bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural Damping Enhancement Via Non-Obstructive Particle Damping Technique
Authors: Panossian H. V.
Published: June 2008

Presented herein is a novel passive vibration damping technique that is referred to as “Non-Obstructive Particle Damping (NOPD).” The NOPD technique consists of making small diameter holes (or cavities) at appropriate locations inside vibrating structures and filling these holes to appropriate levels with particles which yield the maximum damping effectiveness for the desired mode (or modes). Powders, spherical shaped, metallic, non-metallic or liquid particles (or mixtures) with different densities, viscosities and adhesive or cohesive characteristics can be used.
Vibration Damping Through Liquid Sloshing, Part I: A Nonlinear Analysis
Authors: Welt F. , Modi V. J.
Published: June 2008

Energy dissipation due to sloshing liquid in rigid torus shaped containers is studied using the potential flow model in conjunction with the boundary layer correction. The analysis is based on the perturbation method to solve for the nonlinear free surface equation. Special consideration is given to the case of resonant interactions which yield interesting damping characteristics. The results suggest that large diameter ratios and low liquid heights lead to efficient damper design. The concept is applicable to a wide spectrum of problems characterized by low frequency oscillations as encountered in wind, ocean and earthquake engineering.
Computation of Acoustic Shape Design Sensitivity Using a Boundary Element Method
Authors: Smith D. C. , Bernhard R. J.
Published: June 2008

Numerical acoustic radiation prediction schemes have developed to the point where they can be used to reliably predict the acoustic response of a vibrating structure. However, the objective of many of the applications of these techniques is the design of a best structural/acoustic configuration for a given application. Thus, the acoustic radiation prediction scheme must be incorporated into a design methodology. Traditionally, the most difficult design variables to incorporate into numerical prediction schemes have been those which define the geometry of the configuration, normally referred to as shape variables. In this investigation, a technique for computing the design sensitivity of the radiated pressure solution to shape variables is formulated and verified. The method uses a boundary element implementation of the Helmholtz integral equation. The potential problems at the characteristic nonuniqueness frequencies of the Helmholtz integral equation are also addressed. The technique is verified for several pulsating sphere examples where analytical solutions are available.
Bifurcations of a Nonlinear Two-Degree-of-Freedom System Under Narrow-Band Stochastic Excitation
Authors: Lin R. , Huseyin K. , To C. W. S.
Published: June 2008

In this paper, bifurcations of a nonlinear two-degree-of-freedom system subjected to a narrow-band stochastic excitation are investigated. Under the assumption that the correlation time greatly exceeds the relaxation time, a quasi-static approach combined with averaging method is adopted to obtain the bifurcation equations, and the singularity theory is applied to analyze the bifurcations. It is demonstrated that bifurcation patterns jump from one to another due to the influence of a random parameter. The probabilities of the jumping bifurcation patterns are given.
Does Curve Veering Occur in the Eigenvalue Problem of Rotors?
Authors: Jei Yang-Gyu , Lee Chong-Won
Published: June 2008

Recently, Crandall and Yeh (1989) reported an interesting phenomenon in the Campbell diagram of a single spool machine. In their words, “It is interesting that . . . the two modes form a coupled system and the curves repel each other avoiding an intersection” in the Campbell diagram, which is the so-called curve veering in the eigenvalue problem. In this paper, the existence of the curve veering in the eigenvalue problem of general rotor-bearing systems are verified and the criteria of the curve veering is evaluated. The abrupt but continuous changes of mode shapes during veering are also illustrated.
Self-Synchronization of Two Unbalanced Rotors
Authors: Paz M. , Cole J. D.
Published: June 2008

The paper presents an analytical method to ascertain the conditions for self-synchronization of a mechanical system excited by two unbalanced rotors with their axes of rotation arranged in any arbitrary configuration. It is shown that the application of Hamilton’s Principle reveals the existence of two solutions for the angle of self-synchronization of which only one produces a stable condition.
Cross-Feedback Stabilization of the Digitally Controlled Magnetic Bearing
Authors: Okada Y. , Nagai B. , Shimane T.
Published: June 2008

A method of stabilizing a high speed rotor supported by magnetic bearings is presented. The magnetic bearing is controlled by a digital controller with rotationally synchronized interruption. The main problem with the rotating disc is the cross-coupling effect caused by the gyroscopic or inductive forces which sometimes make the high speed rotor unstable. Standard PID control is carried out with constant time interval interruption, while the rotational interrupt subroutine performs the cross-coupling feedback. The cross-feedback in the x-y directions well compensates for the undesirable coupling effect. This scheme is applied to a four-mass, two-bearing rotor system and its capability is tested.
On-Line Thermal Balancing Technique for a Large Turbo-Generator
Authors: Zumbach M. , Schweitzer G. , Schoellhorn K.
Published: June 2008

