Papers by Abdou Njifenjou
The buildings of the city of Douala in Cameroon have been experiencing degradation for several de... more The buildings of the city of Douala in Cameroon have been experiencing degradation for several decades due to the climate characterized by high humidity and oppressive heat. As a result, large grayish or black stains can be observed on these buildings. We sometimes witness the subsidence
of the slab of the balconies, the cracking of the walls and the collapse of the buildings worn by the humidity. These damages are generally caused by infitration and capillary rise. In addition, it has been demonstrated that people living in damp buildings are at risk of illnesses such as asthma and
lung infections. Therefore, the novelty of this work is threefold: (i) it proposes for the very first time a numerical study of the transport of humidity and heat through the porous walls of buildings constructed with concrete material, the main construction material in the city of Douala; (ii) It was determined what level of indoor thermal comfort was appropriate for sleeping inside a real three-dimensional G+1 complex residential building constructed with concrete blocks; and (iii) Using the geographical coordinates, and time data, the sun radiation’s direction of incidence was assessed throughout the simulation. The computation was performed using Comsol Multiphysics 6.0 software. The distributions of temperature, relative humidity as well as moisture level were presented at various periods. It appears from the results that face to this high humidity, the concrete material retains a large quantity of water for a considerable periods of time, which weakens the steel reinforcement of concrete which is corroded by rust. The computation of thermal comfort in the 3D building showed that the various rooms of the building were not comfortable during the night since temperature inside the building increased progressively due to diffusion of heat. In addition, the numerical solutions indicated that the energy stored within the walls diffused from the external walls to the internal walls during the night. It was also demonstrated that the walls of the building were warmer than the windows, doors and
the roof at the computational times, which simply revealed a greater storage capacity of heat in the concrete blocks material. The fidings highlighted that the temperature decreased rapidly in a thickness of 0.06 m of the concrete block during the nine days and this decrease was attenuated in the second part of the thickness of the concrete block (0.14 m).
Research Article, 2024
A new development of Finite Volumes (FV, for short) and its theoretical analysis are the purpose ... more A new development of Finite Volumes (FV, for short) and its theoretical analysis are the purpose of this work. Recall that FV are known as powerful tools to address equations of conservation laws (mass, energy, momentum,...). Over the last two decades investigators have succeeded in putting in place a mathematical framework for the theoretical analysis of FV. A perfect illustration of this progress is the design and mathematical analysis of Discrete Duality Finite Volumes (DDFV, for short). We propose now a new class of DDFV for 2nd order elliptic equations involving discontinuous diffusion coefficients or nonlinearities. A one-dimensional linear elliptic equation is addressed here for illustrating the ideas behind our numerical strategy. The algebraic structure of the discrete system we have got is different from that of standard DDFV. The main novelty is that the so-called diamond mesh elements are confined in homogeneous zones for flow problems governed by piecewise constant coefficients. This is got from our judicious definition of the primal mesh. The gain is that there is no need to compute homogenized coefficients to be allocated to the so-called diamond cells as required to conventional DDFV. Notice that poor homogenized permeability allocated to diamond elements leads to poor approximations of fluxes across grid-block interfaces. Moreover for 1-D flow problems in a porous medium involving permeability discontinuities (piecewise constant permeability for instance) the proposed FV scheme leads to a symmetric positive-definite discrete system that meets the discrete maximum principle; we have shown its second order convergence under relevant assumptions.
European journal of environmental and civil engineering, Apr 13, 2024
Research Square (Research Square), Mar 1, 2024
In this work, we present a stable, efficient, positive, and accurate a new semiimplicit path-cons... more In this work, we present a stable, efficient, positive, and accurate a new semiimplicit path-conservative-based method for an energetically-stable sediment transport model. The model is nonconservative and integrates multiple relevant physical and hydrodynamic parameters that are often neglected. Many previously published works have discussed the calculation of eigenvalues, but the inclusion of certain parameters can complicate the process. To avoid this difficulty, we employ a splitting technique that allows us to obtain two subsystems, each with a simple eigenstructure. This approach eliminates the need for complex calculations. The following paper discusses this technique in detail. The study of hyperbolicity is crucial in analyzing and approximating models or designing numerical schemes, as it can cause numerical methods to fail. To achieve high-order accuracy in space, a nonlinear reconstruction procedure and a semi-implicit algorithm have been developed. The proposed numerical method has been proven to be well-balanced and positivity-preserving through several original and formal demonstrations, which are rare to find in sediment transport papers. We have developed a predictor-corrector splitting method and that has been verified. In several numerical tests, our proposed model yielded encouraging results. The model corrects many failures resulting from sediment transport modeling due to its hyperbolicity. Additionally, it shows promise. This approach can be confidently applied to a wide range of sediment transport models.
