bluuebunny/crossref_metadata_2025 huggingface dataset:
Electric Design of Wind-Electric Water Pumping Systems
Authors: Muljadi E. , Flowers L. , Green J. , Bergey M.
Published: February 2008

Wind turbine technology has been used to pump water since ancient history. Direct mechanically coupled wind turbines are the most common method for pumping water to croplands and livestock. Many more recent wind turbines are electrically coupled, with the water pump connected to the wind turbine via a motor-generator connection. With electrical coupling, the distance and location of the water pump is independent of the location of the wind turbine. Therefore, the wind turbine can be located at an optimal wind energy site while the water pump is close to the water well or water tank. This paper analyzes a water-pumping system consisting of a wind turbine, a permanent magnet synchronous generator, an induction motor, and a centrifugal-type water pump.
A Mathematical Model of Bird Collisions With Wind Turbine Rotors
Authors: Tucker V. A.
Published: February 2008

When a bird flies through the disk swept out by the blades of a wind turbine rotor, the probability of collision depends on the motions and dimensions of the bird and the blades. The collision model in this paper predicts the probability for birds that glide upwind, downwind, and across the wind past simple one-dimensional blades represented by straight lines, and upwind and downwind past more realistic three-dimensional blades with chord and twist. Probabilities vary over the surface of the disk, and in most cases, the tip of the blade is less likely to collide with a bird than parts of the blade nearer the hub. The mean probability may be found by integration over the disk area. The collision model identifies the rotor characteristics that could be altered to make turbines safer for birds.
Variable Speed Operation of Generators With Rotor-Speed Feedback in Wind Power Applications
Authors: Muljadi E. , Butterfield C. P. , Migliore P.
Published: February 2008

The use of induction generators in wind power applications has been common since the early development of the wind industry. Most of these generators operate at fixed frequency and are connected directly to the utility grid. Unfortunately, this mode of operation limits the rotor speed to a specific rpm. Variable speed operation is preferred in order to facilitate maximum energy capture over a wide range of wind speeds. This paper explores variable speed operating strategies for wind turbine applications. The objectives are to maximize energy production, provide controlled start-up, and reduce torque loading. This paper focuses on optimizing the energy captured by operating at maximum aerodynamic efficiency at any wind speed. The control strategy we analyze uses rotor speed and generator power as the feedback signals. In the normal operating region, rotor speed is used to compute a target power that corresponds to optimum operation. With power as the control objective, the power converter and generator are controlled to track the target power at any rpm. Thus, the torque-speed characteristic of the generator is shaped to optimize the energy capture. The target power is continuously updated at any rpm. In extreme areas of the operating envelope, during start-up, shutdown, generator overload, or overspeed, different strategies driven by other system considerations must be used.
Effect of an Inert Gas on Heat and Mass Transfer in a Vertical Channel With Falling Films
Authors: Chen Wei , Vliet G. C.
Published: February 2008

The effect of inert (noncondensable) gases on the heat and mass transfer (absorption) for channel flow of water vapor in conjunction with falling aqueous LiBr films is investigated. The hydrodynamic flow of the gas in the channel is approximated as fully developed. This is a “fair” assumption because of the low Reynolds numbers resulting from the low prevailing absorber pressures. The film flow is also assumed to be hydrodynamically developed. This greatly simplifies the problem, as the momentum equation need not be considered. Otherwise the continuity, species, and thermal energy equations govern the problem. Numerical results for a nominal case are presented for the velocity, temperature, and species distributions in the gas and liquid phase regions, and for the interface absorption rate. The effects of varying several parameters (including inerts concentration) on the above variables are also presented. Comparisons are also made with limited data in the literature.
Economic Feasibility and Performance Study of a Solar-Powered Absorption Cycle Using Some Aqueous Salt Solutions
Authors: Malik I. H. , Siddiqui M. Altamush
Published: February 2008

Economic analyses of solar collectors, for optimizing generator temperatures in the absorption cycle using aqueous solutions of LiBr, LiBr–ZnBr2, LiBr–ZnBr2LiCl, and LiBr–ZnCl2–CaBr2 salts, have been carried out for a wide range of the operating conditions. Ordinary collectors with two glass covers and evacuated-tubular collectors have been selected as the sources of energy for providing hot liquid in the generator of the absorption cycle. Of the four solutions, as the working fluids in the absorption cycles, those having better coefficients of performance are the LiBr/H2O at the low evaporator temperatures, and the (LiBr–ZnBr2–LiCl)/H2O as well as the (LiBr–ZnCl2–CaBr2)/H2O at the high evaporator temperatures. Similarly, costs of the solar collectors are low, at low evaporation temperatures for the LiBr/H2O and at high temperatures for the other two solutions; the (LiBr–ZnBr2)/H2O, on the other hand, have relatively low COP and high operating costs.
Analysis and Performance of Radial Flow Rotary Desiccant Dehumidifiers
Authors: Elsayed M. M. , Chamkha A. J.
Published: February 2008

