Showing papers in "Journal of Vibration and Acoustics in 2003"
TL;DR: In this paper, a stochastic modeling of the vibration signal produced by localized faults in rolling element bearings and its use for diagnostic purposes is presented. And the analysis finally gives sound justification for squared envelope analysis and the type of spectral indicators that should be used with it.
Abstract: This paper addresses the stochastic modeling of the vibration signal produced by localized faults in rolling element bearings and its use for diagnostic purposes. The aim is essentially to provide a better understanding of the recognized envelope analysis technique as classically used in the diagnostics of rolling element bearings, and incidentally give theoretical proofs for the specific features of envelope spectra as obtained from experimental data. The proposed model may also prove useful for simulation purposes. First, the excitation force generated by a defect is modeled as a random point process and its spectral signature is derived analytically. Then its transmission through the bearing is investigated in detail in order to find the spectral characteristics of the resulting vibration signal. The analysis finally gives sound justification for squared envelope analysis and the type of spectral indicators that should be used with it.
194 citations
TL;DR: In this article, a semi-active energy rate multi-modal vibration control technique is developed for a piezoceramic actuator coupled to a switching resistor/inductor shunt.
Abstract: In this paper, a novel semi-active energy rate multi-modal vibration control technique is developed for a piezoceramic actuator coupled to a switching resistor/inductor shunt. The technique works by briefly connecting a resistor/inductor shunt to a piezoceramic actuator in order to apply the necessary signed charge to allow energy dissipation. The switch timing is determined by a control scheme that observes the rate of energy change in controlled modes. The control scheme is developed in the paper, and is simplified to enable practical implementation. This new multi-modal control law is applied to both a simple numerical and an experimental test structure. The results from the numerical and experimental tests show that the energy rate multi-mode control law is able to dissipate energy from one, two and three modes of the flexible structures using a single actuator.
139 citations
TL;DR: In this article, the authors used 3D computational fluid dynamics (CFD) to model the labyrinth seal flow path by solving the Reynolds Averaged Navier Stokes equations.
Abstract: Labyrinth seals are utilized inside turbomachinery to provide noncontacting control of internal leakage. These seals can also play an important role in determining the rotordynamic stability of the machine. Traditional labyrinth seal models are based on bulk-flow assumptions where the fluid is assumed to behave as a rigid body affected by shear stress at the interfaces. To model the labyrinth seal cavity, a single, driven vortex is assumed and relationships for the shear stress and divergence angle of the through flow jet are developed. These models, while efficient to compute, typically show poor prediction for seals with small clearances, high running speed, and high pressure.* In an effort to improve the prediction of these components, this work utilizes three-dimensional computational fluid dynamics (CFD) to model the labyrinth seal flow path by solving the Reynolds Averaged Navier Stokes equations. Unlike bulk-flow techniques, CFD makes no fundamental assumptions on geometry, shear stress at the walls, as well as internal flow structure. The method allows modeling of any arbitrarily shaped domain including stepped and interlocking labyrinths with straight or angled teeth. When only leakage prediction is required, an axisymmetric model is created. To calculate rotordynamic forces, a full 3D, eccentric model is solved. The results demonstrate improved leakage anti rotordynamic prediction over bulk-flow approaches compared to experimental measurements.
105 citations
TL;DR: In this article, the authors proposed a new method for attaining the exact algebraic solution of the H∞ optimization problem of the dynamic vibration absorber in linear systems, and applied this method to the design optimization of a viscous damped (Voigt type) absorber and a hysteretic damped absorber attached to undamped primary systems.
Abstract: H∞ optimization of the dynamic vibration absorbers is a classical optimization problem, and has been already solved more than 50 years ago. It is a well-known solution, but we know this solution is only an approximate one. Recently, one of the authors has proposed a new method for attaining the H∞ optimization of the absorber in linear systems. The new method enables us to obtain the exact algebraic solution of the H∞ optimization problem of the absorber. In this paper we first apply this method to the design optimization of a viscous damped (Voigt type) absorber and a hysteretic damped absorber attached to undamped primary systems. For each absorber, six different transfer functions are taken here as performance indices to vibration suppression or isolation. As a result, we found the closed-form exact solutions to all transfer functions. The solutions obtained here are then compared with those of the approximate ones. Finally, we present the closed-form exact solutions to the hysteretic damped absorber attached to damped primacy systems.
