A. Lakis - Profile on Academia.edu (original) (raw)
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Papers by A. Lakis
[Bourneville tuberous sclerosis and colonic polyposis]
Revue medicale de Bruxelles, 2016
Tuberous sclerosis is an autosomal dominant genetic disorder that is characterized by epilepsy, m... more Tuberous sclerosis is an autosomal dominant genetic disorder that is characterized by epilepsy, mental retardation and facial angiofibromas. Usually, the disease is diagnosed in childhood but there are frustrates form of tuberous sclerosis with or without genetic mutation. This clinical case about a man who is diagnosed a colonic polyposis, a rectal adenocarcinoma and a tuberous sclerosis.
Pulsed Eddy Current (PEC) is a non-destructive testing method used to detect corrosion and cracks... more Pulsed Eddy Current (PEC) is a non-destructive testing method used to detect corrosion and cracks in multilayer aluminum structures which are typically found in aircraft applications. Corrosion and metal loss in thin multi-layer structures are complex and variable phenomena that diminish the reliability of pulsed eddy current measurements. In this paper, the Pulsed Eddy Current signals are processed to improve the accuracy and reliability of these measurements. PEC's results (time domain data) are converted by Time-Frequency Analysis (Rihaczek distribution) to represent data in three dimensions. The Time-Frequency approach generates a large amount of data. Principal component analysis (PCA) is applied as feature extraction to reduce redundant data to provide new features for classifiers. K-means Clustering and Expectation-Maximization are applied to classify data and automatically determine corrosion distribution in each layer. A set of tests has been carried out to detect the d...
Operational Modal Analysis of Non-Stationary Mechanical Systems by Short-Time Autoregressive (Star) Modelling
paginas.fe.up.pt
... OPERATIONAL MODAL ANALYSIS OF NON-STATIONARY MECHANICAL SYSTEMS BY SHORT-TIME AUTOREGRESSIVE ... more ... OPERATIONAL MODAL ANALYSIS OF NON-STATIONARY MECHANICAL SYSTEMS BY SHORT-TIME AUTOREGRESSIVE (STAR ... modeling and identification of an output-only modalanalysis of non ... stationary vibrations, which allows the tracking of modal parameter changes ...
Diagnostic of machine using the timefrequency representation of vibration is becoming widely used... more Diagnostic of machine using the timefrequency representation of vibration is becoming widely used by many companies and experts. In recent years, the preventive maintenance is no more desirable and in fact it has evolved into the prediction maintenance. The current methods of machine diagnosis can not provide detailed diagnostics and condition prediction. This paper proposes the application of Short-Time Fourier Transform (STFT) as a timefrequency method, which can provide more information about a signal in time and in frequency and gives a better representation of the signal than the conventional methods in machinery diagnosis. In this paper, we review the machine diagnosis techniques based on the verification of classical vibration parameters. Then the necessity of using time-frequency analysis in machinery diagnostics is discussed. Finally, the theory of the Short-Time Fourier Transform is briefly studied and its advantages are shown by some practical examples.
A theoretical model for predicting adverse health effects of long-term whole-body random vibration exposure
Study design The effects of harmonic and random vibration as a source of low-back disorders are i... more Study design The effects of harmonic and random vibration as a source of low-back disorders are investigated by considering a dynamic mechanical strain-stress approach of the lumbar spine. Objectives The objective of this study is to propose a new methodology in order to estimate the long-term exposure limit to whole-body vibration before the occurrence of fatigue failures, especially in the case of random excitation. of background data Whole-body vibrations induce stresses which cause the fatigue failure of spine. The most common tool used for estimating the incurred risk to vibration exposure is the guide for the evaluation of human exposure to whole-body vibration. Unfortunately, this guide is not well suited in order to estimate the long-term exposure limit and a fatigue approach must be investigated. Methods The methodology is based on the use of a vertebra S-N fatigue law in order to estimate the number of fatigue cycles as a function of both applied dynamic and static failure stresses. The applied dynamic stress and the cumulative injury criterion from the seat to the lumbar spine are derived from a transfer function of the acceleration levels by using the assumptions concerning human characteristics (lumbar spine stiffness, disk surface). Results The number of cycles before injury is derived from this stress level and the long time exposure limits are proposed by applying fatigue laws. This new tool is presented as charts relating fatigue exposure limits to the measurements of the power spectrum density incurred by the human body, recorded at the seat. Maximum acceleration levels are proposed in order to enable the prevention of possible spine failures. Conclusions The results of this study give a new tool to predict the impact of vibration on low-back pain and are very important in order to complete and revise the actual guide for the evaluation of human exposure to whole-body vibration.