A model is developed to predict the steady periodic performance of a radial flow desiccant wheel. The model is expressed in terms of the same dimensionless parameters that are commonly used in modeling of the conventional axial flow desiccant wheel. In addition a dimensionless geometrical ratio of the volume of the matrix to the volume of the wheel core is found to affect the performance of the wheel. A finite difference technique on staggered grid is used to discretize the governing dimensionless equations. The discretized equations are solved to predict the performance of the desiccant wheel at given values of operation parameters. A sensitivity study is carried out to investigate the effect of changing any of these parameters on the performance of the wheel. The performance of the radial flow desiccant wheel is compared with that of the convential axial flow desiccant wheel having the same values of the operation parameters.
Heat Transfer During Solidification Around a Horizontal Tube With Internal Convective Cooling
Authors: Zhang Yuwen , Chen Zhongqi , Faghri A.
Published: February 2008

Theoretical solution for the solidification process around a horizontal tube with internal convection cooling is described in this paper. Boundary layer analysis and the approximate integral method were applied to obtain the solution for the natural convection in the liquid region and the transient temperature profile in the solid region. Effects of Biot number on the wall temperature and the volume fraction solidified are also discussed in this paper.
Modeling of Solar Receiver for Cracking of Liquid Petroleum Gas
Authors: Segal A. , Epstein M.
Published: February 2008

The paper presents the model of an industrial solar receiver/reactor for thermal cracking of liquid petroleum gas (LPG) at the typical temperature range of 800–850°C. The concentrated solar radiation enters the receiver located on the ground and provided with a compound parabolic concentrator (CPC) at the ceiling. This is achieved with a “reflecting solar tower.” The radiative model uses the classical concept of equivalent gray plane to represent a panel of 40 cracking tubes placed in parallel of a refractory wall of the receiver. The radiative flux distribution on each wall is calculated and the chemistry in each reactor tube is evaluated until convergence is achieved. The design of an industrial size receiver, its behavior, and performance have been evaluated using this model. The computer program based on this model was run for a variety of flow conditions, feed compositions, and pressures.
Developmental Solar Thermal Receiver Studies for the White Cliffs Solar Thermal Power Plant
Authors: Bannister P. , Mayer I. F.
Published: February 2008

The principal features of a computer simulation used as a design tool for solar thermal receivers for paraboloidal dish collectors are described. Performance predictions derived from this model are outlined and compared with experiments on two receiver designs testing the overall thermal performance and the flux distribution. It is shown that predicted performance is in good agreement and that the model is suitable for design purposes.
Chemically Enhanced Sunlight for Killing Bacteria
Authors: Block S. S. , Seng V. P. , Goswami D. W.
Published: February 2008

Solar ultraviolet (UV) photocatalyzed oxidation of chemicals with titanium dioxide (TiO2) has received considerable attention. Much less recognized, however, is the ability of the same system to destroy bacteria. This study examined this phenomenon and the conditions that effect it. Bacteria in aqueous solution were given solar exposure with titanium dioxide and their survival with time was detected. Lamps with a predominantly solar ultraviolet spectrum were also used in the experiments. Without exposure to UV light, TiO2 does not affect the bacteria. However, several common bacteria were killed in just a few minutes on solar exposure in the presence of TiO2. Whereas without TiO2 it took more than an hour to destroy them. A concentration of 0.01 percent TiO2 was most effective in killing bacteria and tenfold concentrations lower or higher were successively less effective. Inorganic and organic compounds in solution, even in small amounts, interfered with the efficiency of killing. An alkaline solution also reduced the bactericidal activity. Circulation and agitation provided by stirring to keep the TiO2 particles suspended reduced the time necessary to kill the bacteria. Time-intensity curves for killing bacteria were the same general shape with or without TiO2. This suggests that TiO2 served merely as a catalyst to increase the rate of the reaction but that the mechanism of action was not changed. The shape of the curves shows that the organisms are sensitized with a minimum intensity of radiation and that an increase doesn’t greatly increase the rate of the kill. Below this critical intensity, however, the time required for killing markedly increases as the intensity is decreased.
Improved Finite Element Modeling of the Turbofan Engine Inlet Radiation Problem
Authors: Danda Roy I. , Eversman W.
Published: February 2008