98 citations
TL;DR: In this article, the authors studied the local contact between the drillstring and the well during drilling operations using a global finite-element model obtained from a specific computer program, and they focused on the bottom-hole assembly (BHA) which is subjected to compression.
Abstract: This article is devoted to the study of the local contact between the drillstring and the well during drilling operations. The study focuses on the Bottom-Hole-Assembly (BHA), which is subjected to compression. The work is motivated by the need to understand the complex behavior of such a system, in order to improve control over their constructive and destructive potentials. The contact zone is first determined using a global finite-element model obtained from a specific computer program. Contact, which is assumed to be located somewhere on the drill-collar or on stabilizers, is prejudicial and leads to premature abrasive wear of the drillstring, reduction of the Rate Of Penetration (ROP) of the tool into the rock and reduction of the Mean Time Between Failure (MTBF). The proposed mathematical model is expressed in terms of four independent degrees of freedom which are radial displacement, rotation of the section considered, bending along the tangential direction and torsion of the string. They include the effects of bending and torsion, the whirling motion of the drillstring as well as friction phenomena occurring between the drillstring and the well. The tangential effect is modeled by using Coulomb's law of friction. The nonlinear equations of movement are derived using Lagrange equations and are solved numerically to obtain the response. Specific attention is paid to the study of friction and a consistent contact model capable of taking into account the rolling of the drillstring, both with and without slip, is included in the model. This paper also presents a parametric study on the influence of the initial position of the string and the friction coefficient of the contact on the dynamic behavior of the structure. The model is validated by an experimental set-up equipped with two opto-electronic devices.
73 citations
TL;DR: In this article, a topology optimization based approach is proposed to study the optimal configuration of stiffeners for the interior sound reduction, which is aimed at reducing the low frequency noise, a coupled acoustic-structural conservative system without damping effect is considered.
Abstract: A topology optimization based approach is proposed to study the optimal configuration of stiffeners for the interior sound reduction. Since our design target is aimed at reducing the low frequency noise, a coupled acoustic-structural conservative system without damping effect is considered. Modal analysis method is used to evaluate the interior sound level for this coupled system. To formulate the topology optimization problem, a recently introduced Microstructure-based Design Domain Method (MDDM) is employed. Using the MDDM, the optimal stiffener configurations problem is treated as a material distribution problem and sensitivity analysis of the coupled system is derived analytically. The norm of acoustic excitation is used as the indicator of the interior sound level. The optimal stiffener design is obtained by solving this topology optimization problem using a sequential convex approximation method. Examples of acoustic box under single frequency excitation and a band of low frequency excitations are presented and discussed. @DOI: 10.1115/1.1569512#
60 citations
TL;DR: In this article, the authors model the vibration and dynamic instability of a car disc brake as a moving load problem in which one component is amenable to analytical treatment while the other component (the pads, calliper and mounting) has to be dealt with by the finite element method.
Abstract: There are many elasto-mechanical systems that involve two components in moving contact where large-amplitude vibration and noise can be excited. This paper models the vibration and dynamic instability of a car disc brake as a moving load problem in which one component (the disc) is amenable to analytical treatment while the other component (the pads, calliper and mounting) has to be dealt with by the finite element method. A method is presented for solving the dynamic instability of the car disc brake as a nonlinear eigenvalue problem. The same approach can tackle other moving load problems.
60 citations
TL;DR: In this paper, the effect of feedback time-delay on the performance of active vibration suppression is considered. But the authors focus on the stability analysis along the delay axis and do not consider the effects of the delay on the control law.
Abstract: Various active vibration suppression techniques, which use feedback control, are implemented on the structures. In real application, time delay can not be avoided especially in the feedback line of the actively controlled systems. The effects of the delay have to be thoroughly understood from the perspective of system stability and the performance of the controlled system. Often used control laws are developed without taking the delay into account. They fulfill the design requirements when free of delay. As unavoidable delay appears, however, the performance of the control changes. This work addresses the stability analysis of such dynamics as the control law remains unchanged but carries the effect of feedback time-delay, which can be varied. For this stability analysis along the delay axis, we follow up a recent methodology of the authors, the Direct Method (DM), which offers a unique and unprecedented treatment of a general class of linear time invariant time delayed systems (LTI-TDS). We discuss the underlying features and the highlights of the method briefly. Over an example vibration suppression setting we declare the stability intervals of the dynamics in time delay space using the DM. Having assessed the stability, we then look at the frequency response characteristics of the system as performance indications.
