Faculte des science de Tunis

Tunis, Tunisia

Faculte des science de Tunis

Tunis, Tunisia
Time filter
Source Type

Gamoun M.,Institute des Regions Arides | Hanchi B.,Faculte des science de Tunis
Arid Ecosystems | Year: 2014

Two treatments were used to evaluate the effect of grazing intensity (continuously grazing and controlled grazing). The schedule is as follows: 1) Rest 3 years (2004 to 2007), 2) Graze 2 months (August and July) 2007, 3) Rest 7 months (September to March) 2007–2008. The grazing impact at the area of 2000 ha was by 1700 sheep and goats during two months, followed by second rest (7 months) maintain resilient rangelands and ensure a sustainable flow of rangelands goods and services to livestock. The control was at grazing system, allows animals unrestricted and uninterrupted access to a grazing unit for all or most of the grazing season. Novelty of this studies is in the new data, which demonstrates that the rest periods allow plants to recover before they are grazed again, and if controlled grazing can avoid overgrazing effects. © 2014, Pleiades Publishing, Ltd.

Essafi W.,Institute National Of Recherche Et Danalyse Physico Chimique | Abdelli A.,Institute National Of Recherche Et Danalyse Physico Chimique | Abdelli A.,University of Carthage | Bouajila G.,Faculte des science de Tunis | Boue F.,CEA Saclay Nuclear Research Center
Journal of Physical Chemistry B | Year: 2012

We investigate in this paper the influence of the improvement of the solvent quality on the structure and the viscous properties of solutions of an hydrophobic polyelectrolyte, poly(styrene-co-sodium styrenesulfonate): PSS. The solvent used is a mixture of water and an organic solvent, THF, which is also slightly polar. We use small angle neutron scattering in the semidilute regime and viscosimetry as a function of concentration in dilute and semidilute unentangled regime. The structure, namely the scattering from all chains, is characterized by a maximum ("polyelectrolyte peak"). Its position and amplitude depends, at a given sulfonation rate of PSS, on the solvent quality through the added amount of organic solvent (THF). These evolutions with the THF amount are more pronounced when the sulfonation rate f is low (more hydrophobic polyelectrolyte) and the amount of added THF is high. Adding THF to hydrophobic PSS (f = 0.50 or f = 0.38), diminishes also the "shoulder" visible in the log I - log q plot and associated with the pearl size. It is therefore proposed that when THF is added to aqueous polyelectrolyte solutions, the pearls are dissolved and the chain conformation evolves from the pearl-necklace shape already reported in pure water toward the string-like conformation in pure water for fully sulfonated PSS. An addition of THF also reduces the important low q upturn found with hydrophobic polyelectrolyte solutions: the large aggregates are dissolved by THF. The upturn can become for PSSNa f = 0.38, after adding enough THF (50%), even smaller than that for the charged hydrophilic case PSSNa f = 0.82, in water. This can mean that in the quasi-fully charged PSS at f = 0.82 there are still hydrophobic effects in water, which is disagreeing with our recent reports, or that the electrostatics contribution to the upturn is reduced due to a lower dielectric permittivity. Concerning the hydrophilic polyelectrolyte, poly(sodium-2-acrylamido-2-methylpropanesulfonate)-co- (acrylamide): AMAMPS, no evolution in structure occurs until 25% THF. The viscosimetry variation with THF fraction is in good agreement with the scattering one up to 25%: though little dependent on THF for AMAMPS, and for hydrophilic PSSNa, it increases for hydrophobic PSSNa in agreement with the chain expansion signaled by scattering. At 50% THF concentration, the hydrophilic polyelectrolyte shows new surprising behaviors: the scattering of PSSNa is no longer characteristic of polyelectrolytes, and AMAMPS solutions display an unexpected viscosity decrease. © 2012 American Chemical Society.