Mechanical Systems and Signal Processing, 2011
This paper presents improvements of a multivariable autoregressive (AR) model for applications in... more This paper presents improvements of a multivariable autoregressive (AR) model for applications in operational modal analysis considering simultaneously the temporal response data of multi-channel measurements. The parameters are estimated by using the least squares method via the implementation of the QR factorization. A new noise ratebased factor called the Noise rate Order Factor (NOF) is introduced for use in the effective selection of model order and noise rate estimation. For the selection of structural modes, an orderwise criterion called the Order Modal Assurance Criterion (OMAC) is used, based on the correlation of mode shapes computed from two successive orders. Specifically, the algorithm is updated with respect to model order from a small value to produce a costeffective computation. Furthermore, the confidence intervals of each natural frequency, damping ratio and mode shapes are also computed and evaluated with respect to model order and noise rate. This method is thus very effective for identifying the modal parameters in case of ambient vibrations dealing with modern output-only modal analysis. Simulations and discussions on a steel plate structure are presented, and the experimental results show good agreement with the finite element analysis.
An unusual recurrent ileocolonic injury
Acta Gastro Enterologica Belgica
Potassium binders (Kayexalate® and Sorbisterit®) are commonly used to treat hyperkaliemia. They a... more Potassium binders (Kayexalate® and Sorbisterit®) are commonly used to treat hyperkaliemia. They are made of sodium or calcium polystyrene sulfonate. Their use is associated with multiple adverse effects including ileocolonic (or more rarely upper digestive tract) injuries which can lead to necrosis or perforations. This side effect is mostly seen in patients with chronic kidney disease or constipation. It presents with abdominal pain, diarrhea or hematochezia. The diagnosis is made when the histo-logical analysis of samples from the erythematous or ulcerated digestive wall finds polystyrene sulfonate crystals embedded in the mucosa. This diagnosis can be suspected by taking a careful initial drug inventory, if the clinician is aware of this rare but serious adverse effect. The lack of specificity of clinical symptoms and endoscopic lesions makes this inventory even more essential. Treatment is mainly supportive and requires cessation of the drug, while surgery is inevitable in the m...
A theoretical model for predicting adverse health effects of long-term whole-body random vibration exposure
Journal of Fluids and Structures, 2005
The present special issue is a celebration by members of the research community in fluid-structur... more The present special issue is a celebration by members of the research community in fluid-structure interaction honouring the 70th birthday of Professor Michael P. Paı¨doussis, the Founder and Editor-in-Chief of the Journal of Fluids and Structures. His anniversary has been highlighted by a series of festive events, including the presentation of a prize at the especially dedicated symposium organized in his honor at the recent 3rd MIT Conference on Computational Fluid and Solid Mechanics held in Cambridge, MA. 1 In this preface we include comments by several of his family, friends and colleagues who have had the privilege of being a part of his life and career over the years.
