Abdelkader Mojtabi - Academia.edu (original) (raw)

Papers by Abdelkader Mojtabi

The European Physical Journal E

Comptes Rendus de l'Académie des Sciences - Series IIB - Mechanics-Physics-Chemistry-Astronomy, 1997

Transport in Porous Media, 2011

We consider convection in a uniform fluid-saturated porous layer which is bounded by conducting p... more We consider convection in a uniform fluid-saturated porous layer which is bounded by conducting plates and heated from below. The primary aim is to determine the identity of the postcritical convection planform as a function of the thicknesses and conductivities of the bounding plates relative to that of the porous layer. This work complements and extends an early paper by Riahi (1983) who considered a situation where the porous layer is bounded by infinitely thick conducting media. We present regions in parameter space wherein convection in the form of rolls is unstable and within which cells with square planform form the preferred pattern.

Journal of Fluid Mechanics, 1999

This work brings new insight to the question of heat transfer in near–critical fluids under Earth... more This work brings new insight to the question of heat transfer in near–critical fluids under Earth gravity conditions. The interplay between buoyant convection and thermoacoustic heat transfer (piston effect) is investigated in a two-dimensional non-insulated cavity containing a local heat source, to reproduce the conditions used in recent experiments. The results were obtained by means of a finite-volume numerical code solving the Navier–Stokes equations written for a low-heat-diffusing near-critical van der Waals fluid. They show that hydrodynamics greatly affects thermoacoustics in the vicinity of the upper thermostated wall, leading to a rather singular heat transfer mechanism. Heat losses through this wall govern a cooling piston effect. Thus, the thermal plume rising from the heat source triggers a strong enhancement of the cooling piston effect when it strikes the middle of the top boundary. During the spreading of the thermal plume, the cooling piston effect drives a rapid th...

International Journal of Heat and Mass Transfer, 2013

We investigate the onset of convection in a uniform, constant-thickness, horizontal porous layer ... more We investigate the onset of convection in a uniform, constant-thickness, horizontal porous layer which is heated from below. The layer is bounded above and below by thermally conducting but impermeable layers. Our aim is to determine the effect on the onset of convection of the interaction between the presence ofthese outer conducting layers and a horizontal background flow. A linear stability analysis is performed and a dispersion relation is derived from which the stability characteristics of the layer are computed. Convection ce lis are found move along the layer at a speed which is lower than that of the imposed flow due to a thermal drag caused by the presence of the bounding solid layers. Neutra! curves and streamline/ isotherm patterns are presented in order to understand the physical rote played by the governing nondimensional parameters. Wh en the diffusivity of the solid layers is much lower than the diffusivity of the porous layer there exists a regime where the neutra! curve can exhibit two minima, and at one point in parameter space there exists a neutra! curve with a quartic minimum.

International Journal of Heat and Mass Transfer, 2004

International Journal of Heat and Mass Transfer, 2011

Comptes Rendus de l'Académie des Sciences - Series IIB - Mechanics, 2001

Leonardo Journal of Sciences, 2008

The objective of this study is the comparison between the model of Darcy-Brinkman and the Navier-... more The objective of this study is the comparison between the model of Darcy-Brinkman and the Navier-Stokes equations modified, in the case of the natural convection. The study is made in a porous vertical square cavity saturated by a Newtonian fluid. A cylindrical heat source maintained at a uniform heat flux is introduced into porous medium. The equations which describe the thermal transfer and the hydrodynamic flow of the two models are solved numerically by means of the software package Femlab 3.2 based on the finite element method. The results obtained are in the form of average kinetic energy per unit mass, the local and the average Nusselt numbers, the pressure and the viscous force per unit area.

Transport in Porous Media, Apr 23, 2018

We present an analytical and numerical stability analysis of Soret-driven convection in a porous ... more We present an analytical and numerical stability analysis of Soret-driven convection in a porous cavity saturated by a binary fluid mixture and subjected to vertical highfrequency and small-amplitude vibrations. Two configurations have been considered and compared: an infinite horizontal layer and a bounded domain with a large aspect ratio. In both cases, the initial temperature gradient is produced by a constant uniform heat flux applied on the horizontal boundaries. A formulation using time-averaged equations is used. The linear stability of the equilibrium solution is carried out for various Soret separation ratios ϕ, vibrational Rayleigh numbers Rv, Lewis numbers Le and normalized porosity. For an infinite horizontal layer, the critical Rayleigh number Ra c is determined analytically. For a steady bifurcation to a one-cell solution (the critical wavenumber is zero), we obtain Ra c = 12/(ϕ(Le + 1) + 1) for all Rv. When the bifurcation is a Hopf bifurcation or when the critical wavenumber is not zero, we use a Galerkin method to compute the critical values. Our study is completed by a nonlinear analysis of the bifurcation to one-cell solutions in an infinite horizontal layer that is compared to numerical simulations in bounded horizontal domains with large aspect ratio.

