The Experts below are selected from a list of 249 Experts worldwide ranked by ideXlab platform
Martin Vetterli - One of the best experts on this subject based on the ideXlab platform.
-
Orthogonal time varying filter banks and wavelet packets
IEEE Transactions on Signal Processing, 1994Co-Authors: Cormac Herley, Martin VetterliAbstract:Considers the construction of Orthogonal time-varying filter banks. By examining the time domain description of the two-channel Orthogonal filter bank the authors find it possible to construct a set of Orthogonal Boundary filters, which allows to apply the filter bank to one-sided or finite-length signals, without redundancy or distortion. The method is constructive and complete. There is a whole space of Orthogonal Boundary solutions, and there is considerable freedom for optimization. This may be used to generate subband tree structures where the tree varies over time, and to change between different filter sets. The authors also show that the iteration of discrete-time time-varying filter banks gives continuous-time bases, just as in the stationary case. This gives rise to wavelet, or wavelet packet, bases for half-line and interval regions. >
-
ICASSP (3) - Time-varying orthonormal tilings of the time-frequency plane
IEEE International Conference on Acoustics Speech and Signal Processing, 1993Co-Authors: Cormac Herley, Jelena Kovacevic, Kannan Ramchandran, Martin VetterliAbstract:Expansions that give arbitrary orthonormal tilings of the time-frequency plane are considered. These differ from the short-time Fourier transform, wavelet transform, and wavelet packet tilings in that they change over time. It is shown how this can be achieved using time-varying Orthogonal tree structures, which preserve Orthogonality, even across transitions. One method is based on lapped Orthogonal transforms, which makes it possible to change the number of channels in the transform. A second method is based on the construction of Orthogonal Boundary filters to construct essentially arbitrary tilings. A double-tree algorithm is presented that, for a given signal, decides on the best binary segmentation and on which tree split to use for each segment. That is, it is a joint optimization of time and frequency splitting. The algorithm is optimal for additive cost functions (e.g., rate distortion), which gives the best time-varying bases. Results of experiments on test signals are shown. >
-
ISCAS - Orthogonal time-varying filter banks and wavelets
1993 IEEE International Symposium on Circuits and Systems, 1Co-Authors: Cormac Herley, Martin VetterliAbstract:The construction of time-varying Orthogonal filter banks is considered. It is shown that implementing an Orthogonal finite impulse response filter bank over a finite signal segment involves finding a set of Orthogonal Boundary filters, and that by carrying out a Gram-Schmidt Orthogonalization procedure Boundary filters are generated that necessarily remain localized in the region of the Boundary. A complete constructive characterization of such boundaries is given for two-channel finite impulse response filter banks. These Boundary constructions allow changing the topology of Orthogonal subband trees at will, by growing or pruning branches at any time. The Boundary filter case can be further generalized to give overlapping transition filters when changing between Orthogonal structures. If the time-varying filter banks are used in an iterated scheme, they converge to continuous-time bases, much as in the non-time-varying case. >
Cormac Herley - One of the best experts on this subject based on the ideXlab platform.
