6,408 research outputs found
Model reduction of biochemical reactions networks by tropical analysis methods
We discuss a method of approximate model reduction for networks of
biochemical reactions. This method can be applied to networks with polynomial
or rational reaction rates and whose parameters are given by their orders of
magnitude. In order to obtain reduced models we solve the problem of tropical
equilibration that is a system of equations in max-plus algebra. In the case of
networks with nonlinear fast cycles we have to solve the problem of tropical
equilibration at least twice, once for the initial system and a second time for
an extended system obtained by adding to the initial system the differential
equations satisfied by the conservation laws of the fast subsystem. The two
steps can be reiterated until the fast subsystem has no conservation laws
different from the ones of the full model. Our method can be used for formal
model reduction in computational systems biology
Lagrangian coherent structures in n-dimensional systems
Numerical simulations and experimental observations reveal that unsteady fluid systems can be divided into regions of qualitatively different dynamics. The key to understanding transport and stirring is to identify the dynamic boundaries between these almost-invariant regions. Recently, ridges in finite-time Lyapunov exponent fields have been used to define such hyperbolic, almost material, Lagrangian coherent structures in two-dimensional systems. The objective of this paper is to develop and apply a similar theory in higher dimensional spaces. While the separatrix nature of these structures is their most important property, a necessary condition is their almost material nature. This property is addressed in this paper. These results are applied to a model of Rayleigh-Bénard convection based on a three-dimensional extension of the model of Solomon and Gollub
Lagrangian Descriptors for Stochastic Differential Equations: A Tool for Revealing the Phase Portrait of Stochastic Dynamical Systems
In this paper we introduce a new technique for depicting the phase portrait
of stochastic differential equations. Following previous work for deterministic
systems, we represent the phase space by means of a generalization of the
method of Lagrangian descriptors to stochastic differential equations.
Analogously to the deterministic differential equations setting, the Lagrangian
descriptors graphically provide the distinguished trajectories and hyperbolic
structures arising within the stochastic dynamics, such as random fixed points
and their stable and unstable manifolds. We analyze the sense in which
structures form barriers to transport in stochastic systems. We apply the
method to several benchmark examples where the deterministic phase space
structures are well-understood. In particular, we apply our method to the noisy
saddle, the stochastically forced Duffing equation, and the stochastic double
gyre model that is a benchmark for analyzing fluid transport
Point vortices on the hyperbolic plane
We investigate some properties of the dynamical system of point vortices on
the hyperboloid. This system has noncompact symmetry SL(2, R) and a coadjoint
equivariant momentum map J. The relative equilibrium conditions are found and
the trajectories of relative equilibria with non-zero momentum value are
described. We also provide the classification of relative equilibria and the
stability criteria for a number of cases, focusing on N=2, 3. Contrary to the
system on the sphere, relative equilibria with non-compact momentum isotropy
subgroup are found, and are used to illustrate the different stability types of
relative equilibria.Comment: To appear in J. Mathematical Physic
Finite-time Lagrangian transport analysis: Stable and unstable manifolds of hyperbolic trajectories and finite-time Lyapunov exponents
We consider issues associated with the Lagrangian characterisation of flow
structures arising in aperiodically time-dependent vector fields that are only
known on a finite time interval. A major motivation for the consideration of
this problem arises from the desire to study transport and mixing problems in
geophysical flows where the flow is obtained from a numerical solution, on a
finite space-time grid, of an appropriate partial differential equation model
for the velocity field. Of particular interest is the characterisation,
location, and evolution of "transport barriers" in the flow, i.e. material
curves and surfaces. We argue that a general theory of Lagrangian transport has
to account for the effects of transient flow phenomena which are not captured
by the infinite-time notions of hyperbolicity even for flows defined for all
time. Notions of finite-time hyperbolic trajectories, their finite time stable
and unstable manifolds, as well as finite-time Lyapunov exponent (FTLE) fields
and associated Lagrangian coherent structures have been the main tools for
characterizing transport barriers in the time-aperiodic situation. In this
paper we consider a variety of examples, some with explicit solutions, that
illustrate, in a concrete manner, the issues and phenomena that arise in the
setting of finite-time dynamical systems. Of particular significance for
geophysical applications is the notion of "flow transition" which occurs when
finite-time hyperbolicity is lost, or gained. The phenomena discovered and
analysed in our examples point the way to a variety of directions for rigorous
mathematical research in this rapidly developing, and important, new area of
dynamical systems theory
Oseledets' Splitting of Standard-like Maps
For the class of differentiable maps of the plane and, in particular, for
standard-like maps (McMillan form), a simple relation is shown between the
directions of the local invariant manifolds of a generic point and its
contribution to the finite-time Lyapunov exponents (FTLE) of the associated
orbit. By computing also the point-wise curvature of the manifolds, we produce
a comparative study between local Lyapunov exponent, manifold's curvature and
splitting angle between stable/unstable manifolds. Interestingly, the analysis
of the Chirikov-Taylor standard map suggests that the positive contributions to
the FTLE average mostly come from points of the orbit where the structure of
the manifolds is locally hyperbolic: where the manifolds are flat and
transversal, the one-step exponent is predominantly positive and large; this
behaviour is intended in a purely statistical sense, since it exhibits large
deviations. Such phenomenon can be understood by analytic arguments which, as a
by-product, also suggest an explicit way to point-wise approximate the
splitting.Comment: 17 pages, 11 figure
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