3,549 research outputs found
Competition of Brazil nut effect, buoyancy, and inelasticity induced segregation in a granular mixture
It has been recently reported that a granular mixture in which grains differ
in their restitution coefficients presents segregation: the more inelastic
particles sink to the bottom. When other segregation mechanisms as buoyancy and
the Brazil nut effect are present, the inelasticity induced segregation can
compete with them. First, a detailed analysis, based on numerical simulations
of two dimensional systems, of the competition between buoyancy and the
inelasticity induced segregation is presented, finding that there is a
transition line in the parameter space that determines which mechanism is
dominant. In the case of neutrally buoyant particles having different sizes the
inelasticity induced segregation can compete with the Brazil nut effect (BNE).
Reverse Brazil nut effect (RBNE) could be obtained at large inelasticities of
the intruder. At intermediate values, BNE and RBNE coexist and large inelastic
particles are found both near the bottom and at the top of the system.Comment: 13 pages, 11 figure
Shear viscosity of a model for confined granular media
The shear viscosity in the dilute regime of a model for confined granular
matter is studied by simulations and kinetic theory. The model consists on
projecting into two dimensions the motion of vibrofluidized granular matter in
shallow boxes by modifying the collision rule: besides the restitution
coefficient that accounts for the energy dissipation, there is a separation
velocity that is added in each collision in the normal direction. The two
mechanisms balance on average, producing stationary homogeneous states.
Molecular dynamics simulations show that in the steady state the distribution
function departs from a Maxwellian, with cumulants that remain small in the
whole range of inelasticities. The shear viscosity normalized with stationary
temperature presents a clear dependence with the inelasticity, taking smaller
values compared to the elastic case. A Boltzmann-like equation is built and
analyzed using linear response theory. It is found that the predictions show an
excellent agreement with the simulations when the correct stationary
distribution is used but a Maxwellian approximation fails in predicting the
inelasticity dependence of the viscosity. These results confirm that transport
coefficients depend strongly on the mechanisms that drive them to stationary
states.Comment: 9 pages, 4 figure; Accepted in Phys. Rev.
Using ATL to define advanced and flexible constraint model transformations
Transforming constraint models is an important task in re- cent constraint
programming systems. User-understandable models are defined during the modeling
phase but rewriting or tuning them is manda- tory to get solving-efficient
models. We propose a new architecture al- lowing to define bridges between any
(modeling or solver) languages and to implement model optimizations. This
architecture follows a model- driven approach where the constraint modeling
process is seen as a set of model transformations. Among others, an interesting
feature is the def- inition of transformations as concept-oriented rules, i.e.
based on types of model elements where the types are organized into a hierarchy
called a metamodel
Rewriting Constraint Models with Metamodels
An important challenge in constraint programming is to rewrite constraint
models into executable programs calculat- ing the solutions. This phase of
constraint processing may require translations between constraint programming
lan- guages, transformations of constraint representations, model
optimizations, and tuning of solving strategies. In this paper, we introduce a
pivot metamodel describing the common fea- tures of constraint models including
different kinds of con- straints, statements like conditionals and loops, and
other first-class elements like object classes and predicates. This metamodel
is general enough to cope with the constructions of many languages, from
object-oriented modeling languages to logic languages, but it is independent
from them. The rewriting operations manipulate metamodel instances apart from
languages. As a consequence, the rewriting operations apply whatever languages
are selected and they are able to manage model semantic information. A bridge
is created between the metamodel space and languages using parsing techniques.
Tools from the software engineering world can be useful to implement this
framework
Set Constraint Model and Automated Encoding into SAT: Application to the Social Golfer Problem
On the one hand, Constraint Satisfaction Problems allow one to declaratively
model problems. On the other hand, propositional satisfiability problem (SAT)
solvers can handle huge SAT instances. We thus present a technique to
declaratively model set constraint problems and to encode them automatically
into SAT instances. We apply our technique to the Social Golfer Problem and we
also use it to break symmetries of the problem. Our technique is simpler, more
declarative, and less error-prone than direct and improved hand modeling. The
SAT instances that we automatically generate contain less clauses than improved
hand-written instances such as in [20], and with unit propagation they also
contain less variables. Moreover, they are well-suited for SAT solvers and they
are solved faster as shown when solving difficult instances of the Social
Golfer Problem.Comment: Submitted to Annals of Operations researc
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