The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform

Farhang Daneshmand - One of the best experts on this subject based on the ideXlab platform.

  • inverse geometry heat conduction analysis of functionally graded materials using smoothed Fixed Grid finite elements
    Inverse Problems in Science and Engineering, 2013
    Co-Authors: Mohammad Javad Kazemzadehparsi, Farhang Daneshmand
    Abstract:

    A shape identification scheme is developed in this article to determine the shape of the inaccessible parts of a two-dimensional object made of functionally graded material using the measured temperatures on its accessible parts. The proposed method is based on minimization of the differences between measured and calculated temperatures. The smoothed Fixed Grid finite element method which is a new approach based on the non-boundary-fitted meshes and the gradient smoothing technique are used for the solution of direct problem and shape sensitivity analysis. The boundary parameterization technique using splines is also adopted to manipulate the shape variations. The conjugate-gradient method is used as the optimization algorithm and some numerical examples are solved to evaluate the applicability of the proposed method in the context of functionally graded materials.

  • three dimensional smoothed Fixed Grid finite element method for the solution of unconfined seepage problems
    Finite Elements in Analysis and Design, 2013
    Co-Authors: Mohammad Javad Kazemzadehparsi, Farhang Daneshmand
    Abstract:

    A three dimensional numerical analysis for unconfined seepage problems in inhomogeneous and anisotropic domains with arbitrary geometry is presented in this paper. The unconfined seepage problems are nonlinear in its nature due to unknown location of the phreatic surface and nonlinear boundary conditions which complicates its solution. The presented method is based on the application of non-boundary-fitted meshes and is an extension of the recently proposed two dimensional smoothed Fixed Grid finite element method. The main objective of using this method is to facilitate solution of variable domain problems and improve the accuracy of the formulation of the boundary intersecting elements. In this method, the gradient smoothing technique is used to obtain the element matrices. This technique simplifies the solution significantly by reducing the volume integrals over the elements into area integrals on the faces of smoothing cells. To locate the free surface, an initial guess for the unknown geometry is selected and modified in each iteration to eventually satisfy nonlinear boundary condition. The application of the proposed technique for three dimensional seepage problems is carried out for different examples including rectangular, trapezoidal and semi-cylindrical dams and the results are compared with those available in the literature.

  • unconfined seepage analysis in earth dams using smoothed Fixed Grid finite element method
    International Journal for Numerical and Analytical Methods in Geomechanics, 2012
    Co-Authors: Mohammad Javad Kazemzadehparsi, Farhang Daneshmand
    Abstract:

    SUMMARY One major difficulty in seepage analyses is finding the position of phreatic surface which is unknown at the beginning of solution and must be determined in an iterative process. The objective of the present study is to develop a novel non-boundary-fitted mesh finite-element method capable of solving the unconfined seepage problem in domains with arbitrary geometry and continuously varied permeability. A new non-boundary-fitted finite element method named as smoothed Fixed Grid finite element method (SFGFEM) is used to simplify the solution of variable domain problem of unconfined seepage. The gradient smoothing technique, in which the area integrals are transformed into the line integrals around edges of smoothing cells, is used to obtain the element matrices. The solution process starts with an initial guess for the unknown boundary and SFGFEM is used to approximate the field variable. The boundary shape is then modified to eventually satisfy nonlinear boundary condition in an iterative process. Some numerical examples are solved to evaluate the applicability of the proposed method and the results are compared with those available in the literature. Copyright © 2011 John Wiley & Sons, Ltd.

  • solution of geometric inverse heat conduction problems by smoothed Fixed Grid finite element method
    Finite Elements in Analysis and Design, 2009
    Co-Authors: Mohammad Javad Kazemzadehparsi, Farhang Daneshmand
    Abstract:

    This paper presents a methodology for solving the nonlinear inverse geometry heat transfer problems where the observations are temperatures measured at points on the external boundary and the unknowns are the shape and the location of a cavity inside the problem domain. The algorithm is based on the minimization of the squared error between the measured temperatures and those calculated by the proposed smoothed Fixed Grid finite element method where a non-boundary-fitted mesh is used to solve the direct problem. This eliminates mesh adaptation and re-meshing processes as needed in the standard finite element method and reduces the analysis cost significantly. The proposed method uses the advantages of smoothed finite element method and Fixed Grid finite element method in which a Fixed computational mesh is used to solve the variable domain problem and the numerical integration is performed through gradient smoothing technique. The present method is more accurate and facilitates integration over boundary intersecting elements. The shape gradients of the shape identification problem are obtained by direct differentiation method. The domain parameterization technique with cubic splines is adopted to manipulate the shape variation of the cavity. To stabilize the optimization process, a geometric regularization function is used. Some numerical examples are solved to evaluate the method. In these examples effects of initial guess, measurement errors, cavity shape and cavity size on the results are examined. It is shown that the results are in good agreement with exact solutions.

