Pattern Formation in Spatially Extended Systems Lecture 4mcc/MiniCourse/TA_lecture4_slides.pdf ·...

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Tel Aviv, January, 2006:Pattern Formation in Spatially Extended Systems - Lecture 4 1

Pattern Formation in Spatially Extended Systems

Lecture 4

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Outline

• Introduction to chaos

• Pattern chaos

• Spatiotemporal chaos

� Definition and characterization

� Transitions to spatiotemporal chaos and between different

chaotic states

� Coarse grained descriptions

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Equilibrium

Uniform state

Patterns

Spatiotemporalchaos

Turbulence

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E=EckhausZ=ZigZagSV=Skew VaricoseO=OscillatorySV

stableband

qS+qS- qN+qN-

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Some Theoretical Highlights

Landau (1944) Turbulence develops by infinite sequence of transitions

adding additional temporal modes and spatial complexity

Lorenz (1963) Discovered chaos in simple model of convection

Ruelle and Takens (1971)Suggested the onset of aperiodic dynamics

from a low dimensional torus (quasiperiodic motion with a small

numberN frequencies)

Feigenbaum (1978)Quantitative universality for period doubling route

to chaos

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Lorenz Model

X = −σ(X − Y )Y = rX − Y −XZZ = b (XY − Z)

“Classic” parameter values areb = 8/3, σ = 10

andr = 27.

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Lyapunov Exponents and Eigenvectors

Quantifying the sensitive dependence on initial conditions

Sλ(t) = ln∣∣∣ δutδu0

∣∣∣ ; λ = lim tf→∞ 1tf−t0Sλ(tf ); δut → eigenvector

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Dimension of the Attractor

• The fractal dimension of the attractor quantifies the number of

chaotic degrees of freedom.

• There are many possible definitions. Most are inaccessible to

experiment and numerics for high dimensional attractors.

• I will discuss theLyapunov dimensionwhich is conjectured to be

the same as theinformation dimension

Line lengths→ eλ1t , Areas→ e(λ1+λ2)t , Volumes→ e(λ1+λ2+λ3)t , . . .

Lyapunov Dimension:

DL = ν + 1

|λν+1|ν∑i=1

whereν is the largest index such that the sum is positive.

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Lyapunov dimension

Defineµ(n) =∑ni=1 λi (λ1 ≥ λ2 · · · ) with λi theith Lyapunov

exponent.

DL is the interpolated value ofn givingµ = 0 (the dimension of the

volume that neither grows nor shrinks under the evolution)

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Small system chaos: some experimental highlights

Ahlers (1974) Transition from time independent flow to aperiodic flow at

R/Rc ∼ 2 (aspect ratio 5)

Gollub and Swinney (1975)Onset of aperiodic flow from time-periodic

flow in Taylor-Couette

Maurer and Libchaber, Ahlers and Behringer (1978) Transition from

quasiperiodic flow to aperiodic flow in small aspect ratio convection

Lichaber, Laroche, and Fauve (1982)Quantitative demonstration of the

Fiegenbaum period doubling route to chaos

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Pattern chaos

• First experiments (1974):0 = 5.27, cryogenic (normal) liquidHe4 as fluid. High

precision heat flow measurements (no flow visualization).

• Onset of aperiodic time dependence in low Reynolds number flow: relevance of

chaos to “real” (continuum) systems.

• Broad power spectrum with power law decrease at largef : P(f ) ∼ f−4

• Aspect ratio dependence of the onset of time dependence (1978)

0 2 5 57

Rt 10Rc 2Rc 1.1Rc

• Flow visualization (Croquette et al. 1986):0 = 7.66, Argon

• Simulation of Generalized Swift-Hohenberg equation (Greenside, MCC, Coughran

• Simulation of full fluid equations (Paul, MCC, Fischer, and Greenside 2001)

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Aperiodic time series

[from Ahlers 1974]

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Broad power spectrum

[from Ahlers and Behringer 1978]

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Aspect ratio dependence

[from Gao and Behringer 1984]

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Generalized Swift-Hohenberg simulations

[from Greenside, MCC, Coughran 1985]

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Fluid simulations

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R = 3127 R = 6949

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Lyapunov Exponent

10 20 30 40 500

dataλ = 0.6

R = 6949 [Scheel and MCC, 2006]

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Spatiotemporal chaos

Chaos in large aspect ratio (formally0→∞) spatially homogeneous

system:

• Break down of pattern to time dependent state

• Collective effect of many coupled chaotic degrees of freedom

Many natural examples of chaotic systems are closer to this idealization

than to low dimensional chaos.

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Systems

• Coupled Maps

x(n+1)i = f (x(n)i )+D × 1

∑δ=n.n.

[f (x(n)i+δ)− f (x(n)i )]

with e.g.f (x) = ax(1− x)• PDE simulations

� Kuramoto-Sivashinsky equation

∂tu = −∂2xu− ∂4

xu− u∂xu

� Amplitude equations, e.g. Complex Ginzburg-LandauEquation

∂tA = A+ (1+ ic1)∇2A− (1− ic3) |A|2A

• Physical systems ( experiment and numerics )

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Challenges

• System-specific questions

• Definition and characterization

• Transitions to spatiotemporal chaos and between different chaotic

states

• Coarse grained descriptions

• Control

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Challenges

states

• Control

Ideas and methods from dynamical systems, statistical mechanics, phase

transition theory …

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Definition and characterization

• Narrow the phenomena

• Decide if theory, simulation, and experiment match

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Characterizing spatiotemporal chaos

Methods from statistical physics: Correlation lengths and times, etc.

