PHY
Professeur
Activité scientifique
Profil chercheur
Enseignant-chercheur à l’Université de Yaoundé I. Grade : Professeur.
Publications scientifiques
Bernard Kalibe, Maurice Lamara, Elisabeth Ngo Nyobe, Elkana Pemha
Chinese Journal of Physics
Thérèse Berceline Ntjam, Maurice Lamara, Noé Richard Makon, Elisabeth Ngo Nyobe, Elkana Pemha
Applied Thermal Engineering
Médard Marcus Nganbe, Elisabeth Ngo Nyobe, Jacques Hona, Elkana Pemha
Chinese Journal of Physics
Serge Benjamin Mbam Mbam, Maurice Lamara, Noé Richard Makon, Elisabeth Ngo Nyobe, Elkana Pemha
Zeitschrift für Naturforschung A
Abstract The steady laminar flow of an incompressible Newtonian fluid in a porous annular duct with accelerated rotating walls is investigated. The flow is located in the polar plane and is driven by suction and injection at the walls. As required by conservation of mass with zero axial velocity, the fluid is injected into one of the cylinders, which becomes the upstream cylinder, and suctioned into the other, which represents the downstream cylinder. Only the case of the downstream cylinder at rest is examined. The duct gap ratio, the velocity ratio comparing the radial and azimuthal velocities, the Reynolds number, and the velocity coefficient that compares the fluid velocity at the upstream and downstream cylinders are the four parameters of the problem. The purpose of this research is to identify the prerequisites for the flow’s existence and determine how the aforementioned variables affect the flow velocity and pressure gradients at a fixed Reynolds number. The Navier–Stokes equations are replaced by the polar-plane vorticity equation, which is solved using the similarity-solutions method. The shooting technique, including the fourth-order Runge–Kutta algorithm, is used to produce numerical solutions. From the findings, physical understandings of the flow are derived. More specifically, we discover an unexpected interior zone where the fluid is perpetually at rest even while flow is present. The only solution found corresponds to the case of the inner cylinder upstream and rotating provided that the velocity ratio does not exceed the threshold of 0.1 with a velocity coefficient greater than 1.
Yvon Gazambeti, Elisabeth Ngo Nyobe, Maurice Lamara, Elkana Pemha
Zeitschrift für Naturforschung A
Abstract This paper is devoted to the pressure-exerted steady laminar flow of an incompressible Newtonian fluid along a parallel-walled horizontal channel with a porous upper wall and an impermeable lower wall. The fluid is sucked or blown through the porous wall, at constant and uniform velocity, orthogonally to the wall. At the same time, an external pressure gradient constant in time is applied between the two ends of the channel. The aim of this work is to determine and analyze the effects of the external pressure gradient on the flow, the suction/blowing velocity being kept constant. The two-dimensional configuration of the flow with zero-divergence velocity field allows the existence of the stream function given by a single nonlinear partial differential equation which replaces the Navier–Stokes equations and is called the vorticity equation. This latter equation is demonstrated by applying an unusual approach which uses the vector momentum equation in its general form. From the similarity-solutions assumption, it is shown that the vorticity equation leads to a two-point boundary value problem whose solutions are computed by means of a numerical shooting technique including the Newton–Raphson optimization algorithm. Physical understandings of the flow under consideration are derived from the results obtained.
Guy Leopold Mbogba, Elisabeth Ngo Nyobe, Maurice Lamara, Yves Christian Mbono Samba, Elkana Pemha
Zeitschrift für Naturforschung A
Abstract In this paper, we examine a steady laminar flow for an incompressible fluid located in a semi porous annular pipe and subjected to a favorable constant pressure gradient applied between the two borders of the pipe. The inner wall is impermeable and the fluid is sucked or injected at the outer wall at constant and uniform velocity, orthogonally to the wall. The problem under study depends on three parameters: the pipe gap ratio, the dimensionless external pressure gradient, and the Reynolds number defined from the sum of the suction or injection velocity and the maximum Hagen–Poiseuille velocity. The conservation of mass induces the zero-divergence velocity field which allows replacing the steady-flow Navier–Stokes equations with a single equation satisfied by the stream function and called the vorticity equation. Assuming the similarity-solution hypothesis, the problem under consideration is reduced to a fourth-order nonlinear ordinary differential equation with two boundary conditions at each wall. The numerical shooting technique including the Runge–Kutta algorithm and the Newton–Raphson optimization method is applied to obtain the solution for the steady flow. For various values of the dimensionless external pressure gradient, the profiles of the velocity components are found and investigations on the wall shear stress for both walls are performed. The results obtained are discussed and physical understandings for the problem studied are derived.