Mohammad Javad Kazemzadehparsi - One of the best experts on this subject based on the ideXlab platform.

  • numerical flow simulation in gated hydraulic structures using smoothed Fixed Grid finite element method
    Applied Mathematics and Computation, 2014
    Co-Authors: Mohammad Javad Kazemzadehparsi
    Abstract:

    Successful design and operation of hydraulic structures require more effective and reliable tools to be used in a variety of practical problems, including bottom outlets, intakes and/or spillways. Until recently, physical modeling has been the principal approach in studying the flow pattern and behavior of such structures. The main concerns might generally be related to estimating of discharge coefficients, frictional losses, details of local flow patterns, position of the free surface, and air entrainment. The application of a new developed numerical modeling for free surface flow simulation in gated tunnels is presented here. Among various parameters, the free surface profile, pressure distribution and discharge of the flow passing through gated hydraulic structures are considered in the present study. The solution of free surface flow problems is usually carried out through an iterative process due to unknown geometry and nonlinear boundary conditions. Therefore, the solution of these problems using traditional numerical methods presents some difficulties since the computational mesh needs to be modified for each iteration. The application of the smoothed Fixed Grid finite element method in the solution of free surface flow problems is investigated in this paper. The main advantage of the proposed method is that it is based on non-boundary-fitted meshes which simplify the solution procedure. In this method, the gradient smoothing technique is used to facilitate formulation of boundary intersecting elements. To evaluate the applicability of the proposed method, three representative examples are solved and the results are compared with experimental and numerical results available in the literature. The results of the present study indicated the power of smoothed Fixed Grid finite element method in solving the free surface potential flows through gated hydraulic structures.

  • inverse geometry heat conduction analysis of functionally graded materials using smoothed Fixed Grid finite elements
    Inverse Problems in Science and Engineering, 2013
    Co-Authors: Mohammad Javad Kazemzadehparsi, Farhang Daneshmand
    Abstract:

    A shape identification scheme is developed in this article to determine the shape of the inaccessible parts of a two-dimensional object made of functionally graded material using the measured temperatures on its accessible parts. The proposed method is based on minimization of the differences between measured and calculated temperatures. The smoothed Fixed Grid finite element method which is a new approach based on the non-boundary-fitted meshes and the gradient smoothing technique are used for the solution of direct problem and shape sensitivity analysis. The boundary parameterization technique using splines is also adopted to manipulate the shape variations. The conjugate-gradient method is used as the optimization algorithm and some numerical examples are solved to evaluate the applicability of the proposed method in the context of functionally graded materials.

  • three dimensional smoothed Fixed Grid finite element method for the solution of unconfined seepage problems
    Finite Elements in Analysis and Design, 2013
    Co-Authors: Mohammad Javad Kazemzadehparsi, Farhang Daneshmand
    Abstract:

    A three dimensional numerical analysis for unconfined seepage problems in inhomogeneous and anisotropic domains with arbitrary geometry is presented in this paper. The unconfined seepage problems are nonlinear in its nature due to unknown location of the phreatic surface and nonlinear boundary conditions which complicates its solution. The presented method is based on the application of non-boundary-fitted meshes and is an extension of the recently proposed two dimensional smoothed Fixed Grid finite element method. The main objective of using this method is to facilitate solution of variable domain problems and improve the accuracy of the formulation of the boundary intersecting elements. In this method, the gradient smoothing technique is used to obtain the element matrices. This technique simplifies the solution significantly by reducing the volume integrals over the elements into area integrals on the faces of smoothing cells. To locate the free surface, an initial guess for the unknown geometry is selected and modified in each iteration to eventually satisfy nonlinear boundary condition. The application of the proposed technique for three dimensional seepage problems is carried out for different examples including rectangular, trapezoidal and semi-cylindrical dams and the results are compared with those available in the literature.