In this paper a new technique is proposed to rebalance automatically a large turbo-generator during operation. The sensitivity of the rotor unbalances to thermal asymmetries in the rotor is exploited by mounting some heating elements and using them as actively controlled actuators. The shaft vibrations are measured and used as input signals of the feedback controller. Unbalances thus can be compensated during rotor operation. A theoretical model of the thermo-elastic rotor has been developed and an appropriate closed-loop control system has been designed. The simulation results are verified on a special test rig with digital control allowing for varioius control strategies and various operating conditions.
Tracking Dynamics of In-line Suspensions in High-Performance Rigid Disk Drives with Rotary Actuators
Authors: Chiou S. S. , Miu D. K.
Published: June 2008

The laser Doppler interferometric modal measurement technique is combined with finite element analysis to study the dynamics of in-line suspensions under actual operating conditions in commercially available small form factor rigid disk drives equipped with rotary actuators. It is shown that the sway mode, which is a result of in-plane bending of the load beam, is the primary source of radial slider motions and contributes significantly to off-track error. It is also shown that the initial deformed geometry of the suspension, which is determined by the boundary condition during large angle deflection and the nature of the pre-bend shape, has direct effects on the in-plane stiffness of the suspension and therefore the sway mode frequency.
Analysis of the Dynamics of a Type 4 Suspension
Authors: Jeans A. H.
Published: June 2008

A finite element model of a suspension for read-write heads in disk drives is developed. The modelled vibration modes are compared with measurements and agree well in shape and frequency. The sway mode, of particular concern in disk drives with rotary actuators and laterally actuated suspensions, is investigated more thoroughly with regard to its sensitivity to stiffness, thickness, and shape of various parts of the load-beam.
Dynamic Analysis of Engine-Mount Systems
Authors: Ashrafiuon H. , Nataraj C.
Published: June 2008

This paper examines the forced response of an airplane engine supported by an elastic foundation. It is assumed that the vibrations of the engine and the foundation are small enough such that the equations of motion are linear. The engine is modeled as a rigid body connected to the foundation by standard industrial rubber mounts which act as three-dimensional springs with a significant amount of hysteresis damping. Three fundamental models of the foundation are considered; rigid, statically flexible, and dynamically flexible. In the flexible cases, the foundation is modeled as a clamped circular plate, infinite plate, or any structure identified by a finite element stiffness matrix. In all cases, the mass, stiffness, and damping matrices of the engine-mount system are constructed and the frequency response to the rotating unbalance is determined. For the infinite and clamped circular plate cases, analytical methods are used to determine the real and imaginary parts of the flexibility matrix at different frequencies in response to the harmonic forces transmitted to the plate through the rubber mounts. It is shown here that the foundation elasticity may have a significant effect on the engine vibration and the mounting forces transmitted from the engine to the structure. It is also shown that only the dynamic model of the foundation is able to capture the correct response of the system at frequencies close to the foundation’s natural frequencies.
Optimal Semi-Active Suspension with Preview Based on a Quarter Car Model
Authors: Hac´ A. , Youn I.
Published: June 2008

This paper deals with the synthesis of an optimal yet practical finite preview controller for a semi-active dissipative suspension system based on a two-degree-of-freedom (2-DOF) vehicle model. The proposed controller utilizes knowledge about approaching road disturbances obtained from preview sensors to minimize the effect of these disturbances. A truly optimal control law, which minimizes a quadratic performance index under passivity constraints, is derived using a variational approach. The optimal closed loop system becomes piecewise linear varying between two passive systems and a fully active one. It is shown that the steady state system response to a periodic input is also periodic and its amplitude is proportional to the amplitude of the input. Therefore, frequency domain characteristics in a classical sense can be generated. The problem formulation and the analytical solution are given in a general form and hence they apply to any bilinear system with system disturbances that are a priori unknown but some preview information is possible. The results of this analysis are applied to a quarter car model with semi-active suspension whose frequency domain and time domain performances are evaluated and compared to those of fully active and passive models. The effect of preview time on the system performance is also examined.
Observations of Subcritical Superharmonic and Chaotic Response in Rotordynamics
Authors: Ehrich F.
Published: June 2008

