A. Lotfi

22835838600

Publications - 21

Numerical Simulation of Coupled Electromagnetic and Thermal Problems in Permanent Magnet Synchronous Machines

A. Lotfi Dániel Marcsa V. Chabannes C. Prudhomme Zoltán Horváth

Publication Name: Lecture Notes in Computational Science and Engineering

Publication Date: 2021-01-01

Volume: 139

Issue: Unknown

Page Range: 693-701

Description:

The main objective of our task is to develop mathematical models, numerical techniques to analyse the thermal effects in electric machines, to implement the developed algorithm in multiprocessor or multi-core environments and to apply them to industrial use cases. In this study, we take into account coupled character of the electromagnetic and thermal features of the physical process. Both thermal and electromagnetic processes are considered transient, solved by means of the FEM method on independent meshes and the time-discretization is realized using time operator splitting. Two examples are presented to assess the accuracy of the developed coupled solvers and the numerical results are compared with the experimental ones, which are obtained from a prototype machine.

Open Access: Yes

DOI: 10.1007/978-3-030-55874-1_68

Numerical solver for analyzing the thermal behaviour of permanent magnet synchronous machines

A. Lotfi

Publication Name: Aip Conference Proceedings

Publication Date: 2019-07-24

Volume: 2116

Issue: Unknown

Page Range: Unknown

Description:

This study investigates a 3D numerical analysis of the thermal behavior of a permanent magnet (PM) motor, performed by Finite Element Method (FEM) by using open source Feel++ software. In the developed FEM model, the windings and the stator are treated as homogeneous medium with equivalent thermal parameters and the effective properties to characterize the thermal behaviour are calculated based on the volume-weighted average over all constituents. In order to check the accuracy of the computed temperatures, experimental tests were performed in the transient-state. The thermal analyses based on the numerical method are compared with the measured results taken from the prototype machine.

Open Access: Yes

DOI: 10.1063/1.5114557

FEEL++ applications to engineering problems

A. Lotfi

Publication Name: Aip Conference Proceedings

Publication Date: 2018-07-10

Volume: 1978

Issue: Unknown

Page Range: Unknown

Description:

Feel++ is a unified C++ implementation of Galerkin methods (finite and spectral element methods) in 1D, 2D and 3D to solve partial differential equations and provide a set of complementary modeling tools that can be applied to a large range of monophysics or multiphysics applications in fluid mechanics, solid mechanics, heat transfer, blood rheology, eye modeling and simulation and electro-magnetism. The software follows closely the mathematical abstractions associated with partial differential equations (PDE) and in particular the finite element mathematical framework and variational formulations. The Feel++ library offers solving strategies that scales up to thousands and even tens of thousands of cores and allows the developers to create C++ complex and typically non-linear multi-physics applications currently in industry, physics and health-care with powerful tools such as BOOST, PETSC or SLEPC which enable a huge backend for huge memory needs or enormous mathematical calculations.

Open Access: Yes

DOI: 10.1063/1.5044138

Solution procedure of dynamical contact problems with friction

A. Lotfi

Publication Name: Aip Conference Proceedings

Publication Date: 2017-07-21

Volume: 1863

Issue: Unknown

Page Range: Unknown

Description:

Dynamical contact is one of the common research topics because of its wide applications in the engineering field. The main goal of this work is to develop a time-stepping algorithm for dynamic contact problems. We propose a finite element approach for elastodynamics contact problems [1]. Sticking, sliding and frictional contact can be taken into account. Lagrange multipliers are used to enforce non-penetration condition. For the time discretization, we propose a scheme equivalent to the explicit Newmark scheme. Each time step requires solving a nonlinear problem similar to a static friction problem. The nonlinearity of the system of equation needs an iterative solution procedure based on Uzawa's algorithm [2][3]. The applicability of the algorithm is illustrated by selected sample numerical solutions to static and dynamic contact problems. Results obtained with the model have been compared and verified with results from an independent numerical method.

