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[en] It has become increasingly apparent that successful, general methods for the solution of the neutral particle transport equation involve a close connection between the spatial-discretization method used and the source-acceleration method chosen. The first form of the transport equation, angular discretization which is discrete ordinates is considered as well as spatial discretization based upon a mesh arrangement. Characteristic methods are considered briefly in the context of future, desirable developments. The ideal spatial-discretization method is described as having the following attributes: (1) positive-positive boundary data yields a positive angular flux within the mesh including its boundaries; (2) satisfies the particle balance equation over the mesh, that is, the method is conservative; (3) possesses the diffusion limit independent of spatial mesh size, that is, for a linearly isotropic flux assumption, the transport differencing reduces to a suitable diffusion equation differencing; (4) the method is unconditionally acceleratable, i.e., for each mesh size, the method is unconditionally convergent with a source iteration acceleration. It is doubtful that a single method possesses all these attributes for a general problem. Some commonly used methods are outlined and their computational performance and usefulness are compared; recommendations for future development are detailed, which include practical computational considerations
[en] An overview of ORNL's three-dimensional neutral particle transport code, TORT, is presented. Special features of the code that make it invaluable for large applications are summarized for the prospective user. Advanced capabilities currently under development and installation in the production release of TORT are discussed; they include: multitasking on Cray platforms running the UNICOS operating system; Adjacent cell Preconditioning acceleration scheme; and graphics codes for displaying computed quantities such as the flux. Further developments for TORT and its companion codes to enhance its present capabilities, as well as expand its range of applications are disucssed. Speculation on the next generation of neutron particle transport codes at ORNL, especially regarding unstructured grids and high order spatial approximations, are also mentioned
[en] Brownian motion plays an important role in the separation of small particles and molecules, but generally leads to undirected motion or intermixing by diffusion. Matthias and Mueller (2003 Nature 424 53-7) reported on the experimental realization of a drift ratchet, a microfluidic particle transport mechanism that utilizes random fluctuations instead, i.e. a Brownian motor. Here, we offer a new interpretation of this previously published work on the drift ratchet. New experiments, which allow us to distinguish between particles of different sizes, as well as a re-examination of the original work, lead to the conclusion that the measured particle transport does not result from a ratchet effect. We demonstrate that the transport is caused by convection instead. While our result challenges one specific type of experiment, we do not assess the feasibility of a drift ratchet in principle. Instead, we identify the experimental conditions that need to be fulfilled for the successful separation of particles.
[en] Swarm phenomena become very complex in gases containing energetic neutral metastable particles. Preliminary observations have stressed the need for more highly time-resolved studies to separate the electron, positive ion and Penning ionization components in the ionization growth. At the same time it is essential to establish the origin and interpretation of the more complex 1/tau vs 1/d2 plots associated with the Molnar analysis and indeed to reassess the validity of the Molnar approach itself. The purpose of the present paper is to outline new techniques that have led to further insights into the parameters controlling metastable particle contributions to swarm phenomena. Both the first ionization coefficient α/N and the spacially dependent secondary coefficient ω-bar(d) are evaluated from observations of the spatial growth of ionization between plane-parallel electrodes at constant E/N (electric field/gas density). A quenching parameter μ/α = α-1 √G/Dsub(m), were G and Dsub(m) are the volume destruction and diffusion coefficient respectively for metastable particles, provides a means for comparing observation of ω-bar(d) with theory. Temporal investigation of the ionization growth lead to values of both G and Dsub(m). The validity of Molnar's analysis was tested and the latter applied to observations in highly pure N2. A new technique has been developed specifically for investigations of the role of metastable particles in swarm experiments. New ionization effects were observed with highly purified samples, they are explained by a sensitive balance between the production and destruction of two metastable particles of the same gas, themselves capable of interacting with each other to yield new sources of free electrons
[en] Highlights: • A model of colloid-facilitated impurity transport in dual porosity medium is presented. • Nonequilibrium impurity distribution between fast and slow regions is considered. • Rate limited impurity exchange between solute and colloids is taken into account. • A set of five regimes including ones of anomalous type describes the transport. - Abstract: In the context of impurity transport study it is shown how complexity of the system leads to emergence of anomalous transport regimes, moreover the power-law behavior of which changes with time. Specifically, a model of nonequillibrium colloid-facilitated transport in statistically homogeneous double porosity media is presented and possible transport regimes are simulated. Depending on the medium and colloids characteristics different sequences of transport regimes can realize. Temporal interval exists where anomalous transport regimes (sub-diffusion and quasi-diffusion) occur. At asymptotically large times the transport is described by the classical advection-diffusion with modified parameters.
[en] We present a simulation of neutrino transport in stellar matter, in which the neutrino gas and the scattering medium are treated as two fluids exchanging impulsion and energy. The method is capable of handling both the optically thick and thin regimes -and, of crucial importance in type II supernovae theories, the intermediate regime. We exhibit some first simple applications, testing the efficiency of the method. A first sketch of the initial phase of neutrino trapping in a collapsing stellar core is also presented
[en] The main objective of NEER grant was to generate highly accurate 2D and 3D time-dependent neutral particle intensity maps from 3D pulsed wire sources through integration of the analytical representation of a time-dependent point source
[en] In this work we present a variational approach to some methods to solve transport problems of neutral particles. We consider a convex domain X (for example the geometry of a slab, or a convex set in the plane, or a convex bounded set in the space) and we use discrete ordinates quadrature to get a system of differential equations derived from the neutron transport equation. The boundary conditions are vacuum for a subset of the boundary, and of specular reflection for the complementary subset of the boundary. Recently some different approximation methods have been presented to solve these transport problems. We introduce in this work the adjoint equations and the conjugate functions obtained by means of the variational approach. First we consider the general formulation, and then some numerical methods such as spherical harmonics and spectral collocation method. (authors)
[en] The flux of particles in a slab of finite thickness with a radiation source at one of its surfaces has been studied. Strong forward-backward scattering in combination with linearly anisotropic scattering has been assumed. It is found that the variation of the transmitted flux with the strength of backscattering is sometimes non-uniform. A comparison is made with Rutherford scattering, where the same behaviour is observed
[en] The equations for relative computing time per history are derived for Monte-Carlo radiation transport calculations using geometrical surface splitting and antithetic transformations. Numerical results are obtained for one group slab geometry transmission problems. The computed relative efficiencies show the distinct advantage of using antithetic transformation with geometrical surface splitting and with weight dependent Russian roulette. (author)