Dynamics and family of equilibria in a population kinetics model with cosymmetry

We study dynamics in the population kinetics model which is given by the system of nonlinear parabolic equations with cosymmetry property. The cosymmetry implies the emergence of continuous families of steady states with variable spectrum of stability. Different scenarios of evolution of families of equilibria and nonstationary regimes are analyzed numerically by a finite-difference scheme which respects the cosymmetry property. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Exact analytical solution for a thermal boundary layer in a saturated porous medium

The forced convection thermal boundary layer in a porous medium as an analytically tractable special case of a mixed convection problem is considered. It is shown that some general features of the mixed convection solutions reported recently by other authors [B. Brighi, J.-D. Hoernel, On the concave and convex solutions of mixed convection boundary layer approximation in a porous medium, Appl. Math. Lett. (published online, 2005); M. Guedda, Multiple solutions of mixed convection boundary layer approximations in a porous medium, Appl. Math. Lett. (published online, 2005)] can already be recovered from this exactly solvable case.     

On Equilibrium Bifurcations in the Cosymmetry Collapse of a Dynamical System

We study the bifurcations that accompany the collapse of a continuous family of equilibria of a cosymmetric dynamical system (or a family of solutions to a cosymmetric operator equation in general) under some perturbation that destroys cosymmetry. Using the Lyapunov–Schmidt method, we expatiate on the cases in which the branching equation is one- or two-dimensional.     

Unsteady convective diffusion of a solute in a Hagen-Poiseuille flow through a tube with permeable wall

Influence of Interphase Mass Transfer (IMT) on the unsteady convective diffusion in a fluid flow through a tube surrounded by a porous medium is examined against the background of no IMT. The three coefficients namely exchange coefficient, convection coefficient, and dispersion coefficient are evaluated asymptotically at large-time. The exchange coefficient exists due to IMT. All-time analysis is made analytically when there is no IMT. The mean concentration distribution is measured at a point inside and outside the slug. The peak of mean concentration is higher than that of pure convection and it is further enhanced with increase of porous parameter. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Cycle-creating bifurcation from a family of equilibria of a dynamical system and its delay

For a dynamical system with cosymmetry, a study is made of the bifurcation in which a cycle branches off from an equilibrium in a continuous one-parameter family of equilibria, as the parameter passes through a critical value. Unlike the classical situation that occurs when the equilibrium is isolated, a self-excited oscillatory mode generally branches off with delay relative to the parameter. Another characteristic difference is the possibility of supercritical branching of an unstable limit cycle.     

A robust algebraic multilevel preconditioner for non‐symmetric M‐matrices

Stable finite difference approximations of convection‐diffusion equations lead to large sparse linear systems of equations whose coefficient matrix is an M‐matrix, which is highly non‐symmetric when the convection dominates. For an efficient iterative solution of such systems, it is proposed to consider in the non‐symmetric case an algebraic multilevel preconditioning method formerly proposed for pure diffusion problems, and for which theoretical results prove grid independent convergence in this context. These results are supplemented here by a Fourier analysis that applies to constant coefficient problems with periodic boundary conditions whenever using an ‘idealized’ version of the two‐level preconditioner. Within this setting, it is proved that any eigenvalue λ of the preconditioned system satisfies for some real constant c such that . This result holds independently of the grid size and uniformly with respect to the ratio between convection and diffusion. Extensive numerical experiments are conducted to assess the convergence of practical two‐ and multi‐level schemes. These experiments, which include problems with highly variable and rotating convective flow, indicate that the convergence is grid independent. It deteriorates moderately as the convection becomes increasingly dominating, but the convergence factor remains uniformly bounded. This conclusion is supported for both uniform and some non‐uniform (stretched) grids. Copyright © 2000 John Wiley & Sons, Ltd.     