-
Orthogonal time varying filter banks and wavelet packets
IEEE Transactions on Signal Processing, 1994Co-Authors: Cormac Herley, Martin VetterliAbstract:Considers the construction of Orthogonal time-varying filter banks. By examining the time domain description of the two-channel Orthogonal filter bank the authors find it possible to construct a set of Orthogonal Boundary filters, which allows to apply the filter bank to one-sided or finite-length signals, without redundancy or distortion. The method is constructive and complete. There is a whole space of Orthogonal Boundary solutions, and there is considerable freedom for optimization. This may be used to generate subband tree structures where the tree varies over time, and to change between different filter sets. The authors also show that the iteration of discrete-time time-varying filter banks gives continuous-time bases, just as in the stationary case. This gives rise to wavelet, or wavelet packet, bases for half-line and interval regions. >
-
ICASSP (3) - Time-varying orthonormal tilings of the time-frequency plane
IEEE International Conference on Acoustics Speech and Signal Processing, 1993Co-Authors: Cormac Herley, Jelena Kovacevic, Kannan Ramchandran, Martin VetterliAbstract:Expansions that give arbitrary orthonormal tilings of the time-frequency plane are considered. These differ from the short-time Fourier transform, wavelet transform, and wavelet packet tilings in that they change over time. It is shown how this can be achieved using time-varying Orthogonal tree structures, which preserve Orthogonality, even across transitions. One method is based on lapped Orthogonal transforms, which makes it possible to change the number of channels in the transform. A second method is based on the construction of Orthogonal Boundary filters to construct essentially arbitrary tilings. A double-tree algorithm is presented that, for a given signal, decides on the best binary segmentation and on which tree split to use for each segment. That is, it is a joint optimization of time and frequency splitting. The algorithm is optimal for additive cost functions (e.g., rate distortion), which gives the best time-varying bases. Results of experiments on test signals are shown. >
-
ISCAS - Orthogonal time-varying filter banks and wavelets
1993 IEEE International Symposium on Circuits and Systems, 1Co-Authors: Cormac Herley, Martin VetterliAbstract:The construction of time-varying Orthogonal filter banks is considered. It is shown that implementing an Orthogonal finite impulse response filter bank over a finite signal segment involves finding a set of Orthogonal Boundary filters, and that by carrying out a Gram-Schmidt Orthogonalization procedure Boundary filters are generated that necessarily remain localized in the region of the Boundary. A complete constructive characterization of such boundaries is given for two-channel finite impulse response filter banks. These Boundary constructions allow changing the topology of Orthogonal subband trees at will, by growing or pruning branches at any time. The Boundary filter case can be further generalized to give overlapping transition filters when changing between Orthogonal structures. If the time-varying filter banks are used in an iterated scheme, they converge to continuous-time bases, much as in the non-time-varying case. >
Marcelo J. S. De Lemos - One of the best experts on this subject based on the ideXlab platform.
-
Turbulent Heat Transport
Turbulence in Porous Media, 2012Co-Authors: Marcelo J. S. De LemosAbstract:This chapter illustrates the procedure for obtaining the macroscopic energy equation for a porous medium starting from the local energy equations (for the fluid and solid phases). Then, time averaging is applied followed by volume averaging (or vice versa) using the local thermal equilibrium hypothesis. This procedure leads to the one-energy equation model. The final expanded form of the macroscopic energy equation for a rigid, homogeneous porous medium saturated with an incompressible fluid does not depend on the averaging order, that is, both procedures lead to the same results. The transport equations at the pore-scale were numerically solved using the SIMPLE method on a non-Orthogonal Boundary-fitted coordinate system. The relaxation process starts with the solution of the two momentum equations, and the velocity field is adjusted in order to satisfy the continuity principle. This adjustment is attained by solving the pressure correction equation. The turbulence model and the energy equations are relaxed to update the κ, ɛ and temperature fields. In many industrial applications, turbulent flow through a packed bed represents an important configuration for efficient heat and mass transfer. A common model used for analyzing such a system is the local thermal equilibrium assumption, where both solid and fluid phase temperatures are represented by a unique value. This model simplifies theoretical and numerical research, but the assumption of local thermal equilibrium between the fluid and the solid is inadequate for a number of problems. Consequently, in many instances it is important to take into account the distinct temperatures for the porous material and for the working fluid.
-
Thermal dispersion in porous media as a function of the solid-fluid conductivity ratio
International Journal of Heat and Mass Transfer, 2008Co-Authors: Marcos H. J. Pedras, Marcelo J. S. De LemosAbstract:Abstract Thermal dispersion in porous media is an import phenomenon in combustion and in steam injection systems for Enhanced Oil Recovery methods, among several others engineering applications. In this work, thermal dispersion tensors were calculated within an infinite porous medium formed by a spatially periodic array of longitudinally-displaced elliptic rods. Two different thermal conductivity ratios between the solid and fluid phases were used for analyzing their effect on the thermal dispersion tensor, following a systematic analysis of several porous media modeled by different unit-cell geometry. As such, just one unit-cell, together with periodic Boundary conditions for mass, momentum and energy equations, was used to represent the medium. The numerical methodology herein employed is based on the control-volume approach. Turbulence was assumed to exist within the fluid phase and a low Reynolds k – e closure was used to model it. The flow equations at the pore-scale were numerically solved using the SIMPLE method on a non-Orthogonal Boundary-fitted coordinate system. Cell-integrated results for the longitudinal dispersion coefficient showed little sensitiveness on porosity, Boundary condition type, medium morphology and solid–fluid conductivity ratio, whereas for the transversal direction, all of these parameters modified the numerical value obtained for the dispersion coefficient.