50 citations
TL;DR: In this article, a coupled piezoelectric circuit was proposed to solve the problem of small imperfections in a periodic structure, which could lead to large vibration in certain regions of the structure and could be harmful to the system.
Abstract: It is known that, when the mechanical coupling between the substructures is weak, small imperfections in a periodic structure can induce vibration localization. This phenomenon could lead to large vibration in certain regions of the structure and could be very harmful to the system. In this study, it is shown that the proposed coupled piezoelectric circuits can greatly relieve or even eliminate such localization problems. Part of the structural vibration energy will be transferred into electrical energy through the piezoelectric materials, and the newly created electro-mechanical wave/energy channel will sustain the energy propagation throughout the structure. The effectiveness and robustness of the coupled piezoelectric circuits on reducing vibration localization is demonstrated through analysis. Design guidelines are also established via approximation techniques and parametric studies.
48 citations
TL;DR: The results indicate that triaxial accelerometers can provide additional information about the frequency content of helicopter gearbox vibrations, and provide researchers and industry with a novel method of capturing and monitoring triAXial changes in the baseline vibration signatures.
Abstract: Research on the nature of the vibration data collected from helicopter transmissions during flight experiments has led to several crucial observations believed to be responsible for the high rates of false alarms and missed detections in aircraft vibration monitoring systems. This work focuses on one such finding, namely, the need to consider additional sources of information about system vibrations. In this light, helicopter transmission vibration data, collected using triaxial accelerometers, are explored in three different directions, analyzed for content, and then combined using Principal Components Analysis (PCA) to analyze changes in directionality. The frequency content of the three different directions is compared and analyzed using time-synchronously averaged vibration data. To provide a method for analysis and monitoring purposes, the triaxial data are decorrelated using a mathematical transformation, and compared to the original axes to determine their differences. The benefits of using triaxial data for vibration monitoring and diagnostics are explored by analyzing the changes in the direction of the principal axis of vibration formed using all three axes of vibration. The statistical variation introduced due to the experimental variables is further analyzed using an Analysis of Variance approach to determine the effect of each variable on the overall signature. The results indicate that triaxial accelerometers can provide additional information about the frequency content of helicopter gearbox vibrations, and provide researchers and industry with a novel method of capturing and monitoring triaxial changes in the baseline vibration signatures.
43 citations
TL;DR: In this article, a family of parametric models is defined to represent the nonlinear response of a system and the selection algorithm estimates the likelihood that each member of the family is appropriate.
Abstract: The development of techniques for identification and updating of nonlinear mechanical structures has received increasing attention in recent years. In practical situations, there is not necessarily a priori knowledge about the nonlinearity. This suggests the need for strategies that allow inference of useful information from the data. The present study proposes an algorithm based on a Bayesian inference approach for giving insight into the form of the nonlinearity. A family of parametric models is defined to represent the nonlinear response of a system and the selection algorithm estimates the likelihood that each member of the family is appropriate. The (unknown) probability density function of the family of models is explored using a simple variant of the Markov Chain Monte Carlo sampling technique. This technique offers the advantage that the nature of the underlying statistical distribution need not be assumed a priori. Enough samples are drawn to guarantee that the empirical distribution approximates the true but unknown distribution to the desired level of accuracy. It provides an indication of which models are the most appropriate to represent the nonlinearity and their respective goodness-of-fit to the data. The methodology is illustrated using two examples, one of which comes from experimental data.
TL;DR: In this paper, the authors considered the steady-state response of a rotor fitted with a system of nearly identical torsional vibration absorbers, and found that the most desirable situation is one in which the relative imperfections are kept as small as possible, and this becomes more crucial when the levels of mistuning and damping are very small.