Improvements have been made in the finite element model of the acoustic radiated field from a turbofan engine inlet in the presence of a mean flow. The problem of acoustic radiation from a turbofan engine inlet is difficult to model numerically because of the large domain and high frequencies involved. A numerical model with conventional finite elements in the near field and wave envelope elements in the far field has been constructed. By employing an irrotational mean flow assumption, both the mean flow and the acoustic perturbation problem have been posed in an axisymmetric formulation in terms of the velocity potential, thereby minimizing computer storage and time requirements. The finite element mesh has been altered in search of an improved solution. The mean flow problem has been reformulated with new boundary conditions to make it theoretically rigorous. The sound source at the fan face has been modeled as a combination of positive and negative propagating duct eigenfunctions. Therefore, a finite element duct eigenvalue problem has been solved on the fan face and the resulting modal matrix has been used to implement a source boundary condition on the fan face in the acoustic radiation problem. In the post processing of the solution, the acoustic pressure has been evaluated at Gauss points inside the elements and the nodal pressure values have been interpolated from them. This has significantly improved the results. The effect of the geometric position of the transition circle between conventional finite elements and wave envelope elements has been studied and it has been found that the transition can be made nearer to the inlet than previously assumed.
Approximation Techniques for Broad-Band Acoustic Radiated Noise Design Optimization Problems
Authors: Hambric S. A.
Published: February 2008

This study investigates the applicability of various approximation methods to broadband radiated noise design optimization problems. Low-order series approximations of dynamic response may be used to replace full numerical system solutions to effect significant computer cost savings during design iterations. Also, the ease of evaluating the approximate functions may be further exploited by using global optimization search methods, such as simulated annealing, at individual design iterations. The combination of approximating radiated noise spectra and evaluating the approximate spectra for all possible design alternatives greatly increases the possibility of finding a truly optimal design. The effectiveness of the approximations is measured by considering optimization accuracy, evaluated by the algorithm’s ability to find a global or near-global minimum independent of the initial design; computational efficiency, based on the number of numerical design analyses required for convergence; and generality, where the method should be relatively independent of the problem type. Finite element models of three test cases with varying performance goals and design parameters were used to evaluate the optimization methods. Shell thicknesses, shell loss factors, and rib stiffener locations were varied to minimize structural weight and manufacturing costs while lowering broad-band radiated noise levels below a specified goal. First-order Taylor and half-quadratic series approximation optimization approaches were compared to traditional local minimization methods (Modified Method of Feasible Directions and Broydon-Fletcher-Goldfarb-Shanno). For all test cases, the approximation approaches found the global optimum design more frequently than the local minimization methods. Also, the half-quadratic method converged using fewer design evaluations than the first-order Taylor method for most test cases.
Torsional Vibrations of Synchronous Motor Driven Trains Using p-Method
Authors: Chen W. J.
Published: February 2008

Synchronous motors produce damaging oscillating torques during startup. If the system is not properly analyzed and designed, the torsional excitation can be very destructive. This paper presents a systematic approach to the dynamic analysis of synchronous motor driven rotating machinery. The p-version of the finite element method is used in the formulation of the equations of motion which provides a great deal of simplicity in the modeling process. The convergence is achieved by increasing the polynomial order of the basis functions of the geometric elements. The system damping matrix can be constructed from the element level or can be calculated by specifying the critical damping factors for a given number of modes of interest. A modified Newmark integration method is employed in the nonlinear transient response calculation. The nonlinearity of the flexible resilient couplings can be easily implemented into this direct numerical integration algorithm. The dynamic stiffness and damping of the resilient couplings are updated at each time step to ensure the dynamic equilibrium. Two examples have been employed to illustrate the validity of the proposed algorithm. The effectiveness, accuracy, and simplicity of the use of p-method on the torsional vibration of synchronous motor driven trains are demonstrated in this paper.
Whirling of Composite-Material Driveshafts including Bending-Twisting Coupling and Transverse Shear Deformation
Authors: Bert C. W. , Kim Chun-Do
Published: February 2008

A simplified theory for predicting the first-order critical speed of a shear deformable, composite-material driveshaft is presented. The shaft is modeled as a Bresse-Timoshenko beam generalized to include bending-twisting coupling. Numerical results are compared with those for both thin and thick walled shell theories and generalized Bernoulli-Euler theory.
Dynamic Response Analysis in End Milling Using Pretwisted Beam Finite Element
Authors: Liao C.-L. , Tsai J.-S.
Published: February 2008

This paper develops an analytical model to estimate the dynamic responses in end milling, i.e., dynamic milling cutter deflections and cutting forces, by using the finite element method along with an adequate end milling cutting force model. The whole cutting system includes spindle, bearings and cutter. The spindle is structurally modeled with the Timoshenko-beam element, the milling cutter with the pretwisted Timoshenko-beam element due to its special geometry, and the bearings with lumped springs and dampers. Because the damping matrix in the resulting finite element equation of motion for the whole cutting system is not of proportional damping due to the presence of bearing damping, we use state-vector approach and convolution integral to find the solution of equations of motion. To assure the accuracy of dynamic response predication, the associated cutting force model should be sufficiently precise. Since the dynamic cutting force is proportional to the chip thickness, a quite accurate algorithm for the calculation of chip thickness variation due to tool geometry, runout and spindle-tool vibration is developed. A number of dynamic cutting forces and tool deflections obtained from the present model for various cutting conditions are compared with the experimental and analytical results available in the literature, and good agreement is demonstrated for these comparisons. Therefore the present model is useful for the prediction of end milling instability. Also, the tool deflections obtained by using the pretwisted beam element are found smaller than those by straight beam elements without pretwist angle. Hence, neglecting the pretwist angle in the structural model of milling cutter may overestimate the tool deflections.
Analytical Sensitivity of Acoustic Power Radiated from Plates
Authors: Salagame R. R. , Belegundu A. D. , Koopmann G. H.
Published: February 2008

