Biomedical subjects

E G Flekkøy

Publications and source records attributed to E G Flekkøy.

8 recordsLinked to original sources

PubMed2006

Pattern formation during air injection into granular materials confined in a circular Hele-Shaw cell.

We investigate the dynamics of granular materials confined in a radial Hele-Shaw cell, during central air injection. The behavior of this granular system, driven by its interstitial fluid, is studied both experimentally and numerically. This allows us to explore the associated pattern formation process, characterize its features and dynamics. We classify different hydrodynamic regimes as function of the injection pressure. The numerical model takes into account the interactions between the granular material and the interstitial fluid, as well as the solid-solid interactions between the grains and the confining plates. Numerical and experimental results are comparable, both to reproduce the hydrodynamical regimes experimentally observed, as well as the dynamical features associated to fingering and compacting.

Journal Article↗

PubMed2004

Hybrid computations with flux exchange.

We discuss a hybrid scheme that is based on flux exchange between a particle and a continuum system which reside in two slightly overlapping volumes in space. The scheme ensures conservation of mass, momentum and energy between a system of Lennard-Jones particles and a continuum description given by the compressible Navier-Stokes equations. An implementation in two dimensions employing the compressible Navier-Stokes equations to describe the continuum, and the Lennard-Jones potential to describe the particles, is discussed. The accuracy of the coupling scheme is tested successfully for the case of homogeneous flow and discussed for the more general case.

Journal Article↗

PubMed2001

Coupling particles and fields in a diffusive hybrid model.

A general scheme to patch together discrete and continuous descriptions of diffusion within the same physical space is studied. In the discrete description, diffusion is described by microscopic random walkers on a lattice; in the continuous description, diffusion is described through the macroscopic diffusion equation. The coupling scheme is based on the mutual exchange of mass flux across the discrete-continuous interface. Detailed tests of the scheme, coupling particle, and field descriptions are particularly illustrative for the diffusion problem. Both the nonequilibrium transport behavior and the equilibrium fluctuations of the combined discrete-continuous system are in agreement with theoretical predictions.

Journal Article↗

PubMed2001

Structure formation and instability in a tube of sand.

A new instability in the combined flow of fine grains and gas is investigated by means of experiments, simulations, and analytic techniques. When a bubble of air rises through a granular packing in a tube, a sequence of smaller bubbles spontaneously forms in front of it. The existence of this instability is shown from the experiments, simulations, and theoretical considerations. Moreover, the simulations and experiments agree on the quantitative level. In particular, when the tube is tilted away from the vertical the experiments and the simulations show the same increase in the speed of the rising bubble.

Journal Article↗

PubMed2001

Bubble propagation in a pipe filled with sand.

Granular flow with strong hydrodynamic interactions has been studied experimentally. Experiments have been carried out to study the movement of a single bubble in an inclined tube filled with glass beads and air. A maximum bubble velocity was found at an inclined angle straight theta(m). The density variations in the sand were measured by capacitance measurements, and a decompactification zone was observed just above the bubble when the inclination angle straight theta was larger than straight theta(m). The length of the decompactification front increased with increasing inclination angle and disappeared for angles smaller than straight theta(m). Both pressure and visualization experiments were carried out and compared with the density measurements.

Journal Article↗

PubMed1999

Reciprocity and cross coupling of two-phase flow in porous media from Onsager theory.

The symmetry properties of the permeability matrix in the macroscopic transport law for two-phase immiscible flow in porous media are investigated. The porous medium is treated as a single, closed thermodynamic system being forced by piston reservoirs. This construction is used to relate the Darcy fluxes to the time derivatives of the piston motion, and to identify the fluxes and forces in the Onsager sense. When the surface-tension forces that develop on the fluid interface are linear in the interface displacement, Onsager's theory is directly applicable and the permeability matrix must be symmetric. This argument is extended to show that reciprocity still holds when surfactants modify interface properties.

Journal Article↗

PubMed1999

Two-phase flow through porous media in the fixed-contact-line regime.

The complete set of equations controlling immiscible two-phase flow through porous media are derived from first principles under the sole restriction that contact lines between the two fluids and the grain surfaces are not allowed to migrate irreversibly. Because rough grain surfaces have the ability to trap contact lines over significant ranges of capillary-pressure variation, such laws are of practical interest. As distinct from previous coarse-graining work, we explicitly allow for the stretching of the fluid interface, which results in considerable nonlinearity at the macroscopic scale. The laws are obtained through an asymptotic analysis and have several new features compared to the standard laws conventionally used in two-phase flow modeling. These include the need to (i) distinguish between measurable fluxes and the volume-averaged flow; (ii) allow for flow induced by the time rate of change of the capillary pressure; and (iii) include quadratic-force terms in the generalized Darcy laws when macroscopic-pressure diffusion is slow (as defined herein).

Journal Article↗

PubMed1999

Force measurements on static granular materials.

A method for measuring normal forces under a granular packing is introduced. A high precision electronic balance is used to measure normal forces on individual beads at the bottom of a granular material. Due to the high sensitivity of this setup the normal forces on individual beads without applying any external load on the system were measured. From these measurements the distribution of forces, the response to small perturbations at grain level and the spatial correlation between normal forces were investigated at the bottom of the packing. The distribution function is consistent with a power law with an exponent alpha=0.3 for small forces. At large forces a crossover to an exponential decay is observed with a decay constant beta=1.8. Small amplitude spatial correlations were observed in the normal forces at the bottom of the pile, and the radial pressure distribution was dependent of the different filling procedures. Finally we report on systematic relaxations in the measured weight as a function of time on single grains at the bottom of the packing after perturbing the system.

Journal Article↗