Médard Marcus Nganbe, Jacques Hona, Elisabeth Ngo Nyobe, Elkana Pemha
The European Physical Journal Plus
Médard Marcus Nganbe, Elisabeth Ngo Nyobe, Jacques Hona, Elkana Pemha
Chinese Journal of Physics
Maurice Lamara, Elisabeth Ngo Nyobe, Elkana Pemha
Progress In Electromagnetics Research M
Medard Marcus Nga, Jacques Hona, Elisabeth Ngo Nyobe, Elkana Pemha
Journal of Applied Sciences
Background and Objective: Fluid flow and heat transfer between two permeable walls are used to model a variety of porous surface mechanisms ubiquitous in engineering and industry. This study deals with a numerical contribution in order to ensure a deeper understanding of phenomena of heat and mass distributions inside a rectangular, porous industrial conduct. The investigation is restricted to find a numerical solution of a two-dimensional flow driven by liquid withdrawal/addition also known as suction/injection through two parallel porous walls which are accelerated and maintained at different temperatures. Materials and Methods: By similarity transformation, the Navier-Stokes equations and the energy equation describing mass and heat distributions inside the channel are reduced to a nonlinear boundary-value problem which is solved applying a numerical integration based on the shooting method. Results: The solution of the problem is expressed in terms of velocity components, temperature and pressure gradients between two opposing permeable surfaces. Conclusion: It is found that positive wall accelerating parameters give the existence of flow reversal.
Jacques Hona, Elisabeth Ngo Nyobe, Elkana Pemha
International Journal of Engineering Systems Modelling and Simulation
The creeping flow occurs through a straight horizontal conduct formed by two coaxial cylinders fixed at different temperatures. The problem is governed by the vorticity equation coupled to the energy equation due to the variation of the dynamic viscosity with temperature. The results about the wall shear stress obtained from the numerical integration reveal the existence of the cusp type bifurcation. Under certain values of control parameters, the suction which manifests itself as the mass withdrawal phenomenon causes important variations of temperature near the walls. Thus, the maxima of thermal gradients are located at the walls. The temperature is almost constant around the centre of the flow region; this behaviour involves in certain circumstances the presence of a large area of inflection in the temperature distribution through the annular conduct. The dominance of flow reversal on wall suction agrees with the behaviour of pressure gradients.
Jacques Hona, Elisabeth Ngo Nyobe, Elkana Pemha
International Journal of Engineering Systems Modelling and Simulation
The horizontal channel consists of two parallel rigid plates. These plates are uniformly porous and fixed at different temperatures. The problem is modelled by means of the Navier-Stokes equations and the energy equation. The similarity technique leads to introduce the stream function in the governing equations. The investigation is then reduced to solve a non-linear two-point boundary-value problem with six boundary conditions by applying a numerical strategy based on the shooting method. The numerical results show that the temperature distribution presents a large area of inflection through the channel under certain values of control parameters. The maxima of thermal gradients are located at the walls due to suction. The high viscosity of the fluid is favourable to the existence of the reverse flow and is adverse to suction. For a given Reynolds number, the normal pressure gradient is very sensitive to the temperature difference between the walls.
Elisabeth Ngo Nyobe, Elkana Pemha, Jacques Hona, Jean Bilong, Maurice Lamara
Optics and Lasers in Engineering
Elkana Pemha, Jacques Hona, Elisabeth Ngo Nyobe
International Journal of Flow Control
The laminar incompressible flow develops within a channel with two permeable walls which undergo expansion. The flow is injected through the two uniformly porous walls kept at different temperatures. The mathematical model of the problem is obtained by applying mass, momentum and energy conservations. Using the similarity transformation technique, the governing equations give rise to two nonlinear ordinary differential equations. The numerical procedure for solving the differential equations of the problem is detailed. The numerical results are analyzed and the influences of the Reynolds number, the Prandtl number, and the wall moving ratio are discussed. The variations of the velocity distribution, thermal gradients, and pressure gradients are obtained. In particular, when low injection occurs concurrently with high expansion, the reverse flow takes place inside the channel. High injection provides oscillatory pressure gradients under different expansion ratios.