Salhi A.,Faculte des science de Tunis | Salhi A.,Aix - Marseille University | Pieri A.B.,CNR Institute of atmospheric Sciences and Climate
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2014

Rotating stratified flows in thermal wind balance are at the center of geophysical fluid dynamics. Recently, endeavors were put on studying the linear response of such flows to potential vorticity perturbations. It has been shown that the initial potential vorticity (PV) distribution is fundamental and is responsible for important transient growth of the perturbation and gravity-wave generation. Using Pfeiffer's theorem [J. Differ. Equat. 11, 145 (1972)JDEQAK0022-039610.1016/0022-0396(72)90085-X], we give the mathematical demonstration of the stability of asymmetric perturbations k1≠0 of a uniform, unbounded flow in thermal wind balance. Incidentally, we prove that both the wave mode (that corresponds to a vanishing PV) and the vortex mode (corresponding to a nonzero PV) are stable. The emphasis is put on the nontrivial behavior of inertia-gravity waves (IGWs) when deformed by a background shear. In particular, we show that in the linear limit, sheared inertia-gravity waves asymptotically oscillate at the inertial waves frequency, but their amplitude is sensitive to shear, stratification, and rotation. Last, we study the development of the IGWs dynamics considering isotropic initial conditions. Computations indicate that both the vortex mode and the wave mode generate IGWs, but the energy of the IGWs generated by the vortex mode is more important than the energy of the IGWs generated by the wave mode. It is also found that, at large times, the energy of the IGWs generated by the vortex mode increases as the ratio kv/kh (initial vertical wavenumber over horizontal wavenumber) increases (like kv2/kh2), while the energy of the IGWs generated by the wave mode oscillates in function of kv/kh. © 2014 American Physical Society.

Jouini A.,Faculte des science de Tunis | Lemarie-Rieusset P.G.,DeVry University
Physica D: Nonlinear Phenomena | Year: 2013

We present in this paper two elementary constructions of multiresolution analyses on the L-shaped domain D. In the first one, we shall describe a direct method to define an orthonormal multiresolution analysis. In the second one, we use the decomposition method for constructing a biorthogonal multiresolution analysis. These analyses are adapted for the study of the Sobolev spaces Hs(D)(s N). © 2013 Elsevier B.V. All rights reserved.

Salhi A.,Faculte des science de Tunis | Cambon C.,École Centrale Lyon
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2010

We study the stability problem of unbounded shear flow, with velocity Ui =S x3 δi1, subjected to a uniform vertical density stratification, with Brunt-Väisälä frequency N, and system rotation of rate Ω about an axis aligned with the spanwise (x2) direction. The evolution of plane-wave disturbances in this shear flow is governed by a nonhomogeneous second-order differential equation with time-dependent coefficients. An analytical solution is found to be described by Legendre functions in terms of the nondimensional parameter σφ2 =R (R+1) sin2 φ+ Ri, where R= (2Ω/S) is the rotation number, φ is the angle between the horizontal wave vector and the streamwise axis, and Ri = N2 / S2 is the Richardson number. The long-time behavior of the solution is analyzed using the asymptotic representations of the Legendre functions. On the one hand, linear stability is analyzed in terms of exponential growth, as in a normal-mode analysis: the rotating stratified shear flow is stable if Ri >1/4, or if 0< Ri <1/4 and R (R+1) >0, or if R (R+1) <0

Salhi A.,Faculte des science de Tunis | Salhi A.,École Centrale Lyon | Lehner T.,University of Paris Descartes | Godeferd F.,École Centrale Lyon | Cambon C.,École Centrale Lyon
Astrophysical Journal | Year: 2013

We examine accretion disk flow under combined radial and vertical stratification utilizing a local Cartesian (or "shearing box") approximation. We investigate both axisymmetric and nonaxisymmetric disturbances with the Boussinesq approximation. Under axisymmetric disturbances, a new dispersion relation is derived. It reduces to the Solberg-Hoïland criterion in the case without vertical stratification. It shows that, asymptotically, stable radial and vertical stratification cannot induce any linear instability; Keplerian flow is accordingly stable. Previous investigations strongly suggest that the so-called bypass concept of turbulence (i.e., that fine-tuned disturbances of any inviscid smooth shear flow can reach arbitrarily large transient growth) can also be applied to Keplerian disks. We present an analysis of this process for three-dimensional plane-wave disturbances comoving with the shear flow of a general rotating shear flow under combined stable radial and vertical rotation. We demonstrate that large transient growth occurs for K 2/k1 ≫ 1 and k3 = 0 or k1 ∼ k3, where k1, K2, and k3 are the azimuthal, radial, and vertical components of the initial wave vector, respectively. By using a generalized "wave-vortex" decomposition of the disturbance, we show that the large transient energy growth in a Keplerian disk is mainly generated by the transient dynamics of the vortex mode. The analysis of the power spectrum of total (kinetic+potential) energy in the azimuthal or vertical directions shows that the contribution coming from the vortex mode is dominant at large scales, while the contribution coming from the wave mode is important at small scales. These findings may be confirmed by appropriate numerical simulations in the high Reynolds number regime. © 2013. The American Astronomical Society. All rights reserved.