When a rotor, excited by unbalance, is operating eccentrically within a clearance and in local contact with the stator it behaves as a bilinear oscillator with a natural periodic motion that resembles bouncing. When excited by unbalance at a subcritical rotative speed which is exactly or nearly 1/Nsuper times its natural frequency, the nonlinear system will respond by bouncing at or nearly at its natural frequency, or superharmonically at a frequency exactly Nsuper times the operating speed or forcing frequency. As in supercritical subharmonic response, there is a zone with characteristics of chaotic behavior in the transition zone between any order of superharmonic response and the next highest order of superharmonic response. There is also an intricate pattern of progressive bifurcations of the orbit on entry into this characteristically chaotic region and a reverse progression on exit from this region. The response is a mirror image or reciprocal set of the more thoughly studied supercritical subharmonic response of the same bilinear oscillator system which, when excited by unbalance at a supercritical rotative speed which is exactly or nearly a whole number Nsub times its natural frequency, the nonlinear system will respond by bouncing at exactly or nearly its natural frequency at a frequency exactly 1/Nsub times the operating speed or forcing frequency. Such supercritical subharmonic response is also characterized by the appearance of characteristically chaotic behavior in the transition zone between successive orders of subharmonic response and by patterns of progressive bifurcations of the orbit on entry into and exit from each region of characteristically chaotic response. Various aspects of subcritical superharmonic response are studied in a numerical model of the nonlinear system, and are compared to data taken on the core spool of an aircraft engine gas turbine. The engine data show many of the unique characteristics of response, wave form, and spectral content predicted by the numerical model of the bilinear oscillator when operating at subcritical rotative speed.
Spectral Analysis of Wave Motion in Plane Solids With Boundaries
Authors: Rizzi S. A. , Doyle J. F.
Published: June 2008

A spectral formulation is employed whereby in-plane stress waves are synthesized from the superposition of components at discrete frequencies and wavenumbers. The summations are performed using the fast Fourier transform and the Fourier series, respectively. Because the components are discrete, the solution to problems (over the entire field) with completely arbitrary loading, both in time and space, is made tractable. Waves generated from a line load acting on an infinite and semiinfinite plane are first considered. A cascade approach is then adopted for the treatment of these waves incident on a free, fixed, and elastic boundary. At each stage, the results are compared with those obtained from the available classical solutions and/or finite element results. These studies will form the basis for the investigation of in-plane stress waves in multiply layered media.
On the Relationship Between Veering of Eigenvalue Loci and Parameter Sensitivity of Eigenfunctions
Authors: Chen Pei-Tai , Ginsberg J. H.
Published: June 2008

The nature of the eigensolution of a general self-adjoint system in which two eigenvalues become nearly equal is studied for the relationship between the eigenvalue loci, which are the plot of eigenvalues versus a system parameter, and the manner in which the eigenfunctions change with changes in that parameter. A perturbation series solution is used to relate the eigenvalues and eigenfunctions for different values of the parameter. Criteria governing the occurrence of veering of these loci, as opposed to intersection, are established. It is proven that in the veering region, the eigenfunctions are linear combinations of the corresponding eigenfunctions at the outer limits of the veering zone, and that those combinations are highly sensitive to the value of the system parameter. A corollary is that in systems whose individual subcomponents are lightly coupled and nearly identical, eigenvalue veering and high mode sensitivity are associated with mode localization, in which some of the system modes are strongly enhanced in a small portion of the domain. An example of a two-span beam with irregular spacing of the supports, whose exact eigensolution has been shown to feature mode localization and high mode sensitivity, demonstrates the accuracy of the perturbation solution. A rectangular clamped membrane is used to illustrate the relationship between approximation error in the derivation of the system equations and eigenvalue veering.
An Application of Torsional Wave Analysis to Turbogenerator Rotor Shaft Response
Authors: Bogacz R. , Szolc T. , Irretier H.
Published: June 2008

In this paper transient torsional vibrations of a steam turbogenerator rotor shaft system due to high speed reclosing of the electric network are investigated. The analysis is performed using torsional elastic wave theory applied to a continuous model in the form of a stepped shaft. Wave solutions of the equations of motion are used in order to determine dynamic torsional elastic moments and vibratory angular velocities in cross-sections of the turbine shafts. The results are illustrated in the form of graphs.
A Modified Model Referenced Adaptive Control Technique for Nonlinear Dynamic Systems
Authors: Sardar H. M. , Ahmadian M.
Published: June 2008

It has been shown that the model referenced adaptive control (MRAC) scheme developed by Dubowsky and DesForges (1979) is effective for controlling nonlinear systems provided that the adjusting mechanism, which modifies the control gains, is selected properly. This study presents a modified MRAC Scheme that includes a technique for effectively designing the adjusting mechanism. A fundamental understanding of the relationship between the control process and the adjusting mechanism is developed. Next, this knowledge is used to establish a systematic procedure for designing the adjusting mechanism through an optimization process. Simulation results for the UCLA arm are presented to demonstrate the effectiveness and validity of the method.
Dynamic Deflection of Paper Emerging from a Channel
Authors: Stolte J. , Benson R. C.
Published: June 2008

In many machines handling lightweight, flexible sheets, the sheet must transit an open space. Examples include magnetic tape drives, xerographic copiers, and sewing machines. The nonlinear theory of the elastica has often been used to model nonlinear, static deflections. Dynamic modeling is more difficult, and far less studied. Recently, however, L. Mansfield and J. G. Simmonds (1987) have considered the dynamic deflection of a sheet emerging from a horizontal channel at constant velocity and subjected to gravity loading. In this paper, we consider a generalization of that problem to include arbitrary exit angles and an accelerating “feed” rate. Gravity loading is retained. The resulting nonlinear equations of motion are tractable and are solved numerically.
Investigation on the Steady-State Responses of Asymmetric Rotors
Authors: Kang Yuan , Shih Yuan-Pin , Lee An-Chen
Published: June 2008