World journal of engineering and technology, 2024
HAL (Le Centre pour la Communication Scientifique Directe), Dec 11, 2022
Springer eBooks, 2008
This work analyzes some mathematical aspects of a new Multi-Point Flux Approximation (MPFA) formu... more This work analyzes some mathematical aspects of a new Multi-Point Flux Approximation (MPFA) formulation for flow problems. This MPFA formulation has been developed in [12, 13] for quadrilateral grids and [10] for unstructured grids. Our MPFA formulation displays capabilities for handling flow problems in geologically complex media modelled by spatially varying full permeability tensor. However in this work, we focus our attention on the case of anisotropic homogeneous porous media. In this framework, the proposed MPFA formulation leads to a well-posed discrete problem which is a linear system whose associated matrix is symmetric and positive definite, even if the permeability tensor governing the flow is only positive definite. Following the spirit of the finite element theory, we have introduced the concept of globally continuous and piecewise linear approximate solution. The convergence analysis of this solution is strongly based upon another concept: the weak approximate solution. Stability and convergence results for the weak approximate solution are proven for L 2-and L ∞-norm, and for a discrete energy norm as well. These results permit to prove some error estimates related to the globally continuous and piecewise linear approximate solution.
African Diaspora Journal of Mathematics. New Series, 2016
Our purpose in this paper is to present a theoretical analysis of the Discrete Duality Finite Vol... more Our purpose in this paper is to present a theoretical analysis of the Discrete Duality Finite Volume method (DDFV method) for 2D-flow problems in anisotropic heterogeneous porous media with full Neumann boundary conditions. We start with the derivation of the discrete problem, and then we give a result of existence and uniqueness of a solution for that problem. Their theoretical properties, namely stability and error estimates in discrete energy norms and L 2-norm are investigated. Numerical tests are provided.
L'Ecran marocain, Sep 26, 2006
ABSTRACT Nous présentons dans ce papier un concept de solution volumes finis continue pour des pr... more ABSTRACT Nous présentons dans ce papier un concept de solution volumes finis continue pour des problèmes de diffusion-convection avec des données régulières. Nous comparons dans certains cas particuliers la solution proposée avec la solution volumes finis classiques (qui est une fonction constante par morceaux) et la solution exacte. La norme L2 de l'erreur est déduite des simulations numériques. Journal des Sciences Pour l'Ingénieur Vol. 6, 2006: 56-64
Revue de l'Institut Français du Pétrole, 1994
Le cadre de ce travail est le calcul des parametres petrophysiques effectifs d'un milieu pore... more Le cadre de ce travail est le calcul des parametres petrophysiques effectifs d'un milieu poreux heterogene pour le simulateur de reservoirs petroliers. Apres le choix d'un modele d'ecoulement dans un milieu poreux heterogene comportant une microstructure periodique nous rappelons brievement les grandes etapes de la methode des echelles multiples pour l'homogeneisation de ce modele. Cela nous conduit a la formule classique d'homogeneisation de la permeabilite absolue. Par la suite nous presentons une demarche originale permettant de passer de cette formule classique a une formule plus simple (d'un point de vue numerique) s'exprimant en termes d'energie dissipee par les forces de viscosite locales et caracterisant le milieu heterogene periodique considere. Nous demontrons ensuite, sous certaines hypotheses, l'egalite entre les energies dissipees par les forces de viscosite associees respectivement a l'ecoulement local et a l'ecoulement macroscopique. Nous terminons par la presentation de quelques resultats numeriques concernant des modeles d'ecoulement diphasique incompressible.
HAL (Le Centre pour la Communication Scientifique Directe), Nov 20, 2022
The finite volume methods are well known as powerful tools to address system of conservation equa... more The finite volume methods are well known as powerful tools to address system of conservation equations. Over the past two decades a lot of effort has been made to put in place a mathematical framework for the theoretical analysis of Finite Volume Methods for second order differential operators. A perfect illustration of this progress is the development of Discrete Duality Finite Volume Methods (DDFV, for short). Following that trend we expose in this work a new approach of Finite Volume Schemes for second order elliptic problems in one dimension space, involving discontinuities in the diffusion coefficients. That new approach is based up on a system of two grids: a primary grid for having a control on localization of discontinuity points of the diffusion coefficient and a control-volume grid (which is not the dual of the primary grid) defined in such a way to get a second order approximation of the fluxes. The algebraic structure of the discrete problem we have got shows that this new approach is not a 1-D version of the DDFV even that both of them are cell-centered and vertex-centered. The proposed scheme leads to a symmetric, positive definite algebraic system and its solution meets the maximum principle. We have shown the second order convergence of the proposed scheme for pure diffusion problems on any primary grid (respecting quasi-uniformity of grid blocs). The second order convergence still holds for diffusion-reaction problems if the primary mesh elements are uniform, even if the diffusion coefficient gets discontinuity points. The ongoing work on the extension of the proposed method to two-dimensional problems is promising in terms of avoiding the computation of the equivalent diffusion coefficient to allocate to the diamond mesh elements as required from application of DDFV.