Salhi A.,Faculte des science de Tunis | Lehner T.,University of Paris Descartes | Godeferd F.,École Centrale Lyon | Cambon C.,École Centrale Lyon
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2012

We present a spectral linear analysis in terms of advected Fourier modes to describe the behavior of a fluid submitted to four constraints: shear (with rate S), rotation (with angular velocity Ω), stratification, and magnetic field within the linear spectral theory or the shearing box model in astrophysics. As a consequence of the fact that the base flow must be a solution of the Euler-Boussinesq equations, only radial and/or vertical density gradients can be taken into account. Ertel's theorem no longer is valid to show the conservation of potential vorticity, in the presence of the Lorentz force, but a similar theorem can be applied to a potential magnetic induction: The scalar product of the density gradient by the magnetic field is a Lagrangian invariant for an inviscid and nondiffusive fluid. The linear system with a minimal number of solenoidal components, two for both velocity and magnetic disturbance fields, is eventually expressed as a four-component inhomogeneous linear differential system in which the buoyancy scalar is a combination of solenoidal components (variables) and the (constant) potential magnetic induction. We study the stability of such a system for both an infinite streamwise wavelength (k 1=0, axisymmetric disturbances) and a finite one (k 1 0, nonaxisymmetric disturbances). In the former case (k 1=0), we recover and extend previous results characterizing the magnetorotational instability (MRI) for combined effects of radial and vertical magnetic fields and combined effects of radial and vertical density gradients. We derive an expression for the MRI growth rate in terms of the stratification strength, which indicates that purely radial stratification can inhibit the MRI instability, while purely vertical stratification cannot completely suppress the MRI instability. In the case of nonaxisymmetric disturbances (k 1 0), we only consider the effect of vertical stratification, and we use Levinson's theorem to demonstrate the stability of the solution at infinite vertical wavelength (k 3=0): There is an oscillatory behavior for τ>1+K 2/k 1, where τ=St is a dimensionless time and K 2 is the radial component of the wave vector at τ=0. The model is suitable to describe instabilities leading to turbulence by the bypass mechanism that can be relevant for the analysis of magnetized stratified Keplerian disks with a purely azimuthal field. For initial isotropic conditions, the time evolution of the spectral density of total energy (kinetic + magnetic + potential) is considered. At k 3=0, the vertical motion is purely oscillatory, and the sum of the vertical (kinetic + magnetic) energy plus the potential energy does not evolve with time and remains equal to its initial value. The horizontal motion can induce a rapid transient growth provided K 2/k 11. This rapid growth is due to the aperiodic velocity vortex mode that behaves like K h/k h where k h(τ)= [k12 +(K 2 -k 1τ )2 ]1 /2 and K h=k h(0). After the leading phase (τ>K 2/k 11), the horizontal magnetic energy and the horizontal kinetic energy exhibit a similar (oscillatory) behavior yielding a high level of total energy. The contribution to energies coming from the modes k 1=0 and k 3=0 is addressed by investigating the one-dimensional spectra for an initial Gaussian dense spectrum. For a magnetized Keplerian disk with a purely vertical field, it is found that an important contribution to magnetic and kinetic energies comes from the region near k 1=0. The limit at k 1=0 of the streamwise one-dimensional spectra of energies, or equivalently, the streamwise two-dimensional (2D) energy, is then computed. The comparison of the ratios of these 2D quantities with their three-dimensional counterparts provided by previous direct numerical simulations shows a quantitative agreement. © 2012 American Physical Society.