Open Access: Yes

DOI: 10.1063/1.4992771

Simulation of an active cooling system for photovoltaic modules

A. Lotfi

Publication Name: Aip Conference Proceedings

Publication Date: 2016-06-08

Volume: 1738

Issue: Unknown

Page Range: Unknown

Description:

Photovoltaic cells are devices that convert solar radiation directly into electricity. However, solar radiation increases the photovoltaic cells temperature [1] [2]. The temperature has an influence on the degradation of the cell efficiency and the lifetime of a PV cell. This work reports on a water cooling technique for photovoltaic panel, whereby the cooling system was placed at the front surface of the cells to dissipate excess heat away and to block unwanted radiation. By using water as a cooling medium for the photovoltaic solar cells, the overheating of closed panel is greatly reduced without prejudicing luminosity. The water also acts as a filter to remove a portion of solar spectrum in the infrared band but allows transmission of the visible spectrum most useful for the PV operation. To improve the cooling system efficiency and electrical efficiency, uniform flow rate among the cooling system is required to ensure uniform distribution of the operating temperature of the PV cells. The aims of this study are to develop a 3D thermal model to simulate the cooling and heat transfer in Photovoltaic panel and to recommend a cooling technique for the PV panel. The velocity, pressure and temperature distribution of the three-dimensional flow across the cooling block were determined using the commercial package, Fluent. The second objective of this work is to study the influence of the geometrical dimensions of the panel, water mass flow rate and water inlet temperature on the flow distribution and the solar panel temperature. The results obtained by the model are compared with experimental results from testing the prototype of the cooling device.

Open Access: Yes

DOI: 10.1063/1.4952279

Numerical investigation of heat transfer in air cooled permanent magnet electrical machines

A. Lotfi

Publication Name: Aip Conference Proceedings

Publication Date: 2015-03-10

Volume: 1648

Issue: Unknown

Page Range: Unknown

Description:

The aim of this study is to present a magneto-thermal analysis of an external-rotor permanent magnet synchronous machine based on finite element method. The developed model can be used to predict temperature distribution inside the studied motor during the rated operation. Electromagnetic computation is carried out with the aid of two 2D finite-element (FE) simulations on the cross-section of the PM motor. In addition, the magnetic core losses of the stator and rotor are modelled based on the results from the electromagnetic analysis and a post-processing formula based on the loss-separation principle. After the loss calculation, the temperatures of the machine are calculated by using 3D finite element method. The results obtained by the model are compared with experimental results from testing the prototype electric motor.

Open Access: Yes

DOI: 10.1063/1.4913164

Multi-physics thermal analysis of permanent magnets motors with exterior rotor

A. Lotfi

Publication Name: Civil Comp Proceedings

Publication Date: 2015-01-01

Volume: Unknown

Issue: Unknown

Page Range: Unknown

Description:

This paper presents a magneto-thermal analysis of an external-rotor permanent magnet synchronous machine based on finite element method. The model developed can be used to predict the temperature distribution inside the motor during the rated operation. Electromagnetic computation is carried out with the aid of two twodimensional finite-element simulations of the cross-section of the permanent magnet motor [1]. In addition, the magnetic core losses of the stator and rotor are modelled based on the results from the electromagnetic analysis and a post-processing formula based on the loss-separation principle. To analyse the process of heat transfer in an electrical machine, empirical correlations are used to describe the convective heat transfer from the different surfaces of the permanent magnet motor. The heat transfer coefficients are determined using dimensionless numbers and the Nusselt number [2]. After the loss calculation, the temperatures of the machine are calculated using a three-dimensional finite element method. The results obtained from the model are compared with the experimental results from testing the prototype electric motor.

Open Access: Yes

DOI: DOI not available

Multi-Physics thermal analysis of permanent magnets motors with exterior rotor

A. Lotfi

Publication Name: Civil Comp Proceedings

Publication Date: 2015-01-01

Volume: 108

Issue: Unknown

Page Range: Unknown

Description:

This paper presents a magneto-thermal analysis of an external-rotor permanent magnet synchronous machine based on finite element method. The model developed can be used to predict the temperature distribution inside the motor during the rated operation. Electromagnetic computation is carried out with the aid of two twodimensional finite-element simulations of the cross-section of the permanent magnet motor [1]. In addition, the magnetic core losses of the stator and rotor are modelled based on the results from the electromagnetic analysis and a post-processing formula based on the loss-separation principle. To analyse the process of heat transfer in an electrical machine, empirical correlations are used to describe the convective heat transfer from the different surfaces of the permanent magnet motor. The heat transfer coefficients are determined using dimensionless numbers and the Nusselt number [2]. After the loss calculation, the temperatures of the machine are calculated using a three-dimensional finite element method. The results obtained from the model are compared with the experimental results from testing the prototype electric motor.