This paper is to generalize the previous work by utilizing finite element formulation to accommodate the effects of both deviatoric inertia and stiffness due to asymmetry of flexible shaft and disk. A Timoshenko beam element is employed to simulate rotor-bearing systems by taking the gyroscopic moment, rotary inertia, shear deformation of shaft and, asymmetry of disk and shaft into account. Internal damping is not included but the extension is straightforward. Eulerian angles are used to describe the orientations of shaft element and disk, by which, in opposite to the vectorial approach, the mathematical formulation will be symmetric for angular displacements in two directions. The effects of the angle between the major axes of shaft and disk, deviatoric inertia of the asymmetric shaft, and characteristics of bearing on synchronous critical and subcritical speeds are estimated in conjunction with the harmonic balance method. Numerical examples show that the resonant speeds, at which peak responses occur, change due to various angles between major axes, asymmetry of shaft, stiffness, and damping of bearing.
A Modal Synthesis Technique Based on the Force Derivative Method
Authors: Suarez L. E. , Matheu E. E.
Published: June 2008

In this paper a new free interface modal synthesis method for the dynamic analysis of linear structures by the finite element method is developed. The technique is based on the so called higher order modal combination methods. In particular, the second force derivative method is used to accurately include the effect of the higher truncated modes in the representation of the substructures’ response. It is shown that the method is capable of providing very accurate estimates of the nature frequencies of the combined structure as well as the associated modes of vibration and elastic forces.
Modeling of a Simple Rotor With a Switching Crack and Its Experimental Verification
Authors: Lee Chong-Won , Yun Jong-Seop , Jun Oh Sung
Published: June 2008

The switching crack model having two different stiffnesses, depending upon the crack status, is developed. The necessary conditions for the crack opening/closing states are then analytically derived from a simple rotor with a switching crack. The validity of the switching crack model and the crack opening/closing conditions is proved through numerical simulations and experiments, and a crack identification scheme based on the analysis is also proposed.
Unstable Self-Excitation of Torsional Vibration in AC Induction Motor Driven Rotational Systems
Authors: Shadley J. R. , Wilson B. L. , Dorney M. S.
Published: June 2008

The most common NEMA Design Classes of AC induction motors have speed-torque characteristics that can give rise to unstable self-excitation of torsional vibration in rotational systems during start-up. A torsional vibration computational model for start-up transients has been developed as a design tool for induction motor applications. Torsional instability can occur at speeds in the positive sloping segment of the motor’s speed-torque curve and is particularly acute when the mass moment of inertia of the load device is more than two times the mass moment of inertia of the motor rotor. The computational model is compared with an exact solution method and with a laboratory test of a motor-driven inertial load. Applications of the computational model to electric submersible pump (ESP) design cases are discussed.
Coriolis Effect on the Vibration of a Cantilever Plate With Time-Varying Rotating Speed
Authors: Young T. H. , Liou G. T.
Published: June 2008

A method for investigating the Corilois effect on the vibration of a cantilever plate rotating at a time-varying speed is presented in this paper. Due to this time-dependent speed, parametric instability occurs in the system. Furthermore, owing to the existence of the Coriolis force, the system equation is transformed, by a special modal analysis procedure, into independent sets of first-order simultaneous differential equations. This set of simultaneous differential equations is solved by the method of multiple scales, yielding the system response and the expressions for the boundaries of the unstable regions. Finally, the Coriolis effect on the changes in the boundaries of the unstable regions is investigated numerically.
Consistent Modeling of Rotating Timoshenko Shafts Subject to Axial Loads
Authors: Choi S. H. , Pierre C. , Ulsoy A. G.
Published: June 2008

The equations of motion of a flexible rotating shaft have been typically derived by introducing gyroscopic moments, in an inconsistent manner, as generalized work terms in a Lagrangian formulation or as external moments in a Newtonian approach. This paper presents the consistent derivation of a set of governing differential equations describing the flexural vibration in two orthogonal planes and the torsional vibration of a straight rotating shaft with dissimilar lateral principal moments of inertia and subject to a constant compressive axial load. The coupling between flexural and torsional vibration due to mass eccentricity is not considered. In addition, a new approach for calculating correctly the effect of an axial load for a Timoshenko beam is presented based on the change in length of the centroidal line. It is found that the use of either a floating frame approach with the small strain assumption or a finite strain beam theory is necessary to obtain a consistent derivation of the terms corresponding to gyroscopic moments in the equations of motion. However, the virtual work of an axial load through the geometric shortening appears consistently in the formulation only when using a finite strain beam theory.
Suboptimum Maximum Likelihood Estimation of Structural Parameters from Multiple-Excitation Vibration Data
Authors: Lee J. E. , Fassois S. D.
Published: June 2008