  • unconfined seepage analysis in earth dams using smoothed Fixed Grid finite element method
    International Journal for Numerical and Analytical Methods in Geomechanics, 2012
    Co-Authors: Mohammad Javad Kazemzadehparsi, Farhang Daneshmand
    Abstract:

    SUMMARY One major difficulty in seepage analyses is finding the position of phreatic surface which is unknown at the beginning of solution and must be determined in an iterative process. The objective of the present study is to develop a novel non-boundary-fitted mesh finite-element method capable of solving the unconfined seepage problem in domains with arbitrary geometry and continuously varied permeability. A new non-boundary-fitted finite element method named as smoothed Fixed Grid finite element method (SFGFEM) is used to simplify the solution of variable domain problem of unconfined seepage. The gradient smoothing technique, in which the area integrals are transformed into the line integrals around edges of smoothing cells, is used to obtain the element matrices. The solution process starts with an initial guess for the unknown boundary and SFGFEM is used to approximate the field variable. The boundary shape is then modified to eventually satisfy nonlinear boundary condition in an iterative process. Some numerical examples are solved to evaluate the applicability of the proposed method and the results are compared with those available in the literature. Copyright © 2011 John Wiley & Sons, Ltd.

  • solution of geometric inverse heat conduction problems by smoothed Fixed Grid finite element method
    Finite Elements in Analysis and Design, 2009
    Co-Authors: Mohammad Javad Kazemzadehparsi, Farhang Daneshmand
    Abstract:

    This paper presents a methodology for solving the nonlinear inverse geometry heat transfer problems where the observations are temperatures measured at points on the external boundary and the unknowns are the shape and the location of a cavity inside the problem domain. The algorithm is based on the minimization of the squared error between the measured temperatures and those calculated by the proposed smoothed Fixed Grid finite element method where a non-boundary-fitted mesh is used to solve the direct problem. This eliminates mesh adaptation and re-meshing processes as needed in the standard finite element method and reduces the analysis cost significantly. The proposed method uses the advantages of smoothed finite element method and Fixed Grid finite element method in which a Fixed computational mesh is used to solve the variable domain problem and the numerical integration is performed through gradient smoothing technique. The present method is more accurate and facilitates integration over boundary intersecting elements. The shape gradients of the shape identification problem are obtained by direct differentiation method. The domain parameterization technique with cubic splines is adopted to manipulate the shape variation of the cavity. To stabilize the optimization process, a geometric regularization function is used. Some numerical examples are solved to evaluate the method. In these examples effects of initial guess, measurement errors, cavity shape and cavity size on the results are examined. It is shown that the results are in good agreement with exact solutions.

Emmanouel Varvarigos - One of the best experts on this subject based on the ideXlab platform.

  • minimizing energy and cost in Fixed Grid and flex Grid networks
    IEEE\ OSA Journal of Optical Communications and Networking, 2015
    Co-Authors: P Papanikolaou, Kostas Christodoulopoulos, P Soumplis, Konstantinos Manousakis, G I Papadimitriou, Georgios Ellinas, Emmanouel Varvarigos
    Abstract:

    Core networks offer high capacities, thanks mainly to the optical technologies they utilize, but they consume a non-negligible amount of energy. The traffic volume in metro and core networks is forecast to grow at very high rates, exceeding 30% per year for the next five years, and if the corresponding energy requirements grow analogously, they will sooner rather than later form a bottleneck for network communications. Thus, energy efficiency in optical networks is mandatory for the sustainability of the future Internet. The objectives of the current work are to identify the main causes of energy consumption for current Fixed-Grid wavelength division multiplexing and future flex-Grid optical networks, and to propose and compare techniques for improving their energy efficiency. Toward this end, we carried out a comparative study of energy efficiency of flex-Grid networks and Fixed-Grid single-line-rate and mixed-line-rate networks. Under realistic network scenarios, we calculated the energy consumption of the different components of the optical layer and demonstrated that by using energy-aware techniques in planning such networks, we can achieve significant power savings. Since energy prices are location dependent, especially in large networks, e.g., over continents, we show that accounting for such information can increase the cost savings.

  • Planning flexible optical networks under physical layer constraints
    IEEE OSA Journal of Optical Communications and Networking, 2013
    Co-Authors: Kostas Christodoulopoulos, P Soumplis, Emmanouel Varvarigos
    Abstract:

    We consider the planning problem of a flexible optical network. Given the traffic matrix and the transponders' feasible configurations that account for the physical layer, we formulate the planning problem considering both the use or not of regenerators. Demands are served for their requested rates by choosing the route, selecting the transmission configuration, breaking the transmissions in more than one connection and placing regenerators, if needed, and allocating the spectrum to them. The objective is to serve the traffic and find a solution that is Pareto optimal with respect to the maximum spectrum used and the cost (number and type) of transponders used. The problem definition and the proposed algorithms are general and applicable to flex-Grid as well as Fixed-Grid networks. We start by presenting algorithms based on integer linear programming formulations for transparent and translucent networks (without or with regenerators) and we continue by presenting heuristic algorithms. Using input driven by transmission studies on optical orthogonal frequency-division multiplexing (OFDM)-based networks we evaluate the performance gains that can be obtained by an OFDM over a mixed line rate Fixed-Grid WDM optical network.