A general formulation to obtain the analytical expressions for the sensitivity of the acoustic power radiated by a vibrating structure to one of its design variables is described. The formulation, which is based on finite elements, is applied to both single frequency and broad band harmonic excitation of plates. The sensitivity coefficients indicate the effect of changing various design or modeling parameters on the acoustic power and can be used to optimize the structure for minimum sound radiation. Analytical sensitivity estimates are compared with finite difference values. Results show that analytical sensitivity analysis is important from both computational time and accuracy points of view.
Active Vibration Control of the Axially Moving String by Wave Cancellation
Authors: Chung C. H. , Tan C. A.
Published: February 2008

Active vibration control of an axially moving string by wave cancellation is presented. The control problem is formulated in the frequency domain. An exact, closed-form expression for the transfer function of the controlled system, consisting of the flexible structure, a feedback control law and the dynamics of the sensing and actuation devices, is derived. It is shown that all vibration modes can be stabilized and that the controlled system has no resonance. Moreover, the designed controller is applicable to the control of the string transverse vibration under various kinds of loading and constraint conditions. Results for the response of the controlled string under different excitations are presented and discussed along with the wave propagation and cancellation characteristics.
Simple Modeling and Analysis for Crankshaft Three-Dimensional Vibrations, Part 1: Background and Application to Free Vibrations
Authors: Okamura H. , Shinno A. , Yamanaka T. , Suzuki A. , Sogabe K.
Published: February 2008

To simplify the analysis of the three-dimensional vibrations of automobile engine crankshafts under firing conditions, the crankshaft was idealized by a set of jointed structures consisting of simple round rods and simple beam blocks of rectangular cross-section. The front pulley, timing gear, and the fly-wheel were idealized by a set of masses and moments of inertia. The main journal bearings were idealized by a set of linear springs and dash-pots. For each constituent member, the dynamic stiffness matrix was derived (in closed form) from the transfer matrix. Then the dynamic stiffness matrix for the total crankshaft system was constructed, and the natural frequencies and mode shapes were calculated. The modeling and analysis procedures were applied to the analysis of free vibrations of four kinds of crankshafts: single cylinder, three-cylinder in-line, four-cylinder in-line, and V-six engines. The different coupling behavior of the three-dimensional vibrations in the planar-structure and the solid-structure crankshaft is discussed, and the influence of the bearing oil film stiffness on the crankshaft natural frequency is also analyzed.
Vibration of Cylinders Partially-Filled with Liquids
Authors: Mistry J. , Menezes J. C.
Published: February 2008

A mathematical model of a coupled cylindrical shell-fluid dynamic interaction problem based on an axisymmetric finite element analysis is presented. The dynamic solution of the problem is separated into axisymmetric and nonaxisymmetric behaviors and a method is presented for the nonaxisymmetric transient response analysis. It is employed to estimate the natural frequencies of the cylinders partially filled with water. The theoretical predictions are compared with the experimental results and it is shown that there eixsts a good correlation between the two sets of results.
Inertia Effects on Compressible Squeeze Films
Authors: Kang Jongmin , Xu Zhaoshun , Akay Adnan
Published: February 2008

In this paper, the combined effects of compressibility and fluid inertia in a squeeze film are considered. The governing equations are derived using an integral method for a one-dimensional case, initially considering a combination of Couette and Poiseuille Flows. Numerical and experimental results are obtained for the case of a pure squeeze film between flat circular disks. Influence of the film geometry was examined by considering a cavity on the surface of one of the disks. The numerical solutions are obtained by use of the Crank-Nicholson method with Lax modification. Comparison of the numerical results for pressure in the film with the experimental results show good agreement. The inertia of the fluid is found to significantly influence the pressure waveform in the film by altering the phase of the pressure developed in the film with respect to the oscillating disk. It is shown that these phase changes lead to “resonances” in the mean bearing force. The results also show that the mean bearing force can be superambient or subambient depending on the squeeze number. Both the damping and the bearing force show a “jump” at a critical squeeze number. Damping due to the fluid layer is shown to be amplitude-dependent.
Instantaneous Directivity in Jet Noise by Multipole Decomposition
Authors: Ghosh A. , Bridges J. , Hussain F.
Published: February 2008