Abstract: In this paper we consider the steady-state response of a rotor fitted with a system of nearly identical torsional vibration absorbers. The absorbers are of the centrifugal pendulum type, which provide an effective mean of attenuating torsional vibrations of the rotor at a given order. The model considered employs absorbers that are tuned close to the order of the excitation, with an intentional mistuning that is selected by design, and imperfections among the absorbers which arise from manufacturing, wear, and other effects. It is shown that these systems can experience localized responses in which the response amplitude of one or more absorbers can become relatively large as compared to the response of the corresponding system with identical absorbers. The results are based on an exact steadystate analysis of the mathematical model, and they show that the strength of the localization depends on the average level of absorber mistuning (a design parameter), the magnitude of the relative imperfections among the absorbers, and the absorber damping. It is found that the most desirable situation is one in which the relative imperfections are kept as small as possible, and that this becomes more crucial when the levels of mistuning and damping are very small. The results of the analysis are confirmed by simulations of the fully nonlinear equations of motion of the rotor/absorber system. It is concluded that the presence of localization should be accounted for in absorber designs, since its presence makes the absorbers less effective in terms of vibration reduction and, perhaps more significantly, it can drastically reduce their operating range, since such absorbers typically have limited rattle space. @DOI: 10.1115/1.1522420#
TL;DR: In this article, a mathematical model of an unbalanced rotor system located in a maneuvering aircraft is derived and the dynamic characteristics of the rotor running at a constant angular speed or a constant acceleration are studied under the assumptions that the aircraft maneuvers only in a vertical plane and the pitching angle and the flight path inclination of the aircraft are equal.
Abstract: This paper shows how the dynamics of a rotor in a maneuvering aircraft changes according to the operation of the aircraft. The mathematical model of an unbalanced rotor system located in the maneuvering aircraft is derived. The dynamic characteristics of the rotor running at a constant angular speed or a constant acceleration are studied under the assumptions that the aircraft maneuvers only in a vertical plane and that the pitching angle and the flight path inclination of the aircraft are equal. The effects of gravity and unbalance parameter are considered. The results show that the unbalanced response of a rotor in an aircraft is obviously influenced by the aircraft's flying status.
TL;DR: In this article, the authors present an experimental study on the active control of self-excited vibrations in a rotor bearing system supported on a pair of externally pressurized thrust bearings in the axial direction and on actively controlled journal gas bearing in the radial direction.
Abstract: This paper presents an experimental study on the active control of self-excited vibrations in a rotor-bearing system supported on a pair of externally pressurized thrust bearings in the axial direction and on actively controlled journal gas bearings in the radial direction. The active journal gas bearings used are of the tilting-pad type with one traditional passive pad and two active pads with embedded piezoelectric actuators. Feedback control systems are constructed with gap sensors for measuring the vibration of the rotor, embedded piezoelectric actuators, and PID controllers. The experimental results show that the self-excited vibration can be effectively suppressed with the designed feedback control system, if the gains of the PID controllers are properly tuned.
TL;DR: In this article, the effect of a thrust active magnetic bearing (TAMB) on the stability and bifurcation of an AMBRS rotor system was investigated. And the results indicated that the TAMB and mass eccentricity have a great influence on nonlinear stability of the T periodic motion of the system.
Abstract: This paper is concerned with the effect of a thrust active magnetic bearing (TAMB) on the stability and bifurcation of an active magnetic bearing rotor system (AMBRS). The shaft is flexible and modeled by using the finite element method that can take the effects of inertia and shear into consideration. The model is reduced by a component mode synthesis method, which can conveniently account for nonlinear magnetic forces and moments of the bearing. Then the system equations are obtained by combining the equations of the reduced mechanical system and the equations of the decentralized PID controllers. This study focuses on the influence of nonlinearities on the stability and bifurcation of T periodic motion of the AMBRS subjected to the influences of both journal and thrust active magnetic bearings and mass eccentricity simultaneously. In the stability analysis, only periodic motion is investigated. The periodic motions and their stability margins are obtained by using shooting method and path-following technique. The local stability and bifurcation behaviors of periodic motions are obtained by using Floquet theory. The results indicate that the TAMB and mass eccentricity have great influence on nonlinear stability and bifurcation of the T periodic motion of system, cause the spillover of system nonlinear dynamics and degradation of stability and bifurcation of T periodic motion. Therefore, sufficient attention should be paid to these factors in the analysis and design of a flexible rotor system equipped with both journal and thrust magnetic bearings in order to ensure system reliability.