Open Journal of Marine Science, 2023
This study proposed the newly-designed Pelagic and demersal trawls for the
fishing vessels opera... more This study proposed the newly-designed Pelagic and demersal trawls for the
fishing vessels operating in Cameroonian waters in pelagic and demersal
fishing grounds. The engineering performances of both trawls were investigated using physical modelling method and analytical method based on the
predicted equations. In a flume tank, a series of physical model tests based on
Tauti’s law were performed to investigate the hydrodynamic and geometrical
performances of both trawls and to assess the applicability of the analytical
methods based on predicted equations. The results showed that in model
scale, the working towing speed and door spread for the pelagic trawl were
3.5 knots and 1.85 m, respectively, and for the bottom trawl net they were 4.0
knots and 1.8 m. At that speed and door spread, the drag force, net opening
height, and wing-end spread of the pelagic model trawl were 36.73 N, 0.89 m,
and 0.86 m, respectively, and the swept area was 0.76 m2. Bottom trawl speed
and door spread were 30.43 N, 0.38 m, and 0.45 m, respectively, and the
swept area was 0.25 m2. The maximum difference between the experimental
and analytical results of hydrodynamic performances was less than 56.22%
and 41.45%, respectively, for pelagic and bottom trawls, the results of the
geometrical performances obtained using predicted equations were close to
the experimental results in the flume tank with a maximum relative error less
than 12.85%. The newly developed pelagic and bottom trawls had advanced
engineering performance for high catch efficiency and selectivity and could
be used in commercial fishing operations in Cameroonian waters.
Keywords:
Cameroonian Waters, Pelagic Trawl, Bottom Trawl, Engineering
Performances, Physical Model Test, Analytical Methods Formatting.
Preprint, 2023
From mathematical sciences point of view, the Maximum Principle is a physically important propert... more From mathematical sciences point of view, the Maximum Principle is a physically important property met by solutions of elliptic partial differential equations (PDE for short) of second order governing most diffusion-reaction phenomena. The respect of this property (named also Positivity-Preserving Property in the literature) at the discrete level is required for any numerical scheme designed for solving such PDE. By means of algebraic arguments it is usually proven that conventional finite volume schemes for second order elliptic PDE meet the discrete maximum principle. In this communication we expose a new technique based upon some geometric arguments for proving that conventional finite volume schemes for diffusion-reaction problems meet the discrete version of Maximum Principle.
Preprint, 2023
Several sediment transport models coupled with shallow water equations have been developed over d... more Several sediment transport models coupled with shallow water equations have been developed over decades in the literature. The free surface water flow in presence of abrupt bottom slopes gives rise to turbulence. Classical Shallow Water equations do not involve neither turbulence effects nor the phase-lag between the water velocity and the sediment velocity. Considering a one-dimensional free surface turbulent water flow with sediment transport we derive a new one-dimensional sediment transport mathematical model. The main novelties are that : (i) The proposed new sediment transport model takes into account the vertical shear (i.e. the distortion of horizontal profile velocity in the vertical direction effects), (ii) In the new model there is a differentiation between the fluid velocity and the sediment velocity. The mathematical model derived is a system of nonlinear hyperbolic partial differential equations approximated by a stable and robust finite volume scheme in path-conservative framework. Some numerical tests are performed to show the efficiency of the new mathematical model.