Dynamic Analysis of Axially Non-Uniform Thin Cylindrical Shells
Journal of Mechanical Engineering Science
Part 1 of this paper presents a new theory for the dynamic and static analysis of axially non-uni... more Part 1 of this paper presents a new theory for the dynamic and static analysis of axially non-uniform, thin cylindrical shells. It is a hybrid of finite element and classical shell theories: the shell is subdivided into cylindrical finite elements, and the displacements within each (expressed in terms of nodal displacements), i.e., the displacement functions, are obtained using Sanders' equations for thin cylindrical shells in full. Sanders' theory gives zero strains for small rigid-body motions, so that displacement functions based on it satisfy the convergence criteria of the finite-element method. Expressions for the mass, stiffness and stress-resultant matrices are obtained, and the method for constructing the equivalent global matrices is given. This paper is supported by Part 2, where the eigenvalues of a number of shells are calculated and compared with other theories and experiments. In Part 2, the free flexural vibration characteristics of thin cylindrical shells ar...
Computational modeling of coupled fluid-structure systems with applications
Structural Engineering and Mechanics, 2008
ABSTRACT This paper outlines the development of a computational model in order to analyze the dyn... more ABSTRACT This paper outlines the development of a computational model in order to analyze the dynamic behaviour of coupled fluid-structure systems such as a) liquid containers, b) a set of parallel or radial plates. In this work a hybrid fluid-solid element is developed, capable of simulating both membrane and bending effects of the plate. The structural mass and stiffness matrices are determined using exact integration of governing equations which are derived using a combination of classical plate theory and a finite element approach. The Bernoulli equation and velocity potential function are used to describe the liquid pressure applied on the solid-fluid element. An impermeability condition assures a permanent contact at the fluid-structure interface. Applications of this model are presented for both parallel and radial plates as well as fluid-filled rectangular reservoir. The effect of physical parameters on the dynamic behaviour of a coupled fluid-structure system is investigated. The results obtained using the presented approach for dynamic characteristics such as natural frequency are in agreement to those calculated using other theories and experiments.
Theoretical model of cylindrical structures containing turbulent flowing fluids
Free vibration and response to pressure field of anisotropic thin cylindrical shells
Acquisition channel for phase and wall pressure prediction in turbulent two-phase flow
... | Lara |; INIST |; CNRS ... voir notice détaillée. Fichiers dans ce document. AR3_1.pdf, 69.5... more ... | Lara |; INIST |; CNRS ... voir notice détaillée. Fichiers dans ce document. AR3_1.pdf, 69.55Kb. Pour citer ce document : URI: http://hdl.handle.net/2042/10500. Titre: Acquisition channel for phase and wall pressure prediction in turbulent two-phase flow. Auteur: SAMI, SM; LAKIS, AA ...
Aerothermoelastic Stability of Functionally Graded Circular Cylindrical Shells
ASME 2010 7th International Symposium on Fluid-Structure Interactions, Flow-Sound Interactions, and Flow-Induced Vibration and Noise: Volume 3, Parts A and B, 2010
[ASME Conference Proceedings 2010, 939 (2010)]. Farhad Sabri, Aouni A. Lakis. Abstract. ... The e... more [ASME Conference Proceedings 2010, 939 (2010)]. Farhad Sabri, Aouni A. Lakis. Abstract. ... The effects of temperature rise and shell internal pressure on the flutter boundaries of FG circular cylindrical shell for different values of power law index are investigated. ...
This paper presents a study on the identification of modal parameters by experimental operational... more This paper presents a study on the identification of modal parameters by experimental operational modal analysis for the assessment of bridge rehabilitation. Modal parameters of the bridge before and after rehabilitation are estimated. Normal traffic flow over the bridge provided the excitation source. A single 3D accelerometer, located at mid-span of the bridge was used to obtain natural frequencies and damping ratios of the vibration modes. Identification of modal parameters was conducted using four methods including the Peak Picking method (PP), the Least Square Complex Exponential method (LSCE), the Autoregressive method (AR) and the Autoregressive Moving Average method (ARMA). During the study, the concrete deck of the bridge was replaced by orthotropic steel in order to increase its load capacity. The effect of this rehabilitation can be clearly observed through estimated vibration parameters in the stability diagrams.
