23 research outputs found
Diffusive transport in two-dimensional nematics
We discuss a dynamical theory for nematic liquid crystals describing the
stage of evolution in which the hydrodynamic fluid motion has already
equilibrated and the subsequent evolution proceeds via diffusive motion of the
orientational degrees of freedom. This diffusion induces a slow motion of
singularities of the order parameter field. Using asymptotic methods for
gradient flows, we establish a relation between the Doi-Smoluchowski kinetic
equation and vortex dynamics in two-dimensional systems. We also discuss moment
closures for the kinetic equation and Landau-de Gennes-type free energy
dissipation
Limit shapes for Gibbs ensembles of partitions
We explicitly compute limit shapes for several grand canonical Gibbs
ensembles of partitions of integers. These ensembles appear in models of
aggregation and are also related to invariant measures of zero range and
coagulation-fragmentation processes. We show, that all possible limit shapes
for these ensembles fall into several distinct classes determined by the
asymptotics of the internal energies of aggregates
Coalescing particle systems and applications to nonlinear Fokker–Planck equations
We study a stochastic particle system with a logarithmically-singular
inter-particle interaction potential which allows for inelastic particle
collisions. We relate the squared Bessel process to the evolution of localized
clusters of particles, and develop a numerical method capable of detecting
collisions of many point particles without the use of pairwise computations, or
very refined adaptive timestepping. We show that when the system is in an
appropriate parameter regime, the hydrodynamic limit of the empirical mass
density of the system is a solution to a nonlinear Fokker-Planck equation, such
as the Patlak-Keller-Segel (PKS) model, or its multispecies variant. We then
show that the presented numerical method is well-suited for the simulation of
the formation of finite-time singularities in the PKS, as well as PKS pre- and
post-blow-up dynamics. Additionally, we present numerical evidence that blow-up
with an increasing total second moment in the two species Keller-Segel system
occurs with a linearly increasing second moment in one component, and a
linearly decreasing second moment in the other component
A computational strategy for multiscale chaotic systems with applications to Lorenz 96 model
Abstract Numerical schemes for systems with multiple spatio-temporal scales and chaotic behavior is investigated. The multiscale schemes use asymptotic results for this type of systems which guarantee the existence of an effective dynamics for some suitably defined modes varying slowly on the largest scales. The multiscale schemes are analyzed for generic large deterministic systems displaying chaotic behavior, then illustrated on a specific example due to E. N. Lorenz. Issues like consistency, accuracy, and efficiency are discussed in detail. The role of possible hidden slow variables as well as additional effects arising on the diffusive time-scale are also investigated. As a byproduct we obtain a rather complete characterization of the effective dynamics in Lorenz model
The effect of stress-induced martensite aging in tension and compression on B2–B19′ martensitic transformation in Ni50.3Ti32.2Hf17.5 high-temperature shape memory alloy
The present study investigates the high-temperature shape memory effect (SME) in heterophase Ni50.3Ti32.2Hf17.5 polycrystals with nanosized H-phase particles after stress-induced martensite (SIM) aging in tension and compression. SIM aging created the conditions for fully reversible two-way SME with a strain of up to 50% of the one-way shape memory strain. SIM aging altered the viscoelastic properties of material, in particular, the elastic moduli of austenite and martensite increased, as did internal friction. Increased interface mobility is suggested as the reason for internal friction growth
A study of blow-ups in the Keller-Segel model of chemotaxis
We study the Keller-Segel model of chemotaxis and develop a composite
particle-grid numerical method with adaptive time stepping which allows us to
accurately resolve singular solutions. The numerical findings (in two
dimensions) are then compared with analytical predictions regarding formation
and interaction of singularities obtained via analysis of the stochastic
differential equations associated with the Keller-Segel model
Anomalous relaxation and self-organization in non-equilibrium processes
We study thermal relaxation in ordered arrays of coupled nonlinear elements
with external driving. We find, that our model exhibits dynamic
self-organization manifested in a universal stretched-exponential form of
relaxation. We identify two types of self-organization, cooperative and
anti-cooperative, which lead to fast and slow relaxation, respectively. We give
a qualitative explanation for the behavior of the stretched exponent in
different parameter ranges. We emphasize that this is a system exhibiting
stretched-exponential relaxation without explicit disorder or frustration.Comment: submitted to PR