  • quantifying spectrum cost and energy efficiency in Fixed Grid and flex Grid networks invited
    IEEE\ OSA Journal of Optical Communications and Networking, 2012
    Co-Authors: Eleni Palkopoulou, Kostas Christodoulopoulos, M Angelou, Dimitrios Klonidis, Axel Klekamp, F Buchali, Emmanouel Varvarigos, Ioannis Tomkos
    Abstract:

    Single and multi-carrier networks offering channel rates up to 400 Gb/s are evaluated under realistic reach parameters. It is found that efficient spectrum utilization and fine bit-rate granularity are essential to achieve cost and energy efficiency. Additionally, the break-even cost of flexible orthogonal frequency division multiplexing transponders is examined under different settings. The break-even cost of a flexible transponder corresponds to the cost value for which the total cost of the network is equal to that of the related single-line-rate network. The impact of the traffic load, the additional cost required for flex-Grid optical cross connects, the cost of spectrum, as well as the cost of Fixed-Grid transponders is examined.

  • spectrum cost and energy efficiency in Fixed Grid and flex Grid networks
    Optical Fiber Communication Conference, 2012
    Co-Authors: M Angelou, Kostas Christodoulopoulos, Dimitrios Klonidis, Axel Klekamp, F Buchali, Emmanouel Varvarigos, Ioannis Tomkos
    Abstract:

    Single and multi-carrier networks offering channel rates up to 400Gb/s are evaluated under realistic reach parameters. It is found that efficient spectrum utilization and fine bit-rate granularity are essential to achieve cost and energy efficiency.

  • spectrally bitrate flexible optical network planning
    European Conference on Optical Communication, 2010
    Co-Authors: Kostas Christodoulopoulos, Ioannis Tomkos, Emmanouel Varvarigos
    Abstract:

    We consider the Routing and Spectrum Allocation (RSA) problem in an OFDM-based optical network with elastic bandwidth allocation. We asses the spectrum utilization gains of this flexible architecture compared to a traditional Fixed-Grid rigid-bandwidth WDM network.

Kostas Christodoulopoulos - One of the best experts on this subject based on the ideXlab platform.

  • minimizing energy and cost in Fixed Grid and flex Grid networks
    IEEE\ OSA Journal of Optical Communications and Networking, 2015
    Co-Authors: P Papanikolaou, Kostas Christodoulopoulos, P Soumplis, Konstantinos Manousakis, G I Papadimitriou, Georgios Ellinas, Emmanouel Varvarigos
    Abstract:

    Core networks offer high capacities, thanks mainly to the optical technologies they utilize, but they consume a non-negligible amount of energy. The traffic volume in metro and core networks is forecast to grow at very high rates, exceeding 30% per year for the next five years, and if the corresponding energy requirements grow analogously, they will sooner rather than later form a bottleneck for network communications. Thus, energy efficiency in optical networks is mandatory for the sustainability of the future Internet. The objectives of the current work are to identify the main causes of energy consumption for current Fixed-Grid wavelength division multiplexing and future flex-Grid optical networks, and to propose and compare techniques for improving their energy efficiency. Toward this end, we carried out a comparative study of energy efficiency of flex-Grid networks and Fixed-Grid single-line-rate and mixed-line-rate networks. Under realistic network scenarios, we calculated the energy consumption of the different components of the optical layer and demonstrated that by using energy-aware techniques in planning such networks, we can achieve significant power savings. Since energy prices are location dependent, especially in large networks, e.g., over continents, we show that accounting for such information can increase the cost savings.

  • Planning flexible optical networks under physical layer constraints
    IEEE OSA Journal of Optical Communications and Networking, 2013
    Co-Authors: Kostas Christodoulopoulos, P Soumplis, Emmanouel Varvarigos
    Abstract:

    We consider the planning problem of a flexible optical network. Given the traffic matrix and the transponders' feasible configurations that account for the physical layer, we formulate the planning problem considering both the use or not of regenerators. Demands are served for their requested rates by choosing the route, selecting the transmission configuration, breaking the transmissions in more than one connection and placing regenerators, if needed, and allocating the spectrum to them. The objective is to serve the traffic and find a solution that is Pareto optimal with respect to the maximum spectrum used and the cost (number and type) of transponders used. The problem definition and the proposed algorithms are general and applicable to flex-Grid as well as Fixed-Grid networks. We start by presenting algorithms based on integer linear programming formulations for transparent and translucent networks (without or with regenerators) and we continue by presenting heuristic algorithms. Using input driven by transmission studies on optical orthogonal frequency-division multiplexing (OFDM)-based networks we evaluate the performance gains that can be obtained by an OFDM over a mixed line rate Fixed-Grid WDM optical network.

