Multiphase flows in porous media are important in many natural and industrial processes. Pore-scale models for multiphase flows have seen rapid development in recent years and are becoming increasingly useful as predictive tools in both academic and industrial applications. However, quantitative comparisons between different pore-scale models, and between these models and experimental data, are lacking. Here, we perform an objective comparison of a variety of state-of-the-art pore-scale models, including lattice Boltzmann, stochastic rotation dynamics, volume-of-fluid, level-set, phase-field, and pore-network models. As the basis for this comparison, we use a dataset from recent microfluidic experiments with precisely controlled pore geometry and wettability conditions, which offers an unprecedented benchmarking opportunity. We compare the results of the 14 participating teams both qualitatively and quantitatively using several standard metrics, such as fractal dimension, finger width, and displacement efficiency. We find that no single method excels across all conditions and that thin films and corner flow present substantial modeling and computational challenges.

Quasi-steady flows of interstellar gas in a spiral gravitational field are followed for the purpose of investigating galactic shocks and the resultant processes of the formation of stars and interstellar clouds. We model the interstellar medium with two stable phases in which thermal balance is maintained through heating by low-energy cosmic rays. The problem, including transitions between the two phases, is given a general formulation but is solved in an approximation which ignores the difference in fluid velocities of the two phases. We also assume that the cosmic-ray flux is uniform in circles about the center of the Galaxy and that the relative abundances of the chemical elements are "normal." For a spiral gravitational field with strength equal to 5 percent that of the axisymmetric field at 10 kpc from the galactic center, the density ratio at maximum and minimum compressions is 9:1 for the intercloud medium while it is 40:1 for the gas in a typical cloud. During the decompression phase of the flow, a small percentage of the mass of the clouds evaporates to become intercloud material, but this small amount is recovered in the shock. As a by-product of phase transitions, the properties of the clouds in the regions between spiral arms are such as to make their detection in 21-cm absorption very difficult. In the absence of the cloud phase, we determine the thickness of the shock layer in the intercloud medium to be typically 50 pc. An interstellar cloud immersed as a test particle in the intercloud medium experiences a dynamic rather than a quasi-static compression as it passes through the shock layer. The critical mass for the gravitational collapse of a cloud is reduced by a large factor because of the compression in the shock. I. INTRODUCTION The problems discussed in this paper are motivated by the desire to understand the detailed mechanisms which trigger the formation of stars in normal spiral galaxies. Central to our discussion are two fundamental ideas: (i) spiral galactic shocks and (ii) the two-phase model of the interstellar medium. Within this context, we concentrate on the roles played by gravitational and thermal mechanisms. We avoid the vexing problem of the magnetic-field geometry by ignoring at the very outset the effects of the interstellar magnetic field. We do this not because we feel these effects to be unimportant, but because we wish to keep the present discussion as simple as possible. a) Basic Concepts On a small scale the main obstacle to star formation is that most of the interstellar clouds would not be even remotely bound by their self-gravitation if the clouds were * Now at the State University of New York at Stony Brook.

No abstract

No abstract

No abstract

A pore-scale model is introduced for two-phase flow in dense packings of polydisperse spheres. The model is developed as a component of a more general hydromechanical coupling framework based on the discrete element method, which will be elaborated in future papers and will apply to various processes of interest in soil science, in geomechanics and in oil and gas production. Here the emphasis is on the generation of a network of pores mapping the void space between spherical grains, and the definition of local criteria governing the primary drainage process. The pore space is decomposed by Regular Triangulation, from which a set of pores connected by throats are identified. A local entry capillary pressure is evaluated for each throat, based on the balance of capillary pressure and surface tension at equilibrium. The model reflects the possible entrapment of disconnected patches of the receding wetting phase. It is validated by a comparison with drainage experiments. In the last part of the paper, a series of simulations are reported to illustrate size and boundary effects, key questions when studying small samples made of spherical particles be it in simulations or experiments. Repeated tests on samples of different sizes give evolution of water content which are not only scattered but also strongly biased for small sample sizes. More than 20,000 spheres are needed to reduce the bias on saturation below 0.02. Additional statistics are generated by subsampling a large sample of 64,000 spheres. They suggest that the minimal sampling volume for evaluating saturation is one hundred times greater that the sampling volume needed arXiv:1601.00840v1 [cond-mat.soft] 5 Jan 2016 for measuring porosity with the same accuracy. This requirement in terms of sample size induces a need for efficient computer codes. The method described herein has a low algorithmic complexity in order to satisfy this requirement.It will be well suited to further developments toward coupled flow-deformation problems in which evolution of the microstructure require frequent updates of the pore network.

No abstract

No abstract