energies. We also investigate the
effect introducing a kappa-distribution has on the global interaction between
the SW and the local interstellar medium (LISM), and find that there is
generally an increase in energy transport from the heliosphere into the LISM,
due to the modified profile of ENA's energies. This results in a termination
shock that moves out by 4 AU, a heliopause that moves in by 9 AU and a bow
shock 25 AU farther out, in the nose direction
On the Reusability and Numeric Efficiency of C++ Packages in Scientific Computing
In this paper, we discuss the reusability and numerical efficiency of selected Object-Oriented numerical packages, serial and parallel, for developing high performance scientific computing applications