Multi-objective-oriented removal of airborne pollutants from a slot-ventilated enclosure subjected to mechanical and multi component buoyancy flows

Slot-ventilated enclosure flows, jointly driven by two component buoyancy forces and forced ventilation, extensively exist in the industrial and civil environment. Inverse fluid flow solutions of the multi-objective oriented removal of airborne pollutants from the slot ventilated enclosure, simultaneously subjected to mechanical forces and multi-component buoyancy forces, are conducted in the present work. A simplified conjugate gradient methodology has been implemented to provide effective convection removal of contaminants, under different flow regimes. In particular, direct and inverse convection problems are subsequently solved in detail. For the direct convection problem, aiding and opposing multi-component buoyancy effects are included to study the convective heat and species transport mechanism. For the inverse optimization problem, a single-objective optimization is firstly implemented, which also provides input parameters for the multi-objective optimization. Following that, a multi-objective function is set up by combining the two single objectives involving the spatial average concentration and the mean radius of diffusion. Multi-objective optimization is then implemented depending on the conjugate gradient procedures. Both the single and multi-objectives could be achieved reasonably through positioning of local heating sources and the free vent outlet. Our solution methodology will be useful for improving room pollutant removal and developing efficient ventilation strategies.     

Numerical analysis of finite element methods for miscible displacements in porous media

Finite element methods are used to solve a coupled system of nonlinear partial differential equations, which models incompressible miscible displacement in porous media. Through a backward finite difference discretization in time, we define a sequentially implicit time-stepping algorithm that uncouples the system at each time-step. The Galerkin method is employed to approximate the pressure, and accurate velocity approximations are calculated via a post-processing technique involving the conservation of mass and Darcy's law. A stabilized finite element ( SUPG ) method is applied to the convection–diffusion equation delivering stable and accurate solutions. Error estimates with quasi-optimal rates of convergence are derived under suitable regularity hypotheses. Numerical results are presented confirming the predicted rates of convergence for the post-processing technique and illustrating the performance of the proposed methodology when applied to miscible displacements with adverse mobility ratios. © 1998 John Wiley & Sons, Inc. Numer Methods Partial Differential Eq 14: 519–548, 1998     

Effects of variable viscosity on non-Darcy MHD free convection along a non-isothermal vertical surface in a thermally stratified porous medium

An analysis is performed for non-Darcy free convection flow of an electrically conducting fluid over an impermeable vertical plate embedded in a thermally stratified, fluid saturated porous medium for the case of power-law surface temperature. The present work examines the effects of non-Darcian flow phenomena, variable viscosity, Hartmann–Darcy number and thermal stratification on free convective transport and demonstrates the variation in heat transfer prediction based on three different flow models. The wall effect on porosity variation is approximated by an exponential function. The effects of thermal dispersion and variable stagnant thermal conductivity are taken into consideration in the energy equation. The resulting non-similar system of equations is solved using a finite difference method. Results are presented for velocity, temperature profiles and local Nusselt number for representative values of different controlling parameters.     

On the use of multioperators in the construction of high-order grid approximations

Examples of using the multioperator technique in order to increase the order of accuracy of some linear operators are given. Formulas for numerical differentiation, approximation of diffusion terms, recalculation, filtration, and extrapolation of grid functions are considered. A new family of multioperator approximations for convective terms of equations is presented. Multioperators of 16th and 32nd orders are analyzed as an example.     

Inhibition or enhancement of chaotic convection via inclined magnetic field

In this paper, we investigate the onset of convection in a horizontal layer of fluid which is heated from the underside. An inclined magnetic field is applied to the layer. The Galerkin truncated approximations were used to obtain a Lorenz-like model. The nonlinear system was solved by the fourth-order Runge–Kutta method. The results show that the Hartmann number and the angle of inclination of the magnetic field could inhibit or enhance the onset of chaotic convection.     

Rayleigh–Benard convection in a chemical equilibrium gas (simulation of surface detonation wave initiation)

The problem of the Rayleigh–Benard convection for a chemical equilibrium gas is solved numerically. The gas is assumed to be incompressible, and the layer boundaries are assumed to be flat, isothermal, and free from shear stress. The Boussinesq model with the coefficient at the buoyancy term depending on the transverse coordinate is used. The resultant nonlinear system of equations is solved by a previously developed numerical method based on the spectral representation of vorticity and temperature fields. According to the idea of splitting, analytical formulas are first used to take into account the linear increase in disturbances, and then the nonlinear convective transfer is calculated by the finite-difference method. Various convection modes are obtained: stationary, periodic, quasi-periodic, and stochastic convection.     

