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catastrophy theory seminar at the department of chemistry, cambridge
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Gradient Dynamical Systems, Bifurcation Theory,
Numerical Methods and Applications
Boris Fa£kovec
David Wales group
Department of Chemistry
University of Cambridge
30st May 2013
1 / 17 Boris Fa£kovec ([email protected]) Bifurcation theory May 30 2013
What is this about?
solution x1(y1, ...c1, ...cm, t), ...xn(y1, ...c1, ...cm, t) to
Fi
(xα..., cβ...
∂kxj
∂ykl, ...
∂rxp∂tr
, ...
∫fγ(xδ, ...cε, ...yζ)dyζ , ...
)= 0
theory of dynamical systemsno spatial derivativesno higher order derivativesno integrals�rst order time derivative separable
dxidt
= f (x1, ...xn, c1, ...cm, t)
bifurcation theory - how the solutions change with cigradient systems - xi = ∂V (x1,...xn,c1,...cn)
∂xiautonomous - right side does not explicitly depend on timegradient - not Hamiltonian systems pi = − ∂H
∂qi, qi = ∂H
∂pi
catastrophy theory - one more approximationstationary → how solutions to systems of non-linear algebraicequations change with ci
2 / 17 Boris Fa£kovec ([email protected]) Bifurcation theory May 30 2013
Where are Fractals?
not in this presentation
self-repeating structures - non-integer Haussdorf dimension
important in dynamical systems but . . .
discrete dynamical systems
in chemistry - basins of attraction
3 / 17 Boris Fa£kovec ([email protected]) Bifurcation theory May 30 2013
What is a Dynamical System?
dynamical system {S, ϕ}dynamics ϕ : S× R→ S
ϕ(σ, 0) = σϕ(ϕ(σ, t), s) = ϕ(σ, t + s)
∀σ ∈ S
phase �ow - in RN , dynamics given by vector �eld
one more condition ∂ϕ(x,t)∂t = v(ϕ(x, t))
system of �rst order ordinary di�erential equations (SODE)
if we can evaluate ϕ fast, we know the dynamics
no memory
autonomousdxidt
= f (x1, ...xn, c1, ...cm)
4 / 17 Boris Fa£kovec ([email protected]) Bifurcation theory May 30 2013
Phase Portrait
solution to SODE - touple of functions (x1(t), ...xn(t))
trajectories
unique (in contrast with con�guration space)singular - stationary point, periodic, homoclinic, heteroclinic
Steady state, periodic, homoclinic, heteroclinic trajectory
Lyapunov stability
asymptotic
5 / 17 Boris Fa£kovec ([email protected]) Bifurcation theory May 30 2013
Linear SODE
linear SODE (homogeneous): x = A x
has an analytical solution: ϕ(x0, τ) = eAτx0
6 / 17 Boris Fa£kovec ([email protected]) Bifurcation theory May 30 2013
Non-linear SODE
general system xi = fi (x1, ...xn, c1, ...cm)
Grobman-Hartman theorem
non-zero Re(eigval)topologically equivalent
stability by eigenvalues of matrix of linearisation
qualitative studies of phase portrait
identify singular trajectoriesdetermine their stability
7 / 17 Boris Fa£kovec ([email protected]) Bifurcation theory May 30 2013
Structural Stability of Vector Fields
SODE x = v(x) is structurally stable if there is ε, for which all
vector �elds u (|u− v| < ε) have qualitatively the same phase
portraits
bifurcation - cannot happen in structurally stable systemssteady stateperiodic trajectory
local bifurcations of 1-parametric system of vector �eldssaddle-node bifurcation (parameter µ )
8 / 17 Boris Fa£kovec ([email protected]) Bifurcation theory May 30 2013
Common types of bifurcations
pitchfork bifurcation
Hopf bifurcation
.
