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arXiv1994

Equations for Turbulent Flood Waves

Long waves in rivers, estuaries and floods are described by the St Venant and Boussinesq equations in classical fluid dynamics. Based on the widely used $k$-$ε$ model for turbulence, we use the techniques of centre manifold theory to show how these classical models can be derived from a unified approach to low-dimensional modelling. The model may be refined to an arbitrary degree of accuracy and predicts interesting interactions between the mean flow and the mean turbulence.

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arXiv1995

Controlling Chaos using an Exponential Control

We demonstrate that chaos can be controlled using a multiplicative exponential feedback control. All three types of unstable orbits - unstable fixed points, limit cycles and chaotic trajectories can be stabilized using this control. The control is effective both for maps and flows. The control is significant, particularly for systems with several degrees of freedom, as knowledge of only one variable on the desired unstable orbit is sufficient to settle the system on to that orbit. We find, that in all the cases studied, the transient time is a decreasing function of the stiffness of control. But increasing the stiffness beyond an optimum value can increase the transient time. The control can also be used to create suitable new stable attractors in a map, which did not exist in the original system.

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arXiv1999

The ambiguous meaning of irreversibility

Irreversibility of spontaneous macroscopic dynamics and its asymmetry with respect to the sign reversal of the variable $t$ is usually interpreted as a genuine property of complex isolated systems. Discussion of the kinetics involved in Joule's experiment concerning spontaneous expansion of a gas shows that the isolation hypothesis results from ambiguous definitions of a number of keywords. Whereas Poincaré's apparent irreversibility result from conservative Hamiltonian dynamics, full relaxation implies interaction with the outside world. Only the latter process leads to entropy change.

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arXiv1995

Chaos induced by Pauli blocking

Dynamics of classical scattering in the system of fermions is studied. The model is based on the coherent state representation and the equations of motion for fermions are derived from the time-dependent variational principle. It is found that the antisymmetrization due to the Pauli exclusion principle, may lead to hyperbolic chaotic scattering even in the absence of interaction between particles. At low bombarding energies, the same effect leads to the screening of the hard, short-ranged component in the two particle interaction and thus regularizes the dynamics.

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arXiv1995

Do Spectral Trace Formulae Converge?

We evaluate the Gutzwiller trace formula for the level density of classically chaotic systems by considering the level density in a bounded energy range and truncating its Fourier integral. This results in a limiting procedure which comprises a convergent semiclassical approximation to a well defined spectral quantity at each stage. We test this result on the spectrum of zeros of the Riemann zeta function, obtaining increasingly good approximations to the level density. The Fourier approach also explains the origin of the convergence problems encountered by the orbit truncation scheme.

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arXiv1993

Field theory and KAM tori

The parametric equations of KAM tori for a quasi integrable system, are shown to be one point Schwinger functions of a suitable euclidean quantum field theory on the torus. KAM theorem is equivalent to a ultraviolet stability theorem. A renormalization group treatment of the field theory leads to a resummation of the formal pertubation series and to an expansion in terms of new parameters (identified as a family of renormalization constants). The new parameters are analytic in the coupling (at small coupling): the breakdown of the tori at large coupling is speculated to be related to the crossing of a "critical" surface at a value where the renormalization constants are still finite. A mechanism for the possible universality of the singularities of parametric equations for the invariant tori, in their parameter dependence as well as in the coupling dependence, is proposed.}

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arXiv1995

The Source of Irreversibility in Macroscopic Dynamics

The dynamics of irreversible relaxation of non-equilibrium macroscopic systems is discussed. Arguments are developed showing that the general process is supported by two independent successive mechanisms. One is mixing and it follows pure conservative dynamics. It does not lead to thermodynamic equilibrium. The second step involves dissipation to the outside world of the information contained in the initial constraints. This step implies interactions with the surroundings. Strict isolation is excluded. The first step is only apparently irreversible. Strict irreversibility concerns only the second one. The first step is isentropic. Entropy change implies the second step. Strictly isolated non-equilibrium systems do not relax towards a new thermodynamic equilibrium.