Jean Bilong, Elisabeth Ngo Nyobe, Jacques Hona, Elkana Pemha
Progress In Electromagnetics Research B
Abstract—Using the geometrical optics approximation, a theoretical prediction of the deflection angle correlation of a laser beam propagating in a hot turbulent jet is found as a functional form of the turbulent spectrum of the refractive index fluctuations. By applying the modified Von Karman model and Tatarskii model, the structure coefficient of the refractive index and the deflection angle correlation of the laser beam are then computed by means of a numerical procedure. Experiments to measure the structure coefficient are performed. A good agreement between the experimental results obtained and the theoretical predictions demonstrates the validity of the theoretical approach. 1.
Elkana Pemha, Elisabeth Ngo Nyobe
Progress In Electromagnetics Research B
Abstract—This paper is devoted to a laser-based diagnostic technique described as a method for solving an applied inverse problem in turbulent media using laser beam propagation. This problem consists of extracting local information about temperature fluctuations inside a hot turbulent jet of air, from the luminous photodiode trace produced by a laser beam, after having traversed the jet. A genetic algorithm is implemented in order to calculate the optimized laser beam directions corresponding to the whole luminous trace. An approximated ray equation which is proved from the geometrical optics is solved numerically by using those directions and enables to determine the variance of temperature fluctuations along the whole path of the laser beam. A good agreement coming from the comparison between the results obtained and the published experimental data proves the validity of the method. 1.
Jacques Hona, Elisabeth Ngo Nyobe, Elkana Pemha
International Journal of Bifurcation and Chaos
In this paper, the axisymmetric flow of a viscous fluid through a porous annular tube with walls kept at different temperatures is studied theoretically. The physical properties of the fluid remain constant, notably its specific mass, its dynamic viscosity and its thermal diffusivity. The nondimensional parameters which the solutions of the problem depend on are defined. A numerical integration using the shooting method is applied for solving the Navier–Stokes equations and the energy equation. Bifurcation diagrams are presented and enable to highlight significant properties of the flow. Some thermal behaviors corresponding to specific values of the parameters are performed. Asymmetric solutions of the steady flow are described and some results about velocity components are also analyzed.
Alim Alim, Elisabeth Ngo Nyobe, Elkana Pemha
Optics Communications
Jacques Hona, Elisabeth Ngo Nyobe, Elkana Pemha
Electromagnetic waves
Abstract—In the view of measuring directional fluctuations of a thin laser beam sent through a heated turbulent jet, an optical method using interference and diffraction with the out coming beam is proposed. The experimental set-up is described. A new technique for separating directional fluctuations of the laser beam is explained. From the measurement of the interference pattern perturbations, are deduced the Rms of the laser beam deflection angle, the spectrum of directional fluctuations of the laser beam, and the value of a scattering coefficient characterizing the heated turbulent jet. The measured spectrum reveals a −8/3 power law and the value obtained for that coefficient is nearly equal to that found in previous works. This agreement enables to conclude that the experimental technique used is efficient and satisfactory. 1.
Elisabeth Ngo Nyobe, Elkana Pemha
Progress In Electromagnetics Research B
Abstract—The paper describes the propagation of a thin laser beam which passes through a hot turbulent jet, perpendicularly to the flow direction, using geometrical optics approximation. From the modelling of the random propagation direction of the laser beam along its whole path, the diffusion coefficient of the turbulent jet is determined by means of a shape optimization technique in which a genetic algorithm is used. The results obtained from the GA are then improved by the Golden Section method. 1.
Elisabeth Ngo Nyobe, Elkana Pemha
Electromagnetic waves
Abstract—The propagation of waves in a random medium is a very complex phenomena which presents numerous difficulties in its experimental approach, and in its theoretical analysis. In this work, the case of a laser beam direction during its random propagation through a hot free jet of air, is considered using geometrical optics. Some experiments are done in the jet and from the hypothesis of the Markovian process, the main stochastic characteristics of the laser beam direction are studied. In addition, the sensitivity of the probability density of the beam random direction with respect to the
Elkana Pemha, B. Gay, A. Tailland
Physics of Fluids A Fluid Dynamics
In order to measure the diffusion coefficient of a heated plane airstream, a single laser beam is passed through the jet, perpendicularly to the flow direction. The thermic turbulence in the airstream causes random fluctuations of the refractive index. Consequently, the beam direction undergoes, in the flow, random perturbations. After having traversed the jet, the beam produces a luminous trace on a photoelectric cell placed outside the jet. An experimental setup for measuring the probabilities of the beam impact positions on the cell is described. From the Markovian process model, applied along the whole random path of the beam, it has been possible to compute these probabilities by solving the Einstein–Fokker–Kolmogorov equation. The diffusion coefficient can be determined by adjusting the numerical solution to agree with the experimental results. In addition, the calculation procedure gives the order of magnitude of an integral scale, characterizing the dimension of the turbulent structures in which the propagation of light can be considered rectilinear. A good agreement between the results and the published data obtained by means of the cold-wire anemometer technique proves the validity of the method.