Open Access: Yes

DOI: DOI not available

A numerical method for creating and reconstructing computer-generated holograms

A. Lotfi

Publication Name: Aip Conference Proceedings

Publication Date: 2012-12-01

Volume: 1479

Issue: 1

Page Range: 2182-2186

Description:

The main purpose of this article is to develop a program package which generates artificial holograms by numerical methods and these computer generated holograms are numerically reconstructed. For the calculation of light propagation needed in the generation of holograms or in the reconstruction of the object from the hologram, we use two strategies. The first is the Fourier-based algorithm where the diffraction integral is approximated as a convolution integral, allowing computation using the fast Fourier transform algorithm. The second uses finite difference discretization to solve the parabolic wave equation. Numerical tests that assess the accuracy of these algorithms are presented. © 2012 American Institute of Physics.

Open Access: Yes

DOI: 10.1063/1.4756625

Domain-decomposition based H1/2 seminorm preconditioners for frictional contact problems

A. Lotfi

Publication Name: Civil Comp Proceedings

Publication Date: 2012-01-01

Volume: 100

Issue: Unknown

Page Range: Unknown

Description:

The bilateral or unilateral contact problem with Coulomb friction between two elastic bodies is considered [1, 12]. An algorithm is introduced to solve the resulting finite element system using a non-overlapping domain decomposition method. This technique enables the transformation of the solution of the global problem to the solution of the elasticity equations for each body separately and the solution of a smaller problem for the contact surface. The solution is obtained by using a successive approximation method, in each step of this algorithm two intermediate problems are solved, the first with prescribed tangential pressure, and the second with prescribed normal pressure[11]. © Civil-Comp Press, 2012.

Open Access: Yes

DOI: DOI not available

Numerical methods for digitally synthetic holograms

A. Lotfi

Publication Name: Civil Comp Proceedings

Publication Date: 2012-01-01

Volume: 100

Issue: Unknown

Page Range: Unknown

Description:

The main purpose of this paper is to develop a program package which generates artificial holograms by numerical methods and these computer generated holograms are numerically reconstructed. For the calculation of light propagation needed in the generation of holograms or in the reconstruction of the object from the hologram, two strategies are used. The first is the Fourier-based algorithm where the diffraction integral is approximated as a convolution integral, allowing computation using the fast Fourier transform algorithm. The second uses finite difference discretization to solve the parabolic wave equation. Numerical tests that assess the accuracy of these algorithms are presented. © Civil-Comp Press, 2012.

Open Access: Yes

DOI: DOI not available

Displacement measurement by digital holographic interferometry

A. Lotfi

Publication Name: Proceedings of the 13th International Conference on Civil Structural and Environmental Engineering Computing

Publication Date: 2011-12-01

Volume: Unknown

Issue: Unknown

Page Range: Unknown

Description:

Digital holographic interferometry is a useful technique for measuring the deformation or displacement of the surface of an object by recording at least two speckle patterns, one before and one after the object is deformed. Holographic interferometry utilizes interference between the speckled image of an object illuminated by a laser and a reference beam derived from the same laser. Any change in the shape of the object results in local changes in the intensity distribution in the holographic fringe pattern. The holograms are generated directly on a charge coupled device (CCD) target and stored electronically. The speckle patterns can be taken quickly and analyzed using holographic interferometry and the Fourier transform method. © Civil-Comp Press, 2011.

