title: Smooth and filamental structures in chaotically advected chemical fields.
abstract: This paper studies the spatial structure of decaying chemical fields generated by a chaotic-advection flow and maintained by a spatially smooth chemical source. Previous work showed that in a regime where diffusion can be neglected (large Péclet number), the structures are filamental or smooth depending on the relative strength of the chemical dynamics and the stirring induced by the flow. The scaling exponent, gamma(q), of the qth -order structure function depends, at leading order, linearly on the ratio of the rate of decay of the chemical processes, alpha , and the average rate of divergence of neighboring fluid parcel trajectories (Lyapunov exponent), h. Under a homogeneous stretching approximation, gamma(q)/q=max[alpha/h,1] which implies that a well-defined filamental-smooth transition occurs at alpha=h. This approximation has been improved by using the distribution of finite-time Lyapunov exponents to characterize the inhomogeneous stretching of the flow. However, previous work focused more on the behavior of the exponents as q varies and less on the effects of alpha and hence the implications for the filamental-smooth transition. Here we set out the precise relation between the stretching rate statistics and the scaling exponents and emphasize that the latter are determined by the distribution of the finite-size (rather than finite-time) Lyapunov exponents. We clarify the relation between the two distributions. We show that the corrected exponents, [symbol: see text] depend nonlinearly on alpha with [formula: see text]. The magnitude of the correction to the homogeneous stretching approximation, [formula: see text], grows as alpha increases, reaching a maximum when the leading-order transition is reached (alpha=h). The implication of these results is that there is no well-defined bulk filamental-smooth transition. Instead it is the case that the chemical field is unambiguously smooth for alpha>h(max), where h(max) denotes the maximum finite-time Lyapunov exponent and unambiguously filamental for alpha<h, with an intermediate character for alpha between these two values. Theoretical predictions are confirmed by numerical results obtained for a linearly decaying chemistry coupled to a renewing type of flow together with careful calculations of the Crámer function.
authors.affiliation: Laboratoire de Météorologie Dynamique, ENS, Paris, France. tzella@lmd.ens.fr
authors.forename: Alexandra
authors.initials: A
authors.lastname: Tzella
authors.forename: Peter H
authors.initials: PH
authors.lastname: Haynes