4,820 research outputs found
Numerical analysis of reversible A + B <-> C reaction-diffusion systems
We develop an effective numerical method of studying large-time properties of
reversible reaction-diffusion systems of type A + B C with initially
separated reactants. Using it we find that there are three types of asymptotic
reaction zones. In particular we show that the reaction rate can be locally
negative and concentrations of species A and B can be nonmonotonic functions of
the space coordinate x, locally significantly exceeding their initial values.Comment: To appear in EPJ B, 5 pages + 6 figure
How to Calculate Tortuosity Easily?
Tortuosity is one of the key parameters describing the geometry and transport
properties of porous media. It is defined either as an average elongation of
fluid paths or as a retardation factor that measures the resistance of a porous
medium to the flow. However, in contrast to a retardation factor, an average
fluid path elongation is difficult to compute numerically and, in general, is
not measurable directly in experiments. We review some recent achievements in
bridging the gap between the two formulations of tortuosity and discuss
possible method of numerical and an experimental measurements of the tortuosity
directly from the fluid velocity field.Comment: 6 pages, 8 figure
Grammar-based Representation and Identification of Dynamical Systems
In this paper we propose a novel approach to identify dynamical systems. The
method estimates the model structure and the parameters of the model
simultaneously, automating the critical decisions involved in identification
such as model structure and complexity selection. In order to solve the
combined model structure and model parameter estimation problem, a new
representation of dynamical systems is proposed. The proposed representation is
based on Tree Adjoining Grammar, a formalism that was developed from linguistic
considerations. Using the proposed representation, the identification problem
can be interpreted as a multi-objective optimization problem and we propose a
Evolutionary Algorithm-based approach to solve the problem. A benchmark example
is used to demonstrate the proposed approach. The results were found to be
comparable to that obtained by state-of-the-art non-linear system
identification methods, without making use of knowledge of the system
description.Comment: Submitted to European Control Conference (ECC) 201
Link between the diversity, heterogeneity and kinetic properties of amorphous ice structures
Based on neutron wide-angle diffraction and small-angle neutron scattering
experiments, we show that there is a correlation between the preparational
conditions of amorphous ice structures, their microscopic structural
properties, the extent of heterogeneities on a mesoscopic spatial scale and the
transformation kinetics. There are only two modifications that can be
identified as homogeneous disordered structures, namely the very high-density
vHDA and the low-density amorphous LDA ice. Structures showing an intermediate
static structure factor with respect to vHDA and LDA are heterogeneous phases.
This holds independently from their preparation procedure, i.e. either obtained
by pressure amorphisation of ice I_h or by heating of vHDA. The degree of
heterogeneity can be progressively suppressed when higher pressures and
temperatures are applied for the sample preparation. In accordance with the
suppressed heterogeneity the maximum of the static structure factor displays a
pronounced narrowing of the first strong peak, shifting towards higher
Q-numbers. Moreover, the less heterogeneous the obtained structures are the
slower is the transformation kinetics from the high--density modifications into
LDA. The well known high-density amorphous structure HDA does not constitute
any particular state of the amorphous water network. It is formed due to the
preparational procedure working in liquid nitrogen as thermal bath, i.e. at
about 77 K
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