2,684 research outputs found
Direct simulation of liquid-gas-solid flow with a free surface lattice Boltzmann method
Direct numerical simulation of liquid-gas-solid flows is uncommon due to the
considerable computational cost. As the grid spacing is determined by the
smallest involved length scale, large grid sizes become necessary -- in
particular if the bubble-particle aspect ratio is on the order of 10 or larger.
Hence, it arises the question of both feasibility and reasonability. In this
paper, we present a fully parallel, scalable method for direct numerical
simulation of bubble-particle interaction at a size ratio of 1-2 orders of
magnitude that makes simulations feasible on currently available
super-computing resources. With the presented approach, simulations of bubbles
in suspension columns consisting of more than fully resolved
particles become possible. Furthermore, we demonstrate the significance of
particle-resolved simulations by comparison to previous unresolved solutions.
The results indicate that fully-resolved direct numerical simulation is indeed
necessary to predict the flow structure of bubble-particle interaction problems
correctly.Comment: submitted to International Journal of Computational Fluid Dynamic
Comparison of multiphase SPH and LBM approaches for the simulation of intermittent flows
Smoothed Particle Hydrodynamics (SPH) and Lattice Boltzmann Method (LBM) are
increasingly popular and attractive methods that propose efficient multiphase
formulations, each one with its own strengths and weaknesses. In this context,
when it comes to study a given multi-fluid problem, it is helpful to rely on a
quantitative comparison to decide which approach should be used and in which
context. In particular, the simulation of intermittent two-phase flows in pipes
such as slug flows is a complex problem involving moving and intersecting
interfaces for which both SPH and LBM could be considered. It is a problem of
interest in petroleum applications since the formation of slug flows that can
occur in submarine pipelines connecting the wells to the production facility
can cause undesired behaviors with hazardous consequences. In this work, we
compare SPH and LBM multiphase formulations where surface tension effects are
modeled respectively using the continuum surface force and the color gradient
approaches on a collection of standard test cases, and on the simulation of
intermittent flows in 2D. This paper aims to highlight the contributions and
limitations of SPH and LBM when applied to these problems. First, we compare
our implementations on static bubble problems with different density and
viscosity ratios. Then, we focus on gravity driven simulations of slug flows in
pipes for several Reynolds numbers. Finally, we conclude with simulations of
slug flows with inlet/outlet boundary conditions. According to the results
presented in this study, we confirm that the SPH approach is more robust and
versatile whereas the LBM formulation is more accurate and faster
Simulations of slip flow on nanobubble-laden surfaces
On microstructured hydrophobic surfaces, geometrical patterns may lead to the
appearance of a superhydrophobic state, where gas bubbles at the surface can
have a strong impact on the fluid flow along such surfaces. In particular, they
can strongly influence a detected slip at the surface. We present two-phase
lattice Boltzmann simulations of a flow over structured surfaces with attached
gas bubbles and demonstrate how the detected slip depends on the pattern
geometry, the bulk pressure, or the shear rate. Since a large slip leads to
reduced friction, our results allow to assist in the optimization of
microchannel flows for large throughput.Comment: 22 pages, 12 figure
Degradation of metallic surfaces under space conditions, with particular emphasis on hydrogen recombination processes
The widespread use of metallic structures in space technology brings risk of
degradation which occurs under space conditions. New types of materials
dedicated for space applications, that have been developed in the last decade,
are in majority not well tested for different space mission scenarios. Very
little is known how material degradation may affect the stability and
functionality of space vehicles and devices during long term space missions.
Our aim is to predict how the solar wind and electromagnetic radiation degrade
metallic structures. Therefore both experimental and theoretical studies of
material degradation under space conditions have been performed. The studies
are accomplished at German Aerospace Center (DLR) in Bremen (Germany) and
University of Zielona G\'{o}ra (Poland). The paper presents the results of the
theoretical part of those studies. It is proposed that metal bubbles filled
with Hydrogen molecular gas, resulting from recombination of the metal free
electrons and the solar protons, are formed on the irradiated surfaces. A
thermodynamic model of bubble formation has been developed. We study the
creation process of -bubbles as function of, inter alia, the metal
temperature, proton dose and energy. Our model has been verified by irradiation
experiments completed at the DLR facility in Bremen. Consequences of the bubble
formation are changes of the physical and thermo-optical properties of such
degraded metals. We show that a high surface density of bubbles (up to
) with a typical bubble diameter of m will
cause a significant increase of the metallic surface roughness. This may have
serious consequences to any space mission. Changes in the thermo-optical
properties of metallic foils are especially important for the solar sail
propulsion technology, ..
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