A. A. Lakis

Free vibration of cylindrical shells partially filled with liquid

Abstract A theory is presented for the determination of the free vibration characteristics of vertical, thin, circular cylindrical shells, partially or completely filled with stationary liquid. The shell may be uniform or non-uniform, provided it is axially symmetric. This is a finite-element theory, using cylindrical finite elements, but the displacement functions are determined by using classical shell theory. The inertial loading of the fluid is taken into account by incorporating the virtual mass of the fluid into the mass matrix of those finite elements which are below the liquid free-surface. Calculations of the natural frequencies and eigenvectors were conducted for one such shell, for which experimental data were available. Agreement between theory and experiment was found to be quite good.

Non-linear free vibration analysis of laminated orthotropic cylindrical shells

Abstract A method to predict the influence of geometric non-linearities on the natural frequencies of an empty laminated orthotropic cylindrical shell is presented in this paper. It is a hybrid of finite element and classical thin shell theories. Sanders—Koiter non-linear and strain-displacement relations are used. Displacement functions are evaluated using linearized equations of motion. Modal coefficients are then obtained for these displacement functions. Expressions for the mass, linear and non-linear stiffness matrices are derived through the finite element method (in terms of the elements of the elasticity matrix). The uncoupled equations are solved with the help of elliptic functions. The frequency variations are first determined as a function of shell amplitudes and then compared with the results in the literature.

Fault diagnosis in power transformers using multi-class logical analysis of data

This paper presents the implementation of a novel multi-class diagnostic technique for the detection and identification of faults based on an approach called logical analysis of data (LAD). LAD is a data mining, artificial intelligence approach that is based on pattern recognition. In the context of condition based maintenance (CBM), historical data containing condition indices and the state of the machine are the inputs to LAD. After training and testing phases, LAD generates patterns that characterize the faulty states according to the type of fault, and differentiate between these states and the normal state. These patterns are found by solving a mixed 0–1 integer linear programming problem. They are then used to detect and to identify a future unknown state of equipment. The diagnostic technique has already been tested on several known machine learning datasets. The results proved that the performance of this technique is comparable to other conventional approaches, such as neural network and support vector machine, with the added advantage of the clear interpretability of the generated patterns, which are rules characterizing the faults’ types. To demonstrate its merit in fault diagnosis, the technique is used in the detection and identification of faults in power transformers using dissolved gas analysis data. The paper reaches the conclusion that the multi-class LAD based fault detection and identification is a promising diagnostic approach in CBM.

Diagnosis of rotor bearings using logical analysis of data

Purpose – The purpose of this paper is to test the applicability and the performance of an approach called logical analysis of data (LAD) on the detection of faults in rotating machinery using vibration signals.Design/methodology/approach – LAD is a supervised learning data mining technique that relies on finding patterns in a binary database to generate decision functions. The hypothesis is that a LAD‐based decision model can be used as an effective tool for automatic detection of faults in rolling element bearings. A novel Multiple Integer Linear Programming approach is used to generate patterns for the LAD decision model. Frequency and time‐based features are extracted from rotor bearing vibration signals and are pre‐processed to be suitable for use with LAD.Findings – The results show good classification accuracy with both time and frequency features.Practical implications – The diagnostic tool implemented in the form of software in a production or operations maintenance environment can be very helpfu...

Dynamic analysis of anisotropic fluid-filled conical shells

Abstract This paper presents a general theory for the dynamic analysis of thin conical anisotropic shells containing flowing fluid. The shell may be uniform or nonuniform, provided that its geometry is axisymmetric. The study is based on finite element theory, using conical finite elements, but the displacement functions are determined by means of classical shell theory. The velocity potential and Bernoulli equation for a liquid finite element yield an expression for fluid pressure, p, as a function of the modal displacements of the element and three forces (inertial, centrifugal and Coriolis) operative in the fluid flow. A number of examples are given to illustrate the theory and the dynamic behaviour of conical shells partially filled with quiescent fluid.