Noise generated by nearly axisymmetric vortex pairing in an excited low Mach number jet was studied experimentally as a prototypical jet noise source using a three-dimensional microphone array. The measured time-dependent sound field was decomposed into its multipole components using general linear regression techniques. Although the vortex interactions were very nearly axisymmetric and thus a nearly axisymmetric lateral quadrupole sound field was expected, the sound field was found to have large departures from axisymmetry. This acoustic sensitivity to symmetry breaking in the vorticity field was also shown in calculations using numerical vortex models and vortex sound theory. The directivity of the far-field sound gets smeared out in the time averaged data due to the tilting of the quadrupoles; this emphasizes the need for studying the jet sound pressure field in four dimensions (x, t) to better understand the mechanisms of noise generation.
Improved Method for In-situ Elastic Constants of Isotropic and Orthotropic Composite Materials Using Plate Modal Data With Trimodal and Hexamodal Rayleigh Formulations
Authors: Ayorinde E. O. , Gibson R. F.
Published: February 2008

This paper relates a further development of an earlier work by the authors, in which they presented a method for deriving the four independent elastic constants (longitudinal and transverse Young’s moduli, in-plane shear modulus and major Poisson’s ratio) of an orthotropic material from resonance data obtained in a modal analysis of a freely-supported plate made out of the material. In the present work, simple averaging, as opposed to the weighted averaging employed in the earlier version, is utilized. The use of three modes and six modes is compared on the basis of results of both the forward and the inverse problems. Results are obtained for materials spanning orthotropy ratios from unity (i.e., isotropic) to about 13. The results suggest that, in comparison with our earlier method, the improved method is easier to use and is just as accurate. The adaptability of the basic method developed by the authors to various levels and types of refinement is also demonstrated, as is the potential of the method for fast characterization of elastic properties of advanced composites.
Breathing Vibrations of a Horizontal Circular Cylindrical Tank Shell, Partially Filled With Liquid
Authors: Amabili M. , Dalpiaz G.
Published: February 2008

A theoretical approach to study breathing vibrations of cylindrical shells with horizontal axis, partially filled with liquid, is delineated and the results of some modal tests conducted on an industrially-manufactured tank are presented and discussed. The good agreement between theoretical and experimental results is preliminarily verified in the case of both an empty and completely full shell, in order to confirm that it is possible to apply the theoretical approach to real structures. The modal properties of a partially-filled shell as a function of liquid level are then experimentally studied, the mode shapes are compared using the Modal Assurance Criterium and a qualitative explanation of the dynamic behavior is proposed.
A Spectral Description of Inertial Effects in Fluid-Loaded Plates
Authors: Ginsberg J. H. , Cunefare K. , Pham H.
Published: February 2008

The wavenumber-based formulation of the surface variational principle (SVP) describes the surface pressure and displacement as a comparatively small set of interacting waves. It enables one to pose questions of parametric sensitivity from a global perspective. The present paper is the first application of such an approach to the question of the level of detail to which a model must be constructed. It considers a two-dimensional problem of an elastic plate in an infinite baffle, with pinned boundary conditions. A study by Feit and Johnson (1991) demonstrated that the signal scattered by the plate is significantly altered by the presence of an attached mass, and that the distribution of mass as well as the total mass, is important. In order to explore these issues, a line mass attached to the plate is replaced in the SVP formulation by a continuous spatial distribution. The functional form of this distribution is described in a spectral manner using Fourier series, whose ascending orders represent successive stages in refinement of the scale to which a model describes inertial effects. The excitation applied to the plate is taken as a concentrated line harmonic force. With the excitation held fixed, the influence of each spectral component of inertial distribution on the surface response and radiated power are assessed. Evaluations carried out for a range of frequencies shed light on how small scale inertial heterogeneities can influence macroscopic radiation features.
Vibration and Damping Analysis of Cylindrical Shells With Constrained Damping Treatment—A Comparison of Three Theories
Authors: Ramesh T. C. , Ganesan N.
Published: February 2008

Cylindrical shells with a constrained damping layer treatment are studied using three theories. The finite element method is made use of in the study. The nondimensional frequencies and loss factors predicted by the three theories are compared and the theories are evaluated. The importance of inclusion of the extensional effects in the core and its effect on the loss factor is brought out in this study.
A Modified MSE Method for Viscoelastic Systems: A Weighted Stiffness Matrix Approach
Authors: Hu B.-G. , Dokainish M. A. , Mansour W. M.
Published: February 2008

The conventional Modal Strain Energy method (MSE is briefly discussed. Several indices are proposed to characterize a viscoelastically damped system. An Overall Error Index is proposed to assess the accuracy of the solution. A Modified MSE method is developed for a better evaluation of modal damping. Instead of neglecting the damping stiffness matrix in the determination of the eigenvectors, as is the case in the conventional MSE method, the authors used a weighted matrix to solve a real eigenvalue problem. With such modification the estimation of the modal damping is often improved. Numerical simulation of multi degree-of-freedom systems are reported using the proposed Modified MSE method and the indices.
Improvement of Observer/Kalman Filter Identification (OKID) by Residual Whitening
Authors: Phan M. , Horta L. G. , Juang J.-N. , Longman R. W.
Published: February 2008