OATAO is an open access repository that collects the work of Toulouse researchers and makes it fr... more OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. hal-00866157, version 1-This is an author-deposited version published in:

La convection thermogravitationnelle, qui génère de s variations locales de la masse volumique du... more La convection thermogravitationnelle, qui génère de s variations locales de la masse volumique du fluide en fonction de la température, conduit final ement à une séparation des espèces dans la cavité. On s’intéresse dans ce travail à la séparation des espèces d’une solution binaire remplissant une cavité parallélépipédique poreuse, placée dans le c hamp de la pesanteur, et soumise au niveau de ses parois horizontales et verticales à des densités de flux uniformes et croisés. Nous déterminons aussi les paramètres de contrôle conduisant à la séparati on optimale. En utilisant l’étude de la stabilité linéaire, on a vérifié que le nombre de Rayleigh cr itique linéaire, , associé à la perte de la stabilité de la solution unicellulaire est supérieur au nombr e de Rayleigh optimal, , conduisant à la séparation des constituants du mélange.

International Journal of Thermal Sciences

In the presence of a gravity field or under microgravity, pure thermo-diffusion leads to very wea... more In the presence of a gravity field or under microgravity, pure thermo-diffusion leads to very weak species separation in binary mixtures. To increase the species separation in the presence of gravity, many authors use thermo-gravitational diffusion in vertical columns (TGC). For a given binary mixture, the species separation between the top and the bottom of these columns depends on the temperature difference, ΔT, imposed between the two vertical walls facing each other, and the thickness, H, between these two walls (annular or parallelepipedic column). These studies show that, for a fixed temperature difference, the species separation is optimal for a thickness, H opt , much smaller than one millimetre. The species separation decreases sharply when the thickness H decreases with respect to this optimum value. It decreases progressively as H increases with respect to H opt. In addition, for mixtures with a negative thermo-diffusion coefficient, the heaviest component migrates towards the upper part of the column and the lightest one towards the lower part. The loss of stability of the configuration thus obtained leads to a brutal homogeneity of the binary solution. The objective of this study in microgravity was to increase the optimum of species separation. For this purpose, the binary fluid motion was provided by uniform velocities imposed on the two walls of the cavity facing each other. This forced flow led to species separation between the two motionless walls of the cavity. In this case, the fluid motion generated in the cavity was not dependent on the imposed temperature difference, ΔT contrarily to the case of thermogravitational column. Under these conditions and for a given column of thickness H, there are three independent control parameters: ΔT and the two velocities of the walls facing each other. Using the parallel flow approximation for a cell of large aspect ratio, the velocity, temperature and mass fraction fields within the cavity were determined analytically. Thus the parameters leading to optimal species separation were calculated. The analytical results were corroborated by direct numerical simulations. The present paper thus proposes a new process for the determination of the Soret coefficient, the thermodiffusion coefficient and mass-diffusion coefficient.

Transport in Porous Media, 2017

The onset of convection in a porous layer heated from below is considered, and we determine how t... more The onset of convection in a porous layer heated from below is considered, and we determine how the presence of two solid but heat-conducting bounding plates of finite thickness alters the manner in which convection ensues. Heat is generated by the lower plate (with an insulating lower boundary), but the upper one is passive with a fixed upper boundary temperature. It is shown that this composite layer may mimic in turn one of the three different types of classical single-layer onset problems which are well-known in the literature. The type which is selected (or indeed whether it corresponds to a transitional case) depends quite critically on the precise values of the relative thickness of the solid layers and their conductivity ratio. It is also shown that care needs to be taken over declaring that the solid plates are thin: extreme values of the conductivity ratio can yield a stability criterion which appears to be different from that suggested by the imposed boundary conditions.

European Journal of Mechanics - B/Fluids

ABSTRACT

Numerical Heat Transfer, Part A: Applications, 1996

Convective oSCiJJaJiollS in porous media are studied nUflUirieatly. A two-dimensional square, dif... more Convective oSCiJJaJiollS in porous media are studied nUflUirieatly. A two-dimensional square, differentially heated cavity, fiUed with a saturated porous medium, is considered subject to linear hannonic oscillations in the vertical direction. The formulation is based on the Darcy-Boussinesq model. The problem includes three nondlmensional parameters: the Royleigh number for porous media Ra, its vibrational analog Ro", and the nondimensional frequem:y f. The time-dependent Darcy-Boussinesq equations have been solved using a pseudo-spectral Chebyshev collocation methad. The instantaneous fields of the established oscillalory regimes are presented. Also, some instanumeous and mean characteristics are studied and discussed. The distinctions from the case of viscous fluid atone are emphasized.

Numerical Heat Transfer Part A-applications - NUMER HEAT TRANSFER PT A-APPL, 2000

Convective oscillations in porous and fluid media are studied numerically. A two-dimensional, squ... more Convective oscillations in porous and fluid media are studied numerically. A two-dimensional, square, differentially heated cavity, filled with a porous medium saturated by a binary fluid or simply by a binary fluid, is considered. This cavity is subjected to linear harmonic oscillations in the vertical direction. The formulation is based on the Darcy-Brinkman-Forchheimer-Boussinesq model. The time dependent Darcy-Brinkman-Forchheimer-Boussinesq equations are solved using a pseudo-spectral Legendre collocation method. The instantaneous and mean characteristics of the flows are studied and discussed. An intensification of the heat and mass transfers is observed at low frequency for sufficiently high vibration intensity. A comparison between the response to the imposed vibrations is made for Darcy numbers varying from Da = 10 to Da = 10.