Prediction of the response of a cylindrical shell to arbitrary or boundary-layer-induced random pressure fields

Abstract A general theory is presented for the response, to an arbitrary random pressure field, of a uniform or axially-non-uniform thin cylindrical shell. The theory is then specialized to the case where the pressure field originates from the turbulent boundary layer of a subsonic internal flow. The basic formulation of the dynamical problem is in terms of a hybrid classical/finite element theory in which the finite elements are cylindrical frusta and the displacement functions are determined from the shell equations; the pressure forces are lumped at the nodes of the finite elements. The cross-correlation spectral density and the mean square value of the displacements of the shell are obtained for an arbitrary pressure field and for a boundary-layer pressure field. Some calculations of the latter case are conducted to illustrate the theory. In one case the theory is compared with experiment, and agreement is found to be quite good.

Shear deformation in dynamic analysis of anisotropic laminated open cylindrical shells filled with or subjected to a flowing fluid

Abstract The free vibration of anisotropic laminated composite, as well as isotropic open or closed, cylindrical shells submerged in and subjected simultaneously to an internal and external incompressible, inviscid fluid are discussed on the basis of a refined shell theory in which transverse shear deformation and rotary inertia effects are taken into account. The shell may be uniform or non-uniform in the circumferential direction. In this approach, displacements and rotations of the shell and the dynamic pressure of the fluid are modeled by a hybrid finite-element method. The displacement functions are derived from the exact solution of refined shell equations based on orthogonal curvilinear coordinates. The velocity potential and Bernoullis equation have been used to describe an expression for fluid pressure which yields three forces (inertial, centrifugal and Coriolis) of the moving fluid. The mass, stiffness and damping matrices due to the fluid effect can be obtained by an analytical integration of the fluid pressure over the liquid element. Extensive results are given of computations carried out to illustrate the theory and dynamic behavior of open and closed cylindrical shells partially or completely filled with liquid, as well as subjected to a flowing fluid. A satisfactory agreement is seen between the numerical results predicted by the present theory and the results of existing available other theories.

Finite Element Method Applied to Supersonic Flutter of Circular Cylindrical Shells

DOI: 10.2514/1.39580 The method of analysis is a combination of Sander’s thin shell theory and the classic finite element method, in which the nodal displacements are found from the exact solution of shell governing equations rather than approximatedbypolynomialfunctions.Pistontheorywithandwithoutacorrectionfactorforcurvatureisappliedto derive aerodynamic damping and stiffness matrices. The influence of stress stiffness due to internal pressure and axial loading is also taken into account. Aeroelastic equations in hybrid finite element formulation are derived and solved numerically. Different boundary conditions of the shell, geometries, and flow parameters are investigated. In all study cases, the shell loses its stability due to coupled-mode flutter and a traveling wave is observed during this dynamicinstability.Theresultsarecomparedwithexistingexperimentaldataandotheranalyticaland finiteelement solutions. The present study shows efficient and reliable results that can be applied to the aeroelastic design and analysis of shells of revolution in aerospace vehicles. Nomenclature � A� = coefficient matrix of shape functions; see Appendix B.1 a1 = freestream speed of sound � B� = coefficient matrix of strain vector; see Eq. (11) � Cf� = global aerodynamic damping matrix � cf�

Optimal synthesis of a planar four-link mechanism used in a hand prosthesis

Abstract The optimal synthesis of a planar four-link mechanism is carried out with reference to n positions of the output and coupling bars. The effects of the number of positions in the optimal dimensions of the mechanism are presented. Through a comparative study, we are able to select the number of points and the position of each which best satisfy the performance criteria. The optimal mechanism obtained is of the balance–balance type, with a minimum acceptable angle of transmission and no dead point. This crossed four-link mechanism is used in the development of a hand prosthesis.

Rogue components: their effect and control using logical analysis of data

There is a small subset of any repairable component population that can develop a failure mode outside the scope of the standard repair and overhaul procedures, which makes them “rogue”. When this happens, a Darwinian-like “natural selection” phenomenon ensures that they will be placed in the most disadvantageous position in the asset management program, negatively affecting multiple aspects of the operational and maintenance organizations. Rogue components have long plagued the airline industry and created havoc in their asset management programs. In this paper, we describe how these rogues develop, outline the natural selection process that leads to their hampering the asset management program, and examine some of the negative impacts that ensue. Then we propose a Condition based maintenance approach to control the development of these components. We explore the use of a supervised learning data mining technique called Logical analysis of data (LAD) in CBM for the purpose of detecting rogues within a population of repairable components. We apply the resulting LAD based decision model on an inventory of turbo compressors belonging to an airline fleet. Finally, we evaluate the applicability of LAD to the rogue component detection problem and review its efficiency as a decision model for this type of problem.