Mnif A.,Faculte des science de Tunis | Ali M.B.S.,Faculte des science de Tunis | Hamrouni B.,Faculte des science de Tunis
Ionics | Year: 2010

In natural waters, fluoride ions are necessary and beneficial for the human being. At higher level of F- in water, it is toxic and detrimental to human health, leading to serious problems such as dental and skeleton fluorosis. According to the World Health Organization, the acceptable concentrations of fluoride in potable water are in the range of 0.7-1.5 mg L-1. Various treatment technologies for fluoride removal from water have been used such as ion exchange, adsorption and membrane processes. In the present study, removal of fluoride ions from aqueous solutions was investigated using a polyamide thin film composite nanofiltration membrane denoted as HL 2514 T from Osmonics Company. Through this membrane, the mechanism of transport was investigated. The Kedem-Katchelsky model was applied in order to determine phenomenological parameters σ and Ps, respectively, the reflection coefficient of the membrane and the solute permeability of ions. The convective and diffusive parts of the mass transfer were quantified. The retention of monovalent and bivalent salts by this membrane shows that it is negatively charged. In the second part, retention of fluoride anions was investigated. Results show that the retention of fluoride by HL membrane exceeds 80%. The influence of the chemical parameters (feed concentration and ionic strength) and the physical parameters (applied pressure and recovery) on the elimination of fluoride was studied. © Springer-Verlag 2009.

Salhi A.,Faculte des science de Tunis | Nasraoui S.,Faculte des science de Tunis
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2013

We study analytically the instability of the subharmonic resonances in magnetogravity waves excited by a (vertical) time-periodic shear for an inviscid and nondiffusive unbounded conducting fluid. Due to the fact that the magnetic potential induction is a Lagrangian invariant for magnetohydrodynamic Euler-Boussinesq equations, we show that plane-wave disturbances are governed by a four-dimensional Floquet system in which appears, among others, the parameter É representing the ratio of the periodic shear amplitude to the vertical Brunt-Väisälä frequency N3. For sufficiently small É and when the magnetic field is horizontal, we perform an asymptotic analysis of the Floquet system following the method of Lebovitz and Zweibel [Astrophys. J. 609, 301 (2004)10.1086/420972]. We determine the width and the maximal growth rate of the instability bands associated with subharmonic resonances. We show that the instability of subharmonic resonance occurring in gravity shear waves has a maximal growth rate of the form Δm=(33/16)É. This instability persists in the presence of magnetic fields, but its growth rate decreases as the magnetic strength increases. We also find a second instability involving a mixing of hydrodynamic and magnetic modes that occurs for all magnetic field strengths. We also elucidate the similarity between the effect of a vertical magnetic field and the effect of a vertical Coriolis force on the gravity shear waves considering axisymmetric disturbances. For both cases, plane waves are governed by a Hill equation, and, when É is sufficiently small, the subharmonic instability band is determined by a Mathieu equation. We find that, when the Coriolis parameter (or the magnetic strength) exceeds N3/2, the instability of the subharmonic resonance vanishes. © 2013 American Physical Society.

Gelloz B.,Tokyo University of Agriculture and Technology | Harima N.,Hyogo University of Teacher Education | Koyama H.,Hyogo University of Teacher Education | Elhouichet H.,Faculte des science de Tunis | Koshida N.,Tokyo University of Agriculture and Technology
Applied Physics Letters | Year: 2010

Nanocomposites consisting of oxidized porous Si (OPSi) impregnated with rhodamine 110 (Rh110) molecules are characterized in terms of luminescence properties. The photoluminescence and its polarization memory strongly indicates a trace of energy transfer from the fast blue luminescence band of OPSi to the green one of Rh110. Time-resolved experiments showed that energy transfer to Rh110 also takes place from the long-lived blue phosphorescence of OPSi. The transfer channel from nonradiative states of OPSi to Rh110 was also found. The ability of OPSi to harvest and transfer absorbed photon energy to a guest is promising for applications in optoelectronics and biology. © 2010 American Institute of Physics.

Loading Faculte des science de Tunis collaborators
Loading Faculte des science de Tunis collaborators