Open Access: Yes

DOI: DOI not available

Optimal design of extrusion dies in metal forming using the finite element method

A. Lotfi

Publication Name: Proceedings of the 13th International Conference on Civil Structural and Environmental Engineering Computing

Publication Date: 2011-12-01

Volume: Unknown

Issue: Unknown

Page Range: Unknown

Description:

In this paper, a method for calculation of the optimal shapes of axisymmetrical converging dies using the finite element method is presented. The shape optimization problem considered in this paper is to find the best shape of the die such that the flow rate will be uniform at the die exit. The optimization problem is to minimize an objective function by varying a part of boundary (ie: the shape of die) subject to constraints imposed by the metal forming problem. In this method, B-spline functions allow us to determine the shape of the die, using its control points as design variables. © Civil-Comp Press, 2011.

Open Access: Yes

DOI: DOI not available

Efficient numerical models for electro-mechanical analysis

A. Lotfi

Publication Name: Aip Conference Proceedings

Publication Date: 2011-11-28

Volume: 1389

Issue: Unknown

Page Range: 1996-1999

Description:

In this paper, we present a numerical procedure that can be used to model the electro-mechanical coupled behavior of the dielectric actuator domain. The equation describing the electrostatical part is given by the reduced form of the Maxwell equation and the electrostatic potential [1]. The mechanical problem is described by the constitutive equations and equilibrium equations. Using the finite element method, this technique is to divide a whole problem into sub-problems. The complexity of the original problem is therefore reduced by focusing only on areas most relevant. A finite element analysis is then performed by applying the electrostatic Maxwell pressure as Neumann boundary conditions to compute the displacements. Once the displacement is computed, the electrostatic domain or the conductor is updated. Electrostatic analysis is performed on the updated geometry and the finite element method is then used to determine the change in potential due to geometric perturbations. Once the surface charge densities are known, the new electrostatic Maxwell pressure is computed. The mechanical and electrostatic analysis is repeated until an equilibrium state is computed. The procedure is demonstrated in the paper by the solution of some two-dimensional and three-dimensional problems. © 2011 American Institute of Physics.

Open Access: Yes

DOI: 10.1063/1.3637006

Displacement determination by digital holographic interferometry

A. Lotfi

Publication Name: Aip Conference Proceedings

Publication Date: 2010-12-01

Volume: 1281

Issue: Unknown

Page Range: 2089-2094

Description:

Digital Holographic Interferometry is a useful technique for measuring the deformation or displacement of the surface of an object by recording at least two speckle patterns, one before and one after the object is deformed. Holographic interferometry utilizes interference between the speckled image of an object illuminated by a laser and a reference beam derived from the same laser. Any change in the shape of the object results in local changes in the intensity distribution in the holographic fringe pattern. This optical technique allows detecting deformation with sensitivity smaller than the wavelength of light. The holograms are generated directly on a charge coupled device (CCD) target and stored electronically. The speckle patterns can be taken quickly analyzed using Holographic interferometry and Fourier transform method. © 2010 American Institute of Physics.

Open Access: Yes

DOI: 10.1063/1.3498364

Optimal die design in extrusion process using adaptive finite element method

A. Lotfi G. Molnárka

Publication Name: Aip Conference Proceedings

Publication Date: 2009-11-26

Volume: 1168

Issue: Unknown

Page Range: 324-328

Description:

In this work, a method for calculation of the optimal shapes of axisymmetrical converging dies by an adaptive finite element method is presented. The shape optimization problem considered in this paper is to find the best shape of the die such that the flow rate will be uniform at the die exit.The optimization problem is to minimize an objective function by varying a part of boundary (ie: the shape of die) subject to constraints imposed by the metal forming problem. In this method, the B-spline functions allow us to determine the shape of the die, using its control points as design variables. An adaptive solution procedure is adapted to control the error due to the finite element approximation. The mesh adaptation is performed using the Zienkiewicz - Zhu (Z2) type error estimator. © 2009 American Institute of Physics.