This paper presents a time-domain method to identify a state space model of a linear system and its corresponding observer/Kalman filter from a given set of general input-output data. The identified filter has the properties that its residual is minimized in the least squares sense, orthogonal to the time-shifted versions of itself, and to the given input-output data sequences. The connection between the state space model and a particular auto-regressive moving average description of a linear system is made in terms of the Kalman filter and a deadbeat gain matrix. The procedure first identifies the Markov parameters of an observer system, from which a state space model of the system and the filter gain are computed. The developed procedure is shown to improve results obtained by an existing observer/Kalman filter identification method, which is based on an auto-regressive model without the moving average terms. Numerical and experimental results are presented to illustrate the proposed method.
Accurate Vibration Analysis of Simply Supported Rhombic Plates by Considering Stress Singularities
Authors: Huang C. S. , McGee O. G. , Leissa A. W. , Kim J. W.
Published: February 2008

This is the first known work which explicitly considers the bending stress singularities that occur in the two opposite, obtuse corner angles of simply supported rhombic plates undergoing free, transverse vibration. The importance of these singularities increases as the rhombic plate becomes highly skewed (i.e., the obtuse angles increase). The analysis is carried out by the Ritz method using a hybrid set consisting of two types of displacement functions, e.g., (1) algebraic polynomials and (2) corner functions accounting for the singularities in the obtuse corners. It is shown that the corner functions accelerate the convergence of solution, and that these functions are required if accurate solutions are to be obtained for highly skewed plates. Accurate nondimensional frequencies and normalized contours of the vibratory transverse displacement are presented for simply supported rhombic plates with skew angles ranging to 75 deg. (i.e., obtuse angles of 165 deg.). Frequency and mode shapes of isosceles and right triangular plates with all edges simply supported are also available from the data presented.
Measured Transitions Between Sticking and Slipping at Lubricated Line Contacts
Authors: Polycarpou A. A. , Soom A.
Published: February 2008

Sets of unsteady friction experiments are presented. The data allow an interpretation to be made of the principal physical phenomena that occur during the transitions that take place during impending slip or stick in interrupted sliding. The test geometry is that of a lubricated line contact, to which different patterns of velocity oscillation are applied. During acceleration from a rest or near-rest condition, a tangential contact stiffness and a microslip regime are observed before macroscopic sliding takes place. We also note a region of “microstick,” somewhat analogous to microslip, which makes it difficult to unambiguously identify a particular point where sticking actually begins. High frequency vibrations were also monitored, and while interesting, do not appreciably affect the main friciton behavior. The friction behavior is considerably more complicated than for dry contacts. Implications to the modeling of unsteady lubricated friction are discussed.
A Forced Response Analysis and Application of Impact Dampers to Rotordynamic Vibration Suppression in a Cryogenic Environment
Authors: Moore J. J. , Palazzolo A. B. , Gadangi R. , Nale T. A. , Klusman S. A. , Brown G. V. , Kascak A. F.
Published: February 2008

A high speed damper test rig has been assembled at Texas A&M University to develop rotordynamic dampers for rocket engine turbopumps that operate at cryogenic temperatures, such as those used in the space shuttle main engines (SSMEs). Damping is difficult to obtain in this class of turbomachinery due to the low temperature and viscosity of the operating fluid. An impact damper has been designed and tested as a means to obtain effective damping in a rotorbearing system. The performance and behavior of the impact damper is verified experimentally in a cryogenic test rig at Texas A&M. Analytical investigations indicate a strong amplitude dependence on the performance of the impact damper. An optimum operating amplitude exists and is determined both analytically and experimentally. In addition, the damper performance is characterized by an equivalent viscous damping coefficient. The test results prove the impact damper to be a viable means to suppress vibration in a cryogenic rotorbearing system.
A Time-Domain Computational Simulation of Acoustic Silencers
Authors: Selamet A. , Dickey N. S. , Novak J. M.
Published: February 2008

A one-dimensional finite difference numerical method is applied for the simulation of sound attenuation in reactive silencers placed in an impedance tube configuration. This time-domain approach solves the nonlinear fundamental balance equations of mass, momentum and energy, and the equation of state. The temporal solution is first converted to the frequency domain by the Fast Fourier Transform, which is then processed by the two-microphone technique to yield transmission loss. Results are presented for the attenuation of acoustic disturbances in four basic configurations including the expansion chamber, Helmholtz resonator, quarter-wave resonator and Herschel-Quincke tube. A comparison to available experimental data and linearized acoustic theory shows excellent agreement for silencers with both anechoic and echoic terminations.
Fundamental Structural-Acoustic Idealizations for Structures with Fuzzy Internals
Authors: Pierce A. D. , Sparrow V. W. , Russell D. A.
Published: February 2008