Stationary convection–diffusion equation in an infinite cylinder

We study the existence and uniqueness of a solution to a linear stationary convection–diffusion equation stated in an infinite cylinder, Neumann boundary condition being imposed on the boundary. We assume that the cylinder is a junction of two semi-infinite cylinders with two different periodic regimes. Depending on the direction of the effective convection in the two semi-infinite cylinders, we either get a unique solution, or one-parameter family of solutions, or even non-existence in the general case. In the latter case we provide necessary and sufficient conditions for the existence of a solution.     

Numerical study of mixed convection flow of a micropolar fluid towards permeable vertical plate with convective boundary condition

In this article, the mixed convective flow of a micropolar fluid along a permeable vertical plate under the convective boundary condition is analyzed. The scaling group of transformations is applied to get the similarity representation of the system of partial differential equations of the problem and then the resulting equations are solved by using Spectral Quasi-Linearisation Method. This study reveals that the dual solutions exists for certain values of mixed convection parameter. The outcomes are analyzed with dual solutions in detail. Effects of micropolar parameter, Biot number and suction/injection parameters on different flow profiles are discussed and depicted graphically.     

Mathematical modeling of thermal convection in a 3D model of the “DAKON” convection sensor

A three-dimensional time-dependent mathematical model of thermal convection in a cubic convection sensor is developed. An efficient numerical algorithm is designed for computing convective flows on the basis of quasi-hydrodynamic equations. Analysis of the calculation results suggests that, in the range of real microaccelerations, the 3D model does not add new effects to the structure of convective motion compared with simplified two-dimensional models. The conclusion is that simplified models can be beneficially used for the interpretation of measurements carried out with the DAKON convection sensor.     

Selection of steady states in planar Darcy convection

The planar natural convection of incompressible fluid in a porous medium is considered. We study selection of steady states under temperature perturbations on the boundary. A selection map is introduced to analyze the selection of steady state from a continuous family of equilibria which exists under zero boundary conditions. The results of finite-difference modeling for the rectangular enclosure are presented. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermomagnetic convection induced by a time-modulated driving force in magnetic fluids

It is well-known that the flow properties of magnetic fluids – so called ferrofluids – can be modified by applying an external magnetic field. Under certain conditions, the magnetic force induced by this external field causes a convective flow. What has yet to be investigated is what happens when this driving force is modulated in time. For this purpose, a horizontal ferrofluid layer has been exposed to an intermittent magnetic field, which causes a time-modulated force. This force depends on the strength of the external magnetic field and the fluid temperature, and therefore the flow phenomenon generated is called thermomagnetic convection. In addition, if the fluid layer is heated from below, the classical thermal convection contributes to the flow system. In our studies, both effects – thermomagnetic and thermal – contribute together to the convection. The experimental results presented here confirm previous theoretical investigations about the influence of the frequency of the driving force on the strength of the convective flow, which reach minimum values at certain frequencies. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

The forced convection flow of a uniform stream over a flat surface with a convective surface boundary condition

The forced convection heat transfer resulting from the flow of a uniform stream over a flat surface on which there is a convective boundary condition is considered. In previous papers [5], [6], [7], [8] it was assumed that the convective heat transfer parameter h_f associated with the hot surface depended on x, where x measures distance along the surface, so that problem could be reduced to similarity form. Here it is assumed that this heat transfer parameter h_f is a constant, with the result that the temperature profiles and overall heat transfer characteristics evolve as the solution develops from the leading edge. The heat transfer near the leading edge (small x), which we find to be dominated by the surface heat flux, the solution at large distances along the surface (large x), which dominated by the surface temperature, are discussed. A numerical solution to the full problem is then obtained for a range of values of the Prandtl number to join these two solution regimes.     

Numerical simulation of solutal Rayleigh-Bénard-Marangoni convection in a layered two-phase system

We present a two-dimensional simulation of solutal Rayleigh-Bénard-Maragoni convection in a layered system. In the initial state, the solute concentration is homogeneous in each layer but not in partition equilibrium. Diffusive transfer of solute leads to convective instability. Marangoni convection dominates initially as it operates on a smaller length scale. Rayleigh convection appears later as an instability of the mixed unstably stratified fluid near the interface. Compared to pure Marangoni convection the dynamics is more disordered due to additional flow in the bulk. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)