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A continuum approximation of the Fermi-Pasta-Ulam-Tsingou model with Langevin dynamics
KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Mathematical Statistics.
2017 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesisAlternative title
En kontinuumapproximation av Fermi-Pasta-Ulam-Tsingou modellen med Langevindynamik (Swedish)
Abstract [en]

In continuum mechanics, the conservation laws for mass, momentum and energy coupled with the constitutive relations of the stress and heat flux could be a powerful solution method for continuum systems. However, it is required that the equations form a closed system, i.e. that the stress and heat flux are formulated as functions of the conserved variables.

This thesis studies the constitutive relations of the stress and heat flux in the Fermi-Pasta-Ulam-Tsingou model with Langevin dynamics, by the means of molecular dynamics simulations. In essence, the model consists of a many particle system in the presence of a heat bath, where each particle is chained to its two neighbors by a nonlinear quadratic spring force.

A numerical method is implemented to propagate the particle dynamics. Following Hardy [7], formulas relating the macroscopic entities to the particle dynamics are employed in order to study the behavior of the stress and heat flux in relation to the mass, momentum and energy. In fact, the numerical result show that the stress appears as a linear function of the energy.

Abstract [sv]

En värdefull lösningsmetod för kontinuumsystem är att lösa det system av partiella differentialekvationer som utgörs av konserveringslagarna för massa, rörelsemängd och energi tillsammans med två konstitutiva relationer för spänning och värmeflöde. För detta krävs det att ekvationerna bildar ett slutet system, dvs. att spänningen och värmeflödet är funktioner av de konserverade storheterna.

Detta examensarbete studerar de konstitutiva relationerna för spänningen och värmeflödet i Fermi-Pasta-Ulam-Tsingou modellen med Langevindynamik, som bestäms genom simulering av molekyldynamiken. Modellen består väsentligen av många partiklar som är kopplade till ett värmebad, där varje partikel är länkad till sina två grannar genom en ickelinjär fjäderkraft.

En numerisk metod implementeras för att propagera partikeldynamiken. Baserat på Hardys arbete [7] bestäms först spänningen och värmeflödet, sedan studeras deras beroende av konserveringsvariablerna. De numeriska resultaten visar att spänningen är en linjär funktion av energin.

Place, publisher, year, edition, pages
2017.
Series
TRITA-MAT-E ; 2017:79
National Category
Computational Mathematics
Identifiers
URN: urn:nbn:se:kth:diva-220353OAI: oai:DiVA.org:kth-220353DiVA, id: diva2:1169942
Subject / course
Mathematical Statistics
Educational program
Master of Science - Applied and Computational Mathematics
Supervisors
Examiners
Available from: 2017-12-31 Created: 2017-12-31 Last updated: 2017-12-31Bibliographically approved

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