Maminirina Joelson

Doppler Spectra From a Two-Dimensional Ocean Surface at L-Band

An approximate time-harmonic three-dimensional electromagnetic boundary-integral method, the small-slope integral equation, is combined with a series expansion of the Creamer surface representation at second order with respect to the height, denoted by Creamer (2). The resulting model provides at low numerical cost simulations of the nonlinear ocean surface Doppler spectrum at L-band. As a result of approximations, the model is designed for a low-wind speed, typically up to 5 m/s. It is shown that applying directly a second-order model such as Creamer (2) to a semiempirical sea surface spectrum induces an unrealistic magnification of small-scale roughness that is involved in the scattering process at microwave frequencies. This paper thus proposes an undressed version of the Pierson-Moskowitz spectrum that corrects this artifact. Full-polarized Doppler simulations at L-band and 70deg incidence are presented. Effects of the surface nonlinearities are outlined, and the simulated Doppler spectra show correct variations with respect to wind speed and direction

Non Fickian flux for advection-dispersion with immobile periods

The fractal mobile–immobile model (MIM) is intermediate between advection–dispersion (ADE) and fractal Fokker–Planck (FFKPE) equations. It involves two time derivatives, whose orders are 1 and γ (between 0 and 1) on the left-hand side, whereas all mentioned equations have identical right-hand sides. The fractal MIM model accounts for non-Fickian effects that occur when tracers spread in media because of through-flow, and can get trapped by immobile sites. The solid matrix of a porous material may contain such sites, so that non-Fickian spread is actually observed. Within the context of the fractal MIM model, we present a mapping that allows the computation of fluxes on the basis of the density of spreading particles. The mapping behaves as Fickian flux at early times, and tends to a fractional derivative at late times. By means of this mapping, we recast the fractal MIM model into conservative form, which is suitable to deal with sources and bounded domains. Mathematical proofs are illustrated by comparing the discretized fractal p.d.e. with Monte Carlo simulations.

Mass transport subject to time-dependent flow with nonuniform sorption in porous media.

We address the description of solutes flow with trapping processes in porous media. Starting from a small-scale model for tracer particle trajectories, we derive the corresponding governing equations for the concentration of the mobile and immobile phases within a fractal mobile-immobile model approach. We show that this formulation is fairly general and can easily take into account nonconstant coefficients and in particular space-dependent sorption rates. The transport equations are solved numerically and a comparison with Monte Carlo particle-tracking simulations of spatial contaminant profiles and breakthrough curves is proposed, so as to illustrate the obtained results.

On alpha stable distribution of wind driven water surface wave slope.

We propose a new formulation of the probability distribution function of wind driven water surface slope with an alpha-stable distribution probability. The mathematical formulation of the probability distribution function is given under an integral formulation. Application to represent the probability of time slope data from laboratory experiments is carried out with satisfactory results. We compare also the alpha-stable model of the water surface slopes with the Gram-Charlier development and the non-Gaussian model of Liu et al. [J. Phys. Oceanogr. 27, 782 (1997)]. Discussions and conclusions are conducted on the basis of the data fit results and the model analysis comparison.

All order moments and other functionals of the increments of some non-Markovian processes

We propose a theoretical framework to analyze nuclear magnetic resonance (NMR) experiments for the description of dispersion processes featuring memory effects. Memory effects, addressed here, can be represented by subordinated Brownian motions with random time changes that invert Levy time processes, with stable densities of exponent between 0 and 1. According to whether the Levy process has a drift equal to zero or not, the subordinated motion has a p.d.f that solves the fractional Fokker–Planck equation or the fractal mobile/immobile model. NMR experiments can measure the characteristic function of displacements of water molecules and facilitate their interpretation in media showing memory effects. We give mathematical expressions for the moments and averaged exponentials of the increment of subordinated Brownian motions within the framework of fractal MIM and FFPE. The results are illustrated on the basis of a numerical method.