  • quantifying spectrum cost and energy efficiency in Fixed Grid and flex Grid networks invited
    IEEE\ OSA Journal of Optical Communications and Networking, 2012
    Co-Authors: Eleni Palkopoulou, Kostas Christodoulopoulos, M Angelou, Dimitrios Klonidis, Axel Klekamp, F Buchali, Emmanouel Varvarigos, Ioannis Tomkos
    Abstract:

    Single and multi-carrier networks offering channel rates up to 400 Gb/s are evaluated under realistic reach parameters. It is found that efficient spectrum utilization and fine bit-rate granularity are essential to achieve cost and energy efficiency. Additionally, the break-even cost of flexible orthogonal frequency division multiplexing transponders is examined under different settings. The break-even cost of a flexible transponder corresponds to the cost value for which the total cost of the network is equal to that of the related single-line-rate network. The impact of the traffic load, the additional cost required for flex-Grid optical cross connects, the cost of spectrum, as well as the cost of Fixed-Grid transponders is examined.

  • spectrum cost and energy efficiency in Fixed Grid and flex Grid networks
    Optical Fiber Communication Conference, 2012
    Co-Authors: M Angelou, Kostas Christodoulopoulos, Dimitrios Klonidis, Axel Klekamp, F Buchali, Emmanouel Varvarigos, Ioannis Tomkos
    Abstract:

    Single and multi-carrier networks offering channel rates up to 400Gb/s are evaluated under realistic reach parameters. It is found that efficient spectrum utilization and fine bit-rate granularity are essential to achieve cost and energy efficiency.

  • Dynamic bandwidth allocation in flexible OFDM-based networks
    2011 Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference, 2011
    Co-Authors: Kostas Christodoulopoulos, Ioannis Tomkos
    Abstract:

    We propose a general policy to allocate subcarriers to time-varying traffic in a flexible OFDM optical network. We compare the OFDM network performance to that of a Fixed-Grid WDM network using simulations.

Ioannis Tomkos - One of the best experts on this subject based on the ideXlab platform.

  • quantifying spectrum cost and energy efficiency in Fixed Grid and flex Grid networks invited
    IEEE\ OSA Journal of Optical Communications and Networking, 2012
    Co-Authors: Eleni Palkopoulou, Kostas Christodoulopoulos, M Angelou, Dimitrios Klonidis, Axel Klekamp, F Buchali, Emmanouel Varvarigos, Ioannis Tomkos
    Abstract:

    Single and multi-carrier networks offering channel rates up to 400 Gb/s are evaluated under realistic reach parameters. It is found that efficient spectrum utilization and fine bit-rate granularity are essential to achieve cost and energy efficiency. Additionally, the break-even cost of flexible orthogonal frequency division multiplexing transponders is examined under different settings. The break-even cost of a flexible transponder corresponds to the cost value for which the total cost of the network is equal to that of the related single-line-rate network. The impact of the traffic load, the additional cost required for flex-Grid optical cross connects, the cost of spectrum, as well as the cost of Fixed-Grid transponders is examined.

  • spectrum cost and energy efficiency in Fixed Grid and flex Grid networks
    Optical Fiber Communication Conference, 2012
    Co-Authors: M Angelou, Kostas Christodoulopoulos, Dimitrios Klonidis, Axel Klekamp, F Buchali, Emmanouel Varvarigos, Ioannis Tomkos
    Abstract:

    Single and multi-carrier networks offering channel rates up to 400Gb/s are evaluated under realistic reach parameters. It is found that efficient spectrum utilization and fine bit-rate granularity are essential to achieve cost and energy efficiency.

  • Dynamic bandwidth allocation in flexible OFDM-based networks
    2011 Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference, 2011
    Co-Authors: Kostas Christodoulopoulos, Ioannis Tomkos
    Abstract:

    We propose a general policy to allocate subcarriers to time-varying traffic in a flexible OFDM optical network. We compare the OFDM network performance to that of a Fixed-Grid WDM network using simulations.

  • spectrally bitrate flexible optical network planning
    European Conference on Optical Communication, 2010
    Co-Authors: Kostas Christodoulopoulos, Ioannis Tomkos, Emmanouel Varvarigos
    Abstract:

    We consider the Routing and Spectrum Allocation (RSA) problem in an OFDM-based optical network with elastic bandwidth allocation. We asses the spectrum utilization gains of this flexible architecture compared to a traditional Fixed-Grid rigid-bandwidth WDM network.