9 / 17 Boris Fa£kovec ([email protected]) Bifurcation theory May 30 2013
Bifurcation Diagram
diagram of stationary solutions
a quantity as a function of a parameterbranching, critical points, bifurcation points, double, triple BP,HB points
bifurcation diagram
more than 1 parametercurves of bifurcation points
10 / 17 Boris Fa£kovec ([email protected]) Bifurcation theory May 30 2013
Bifurcation Diagrams
LEFT diagram of stationary states - periodic trajectories for
lorenz model
RIGHT - bifurcation diagram for CSTR1EXO model
11 / 17 Boris Fa£kovec ([email protected]) Bifurcation theory May 30 2013
Constructing the Diagram of Stationary Solutions
�nding stationary solutions
parameter mapping
not e�cient
DERPAR
introducing an arti�cial parameter (parametrisation of thecurve in the diagram of stationary solutions)∑n
i=1
(dxi
dz
)2+(dαdz
)2= 1
in stationary point dfidz
= 0∂fi∂α
dαdz
+∑n
j=1
∂fi∂xj
dxj
dz= 0
underdetermined systemGauss-Jordan elimination using maximum pivotalgorithm must remember signs of derivatives
predictor-corrector
can continuate the whole curvecannot �nd isolated curve of solutions
periodic trajectories - DERPER
12 / 17 Boris Fa£kovec ([email protected]) Bifurcation theory May 30 2013
Case 1 - CSTR1EXO
stirred tank reactor with an exothermic reaction
toy system for bifurcation analyses with connection to chem
eng reality
reduction of parameters by using dimensionless quantities
x - conversion, Θ - temperature
Da - Damköhler number - �ow
Θc - temp of cooling medium, γ - activation energy, β - heat
exchange param, Λ - recycle, B - heat production param
dxdt
= −Λx + Da(1− x)eΘ
1+Θ/γ
dΘdt
= −ΛΘ + DaB(1− x)eΘ
1+Θ/γ − β(Θ−Θc)
13 / 17 Boris Fa£kovec ([email protected]) Bifurcation theory May 30 2013
Case 1 - Results
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.2 0.4 0.6 0.8 1 1.2
x
Da
stabunstab
HBLB
14 / 17 Boris Fa£kovec ([email protected]) Bifurcation theory May 30 2013
Case 2 - Cluster of 3 Charged Atoms
gradual increase of LJ energy
x1 = y1 = x2 = 0r13 =
√x23
+ y23
r23 =√
x23
+ (y3 − y2)22-
++
2-
+
+
SODE (6-dimensional phase space):
y2 = vy2x3 = vx3y3 = vy3
vy2 = −[
2y22− y2−y3
r323+ 6ε
(− 2
y132− 2(y2−y3)
r1423+ 1
y72+ (y2−y3)
r823
)]vx3 = −
[2x3r313− x3
r323+ 6εx3
(− 2
r1413− 2
r1423+ 1
r813+ 1
r823
)]vy3 = −
[2y3r313− y3−y2
r323+ 6ε
(−2y3
r1413− 2(y3−y2)
r1423+ y3
r813+ (y3−y2)
r823
)]15 / 17 Boris Fa£kovec ([email protected]) Bifurcation theory May 30 2013
Case 2 - Results
-1.5
-1
-0.5
0
0.5
1
1.5
0 2 4 6 8 10
xco
ordi
nate
ofth
eth
irdat
om
energy ratio LJ/elstat
16 / 17 Boris Fa£kovec ([email protected]) Bifurcation theory May 30 2013
References
M. Holodniok, A. Klic, M. Kubicek, M. Marek, Metody
analyzy nelinearnich dynamickych modelu, 1986
M. Kubicek, Algorithm 502: Dependence of Solution of
Nonlinear Systems on a Parameter. ACM Trans. Math.
Software, (2):98, 1976
A. Klic, M. Dubcova, L. Buric, Soustavy obycejnych
diferencialnich rovnic, 2009
R. Gilmore, Catastrophe theory for scientists and engineers,
1981
V.I. Arnold, Catastrophe theory, 1986
17 / 17 Boris Fa£kovec ([email protected]) Bifurcation theory May 30 2013