-
Chapter 5 – Turbulent Heat Transport
Turbulence in Porous Media, 2006Co-Authors: Marcelo J. S. De LemosAbstract:Publisher Summary This chapter illustrates the procedure for obtaining the macroscopic energy equation for a porous medium starting from the local energy equations (for the fluid and solid phases). Then, time averaging is applied followed by volume averaging (or vice versa) using the local thermal equilibrium hypothesis. This procedure leads to the one-energy equation model. The final expanded form of the macroscopic energy equation for a rigid, homogeneous porous medium saturated with an incompressible fluid does not depend on the averaging order, that is, both procedures lead to the same results. The transport equations at the pore-scale were numerically solved using the SIMPLE method on a non-Orthogonal Boundary-fitted coordinate system. The relaxation process starts with the solution of the two momentum equations, and the velocity field is adjusted in order to satisfy the continuity principle. This adjustment is attained by solving the pressure correction equation. The turbulence model and the energy equations are relaxed to update the κ, ɛ and temperature fields. In many industrial applications, turbulent flow through a packed bed represents an important configuration for efficient heat and mass transfer. A common model used for analyzing such a system is the local thermal equilibrium assumption, where both solid and fluid phase temperatures are represented by a unique value. This model simplifies theoretical and numerical research, but the assumption of local thermal equilibrium between the fluid and the solid is inadequate for a number of problems. Consequently, in many instances it is important to take into account the distinct temperatures for the porous material and for the working fluid.
-
Mass Dispersion Coefficients for Turbulent Flow in an Infinite Porous Medium
Volume!, 2004Co-Authors: Maximilian S. Mesquita, Marcelo J. S. De LemosAbstract:In this work, mass dispersion tensors were calculated within an infinite porous medium formed by a spatially periodic array of longitudinally-displaced cylindrical rods. For the sake of simplicity, just one unit-cell, together with periodic Boundary conditions for mass and momentum equations, and Neumann conditions for the mass concentration, was used to represent such medium. The numerical methodology herein employed is based on the control volume approach. Turbulence is assumed to exist within the fluid phase. High and low Reynolds k-e models were used to model such non-linear effects. The flow equations at the pore-scale were numerically solved using the SIMPLE method applied to a non-Orthogonal Boundary-fitted coordinate system. Integrated mass fraction results were compared with existing data in the literature.Copyright © 2004 by ASME
-
Macroscopic Modeling of Turbulent Mass Transport in Heterogeneous Porous Media
Heat Transfer Volume 1, 2004Co-Authors: Maximilian S. Mesquita, Marcelo J. S. De LemosAbstract:In this work, results for a macroscopic mass transport model are presented for a parallel plate channel filled with a fluid saturated heterogeneous porous medium. The numerical methodology herein employed is based on the control volume approach. Turbulence is assumed to exist within the fluid phase. High and low Reynolds k-e models were used to model such non-linear effects. The flow equations at the pore-scale were numerically solved using the SIMPLE method applied to a non-Orthogonal Boundary-fitted coordinate system. Integrated mass fraction results were compiled leading to correlations for the mass dispersion coefficients in the x and y directions. Application of the macroscopic model using the proposed correlations showed the role of dispersion mechanism in the overall transport in porous media.Copyright © 2004 by ASME
Nuria González Prelcic - One of the best experts on this subject based on the ideXlab platform.
-
Linear Boundary extensions for finite length signals and paraunitary two-channel filterbanks
IEEE Transactions on Signal Processing, 2004Co-Authors: M.e.d. Jimenez, Nuria González PrelcicAbstract:In this paper, we introduce a novel and general matrix formulation of artificial linear Boundary extension methods for removing border effects inherent to any paraunitary two-channel size-limited filterbank. This new characterization of the transformation operator allows us to prove that perfect reconstruction (PR) of finite signals can be ensured under some conditions without using extra subband coefficients; in other words, we characterize the signal extension methods that lead to nonexpansive transforms. The necessary and sufficient condition we find allows us to show that some traditional extension techniques that are being used in an expansive way, such as the polynomial extension, lead in fact to nonexpansive invertible transforms; moreover, we can also prove that in contradiction to previous literature, not every transformation matrix associated with a linear extension is invertible even if using prototype filters of the same length. Apart from these invertibility criteria, we propose the first algorithm for the design of all linear extensions and their associated biOrthogonal Boundary filters that lead to nonexpansive and invertible transforms. Analogously, we provide the first method for the design of all linear extensions that yield Orthogonal transforms: We construct an infinite number of Orthogonal extensions, apart from the commonly used periodic extension, and their associated Orthogonal Boundary filters. The final contribution of the paper is a new algorithm for the design of smooth Orthogonal extensions, which keep the Orthogonality property and overcome the main drawback of periodization, that is, the introduction of subband coefficients of great amplitude near the boundaries in the transform domain.