HAL (Le Centre pour la Communication Scientifique Directe), Jul 20, 2022
This paper propose an novel efficient and robust finite volume method based on unstructured meshe... more This paper propose an novel efficient and robust finite volume method based on unstructured meshes to reproduce the entire sediment processes in coastal or estuaries environment. The primary objective of this paper is to develop a fully coupled hydro-sediment-morphodynamic model integrating multiple physical processes. The proposed model is a non-equilibrium sediment transport equation obtained by coupling Shallow Water/Sediment equations incorporating friction and topography, with sediment concentration equation and Exner-based model. The model is developed to predict sediment-concentration variation and bed evolution in the Shallow Water system. The secondary objective is to develop a high order multi-dimensional Path-Conservative Central-Upwind (PCCU) scheme on unstructured meshes to solve the proposed model. In this paper, we written the multi-dimensional CU scheme in a version path-conservative. We rewritten also 1D and 2D PCCU-AENO scheme on uniform grids. Next, a finite-volume path-conservative based scheme on triangular grids is designed. The proposed method is proven well-balanced preserving-positivity and shockcapturing. We use a polynomial reconstruction procedure to extend the first order scheme from the high order accuracy. We designed here a predictor-corrector method with high order accuracy both in time and in space. The method is verified on a number of numerical tests. The results obtained are encouraging.
HAL (Le Centre pour la Communication Scientifique Directe), Oct 4, 2022
The classical sediment transport models based on shallow water equations (SWE) describes the hydr... more The classical sediment transport models based on shallow water equations (SWE) describes the hydro-morphodynamic process without horizontal velocity shear along the vertical and considers that the fluid velocity is equal to sediment velocity. The classical shear shallow water (SSW) with friction and topography source terms assumes that the fluid density is uniform in all the space. Nevertheless, for the coastal flows with sediment transport we are interested in it seems essential to consider these shear effects, the phase lag effect and the nonhomogenous ness of fluid density. In this paper, we develop new sediment transport models incorporating the shear velocity along the vertical, the phase lag effect and the spatial variation of the fluid density. The starting point is the 2D equations for the evolution of mixing quantities and sediment volume rate. These equations describe the motion of fluid mixing in a domain bounded by a dynamical water surface and water bed. Contrary to the classical sediment transport models, the second-order vertical fluctuations of the horizontal velocity are considered. Considering the kinematic conditions on the moving surfaces, we apply an average along the depth on the three-dimensional equations to obtain simplified equations. The resulting model has a wider range of validity and integrates the morphodynamic processes proposed in the literature. The proposed mathematical derivation is in the context of recent developments with the additional presence of sediment and a dynamic bed.
WSEAS TRANSACTIONS ON FLUID MECHANICS
The classical Exner model coupled with a bed-load sediment flux formula is widely used to describ... more The classical Exner model coupled with a bed-load sediment flux formula is widely used to describe the morphodynamics of coastal environments. However, the main drawbacks of this model are (i) Lack of robustness, (ii) Lack of differentiation between sediment and fluid velocities, and (iii) Generation of instabilities when the interactions between sediment and fluid flow become more important. Moreover, Exner's model does not allow us to know with which characteristic velocity the bottom is moving. This set of drawbacks weakens the effectiveness of most sediment transport models proposed in the literature, particularly the Exner model. In this work, we reformulate the bed-load equation and we propose a new averaged sediment transport model for application in coastal or estuarine environments. The proposed model incorporates phase shift effects into the bed-load equation. The bedform's characteristic velocity, sediment, and fluid velocity are differentiated. We developed a new...
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Papers by Abdou Njifenjou
of the slab of the balconies, the cracking of the walls and the collapse of the buildings worn by the humidity. These damages are generally caused by infitration and capillary rise. In addition, it has been demonstrated that people living in damp buildings are at risk of illnesses such as asthma and
lung infections. Therefore, the novelty of this work is threefold: (i) it proposes for the very first time a numerical study of the transport of humidity and heat through the porous walls of buildings constructed with concrete material, the main construction material in the city of Douala; (ii) It was determined what level of indoor thermal comfort was appropriate for sleeping inside a real three-dimensional G+1 complex residential building constructed with concrete blocks; and (iii) Using the geographical coordinates, and time data, the sun radiation’s direction of incidence was assessed throughout the simulation. The computation was performed using Comsol Multiphysics 6.0 software. The distributions of temperature, relative humidity as well as moisture level were presented at various periods. It appears from the results that face to this high humidity, the concrete material retains a large quantity of water for a considerable periods of time, which weakens the steel reinforcement of concrete which is corroded by rust. The computation of thermal comfort in the 3D building showed that the various rooms of the building were not comfortable during the night since temperature inside the building increased progressively due to diffusion of heat. In addition, the numerical solutions indicated that the energy stored within the walls diffused from the external walls to the internal walls during the night. It was also demonstrated that the walls of the building were warmer than the windows, doors and
the roof at the computational times, which simply revealed a greater storage capacity of heat in the concrete blocks material. The fidings highlighted that the temperature decreased rapidly in a thickness of 0.06 m of the concrete block during the nine days and this decrease was attenuated in the second part of the thickness of the concrete block (0.14 m).