In this paper an effective stochastic and multiple-excitation single-response approach to structural dynamics identification is introduced. The proposed approach accounts for many previously unaccounted for aspects of the problem, as it is based on: A proper, special-form, scalar ARMAX-type representation of the structural and noise dynamics; a new Suboptimum Maximum Likelihood (SML) discrete estimation algorithm (Fassois and Lee, 1990); systematic and efficient modeling strategy and model validation procedures; as well as accurate modal parameter extraction that is compatible with the employed model structure and excitation signal forms. In addition to its comprehensiveness, the proposed approach overcomes the well-known limitations of deterministic time-domain methods in dealing with noise-corrupted data records, while also circumventing some of the major difficulties of existing stochastic schemes by featuring guaranteed algorithmic stability, elimination of wrong convergence problems, very modest computational complexity, and minimal operator intervention. The effectiveness of the approach is verified through numerical simulations with noise-corrupted vibration data, and structural systems characterized by well-separated and closely-spaced vibrational modes. Comparisons with the classical Frequency Domain Method (FDM) are also made, and the approach’s advantages over deterministic methods are demonstrated through comparisons with the Eigensystem Realization Algorithm (ERA). Experimental results, where the proposed approach is used for the modal analysis of a flexible beam from laboratory data, are also presented.
Pressure Pulsations in Centrifugal Pumps
Authors: Guelich J. F. , Bolleter U.
Published: June 2008

Pressure pulsations in centrifugal pumps are created by the wake flow from the impeller blade trailing edge and by large-scale turbulence and vortices generated by flow separation and flow recirculation at part load. The physical mechanisms causing pressure pulsations and the design parameters having an influence on the level of the pressure pulsations are reviewed. The system in which a pump is operated has a strong impact on the level of pressure pulsations measured, which also depends on the detailed design of the impeller and the diffuser. These effects cannot be predicted theoretically. Therefore, statistical data of 36 tests with different pump configurations are given in order to document the level of pulsations which can be achieved.
Optimal Design of Vibration Absorber Systems Supported by Elastic Base
Authors: Ashrafiuon H.
Published: June 2008

This paper presents the effect of foundation flexibility on the optimum design of vibration absorbers. Flexibility of the base is incorporated into the absorber system equations of motion through an equivalent damping ratio and stiffness value in the direction of motion at the connection point. The optimum values of the uncoupled natural frequency and damping ratio of the absorber are determined over a range of excitation frequencies and the primary system damping ratio. Optimal design parameters are computed and compared for the rigid, and flexible models of the base as well as different levels of base flexibility.
Stability Analysis of a 2” Floppy Disk Drive System and the Optimum Design of the Disk Stabilizer
Authors: Chonan S. , Jiang Z. W. , Shyu Y. J.
Published: June 2008

This paper presents a study on the stability of a 2″ floppy disk drive system. A design method of the disk stabilizer that makes the rotating disk stable is presented. The stabilizer and the read/write head are both modeled by springs with high axial stiffnesses. The stiffness of the air film surrounding the disk is determined from the Navier-Stokes equation as a function of the flow rate of the air within the disk jacket. The solution is obtained by using the multi-modal expansion approximation and applying the Galerkin method to the resulting equations. Numerical results show that the 2″ floppy disk rotating at 3600 rpm is unstable without the stabilizer. Further, it is shown that the stability of the disk is much affected by the geometrical configuration of the stabilizer attached to the rotating disk. A stabilizer that contacts the disk at four points was found quite effective in stabilizing the 2″ disk working at 3600 rpm.
Damping and Vibration of Beams With Various Types of Frictional Support Conditions
Authors: Ferri A. A. , Bindemann A. C.
Published: June 2008

The damping characteristics of a flexible beam with various types of frictional supports is considered. For each configuration studied, the only source of damping is dry friction at one support. The study considers supports with constant normal forces and supports for which the normal force varies with beam displacement. The case of transverse slipping is contrasted with that of in-plane slipping. The beam is analyzed using a single spacial mode and first-order harmonic balance is used to study the qualitative features of the damping and vibratory properties. It is seen that the nature of the damping depends on whether the frictional interface is transverse to or in plane with the beam. It is also seen that the equivalent viscous damping of the system can be inversely proportional to, invariant with respect to, directly proportional to, or directly proportional to the square of the displacement amplitude.
Analysis of a Coupling Made of Shape Memory Alloy and Its Dynamic Response Due to Impacts
Authors: Nagaya K. , Hirata Y.
Published: June 2008

This article presents a method for designing a shape memory alloy coupling between rods subjected to axial or torsional forces. The design formula is obtained analytically by using the stress-strain relation of the shape memory alloy. Since the coupling work is simple, the coupling has advantages for building members in dangerous zones. However, the coupling force is not enough, so that it has disadvantages for impact loads. To design the coupling under the consideration of impacts, this paper also discusses a transient response of the coupling for various impact loads. Numerical calculations were carried out for some sample problems.
Effects of Nonlinearities and Damping on the Dynamic Response of a Centrifugal Pendulum Vibration Absorber
Authors: Sharif-Bakhtiar M. , Shaw S. W.
Published: June 2008