-
ICASSP - Smooth Orthogonal signal extensions for paraunitary tree-structured filter banks
IEEE International Conference on Acoustics Speech and Signal Processing, 2002Co-Authors: M.e. Dominguez Jimenez, Nuria González PrelcicAbstract:This work is concerned with subband processing of finite length signals using tree-structured filter banks. In many applications the subband transform is required to be Orthogonal. For this purpose, the filter bank must be paraunitary, and the signal borders must be processed via either Orthogonal Boundary filters or via Orthogonal signal extension methods. Periodization is an Orthogonal extension technique, but introduces undesired artificial discontinuities or spurious high frequencies in the transform vector. Boundary filter design techniques do not solve this problem either. The solution we provide is a new algorithm for the generation of alternative Orthogonal signal extensions which do not introduce spurious high frequencies in the subband signals. Some experimental results illustrate the effectiveness of the proposed design method in comparison to other existing techniques.
-
EUSIPCO - New Orthogonal extension methods for tree-structured filter banks
2000Co-Authors: M.e.d. Jimenez, Nuria González PrelcicAbstract:When processing finite length sequences via paraunitary filter banks, a commonly used technique consists of artificially extending the signal before the analysis stage. The use of the extension methods avoids the border effects, and perfect reconstruction, even Orthogonal, size-limited filter banks can be defined. In this paper we first characterize and generate all signal extension methods which yield Orthogonal subband transforms. Secondly, the particular case of non-circular Orthogonal extension methods is investigated, and the first general design method of non-circular Orthogonal extensions is derived. Finally, the computational cost of the design algorithm is evaluated and compared to that of Orthogonal Boundary filter generation methods.
Ingo Münch - One of the best experts on this subject based on the ideXlab platform.
-
A variant of the linear isotropic indeterminate couple-stress model with symmetric local force-stress, symmetric nonlocal force-stress, symmetric couple-stresses and Orthogonal Boundary conditions
Mathematics and Mechanics of Solids, 2016Co-Authors: Ionel-dumitrel Ghiba, Patrizio Neff, Angela Madeo, Ingo MünchAbstract:In this paper we venture a new look at the linear isotropic indeterminate couple stress model in the general framework of second gradient elasticity and we propose a new alternative formulation which obeys Cauchy-Boltzmann's axiom of the symmetry of the force stress tensor. For this model we prove the existence of solutions for the equilibrium problem. Relations with other gradient elastic theories and the possibility to switch from a {4th order} (gradient elastic) problem to a 2nd order micromorphic model are also discussed with a view of obtaining symmetric force-stress tensors. It is shown that the indeterminate couple stress model can be written entirely with symmetric force-stress and symmetric couple-stress. The difference of the alternative models rests in specifying traction Boundary conditions of either rotational type or strain type. If rotational type Boundary conditions are used in the partial integration, the classical anti-symmetric nonlocal force stress tensor formulation is obtained. Otherwise, the difference in both formulations is only a divergence--free second order stress field such that the field equations are the same, but the traction Boundary conditions are different. For these results we employ a novel integrability condition, connecting the infinitesimal continuum rotation and the infinitesimal continuum strain. Moreover, we provide the complete, consistent traction Boundary conditions for both models.
-
A variant of the linear isotropic indeterminate couple-stress model with symmetric local force-stress, symmetric nonlocal force-stress, symmetric couple-stresses and Orthogonal Boundary conditions:
Mathematics and Mechanics of Solids, 2016Co-Authors: Ionel-dumitrel Ghiba, Patrizio Neff, Angela Madeo, Ingo MünchAbstract:In this paper we venture a new look at the linear isotropic indeterminate couple-stress model in the general framework of second-gradient elasticity and we propose a new alternative formulation whi...