29,543 research outputs found
Unified Mechanical Erosion Model for Multi-phase Mass Flows
Erosion poses a great challenge in multi-phase mass flows as it drastically
changes flow behavior and deposition pattern by dramatically increasing their
masses, adversely affecting population and civil structures. There exists no
mechanically-explained, unified multi-phase erosion model. We constitute a
novel, unified and comprehensive mechanical erosion rates for solid and fluid
phases and demonstrate their richness and urgency. This is achieved by
seminally introducing interacting stresses across erosion-interface. Shear
resistances from the bed against shear stresses from the landslide are based on
consistent physical principles. Proposed multi-phase interactive shear
structures are mechanically superior and dynamically flexible. Total erosion
rate is the sum of solid and fluid erosion rates which are mechanically
extensive and compact. Erosion rates consistently take solid and fluid
fractions from the bed and customarily supply to solid and fluid components in
the flow. This overcomes severe limitations inherited by existing models. For
the first time, we physically correctly construct composite, intricate erosion
velocities of particle and fluid from the bed and architect the complete net
momentum productions that include all interactions between solids and fluids in
the landslide and bed. We invent stress correction, erosive-shear-velocity,
super-erosion-drift and erosion-matrix characterizing complex erosion
processes. By embedding well constrained extensive erosion velocities, unified
erosion rates and net momentum productions including erosion-induced inertia
into mass and momentum balances, we develop a novel, mechanically-explained,
comprehensive multi-phase model for erosive mass flows. As new model covers a
broad spectrum of natural processes it offers great opportunities for
practitioners in solving technical, engineering problems related to erosive
multi-phase mass flows
Transition from viscous to inertial regime in dense suspensions
Non-Brownian suspensions present a transition from Newtonian behavior in the
zero-shear limit to a shear thickening behaviour at a large shear rate, none of
which is clearly understood so far. Here, we carry out numerical simulations of
such an athermal dense suspension under shear, at an imposed confining
pressure. This set-up is conceptually identical to the recent experiments of
Boyer and co-workers [Phys. Rev. Lett. 107,188301 (2011)]. Varying the
interstitial fluid viscosities, we recover the Newtonian and Bagnoldian regimes
and show that they correspond to a dissipation dominated by viscous and contact
forces respectively. We show that the two rheological regimes can be unified as
a function of a single dimensionless number, by adding the contributions to the
dissipation at a given volume fraction.Comment: 4 pages, 3 figure
Unified Superfluid Dark Sector
We present a novel theory of a unified dark sector, where late-time cosmic
acceleration emerges from the dark matter superfluid framework. The system is
described by a superfluid mixture consisting of two distinguishable states with
a small energy gap, such as the ground state and an excited state of dark
matter. Given their contact in the superfluid, interaction between those states
can happen, converting one state into the other. This long range interaction
within the superfluid couples the two superfluid phonon species through a
cosine potential motivated by Josephson/Rabi interactions. As a consequence of
this potential, a new dynamics of late-time accelerated expansion emerges in
this system, without the need of dark energy, coming from a universe containing
only this two-state DM superfluid. Because the superfluid species are
non-relativistic, their sound speeds remain suitably small throughout the
evolution. We calculate the expansion history and growth of linear
perturbations, and compare the results to CDM cosmology. For the
fiducial parameters studied here, the predicted expansion and growth function
are close to those of CDM, but the difference in the predicted growth
rate is significant at late times. The present theory nicely complements the
recent proposal of dark matter superfluidity to explain the empirical success
of MOdified Newtonian Dynamics (MOND) on galactic scales, thus offering a
unified framework for dark matter, dark energy, and MOND phenomenology.Comment: 27 pages, 4 figures. v2: Version accepted in JCA
Diffusion-Based Coarse Graining in Hybrid Continuum-Discrete Solvers: Applications in CFD-DEM
In this work, a coarse-graining method previously proposed by the authors in
a companion paper based on solving diffusion equations is applied to CFD-DEM
simulations, where coarse graining is used to obtain solid volume fraction,
particle phase velocity, and fluid-particle interaction forces. By examining
the conservation requirements, the variables to solve diffusion equations for
in CFD-DEM simulations are identified. The algorithm is then implemented into a
CFD-DEM solver based on OpenFOAM and LAMMPS, the former being a
general-purpose, three-dimensional CFD solver based on unstructured meshes.
Numerical simulations are performed for a fluidized bed by using the CFD-DEM
solver with the diffusion-based coarse-graining algorithm. Converged results
are obtained on successively refined meshes, even for meshes with cell sizes
comparable to or smaller than the particle diameter. This is a critical
advantage of the proposed method over many existing coarse-graining methods,
and would be particularly valuable when small cells are required in part of the
CFD mesh to resolve certain flow features such as boundary layers in wall
bounded flows and shear layers in jets and wakes. Moreover, we demonstrate that
the overhead computational costs incurred by the proposed coarse-graining
procedure are a small portion of the total costs in typical CFD-DEM simulations
as long as the number of particles per cell is reasonably large, although
admittedly the computational overhead of the coarse graining often exceeds that
of the CFD solver. Other advantages of the present algorithm include more
robust and physically realistic results, flexibility and easy implementation in
almost any CFD solvers, and clear physical interpretation of the computational
parameter needed in the algorithm. In summary, the diffusion-based method is a
theoretically elegant and practically viable option for CFD-DEM simulations
Dynamical neutrino masses in the generalized Chaplygin gas scenario with mass varying CDM
Neutrinos coupled to an underlying scalar field in the scenario for
unification of mass varying dark matter and cosmon-{\em like} dark energy is
examined. In the presence of a tiny component of mass varying neutrinos, the
conditions for the present cosmic acceleration and for the stability issue are
reproduced. It is assumed that {\em sterile} neutrinos behave like mass varying
dark matter coupled to mass varying {\em active} neutrinos through the {\em
seesaw} mechanism, in a kind of {\em mixed} dark matter sector. The crucial
point is that the dark matter mass may also exhibit a dynamical behavior driven
by the scalar field. The scalar field mediates the nontrivial coupling between
the mixed dark matter and the dark energy responsible for the accelerated
expansion of the universe. The equation of state of perturbations reproduce the
generalized Chaplygin gas (GCG) cosmology so that all the effective results
from the GCG paradigm are maintained, being perturbatively modified by
neutrinos.Comment: 19 pages, 5 figure
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