Jmva Vianney Koelman

Simulating Microscopic Hydrodynamic Phenomena with Dissipative Particle Dynamics

We present a novel method for simulating hydrodynamic phenomena. This particle-based method combines features from molecular dynamics and lattice-gas automata. It is shown theoretically as well as in simulations that a quantitative description of isothermal Navier-Stokes flow is obtained with relatively few particles. Computationally, the method is much faster than molecular dynamics, and the at same time it is much more flexible than lattice-gas automata schemes.

A Simple Lattice Boltzmann Scheme for Navier-Stokes Fluid Flow

A lattice Boltzmann algorithm for the computation of incompressible Navier-Stokes flow is presented. This algorithm can be written down in a simple, explicit form for a general class of lattices. By working consistently within a Boltzmann description, the model is free from ambiguities and inconsistencies. It automatically gives rise to isotropic, Galilean-invariant flow behaviour for any regular 2D and 3D lattice of sites.

Renormalization group theory outperforms other approaches in statistical comparison between upscaling techniques for porous media

Determining the pressure differential required to achieve a desired flow rate in a porous medium requires solving Darcys law, a Laplace-like equation, with a spatially varying tensor permeability. In various scenarios, the permeability coefficient is sampled at high spatial resolution, which makes solving Darcys equation numerically prohibitively expensive. As a consequence, much effort has gone into creating upscaled or low-resolution effective models of the coefficient while ensuring that the estimated flow rate is well reproduced, bringing to the fore the classic tradeoff between computational cost and numerical accuracy. Here we perform a statistical study to characterize the relative success of upscaling methods on a large sample of permeability coefficients that are above the percolation threshold. We introduce a technique based on mode-elimination renormalization group theory (MG) to build coarse-scale permeability coefficients. Comparing the results with coefficients upscaled using other methods, we find that MG is consistently more accurate, particularly due to its ability to address the tensorial nature of the coefficients. MG places a low computational demand, in the manner in which we have implemented it, and accurate flow-rate estimates are obtained when using MG-upscaled permeabilities that approach or are beyond the percolation threshold.

Hyperfine Contribution to Spin-Exchange Frequency Shifts in the Hydrogen Maser

Summary We have recalculated shifts of the ground state AmF=O transition of a gas of hydrogen atoms in low magnetic field due to electron spin-exchange collisions between the atoms. We predict a new source of frequency shifts not compensated for by the usual methods of tuning the microwave cavities of oscillating hydrogen maser frequency standards. Near room temperature these additional shifts are small, but large enough to affect stability at the level of 60/w = At very low temperatures these shifts are large compared to the potential thermal instabilities of cryogenic hydrogen maser standards. Above 5 Kelvin they decrease rapidly and so are less severe when using neon storage surfaces near 10 K than when using liquid helium storage surfaces near 0.5 K.

Spin-polarized deuterium : stabilization in magnetic traps

We report on a calculation of the spin-exchange two-body rate constants associated with the population dynamics of the hyperfine levels of atomic deuterium as a function of magnetic field in the Boltzmann zero temperature limit. We find that a gas of low field seeking deuterium atoms trapped in a static magnetic field minimum decays rapidly into an ultra stable gas of doubly spin-polarized deuterium.

Hyperfine Contribution to Spin-Exchange Frequency Shifts in Cryogenic Hydrogen Masers

We have rigorously included hyperfine interactions during electron-spin-exchange collisions between ground state hydrogen atoms. We predict additional frequency shifts which are not compensated for by the usual methods of tuning maser cavities. These shifts are large compared to the potential thermal instabilities of cryogenic masers and are very sensitive to details of the interactions, especially the treatment of non-adiabatic effects.