Safizadeh,Lakis,Thomas,2002
[Bourneville tuberous sclerosis and colonic polyposis]
Revue medicale de Bruxelles, 2016
Tuberous sclerosis is an autosomal dominant genetic disorder that is characterized by epilepsy, m... more Tuberous sclerosis is an autosomal dominant genetic disorder that is characterized by epilepsy, mental retardation and facial angiofibromas. Usually, the disease is diagnosed in childhood but there are frustrates form of tuberous sclerosis with or without genetic mutation. This clinical case about a man who is diagnosed a colonic polyposis, a rectal adenocarcinoma and a tuberous sclerosis.
Pulsed Eddy Current (PEC) is a non-destructive testing method used to detect corrosion and cracks... more Pulsed Eddy Current (PEC) is a non-destructive testing method used to detect corrosion and cracks in multilayer aluminum structures which are typically found in aircraft applications. Corrosion and metal loss in thin multi-layer structures are complex and variable phenomena that diminish the reliability of pulsed eddy current measurements. In this paper, the Pulsed Eddy Current signals are processed to improve the accuracy and reliability of these measurements. PEC's results (time domain data) are converted by Time-Frequency Analysis (Rihaczek distribution) to represent data in three dimensions. The Time-Frequency approach generates a large amount of data. Principal component analysis (PCA) is applied as feature extraction to reduce redundant data to provide new features for classifiers. K-means Clustering and Expectation-Maximization are applied to classify data and automatically determine corrosion distribution in each layer. A set of tests has been carried out to detect the d...
Operational Modal Analysis of Non-Stationary Mechanical Systems by Short-Time Autoregressive (Star) Modelling
paginas.fe.up.pt
... OPERATIONAL MODAL ANALYSIS OF NON-STATIONARY MECHANICAL SYSTEMS BY SHORT-TIME AUTOREGRESSIVE ... more ... OPERATIONAL MODAL ANALYSIS OF NON-STATIONARY MECHANICAL SYSTEMS BY SHORT-TIME AUTOREGRESSIVE (STAR ... modeling and identification of an output-only modalanalysis of non ... stationary vibrations, which allows the tracking of modal parameter changes ...
Diagnostic of machine using the timefrequency representation of vibration is becoming widely used... more Diagnostic of machine using the timefrequency representation of vibration is becoming widely used by many companies and experts. In recent years, the preventive maintenance is no more desirable and in fact it has evolved into the prediction maintenance. The current methods of machine diagnosis can not provide detailed diagnostics and condition prediction. This paper proposes the application of Short-Time Fourier Transform (STFT) as a timefrequency method, which can provide more information about a signal in time and in frequency and gives a better representation of the signal than the conventional methods in machinery diagnosis. In this paper, we review the machine diagnosis techniques based on the verification of classical vibration parameters. Then the necessity of using time-frequency analysis in machinery diagnostics is discussed. Finally, the theory of the Short-Time Fourier Transform is briefly studied and its advantages are shown by some practical examples.
A theoretical model for predicting adverse health effects of long-term whole-body random vibration exposure
Study design The effects of harmonic and random vibration as a source of low-back disorders are i... more Study design The effects of harmonic and random vibration as a source of low-back disorders are investigated by considering a dynamic mechanical strain-stress approach of the lumbar spine. Objectives The objective of this study is to propose a new methodology in order to estimate the long-term exposure limit to whole-body vibration before the occurrence of fatigue failures, especially in the case of random excitation. of background data Whole-body vibrations induce stresses which cause the fatigue failure of spine. The most common tool used for estimating the incurred risk to vibration exposure is the guide for the evaluation of human exposure to whole-body vibration. Unfortunately, this guide is not well suited in order to estimate the long-term exposure limit and a fatigue approach must be investigated. Methods The methodology is based on the use of a vertebra S-N fatigue law in order to estimate the number of fatigue cycles as a function of both applied dynamic and static failure stresses. The applied dynamic stress and the cumulative injury criterion from the seat to the lumbar spine are derived from a transfer function of the acceleration levels by using the assumptions concerning human characteristics (lumbar spine stiffness, disk surface). Results The number of cycles before injury is derived from this stress level and the long time exposure limits are proposed by applying fatigue laws. This new tool is presented as charts relating fatigue exposure limits to the measurements of the power spectrum density incurred by the human body, recorded at the seat. Maximum acceleration levels are proposed in order to enable the prevention of possible spine failures. Conclusions The results of this study give a new tool to predict the impact of vibration on low-back pain and are very important in order to complete and revise the actual guide for the evaluation of human exposure to whole-body vibration.