Fundamental issues relative to structural vibration and to scattering of sound from structures with imprecisely known internals are explored, with the master structure taken as a rectangular plate in a rigid baffle, which faces an unbounded fluid medium on the external side. On the internal side is a fuzzy structure, consisting of a random array of point-attached spring-mass systems. The theory predicts that the fuzzy internal structure can be approximated by a statistical average in which the only relevant property is a function mF(Ω) which gives a smoothed-out total mass, per unit plate area, of all those attached oscillators which have their natural frequencies less than a given value Ω. The theory also predicts that the exact value of the damping in the fuzzy structure is of little importance, because the structure, even in the limit of zero damping, actually absorbs energy with an apparent frequency-dependent damping constant proportional to dmF(W)/dω incorporated into the dynamical description of the master structure. A small finite value of damping within the internals will cause little appreciable change to this limiting value.
Active Attenuation of Periodic Vibration in Nonlinear Systems Using an Adaptive Harmonic Controller
Authors: Sutton T. J. , Elliott S. J.
Published: February 2008

The article describes a nonlinear adaptive controller for the attenuation of harmonic disturbances in nonlinear systems. The steepest descent algorithm is used to adapt the controller coefficients and synthesize the optimum periodic waveform at the input of the nonlinear system, giving the best attenuation of the disturbance. A simple frequencydomain model of the system is required to implement the steepest descent approach. The scheme is particularly applicable where the secondary actuator used for active vibration control is a nonlinear device such as a magnetostrictive actuator. Simulation studies show that an adaptive scheme of this type is robust to the choice of frequency-domain model when the nonlinearity is hysteretic, but much more sensitive when the nonlinearity is a saturation function. A real-time adaptive harmonic controller has been built and used to control the motion of a magnetostrictive actuator. With dc and 7 harmonics under control the system was able to overcome the inherent nonlinearity of the actuator at fundamental frequencies in excess of 3 kHz.
Variable Residue Method for Modal Parameter Estimation
Authors: Lin R. M. , Lim M. K. , Liew K. M.
Published: February 2008

A new method for the estimation of modal parameters is presented in this paper. Unlike the majority of the existing methods which involve complicated curve fitting and interpolative procedures, the proposed method calculates the modal parameters by using different sets of frequency response function (FRF) data points around resonance and assumes that the residue (contribution of other modes) of the mode to be analyzed can vary with different frequency. Such features can make the proposed method be very accurate and versatile, especially in the case where modal parameters of close modes are to be identified. The method is further developed to cope with the practical case where FRF data are measured from nonlinear structures and has been found to be very effective. To demonstrate its practical applicability, the method has been applied to data measured from real engineering structures, both linear and nonlinear, and promising results have been obtained.
Design and Analysis of Fiber Enhanced Viscoelastic Damping Polymers
Authors: Alberts T. E. , Xia Houchun
Published: February 2008

A new composite damping material is investigated, which consists of a viscoelastic matrix and high elastic modulus fiber inclusions. This fiber enhanced viscoelastic damping polymer is intended to be applied to lightweight flexible structures as a surface treatment for passive vibration control A desirable packing geometry for the composite material is proposed, which is expected to produce maximum shear strain in the viscoelastic damping matrix. Subsequently, a micromechanical model is established in which the effect of fiber segment length and relative motion between neighboring fibers are taken into account. Based on this model, closed form expressions for the effective storage and loss properties of the damping material are developed, and an optimal relation between design parameters, such as the length, diameter, spacing, and Young’s modulus of fibers and the shear modulus of viscoelastic matrix, is derived for achieving maximum damping performance. To address the verification of the development, the theoretical results are compared with NASTRAN finite element results. Upon comparison of an enhanced viscoelastic damping treatment with a conventionally constrained layer damping treatment, it is found that the enhanced polymer provides a significant improvement in damping performance.
Time Domain Modeling of Linear Viscoelasticity Using Anelastic Displacement Fields
Authors: Lesieutre G. A. , Bianchini E.
Published: February 2008

A time domain model of linear viscoelasticity is developed based on a decomposition of the total displacement field into two parts: one elastic, the other anelastic. The anelastic displacement field is used to describe that part of the strain that is not instantaneously proportional to stress. General coupled constitutive equations for (1) the total and (2) the anelastic stresses are developed in terms of the total and anelastic strains, and specialized to the case of isotropic materials. A key feature of the model is the absence of explicit time dependence in the constitutive equations. Apparent time-dependent behavior is described instead by differential equations that govern (1) the motion of mass particles and (2) the relaxation of the anelastic displacement field. These coupled governing equations are developed in a parallel fashion, involving the divergence of appropriate stress tensors. Boundary conditions are also treated: the anelastic displacement field is effectively an internal field, as it is driven exclusively through coupling to the total displacement, and cannot be directly affected by applied loads. In order to illustrate the use of the method, model parameters for a commonly-used high damping polymer are developed from available complex modulus data.
Wave Propagation in Strings with Rigid Bodies
Authors: Li Li , Vandiver J. Kim
Published: February 2008