Dans le but de mesurer les fluctuations de temperature et, plus generalement, de masse volumique, dans un jet turbulent plan chauffe, on fait traverser celui-ci par un rayon laser, perpendiculairement a la direction de l'ecoulement. Les travaux presentes font appel aux seuls changements de direction du rayon lumineux alors que la plupart des techniques optiques utilisent soit l'amplitude, soit la frequence, soit la phase, soit des combinaisons de ces parametres de l'onde transportee dans le milieu etudie. La relation entre les valeurs efficaces des fluctuations de temperature, et un coefficient de diffusion du jet relatif a l'indice de refraction est alors mise en evidence et doit permettre de calculer ces valeurs efficaces par l'intermediaire des fluctuations d'angle de sortie du rayon laser. Un code de calcul base sur l'hypothese de processus aleatoire de Markov pour la direction du rayon laser au cours de la propagation (comme l'a suggere Chernov), permet de calculer ce coefficient de diffusion et apporte ainsi une contribution a la mesure des fluctuations de temperature. Les resultats obtenus montrent que l'approximation de l'optique geometrique et le modele de Markov se recoupent, a condition que les dimensions des structures turbulentes ne depassent pas une certaine echelle restituee par le code de calcul.
Noé Richard Makon, Elisabeth Ngo Nyobe, Maurice Lamara, Yves Christian Mbono Samba, Elkana Pemha
Alexandria Engineering Journal
Despite the extensive research already done on laminar flows with porous boundaries, polar flows bounded by cylinders have not yet been studied. This paper investigates the polar laminar flow for an incompressible fluid located in a porous annular pipe and driven by suction-injection at the walls. The fluid being confined between the cylinders with zero axial velocity, it is proven that the flow takes place in the polar plane with conservation of mass if an incoming flow exists in the same plane to compensate the mass of fluid extorted by suction. So, one of the cylinders undergoes the suction, and the other the injection. Suitable boundary conditions for both cylinders are then found. The problem depends on the Reynolds number, the pipe gap ratio, and the pore density and surface ratios. The method of solution utilizes the shooting technique including the Runge-Kutta and Newton-Raphson algorithms. Radial flows are found as a possible solution. When the flow is not radial, the patterns of the acceptable streamlines highlight a particular zone in which the fluid is at rest, bounded by two singular streamlines and the downstream cylinder. The flow velocity is determined. Physical understandings of the flow are derived.
Valjacques Nyemb Nsoga, Jacques Hona, Elkana Pemha
International Journal of Nonlinear Sciences and Numerical Simulation
Abstract This paper is a contribution to a better understanding of heat transfer through porous channels used for mechanical sieving and filtration of liquids. The problem modeled by means of the Navier–Stokes equations and the energy equation is similar to a viscous flow between two uniformly permeable walls fixed at different temperatures. Thermal behaviors are determined through three branches denoted solutions of types I, II and III of a diagram of bifurcations presenting the values of the wall shear stress as the Reynolds number varies. We found that the distribution of temperature is similar through branches I and II where a large horizontal inflection area is observed as the Péclet number increases. This large horizontal inflection area inside the channel denotes the presence of thermal boundary layers which more precisely occur across branches I and II when the Péclet number approaches the value of 10. On the other hand, along branch III, thermal boundary layers do not exist and temperature presents a different behavior compared to those of branches I and II.
Jacques Hona, Elkana Pemha, Elisabeth Ngo Nyobe
The International Journal of Multiphysics
In this paper, a flow of a high viscous fluid with temperature-dependent viscosity through a porous industrial conduct is investigated by means of similarity transformation technique. The problem is modeled using mass, momentum and energy conservations. The variation of viscosity as function of temperature couples the vorticity equation to the energy equation. The numerical procedure for solving the differential equations of the problem is detailed. For low values of the main control parameters, the analytical solution of the problem is yielded. It appears from the numerical results of the problem that the variations of temperature are stopped in a large area around the middle of the flow domain. The maxima of thermal gradients are situated at the walls due to suction. The dominance of flow reversal agrees with the behavior of the normal pressure gradient inside the annular conduct.
Profil chercheur
Publications scientifiques
2 publications
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