Mechanical Systems and Signal Processing, 2011
This paper presents improvements of a multivariable autoregressive (AR) model for applications in... more This paper presents improvements of a multivariable autoregressive (AR) model for applications in operational modal analysis considering simultaneously the temporal response data of multi-channel measurements. The parameters are estimated by using the least squares method via the implementation of the QR factorization. A new noise ratebased factor called the Noise rate Order Factor (NOF) is introduced for use in the effective selection of model order and noise rate estimation. For the selection of structural modes, an orderwise criterion called the Order Modal Assurance Criterion (OMAC) is used, based on the correlation of mode shapes computed from two successive orders. Specifically, the algorithm is updated with respect to model order from a small value to produce a costeffective computation. Furthermore, the confidence intervals of each natural frequency, damping ratio and mode shapes are also computed and evaluated with respect to model order and noise rate. This method is thus very effective for identifying the modal parameters in case of ambient vibrations dealing with modern output-only modal analysis. Simulations and discussions on a steel plate structure are presented, and the experimental results show good agreement with the finite element analysis.
An unusual recurrent ileocolonic injury
Acta Gastro Enterologica Belgica
Potassium binders (Kayexalate® and Sorbisterit®) are commonly used to treat hyperkaliemia. They a... more Potassium binders (Kayexalate® and Sorbisterit®) are commonly used to treat hyperkaliemia. They are made of sodium or calcium polystyrene sulfonate. Their use is associated with multiple adverse effects including ileocolonic (or more rarely upper digestive tract) injuries which can lead to necrosis or perforations. This side effect is mostly seen in patients with chronic kidney disease or constipation. It presents with abdominal pain, diarrhea or hematochezia. The diagnosis is made when the histo-logical analysis of samples from the erythematous or ulcerated digestive wall finds polystyrene sulfonate crystals embedded in the mucosa. This diagnosis can be suspected by taking a careful initial drug inventory, if the clinician is aware of this rare but serious adverse effect. The lack of specificity of clinical symptoms and endoscopic lesions makes this inventory even more essential. Treatment is mainly supportive and requires cessation of the drug, while surgery is inevitable in the m...
A theoretical model for predicting adverse health effects of long-term whole-body random vibration exposure
Journal of Fluids and Structures, 2005
The present special issue is a celebration by members of the research community in fluid-structur... more The present special issue is a celebration by members of the research community in fluid-structure interaction honouring the 70th birthday of Professor Michael P. Paı¨doussis, the Founder and Editor-in-Chief of the Journal of Fluids and Structures. His anniversary has been highlighted by a series of festive events, including the presentation of a prize at the especially dedicated symposium organized in his honor at the recent 3rd MIT Conference on Computational Fluid and Solid Mechanics held in Cambridge, MA. 1 In this preface we include comments by several of his family, friends and colleagues who have had the privilege of being a part of his life and career over the years.