This paper studies wave propagation in strings with rigid bodies using the method of transfer matrices. The transmission property of a single rigid body is investigated. It is found that when the size of a rigid body is included, a symmetrically defined rigid body will transmit wave energy completely at a non-zero frequency defined by the tension, the length of the body, the mass of the string replaced by the body, and the mass of the body. Using the concept of impedance matching, the effect of a discontinuity on wave transmission in an infinite string system is revealed. The same idea is extended to the study of wave propagation in a string with multiple, equally-spaced rigid bodies (a periodic structure). The input impedance of such a system and the conditions of complete transmission are expressed in terms of the transfer matrix. The input impedance is used to identify the frequencies at which there is complete wave transmission. These frequencies are related to the natural frequencies of the corresponding finite system and constitute the so-called propagation zones. The results of this work may be applied to the propagation of vibration in complex cable systems such as oceanographic moorings.
Geometrically Nonlinear Formulations of Beams in Flexible Multibody Dynamics
Authors: Mayo J. , Dominguez J. , Shabana A. A.
Published: February 2008

In this paper, the equations of motion of flexible multibody systems are derived using a nonlinear formulation which retains the second-order terms in the strain-displacement relationship. The strain energy function used in this investigation leads to the definition of three stiffness matrices and a vector of nonlinear elastic forces. The first matrix is the constant conventional stiffness matrix; the second one is the first-order geometric stiffness matrix; and the third is a second-order stiffness matrix. It is demonstrated in this investigation that accurate representation of the axial displacement due to the foreshortening effect requires the use of large number or special axial shape functions if the nonlinear stiffness matrices are used. An alternative solution to this problem, however, is to write the equations of motion in terms of the axial coordinate along the deformed (instead of undeformed) axis. The use of this representation yields a constant stiffness matrix even if higher order terms are retained in the strain energy expression. The numerical results presented in this paper demonstrate that the proposed new approach is nearly as computationally efficient as the linear formulation. Furthermore, the proposed formulation takes into consideration the effect of all the geometric elastic nonlinearities on the bending displacement without the need to include high frequency axial modes of vibration.
Modal Controllability and Observability of General Mechanical Systems
Authors: Yang B.
Published: February 2008

Controllability and observability are studied for general mechanical systems with combined effects of damping, gyroscopic and circulatory forces. A new modal analysis is proposed to represent the system transfer functions by the nonorthogonal eigenvectors that are associated with the original equations of motion. Investigation of linear independence of the rows and columns of the transfer functions yields the modal controllability and observability conditions. Because of their explicit relationships with the vibration modes, the controllability and observability tests require less computation than the conventional criteria, avoid trial and error in selection and positioning of actuators and sensors, and can be applied to systems with unidentified parameters. Moreover, the closed-loop root locus sensitivity coefficients are examined to give insights into modal controllability and observability, and to provide useful guidance for active controller design.
Limit Loads for Pipe Elbows Subjected to In-Plane Opening Moments and Internal Pressure
Authors: Shalaby M. A. , Younan M. Y. A.
Published: February 2008

The purpose of this study is to determine limit loads for pipe elbows subjected to inplane bending moments that tend to open the elbow (i.e., increase its radius of curvature), and the influence of internal pressure on the value of the limit load. Load-deflection curves were obtained, and from these curves plastic collapse and instability loads at various values of internal pressure were determined. This was done for different pipe bend factors (h = Rt/r2) using the nonlinear finite element analysis code (ABAQUS) with its special elbow element. A set of limit curves was generated from the results. These curves show the variation of collapse and instability loads with internal pressure for different elbows. Collapse loads were found to increase and then decrease with increasing pressure for all the elbow geometries studied. Instability loads were difficult to reach because of the large stiffening effect of the elbow cross-sectional deformation, and they were generally found to decrease with increasing pressure.
On the Evaluation of Dynamic Stresses in Pipelines Using Limited Vibration Measurements and FEA in the Time Domain
Authors: Moussa W. A. , AbdelHamid A. N.
Published: February 2008

A practical technique is investigated for the determination of dynamic stresses in pipelines through the use of finite element method (FEM) and field measurement vibrations at selected points. Numerical simulation of a randomly loaded pipeline structure is used to establish the validity of the technique in the time domain. The analysis is carried out for a fixed-hinged pipe model. The results show that lack of coincidence between the vibration measurement points (VMPs) and the exciting force, or the use of only translational vibration measurements (TVMs) produce an approximate stress picture. The extent of the “error” in these cases is found to depend on the density of the VMPs and the proximity between these points and the exciting force location. A safety-related risk assessment is applied to find the minimum distance between measuring points that is needed to meet design codes reliability specifications.