Synthetizing hydrodynamic turbulence from noise: formalism and applications to plankton dynamics

We present an analytical scheme, easily implemented numerically, to generate synthetic Gaussian 2D turbulent flows by using linear stochastic partial differential equations, where the noise term acts as a random force of well-prescribed statistics. This methodology leads to a divergence-free, isotropic, stationary and homogeneous velocity field, whose characteristic parameters are well reproduced, in particular the kinematic viscosity and energy spectrum. This practical approach to tailor a turbulent flow is justified by its versatility when analizing different physical processes occurring in advectely mixed systems. Here, we focuss on an application to study the dynamics of Planktonic populations in the ocean

On reaction-subdiffusion equations

To analyze possible generalizations of reaction-diffusion schemes for the case of subdiffusion we discuss a simple monomolecular conversion A --> B. We derive the corresponding kinetic equations for local A and B concentrations. Their form is rather unusual: The parameters of reaction influence the diffusion term in the equation for a component A, a consequence of the nonmarkovian nature of subdiffusion. The equation for a product contains a term which depends on the concentration of A at all previous times. Our discussion shows that reaction-subdiffusion equations may not resemble the corresponding reaction-diffusion ones and are not obtained by a trivial change of the diffusion operator for a subdiffusion one

Turbulent Advection of Reacting Substances

Abstract The turbulent advection of a reacting substance is considered. The existence of two different states, one smooth and the other filamental, is discussed. The transition between these states is characterized in terms of the competition of the stretching and folding mechanism of the turbulent field and the tendency of the scalar field to relax to the non-homogeneous steady state. Key words: Turbulent mixing, Synthetic flows, Stretching rate, Pattern formation PACS: 47.27. Qb, 47.70.Fw, 47.54.+r Mixing in fluids has attracted much interest in recent years and particular considerable progress has been made concerning chaotic advection We consider here the turbulent advection of a reacting scalar field C( r, t). The scalar field is assumed to be passive, i.e. it does not affect the turbulent velocity field and its dynamics to be stable, that is, in the absence of the advecting field the system relaxes to a non-homogeneous steady state C 0 ( r). The reaction-advection equation for the scalar field C( r, t) reads ∂C ∂t + v( r, t) · ∇C = α(C 0 ( r) − C)

Dynamics of Turing patterns under spatio-temporal forcing

We study, both theoretically and experimentally, the dynamical response of Turing patterns to a spatio-temporal forcing in the form of a travelling wave modulation of a control parameter. We show that from strictly spatial resonance, it is possible to induce new, generic dynamical behaviors, including temporally-modulated travelling waves and localized travelling soliton-like solutions. The latter make contact with the soliton solutions of P. Coullet Phys. Rev. Lett. {\bf 56}, 724 (1986) and provide a general framework which includes them. The stability diagram for the different propagating modes in the Lengyel-Epstein model is determined numerically. Direct observations of the predicted solutions in experiments carried out with light modulations in the photosensitive CDIMA reaction are also reported.Comment: 6 pages, 5 figure

Reaction-Subdiffusion Equations for the A <--> B Reaction

We consider a simple linear reversible isomerization reaction A B under subdiffusion described by continuous time random walks (CTRW). The reactants' transformations take place independently on the motion and are described by constant rates. We show that the form of the ensuing system of mesoscopic reaction-subdiffusion is somewhat unusual: the equation giving the time derivative of one reactant concentration, say A(x,t), contains the terms depending not only on Laplacian A, but also on Laplacian B, i.e. depends also on the transport operator of another reactant. Physically this is due to the fact that several transitions from A to B and back may take place at one site before the particle jumps

Mesoscopic description of reactions under anomalous diffusion: A case study

Reaction-diffusion equations deliver a versatile tool for the description of reactions in inhomogeneous systems under the assumption that the characteristic reaction scales and the scales of the inhomogeneities in the reactant concentrations separate. In the present work we discuss the possibilities of a generalization of reaction-diffusion equations to the case of anomalous diffusion described by continuous-time random walks with decoupled step length and waiting time probability densities, the first being Gaussian or Levy, the second one being an exponential or a power-law lacking the first moment. We consider a special case of an irreversible or reversible A ->B conversion and show that only in the Markovian case of an exponential waiting time distribution the diffusion- and the reaction-term can be decoupled. In all other cases, the properties of the reaction affect the transport operator, so that the form of the corresponding reaction-anomalous diffusion equations does not closely follow the form of the usual reaction-diffusion equations

Front propagation in A+B -> 2A reaction under subdiffusion

We consider an irreversible autocatalytic conversion reaction A+B -> 2A under subdiffusion described by continuous time random walks. The reactants' transformations take place independently on their motion and are described by constant rates. The analog of this reaction in the case of normal diffusion is described by the Fisher-Kolmogorov-Petrovskii-Piskunov (FKPP) equation leading to the existence of a nonzero minimal front propagation velocity which is really attained by the front in its stable motion. We show that for subdiffusion this minimal propagation velocity is zero, which suggests propagation failure

Does ohmic heating influence the flow field in thin-layer electrodeposition?

In thin-layer electrodeposition the dissipated electrical energy leads to a substantial heating of the ion solution. We measured the resulting temperature field by means of an infrared camera. The properties of the temperature field correspond closely with the development of the concentration field. In particular we find, that the thermal gradients at the electrodes act like a weak additional driving force to the convection rolls driven by concentration gradients.Comment: minor changes: correct estimation of concentration at the anode, added Journal-re