fishing vessels operating in Cameroonian waters in pelagic and demersal
fishing grounds. The engineering performances of both trawls were investigated using physical modelling method and analytical method based on the
predicted equations. In a flume tank, a series of physical model tests based on
Tauti’s law were performed to investigate the hydrodynamic and geometrical
performances of both trawls and to assess the applicability of the analytical
methods based on predicted equations. The results showed that in model
scale, the working towing speed and door spread for the pelagic trawl were
3.5 knots and 1.85 m, respectively, and for the bottom trawl net they were 4.0
knots and 1.8 m. At that speed and door spread, the drag force, net opening
height, and wing-end spread of the pelagic model trawl were 36.73 N, 0.89 m,
and 0.86 m, respectively, and the swept area was 0.76 m2. Bottom trawl speed
and door spread were 30.43 N, 0.38 m, and 0.45 m, respectively, and the
swept area was 0.25 m2. The maximum difference between the experimental
and analytical results of hydrodynamic performances was less than 56.22%
and 41.45%, respectively, for pelagic and bottom trawls, the results of the
geometrical performances obtained using predicted equations were close to
the experimental results in the flume tank with a maximum relative error less
than 12.85%. The newly developed pelagic and bottom trawls had advanced
engineering performance for high catch efficiency and selectivity and could
be used in commercial fishing operations in Cameroonian waters.
Keywords:
Cameroonian Waters, Pelagic Trawl, Bottom Trawl, Engineering
Performances, Physical Model Test, Analytical Methods Formatting.
of the slab of the balconies, the cracking of the walls and the collapse of the buildings worn by the humidity. These damages are generally caused by infitration and capillary rise. In addition, it has been demonstrated that people living in damp buildings are at risk of illnesses such as asthma and
lung infections. Therefore, the novelty of this work is threefold: (i) it proposes for the very first time a numerical study of the transport of humidity and heat through the porous walls of buildings constructed with concrete material, the main construction material in the city of Douala; (ii) It was determined what level of indoor thermal comfort was appropriate for sleeping inside a real three-dimensional G+1 complex residential building constructed with concrete blocks; and (iii) Using the geographical coordinates, and time data, the sun radiation’s direction of incidence was assessed throughout the simulation. The computation was performed using Comsol Multiphysics 6.0 software. The distributions of temperature, relative humidity as well as moisture level were presented at various periods. It appears from the results that face to this high humidity, the concrete material retains a large quantity of water for a considerable periods of time, which weakens the steel reinforcement of concrete which is corroded by rust. The computation of thermal comfort in the 3D building showed that the various rooms of the building were not comfortable during the night since temperature inside the building increased progressively due to diffusion of heat. In addition, the numerical solutions indicated that the energy stored within the walls diffused from the external walls to the internal walls during the night. It was also demonstrated that the walls of the building were warmer than the windows, doors and
the roof at the computational times, which simply revealed a greater storage capacity of heat in the concrete blocks material. The fidings highlighted that the temperature decreased rapidly in a thickness of 0.06 m of the concrete block during the nine days and this decrease was attenuated in the second part of the thickness of the concrete block (0.14 m).
fishing vessels operating in Cameroonian waters in pelagic and demersal
fishing grounds. The engineering performances of both trawls were investigated using physical modelling method and analytical method based on the
predicted equations. In a flume tank, a series of physical model tests based on
Tauti’s law were performed to investigate the hydrodynamic and geometrical
performances of both trawls and to assess the applicability of the analytical
methods based on predicted equations. The results showed that in model
scale, the working towing speed and door spread for the pelagic trawl were
3.5 knots and 1.85 m, respectively, and for the bottom trawl net they were 4.0
knots and 1.8 m. At that speed and door spread, the drag force, net opening
height, and wing-end spread of the pelagic model trawl were 36.73 N, 0.89 m,
and 0.86 m, respectively, and the swept area was 0.76 m2. Bottom trawl speed
and door spread were 30.43 N, 0.38 m, and 0.45 m, respectively, and the
swept area was 0.25 m2. The maximum difference between the experimental
and analytical results of hydrodynamic performances was less than 56.22%
and 41.45%, respectively, for pelagic and bottom trawls, the results of the
geometrical performances obtained using predicted equations were close to
the experimental results in the flume tank with a maximum relative error less
than 12.85%. The newly developed pelagic and bottom trawls had advanced
engineering performance for high catch efficiency and selectivity and could
be used in commercial fishing operations in Cameroonian waters.
Keywords:
Cameroonian Waters, Pelagic Trawl, Bottom Trawl, Engineering
Performances, Physical Model Test, Analytical Methods Formatting.