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arXiv1995

Pitch Perception of Complex Sounds: Nonlinearity Revisited

The ability of the auditory system to perceive the fundamental frequency of a sound even when this frequency is removed from the stimulus is an interesting phenomenon related to the pitch of complex sounds. This capability is known as ``residue'' or ``virtual pitch'' perception and was first reported last century in the pioneering work of Seebeck. It is residue perception that allows one to listen to music with small transistor radios, which in general have a very poor and sometimes negligible response to low frequencies. The first attempt, due to Helmholtz, to explain the residue as a nonlinear effect in the ear considered it to originate from difference combination tones. However, later experiments have shown that the residue does not coincide with a difference combination tone. These results and the fact that dichotically presented signals also elicit residue perception have led to nonlinear theories being gradually abandoned in favour of central processor models. In this paper we use recent results from the theory of nonlinear dynamical systems to show that physical frequencies produced by generic nonlinear oscillators acted upon by two independent periodic excitations can reproduce with great precision most of the experimental data about the residue without resorting to any kind of central processing mechanism.

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arXiv1995

Disorder in the 1D spinless Holstein model

We investigate a spinless fermion system on a one dimensional lattice interacting locally with the optical modes of a quantized phonon field: the Holstein model. The system is shown to have a disordered ground state, for small enough coupling, at any density. This is in contrast to the non quantized phonon case, the static Holstein model, which at half filling has an ordered ground state for all couplings.

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arXiv1995

Enhancing Efficiency of Mixing in Chaotic Flows

We propose a mechanism by which the efficiency of mixing in chaotic flows can be enhanced. Our mechanism consists of introducing small changes in the system parameters in regions of phase space where the local Lyapunov exponent falls substantially below its average value. We have applied our mechanism to several typical chaotic maps and flows including a system of chemical reactions. We find that our method is quite efficient as it gives a substantial enhancement of the rate of mixing with small changes in system parameters, without disturbing the attractor significantly.

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arXiv1995

Exactly Conservative Integrators

Traditional numerical discretizations of conservative systems generically yield an artificial secular drift of any nonlinear invariants. In this work we present an explicit nontraditional algorithm that exactly conserves these invariants. We illustrate the general method by applying it to the three-wave truncation of the Euler equations, the Lotka--Volterra predator--prey model, and the Kepler problem. This method is discussed in the context of symplectic (phase space conserving) integration methods as well as nonsymplectic conservative methods. We comment on the application of our method to general conservative systems.

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arXiv1995

Direct Interaction Approximation of Magnetohydrodynamic Turbulence

In this paper we apply Kraichnan's direct interaction approximation, which is a one loop perturbation expansion, to magnetohydrodynamic turbulence. By substituting the energy spectra both from kolmogorov-like MHD turbulence phenomenology and a generalization of Dobrowolny et al.'s model we obtain Kolmogorov's and Kraichnan's constant for MHD turbulence. We find that the constants depend of the Alfvén ratio and normalized cross helicity; the dependence has been studied here. We also demonstrate the inverse cascade of magnetic energy for Kolmogorov-like models. Our results are in general agreement with the earlier simulation results except for large normalized cross helicity.

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arXiv1995

The Limits of Mathematics---Tutorial Version

The latest in a series of reports presenting the information-theoretic incompleteness theorems of algorithmic information theory via algorithms written in specially designed versions of LISP. Previously in this LISP code only one-character identifiers were allowed, and arithmetic had to be programmed out. Now identifiers can be many characters long, and arithmetic with arbitrarily large unsigned decimal integers is built in. This and many other changes in the software have made this material much easier to understand and to use.

chao-dyn↗
arXiv1995

Recursive Proportional Feedback and its Use to Control Chaos in an Electrochemical System

The recursive proportional feedback (RPF) algorithm for controlling chaos is described and applied to control chemical chaos observed during the electrodissolution of a rotating copper disk in a sodium acetate/acetic acid buffer. Experimental evidence is presented to indicate why the RPF method was used and the theoretical robustness of the algorithm is discussed. (This paper appears in the "Proceedings of the 2nd Conference on EXPERIMENTAL CHAOS," World Scientific Press, River Ridge, NJ, 1995)

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arXiv1995

Infinite dimensional SRB measures

We review the basic steps leading to the construction of a Sinai-Ruelle-Bowen (SRB) measure for an infinite lattice of weakly coupled expanding circle maps, and we show that this measure has exponential decay of space-time correlations. First, using the Perron-Frobenius operator, one connects the dynamical system of coupled maps on a $d$-dimensional lattice to an equilibrium statistical mechanical model on a lattice of dimension $d+1$. This lattice model is, for weakly coupled maps, in a high-temperature phase, and we use a general, but very elementary, method to prove exponential decay of correlations at high temperatures.

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