The nonlinear dynamic response of a centrifugal pendulum vibration absorber with damping in both the primary system and the pendulum is analyzed using the methods of harmonic balance and Floquet theory. Periodic solutions are approximated by the first harmonic of the response and it is shown that for low and moderate response amplitudes the resulting frequency response curves agree well with results from simulations of the full nonlinear equations of motion. Particular attention is paid to the response at the anti-resonance frequency, that is, the operating frequency for which the absorber is tuned. Cases are demonstrated for which there exists more than one stable steady-state periodic motion of the system at the anti-resonance frequency; this particular property of the system is due to nonlinear effects and cannot be captured through the traditional linear analysis. Furthermore, it is shown that for certain ranges of parameter values the only stable periodic response of the system at the anti-resonance frequency is one of large amplitude, and it cannot be predicted by linear analysis. The effects of system parameters on the shifting of the anti-resonance frequency and on the corresponding carrier amplitude are also considered.
Effect of a Vibrating Substructure on Acoustic Radiation from a Cylindrical Shell
Authors: Achenbach J. D. , Bjarnason J. , Igusa T.
Published: June 2008

The presence of a substructure in a cylindrical shell gives rise to dynamic interactions, including resonance effects, which have an influence on the radiated acoustic field. The lowest mode of the substructure may interact vigorously with a higher order mode of the shell. In this paper a Lagrangian formulation is used to analyze the forced motion of the immersed shell/substructure system. In addition, the frequency window method is used to reduce the complexity of the resulting expressions and to obtain approximate solutions in a frequency range of interest. The particular example is concerned with a spring-mass system, which is attached to the shell. Three cases of forced harmonic motion are considered. The radiated pressure is computed as a function of the frequency. The radiated pressure for the shell/substructure system is compared with that for a shell without an oscillator subjected to the same input force.
Simplified Determination of Stability Bounds in Nonlinear Vibration Systems
Authors: Peleg K. , Hinga S.
Published: June 2008

Even slight nonlinearities in vibrating systems introduce instability frequency bands and unstable amplitudes. In many vibration problems it is desirable to know precisely the bounds of the instability frequencies and the associated amplitude ranges. Using a general nonlinear single degree of freedom system, based on a Coulomb friction augmented Duffing model, simplified graphical/analytical modus operandi was developed for computing singular amplitudes in the frequency domain. The method may be used in lieu of the usual phase plane approach wherein the physical meaning of the different vibration modes is obscured. In previous publications approximate estimation of instability frequency bands of the Duffing system was achieved by assuming variations about the steady state solution. The new method presented herein allows accurate determination of instability frequency ranges in a more general class of vibration systems, while quantifying “Horizontal Tangents Amplitudes,” in addition to the usual “Resonance Amplitudes” and “Vertical Tangents Amplitudes.”
Dynamic Stability of a Cantilever Shaft-Disk System
Authors: Chen Lien-Wen , Ku Der-Ming
Published: June 2008

The dynamic stability behavior of a cantilever shaft-disk system subjected to axial periodic forces varying with time is studied by the finite element method. The equations of motion for such a system are formulated using deformation shape functions developed from Timoshenko beam theory. The effects of translational and rotatory inertia, gyroscopic moment, bending and shear deformation are included in the mathematical model. Numerical results show that the effect of the gyroscopic term is to shift the boundaries of the regions of dynamic instability outwardly and, therefore, the sizes of these regions are enlarged as the rotational speed increases.
Longitudinal Vibration and Damping Analysis of Adhesively Bonded Double-Strap Joints
Authors: He S. , Rao M. D.
Published: June 2008

A mathematical model to study the longitudinal vibration of an adhesively bonded double-strap joint is presented in this paper. Energy method and Hamilton’s principle are used to derive the governing equations of motion and natural boundary conditions of the joint system. The adhesive is modeled as a viscoelastic material using complex modulus approach. Both the shear and longitudinal deformation in the adhesive layer are included in the analysis. The equations to predict the system resonance frequencies and loss factors are derived from the system natural and forced boundary conditions for the case of simply supported boundary conditions. A special searching strategy for finding the zeros of a complex determinant has been utilized to obtain the numerical results. The effects of the adhesive shear modulus and structural parameters such as lap ratio, adhesive and strap thickness on the system resonance frequencies and loss factors are also studied.
Active Control of Radiated Sound from Ducts
Authors: Hall H. R. , Ferren W. Brent , Bernhard R. J.
Published: June 2008