Dynamic Analysis of Axially Non-Uniform Thin Cylindrical Shells
Journal of Mechanical Engineering Science
Part 1 of this paper presents a new theory for the dynamic and static analysis of axially non-uni... more Part 1 of this paper presents a new theory for the dynamic and static analysis of axially non-uniform, thin cylindrical shells. It is a hybrid of finite element and classical shell theories: the shell is subdivided into cylindrical finite elements, and the displacements within each (expressed in terms of nodal displacements), i.e., the displacement functions, are obtained using Sanders' equations for thin cylindrical shells in full. Sanders' theory gives zero strains for small rigid-body motions, so that displacement functions based on it satisfy the convergence criteria of the finite-element method. Expressions for the mass, stiffness and stress-resultant matrices are obtained, and the method for constructing the equivalent global matrices is given. This paper is supported by Part 2, where the eigenvalues of a number of shells are calculated and compared with other theories and experiments. In Part 2, the free flexural vibration characteristics of thin cylindrical shells ar...
Computational modeling of coupled fluid-structure systems with applications
Structural Engineering and Mechanics, 2008
ABSTRACT This paper outlines the development of a computational model in order to analyze the dyn... more ABSTRACT This paper outlines the development of a computational model in order to analyze the dynamic behaviour of coupled fluid-structure systems such as a) liquid containers, b) a set of parallel or radial plates. In this work a hybrid fluid-solid element is developed, capable of simulating both membrane and bending effects of the plate. The structural mass and stiffness matrices are determined using exact integration of governing equations which are derived using a combination of classical plate theory and a finite element approach. The Bernoulli equation and velocity potential function are used to describe the liquid pressure applied on the solid-fluid element. An impermeability condition assures a permanent contact at the fluid-structure interface. Applications of this model are presented for both parallel and radial plates as well as fluid-filled rectangular reservoir. The effect of physical parameters on the dynamic behaviour of a coupled fluid-structure system is investigated. The results obtained using the presented approach for dynamic characteristics such as natural frequency are in agreement to those calculated using other theories and experiments.
Theoretical model of cylindrical structures containing turbulent flowing fluids
Free vibration and response to pressure field of anisotropic thin cylindrical shells
Acquisition channel for phase and wall pressure prediction in turbulent two-phase flow
... | Lara |; INIST |; CNRS ... voir notice détaillée. Fichiers dans ce document. AR3_1.pdf, 69.5... more ... | Lara |; INIST |; CNRS ... voir notice détaillée. Fichiers dans ce document. AR3_1.pdf, 69.55Kb. Pour citer ce document : URI: http://hdl.handle.net/2042/10500. Titre: Acquisition channel for phase and wall pressure prediction in turbulent two-phase flow. Auteur: SAMI, SM; LAKIS, AA ...
Aerothermoelastic Stability of Functionally Graded Circular Cylindrical Shells
ASME 2010 7th International Symposium on Fluid-Structure Interactions, Flow-Sound Interactions, and Flow-Induced Vibration and Noise: Volume 3, Parts A and B, 2010
[ASME Conference Proceedings 2010, 939 (2010)]. Farhad Sabri, Aouni A. Lakis. Abstract. ... The e... more [ASME Conference Proceedings 2010, 939 (2010)]. Farhad Sabri, Aouni A. Lakis. Abstract. ... The effects of temperature rise and shell internal pressure on the flutter boundaries of FG circular cylindrical shell for different values of power law index are investigated. ...
This paper presents a study on the identification of modal parameters by experimental operational... more This paper presents a study on the identification of modal parameters by experimental operational modal analysis for the assessment of bridge rehabilitation. Modal parameters of the bridge before and after rehabilitation are estimated. Normal traffic flow over the bridge provided the excitation source. A single 3D accelerometer, located at mid-span of the bridge was used to obtain natural frequencies and damping ratios of the vibration modes. Identification of modal parameters was conducted using four methods including the Peak Picking method (PP), the Least Square Complex Exponential method (LSCE), the Autoregressive method (AR) and the Autoregressive Moving Average method (ARMA). During the study, the concrete deck of the bridge was replaced by orthotropic steel in order to increase its load capacity. The effect of this rehabilitation can be clearly observed through estimated vibration parameters in the stability diagrams.
Safizadeh,Lakis,Thomas,2002