Open Access: Yes

DOI: 10.1063/1.3241460

Half-magnitude extensions of resolution and field of view in digital holography by scanning and magnification

A. Lotfi Attila Tibor Nagy István Harmati Béla Ráczkevi György Molnar Dezsö Szigethy Attila Nagy G. Molnárka Venczel Borbély Ferenc Gyímesi Aladar Czitrovszky Zoltán Füzessy

Publication Name: Applied Optics

Publication Date: 2009-11-01

Volume: 48

Issue: 31

Page Range: 6026-6034

Description:

Digital holography replaces the permanent recording material of analog holography with an electronic light sensitive matrix detector, but besides the many unique advantages, this brings serious limitations with it as well. The limited resolution of matrix detectors restricts the field of view, and their limited size restricts the resolution in the reconstructed holographic image. Scanning the larger aerial hologram (the interference light field of the object and reference waves in the hologram plane) with the small matrix detector or using magnification for the coarse matrix detector at the readout of the fine-structured aerial hologram, these are straightforward solutions but have been exploited only partially until now. We have systematically applied both of these approaches and have driven them to their present extremes, over half a magnitude in extensions. © 2009 Optical Society of America.

Open Access: Yes

DOI: 10.1364/AO.48.006026

Solving contact problems using the domain decomposition method with an interface preconditioner

A. Lotfi G. Molnárka

Publication Name: Proceedings of the 6th International Conference on Engineering Computational Technology

Publication Date: 2008-12-01

Volume: Unknown

Issue: Unknown

Page Range: Unknown

Description:

The present paper is concerned with the frictionless bilateral or unilateral contact problem between two elastic bodies. An algorithm is introduced to solve the resulting finite element system by a non-overlapping domain decomposition method. This technique enable us to transform the solution of the global problem to the solutions of the elasticity equations for each body separately and the solution of the Schur complement problem on the contact surface. The main goal of this work is the construction of the interface preconditioner for the Schur complement problem. The solution is obtained by using a successive approximation method. Finally, some numerical results of the proposed method are given. © 2008 Civil-Comp Press.

Open Access: Yes

DOI: DOI not available

Domain decomposition method with an interface preconditioner for frictionless contact problem

A. Lotfi

Publication Name: Aip Conference Proceedings

Publication Date: 2008-10-22

Volume: 1048

Issue: Unknown

Page Range: 359-363

Description:

The present paper is concerned with the frictionless bilateral or unilateral contact problem between two elastic bodies. An algorithm is introduced to solve the resulting finite element system by a non-overlapping domain decomposition method. This technique enable us to transform the solution of the global problem to the solutions of the elasticity equations for each body separately and the solution of the Schur complement problem on the contact surface. The main goal of this work is the construction of the interface preconditioner for the Schur complement problem. The solution is obtained by using a successive approximation method. Finally, some numerical results of the proposed method are given. © 2008 American Institute of Physics.

Open Access: Yes

DOI: 10.1063/1.2990933

Numerical method for computer generated hologram

A. Lotfi

Publication Name: Aip Conference Proceedings

Publication Date: 2007-10-23

Volume: 936

Issue: Unknown

Page Range: 351-355

Description:

The main purpose of this article is to develop a program package which generates artificial holograms by numerical methods and these computer generated holograms are numerically reconstructed. For the calculation of light propagation needed in the generation of holograms or in the reconstruction of the object from the hologram, we use two strategies, the first is the Fourier-based algorithm and the second uses finite difference discretization, and numerical tests that assess the accuracy of these algorithms are presented. © 2007 American Institute of Physics.

Open Access: Yes

DOI: 10.1063/1.2790149

The method of asymptotic expansion for plate problem in the linear theory of viscoelasticity

A. Lotfi G. Molnárka

Publication Name: ZAMM Zeitschrift Fur Angewandte Mathematik Und Mechanik

Publication Date: 2000-01-01

Volume: 80

Issue: 4 SUPPL. 2

Page Range: S391-S392

Description:

The method of asymptotic expansion originally developed for elasticity problems is generalized to linear viscoelastic problems. Three-dimensional, unsteady problems corresponding to a class of viscoelastic plates are considered. By employing the Laplace transform, the original problem is converted to an equivalent elastic problem, where the asymptotic expansion method is used. An application of this method to the Maxwell model is also presented.

Open Access: Yes

DOI: 10.1002/zamm.20000801467