• Easy to measure, but perhaps not very insightful

Methods from dynamical systems:Lyapunov exponents and attractor

dimensions.

• Inaccessible in experiment, but can be measured in simulations

• Ruelle suggested that Lyapunov exponents should beintensive,

and the dimension should beextensive∝ Ld

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Lyapunov spectrum and dimension for spiral defect chaos

(Egolf et al. 2000)

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Microextensivity for the 1d Kuramoto-Sivashinsky equation

78 83 88 93L

(from Tajima and Greenside 2000)

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Spatial aspects of sensitivity to initial conditions

Lyapunov vector for spiral defect chaos (Chiam, 2003, after Egolf et al.)

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Challenges

states

• Control

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Transitions

In thermodynamic equilibrium systems the behavior may be simpler near

phase transitions.

Is there universal behavior near transitions in spatiotemporal chaos

(transition to STC, transitions within STC)?

If so, is the universality the same as in corresponding equilibrium systems?

Examples:

• Chaotic Ising map

• Rotating convection

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Chaotic Ising Map

J. Miller and D. Huse [Phys. Rev.E48, 2528 (1993)]

D. Egolf [Science287, 101 (2000)]

x(n+1)i = f (x(n)i )+ g∑δ(x

(n)i+δ − x(n)i )

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Universality

Universality apparently the same as for thermodynamic Ising system:

[Miller and Huse, Phys. Rev.E48, 2528 (1993)]

Also RNG treatment suggests non-equilibrium correction terms are

irrelevant [Bennett and Grinstein, Phys. Rev. Lett.55, 657 (1985)]

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Spiral and domain chaos in Rayleigh-Bénard convection

Rotation Rate

no pattern(conduction)

stripes(convection)

spiralchaos

domainchaos

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Amplitude equation description(Tu and MCC, 1992)]

Amplitudes of rolls at 3 orientationsAi(r , t), i = 1 . . .3

∂tA1 = εA1+ ∂2x1A1− A1(A

21+ g+A2

2+ g−A23)

∂tA2 = εA2+ ∂2x2A2− A2(A

22+ g+A2

3+ g−A21)

∂tA3 = εA3+ ∂2x3A3− A3(A

23+ g+A2

1+ g−A22)

whereε = (R − Rc(�)/Rc(�)Length scale ξ ∼ ε−1/2

Time scale τ ∼ ε−1

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Chaotic dynamics

Suggests characteristic lengths and times scaling asξ ∼ ε−1/2 and

τ ∼ ε−1

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Generalized Swift-Hohenberg simulations of domain chaos

∂t= εψ + (∇2+ 1)2ψ − g1ψ

+ g2z·∇ × [(∇ψ)2∇ψ ] + g3∇·[(∇ψ)2∇ψ ]

e.g.g1 = 1, g2 = 2.6, g3 = 1,5, ε = 0.1 (MCC, Meiron and Tu, 1994)

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GSH stripes

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GSH orientations

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cf. amplitude simulations

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GSH orientations

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GSH domain walls

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Visual demonstration of scaling

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Challenges

states

• Control

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Coarse grained description

• Can we find simplified descriptions of spatiotemporal chaotic

systems atlarge length scales?

� conserved quantity (cf. hydrodynamics)

� near continuous transition

� collective motion such as defects

• Is the simplified description analogous to a thermodynamic

equilibrium system?

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Rough argument

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Rough argument

Expect a Langevin description at large scales

∂ty = D(y)+ η

y is vector of large length scale variables,D is some effective deterministic dynamics,

andη is noise coming from small scale chaotic dynamics.

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Rough argument

∂ty = D(y)+ η

Sinceη represents the effect of many small scale fast chaotic degrees of freedom acting

on the large scales we might expect it to be Gaussian and white⟨ηi(r , t)ηj (r ′, t ′)

⟩ = �ij δ(r − r ′)δ(t − t ′)

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Rough argument

∂ty = D(y)+ η

⟩ = �ij δ(r − r ′)δ(t − t ′)

In systems deriving from a microscopicHamiltoniandynamicsconstraintsrelate the

noise�ij and the deterministic termsD (the fluctuation-dissipation theorem).

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Rough argument

∂ty = D(y)+ η

⟩ = �ij δ(r − r ′)δ(t − t ′)

In systems deriving from a microscopicHamiltoniandynamicsconstraintsrelate the

noise�ij and the deterministic termsD (the fluctuation-dissipation theorem).

In systems based on adissipativesmall scale dynamics,if the dominant macroscopic

dynamics is sufficiently simple, or sufficiently constrained by symmetries, these

relationships mayhappento occur.

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Examples

• Chaotic Kuramoto-Sivashinsky dynamics reduces to noisy

Burgers equation [Zaleski (1989)]

• Chaotic Ising map model near the transition

� Langevin equation for dynamics of domain walls same as in

equilibrium system [Miller and Huse, (1993)]

� Coarse grained configurations satisfy detailed balance and

have a distribution given by an effective free energy [Egolf,

Science287, 101 (2000)]

• Defect dynamics description of 2D Complex Ginzburg Landau

chaos [Brito et al., Phys. Rev. Lett.90, 063801 (2003)]

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Conclusions

In this lecture I introduced some of the basic ideas of chaos, and discussed

the application of these ideas to pattern forming systems.

I discussed one of the first experiments on chaos in continuum systems

where the chaotic dynamics involves many of the ideas discussed in the

previous lectures.

I then introduced spatiotemporal chaos, which remains a poorly

characterized and understood phenomenon, and discussed

• Transitions

Pattern Formation in Spatially Extended Systems Lecture 4mcc/MiniCourse/TA_lecture4_slides.pdf ·...

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