The noise radiated by duct and pipe systems is modeled in the laboratory using a circular duct driven by a speaker at one end. Active control is achieved using a control speaker located adjacent to the open end of the duct. The objective of the investigation was to minimize the total sound power radiated by the duct and a single secondary source. However, the adaptive algorithm used by the controller for this investigation seeks to cancel the acoustic pressure only at a single location; that is, the location of the “error” microphone. Analytical studies predict that in order for the total sound power radiated by primary and secondary sources to be minimized, a single secondary source must radiate sound which is of approximately equal magnitude and opposite phase to the noise source and the error microphone must be placed somewhere in the plane of minimum pressure of an ideal acoustic dipole. These analytical results were verified in part, for the case of the secondary source facing the same direction as the primary source in the plane of the pipe outlet. Other cases were studied where the control speaker was located outside the plane of the duct outlet. The performance improved for these alternative orientations for a wide range of error microphone positions. Measured sound power with and without active control is shown for a range of frequencies and error microphone locations for three configurations of the duct and control source.
Development of Porous Squeeze Film Damper Bearings for Improving the Blade Loss Dynamics of Rotor-Support Systems
Authors: Zhang Shiping , Yan Litang , Li Qihan
Published: June 2008

An efficient oil film damper known as porous squeeze film damper (PSFD) is developed based on conventional squeeze film damper (SFD) for more effective and reliable rotor vibration control and especially for improving the blade loss dynamics for rotor support system. The permeability of the outer race of PSFD could remarkably improve the squeeze film damping properties. The transient response of a simple rigid rotor and flexible Jeffcott’s rotor supported on PSFD and SFD subjected to sudden unbalance of blade loss are investigated. Time transient simulation show that PSFD could operate effectively under much greater unbalance as compared with SFD, especially under relative large impact loading of blade loss. Furthermore, the effective eccentricities of PSFD with small transmissibilities (T<1.0) extend to a range of ε<0.9, and optimum film stiffness and damping distribution within the whole film clearance could be achieved.
Controllable Squeeze Film Damper (An Application of Electro-Rheological Fluid)
Authors: Morishita Shin , Mitsui Jun’ichi
Published: June 2008

A notable characteristic of electro-rheological (ER) fluid is the variation of its apparent viscosity with the application of an electric field. The application of this characteristic to the performance of squeeze film damper of a flexible rotor is investigated in this paper. It is shown experimentally that by controlling the supporting damping capacity continuously, rotor vibration can be reduced remarkably a wide range of rotating speed. Moreover, the study indicates experimentally that there exists an optimum supporting damping for every vibration mode.
Mass, Stiffness, and Damping Matrix Identification: An Integrated Approach
Authors: Roemer M. J. , Mook D. J.
Published: June 2008

Accurate estimates of the mass, stiffness, and damping characteristics of a structure are necessary for determining the control laws best suited for active control methodologies. There are several modal identification techniques available for determining the frequencies, damping ratios, and mode shapes of a structure. However, modal identification methods in both the frequency and time domains have difficulties for certain circumstances. Frequency domain techniques which utilize the steady-state response from various harmonic inputs often encounter difficulties when the frequencies are closely distributed, the structure exhibits a high degree of damping, or the steady-state condition is hard to establish. Time domain techniques have produced successful results, but lack robustness with respect to measurement noise. In this paper, two identification techniques and an estimation method are combined to form a time-domain technique to accurately identify the mass, stiffness, and damping matrices from noisy measurements.
Stable Variational Coupling Method for Fluid-Structure Interaction in Semi-Infinite Media
Authors: Zeng Xiaogang , Bielak J. , MacCamy R. C.
Published: June 2008

An energy-based symmetric coupled finite element and boundary integral method valid for all frequencies has been developed recently by the authors (Zeng et al., 1990; Bielak et al., 1991), for analyzing scattering problems for an inhomogeneous deformable body immersed in an infinite acoustic medium. Here we extend the methodology to a halfspace with a free surface via the method of images. Numerical examples are presented for an infinitely long radially inhomogeneous elastic cylinder with its centroidal axis parallel to the free surface and subjected to an incident plane wave perpendicular to this axis, in order to illustrate the applicability of the new procedure; its accuracy at critical frequencies is assessed with a rigid cylinder.
On the Impact Intensity of Vibrating Axially Moving Roller Chains
Authors: Wang K. W. , Liu S. P. , Hayek S. I. , Chen F. H. K.
Published: June 2008

Experimental observation has shown that the most significant noise source in roller chain drives is from the impacts between the chain and the sprocket during their meshing process. Despite its importance, studies have not been made to thoroughly analyze the chain/sprocket impact dynamics and their interaction with the vibrating, axially moving chain structure. This paper presents a novel analysis which integrates the local meshing phenomena with the global system. An axially moving chain interacting with local impacts has been modelled and the momentum balance method is employed to derive the impulse function. A study is carried out to quantify the intensity of subsequent impacts. It is found that the impact intensity is significantly affected by the vibration characteristics and response of the moving chain, and vice versa. The classical quasi-static approach will create errors in predicting the impulse magnitude and system response. Meshing frequencies that will cause maximum and minimum impulses are analytically predicted. This fundamental investigation provides new insight into roller chain dynamics, which is an essential step toward the design of quiet chain drives.
Experimental Verification of the Nonself-Adjoint State Space Duct Model
Authors: Hull A. J. , Radcliffe C. J.
Published: June 2008

Experimental verification of the nonself-adjoint state space duct model obtained previously by the authors is presented. Both pressure excitation at the end and mass flow excitation in the domain are analyzed and the resulting models are experimentally verified. Frequency response validations for both constant and frequency varying nonconstant impedance at the termination end of the duct are presented. The laboratory duct’s response to an impulsive pressure input is shown to be well predicted by the plane wave, state space model.
Comparison of the Effects of Density and Sound Speed Variation in the Propagation of Planar Transient Waves
Authors: Ginsberg J. H.
Published: June 2008

When expressed in the form of characteristic differential equations, the laws governing propagation of linear one-dimensional waves through heterogeneous media show that the only properties of significance are the sound speed c and the acoustic impedance ρc, either of which may vary spatially. The former occurs in the differential equations governing the (curved) characteristics, while the latter appears in the differential equations governing the evolution of particle velocity and stress along the characteristics. The present study employs an inherently stable finite difference representation of the characteristic equations, in which the spatial grid is obtained by evaluating the intersections in space-time of constant time lines with comparable increments of the characteristic variables. The numerical procedure is used to follow the propagation of a single-lobe sine pulse in cases where only ρ or c fluctuates spatially about a mean value while the other property is constant, and compares those results to the case were both material properties vary. Nonconstancy of c is shown to cause temporal shifts in waveforms, while spatial variation of ρc causes attenuation and distortion of the waveform.
Effects of Viscous and Thermal Losses on Radiation from an Infinite Plate
Authors: Latcha M. A. , Akay A.
Published: June 2008

This paper is concerned with the behavior of sound radiation from bending waves of an infinite plate in a fluid with viscous and thermal losses. The radiation field variables are obtained through a solution of the equation of motion for viscothermal fluids. Acoustic energy flux in the fluid is investigated, with particular emphasis on its behavior near the plate surface. Energy flux is shown to change direction in a region near the plate. The motion of the particles orbiting in elliptical patterns also exhibits a similar change of direction as a result of viscous and thermal losses. A notable effect of the viscothermal losses is the radiation of acoustic power from the plate below the critical frequency.
Cylindrical and Spherical Coordinate Versions of NPE for Transient and Steady-State Sound Beams
Authors: Too Gee-Pinn James , Ginsberg J. H.
Published: June 2008

The NPE (nonlinear progressive wave equation) and associated computer program is a time-domain representation of acoustic propagation in waveguides that includes the effect of nonlinearity. In that approach a spatial window is initialized with the original waveform and then convected in the primary propagation direction as time evolves. Previous work modified NPE by using cylindrical coordinates to describe a paraxial approximation suitable for axisymmetric sound beams. The present development further modifies NPE by using spherical coordinates. The matter of interfacing it with the cylindrical coordinate version, in order to describe the far field of a sound beam, is described. This simulation technique is used to evaluate the long range propagation of the signal radiating from a piston in an infinite baffle that is subjected to harmonic excitation. It is also applied to a focussed sound beam generated by transient excitation of a concave projector. Comparison of the results with experimental data shows good overall agreement, with the main source of error apparently being due to dissipation, which is not addressed in the present models.
Solution of the End Problem of a Liquid-Filled Cylindrical Acoustic Waveguide Using a Biorthogonality Principle
Authors: Albers W. F. , Scarton H. A.
Published: June 2008

This paper treats the forced motion of an isothermal, Newtonian liquid in a semi-infinite cylindrical waveguide. Its bounding wall is assumed rigid, allowing neither normal nor tangential fluid velocities at its inner surface. Small amplitude acoustic waves are considered to be driven by a steady periodic motion due to the rotationally symmetric deflection of an end plate or membrane. A series expansion of the waveguide eigenmodes is used to construct the solution for the motion anywhere within the guide. Based on a biorthogonality property of the eigenmodes, each coefficient of the series is shown to be directly calculable in terms of axial velocity and radial shear stress at the driver face. Also, results of Galerkin solutions, based on driver axial velocity and zero radial velocity, are given for comparison.
Wet Modes of Submerged Structures—Part 1: Theory
Authors: Ettouney M. M. , Daddazio R. P. , DiMaggio F. L.
Published: June 2008

A method for determining the acoustic radiation and scattering response induced by harmonic excitation of a submerged elastic structure is presented. It is based on expansion vectors, called wet modes, which are the vector components of the spatial part of free harmonic vibration at the forcing frequency of the fluid-structure system. Advantages of this formulation are suggested. Numerical results for a spherical shell obtained by this method are compared with those from a series solution and with methods based on the use of in-vacuo structural modes as expansion vectors. An in-depth study of the application of the methodology to a more complex submerged structure is presented in the companion paper (Daddazio et al., 1992).