TL;DR: In this article, a model for both vertical and horizontal analysis is developed for rotor-hydrodynamic bearing system with external flexible damped bearing supports, which can provide a means to improve the stability of the rotor system.
Abstract: A rotor-hydrodynamic bearing system having external flexible damped bearing supports is more complicated than that discussed in Part 1 but it can provide a means to improve the stability of the rotor system. A model for both vertical and horizontal analysis is developed first. Then, the analytical study on the vertical rotor is conducted. The results show that there can be up to four threshold speeds in this configuration that form a consecutive regional pattern, taking turns by stable or unstable regions. Furthermore, the numerical calculation by MATLAB is carried out to obtain the results for the horizontal system. The stability maps for various parametric configurations are presented. It has been shown that the value of support damping has a strong effect on the first several lower threshold speeds. But it has little effect on the last top threshold speed which is mainly determined by the portion of journal mass. Within a certain range of external damping value, the first several regions of instability can be reduced or eradicated. As far as the entire stability map is concerned, there is an optimum range of support damping that can make the rotor have only one top threshold speed over the entire running speed range. When the support stiffness is increased, the stem stability map becomes narrow which means a small support stiffness is good for broadening the range of optimum external damping.
TL;DR: In this article, a spatial shape function of the piezoelastic actuators is sought such that flexural vibrations induced by external forces can be completely nullified, for the case of clamped circular plates with a spatially constant force loading.
Abstract: Vibrations of smart elastic plates with integrated piezoelectric actuators are considered. Piezoelastic layers are used to generate a distributed actuation of the plate. A spatial shape function of the piezoelastic actuators is sought such that flexural vibrations induced by external forces can be completely nullified. An analytic solution of this problem is worked out for the case of clamped circular plates with a spatially constant force loading. The Kirchhoff theory of thin plates is used to derive this analytic solution. Our result is successfully validated by means of coupled 3-dimensional finite-element computations.
TL;DR: Vibration and Acoustic Labs Mechanical Engineering Virginia Tech, Blacksburg, VA 24061-0238 as mentioned in this paper, has been used for the development of a wireless sensor network.
Abstract: Vibration and Acoustic Labs Mechanical Engineering Virginia Tech, Blacksburg, VA 24061-0238
TL;DR: In this paper, the effect of a two-degree-of-freedom system kinematically connected to the beam is represented exactly by replacing it with equivalent stiffness coefficients, which are added to the appropriate stiffness coefficients of the bare beam.
Abstract: This paper is concerned with the dynamic stiffness formulation and its application for a Bernoulli-Euler beam carrying a two degree-of-freedom spring-mass system. The effect of a two degree-of-freedom system kinematically connected to the beam is represented exactly by replacing it with equivalent stiffness coefficients, which are added to the appropriate stiffness coefficients of the bare beam. Numerical examples whose results are obtained by applying the Wittrick-Williams algorithm to the total dynamic stiffness matrix are given and compared with published results. Applications of the theory include the free vibration analysis of frameworks carrying two degree-of-freedom spring-mass systems.
TL;DR: In this paper, a new time domain method is presented to identify moving loads on a bridge deck based on the measured responses, where the bridge deck is modeled as an orthotropic plate and the loads are modeled as a group of four loads moving on top of the bridge board at fixed distance apart.
Abstract: A new time domain method is presented to identify moving loads on a bridge deck based on the measured responses. The bridge deck is modeled as an orthotropic plate and the loads are modeled as a group of four loads moving on top of the bridge deck at fixed distance apart. Dynamic behavior of the bridge deck is analyzed by the orthotropic plate theory and mode superposition technique. Like all inverse problems, this identification is an ill-conditioned problem, and a regularization technique is employed to stabilize the computations. The identified loads moving at different eccentricities are presented. Laboratory work on the force identification is also presented. The effect of incomplete measured modes in the responses is discussed, and an underestimation in the loads may result if the number of vibration mode for identification is larger than that in the responses. Computational simulations and laboratory tests show that the method is effective and practical for identification of individual wheel loads on bridge decks.
TL;DR: In this article, the stability boundary of the in-phase mode of two identical oscillators in terms of the two coupling parameters is determined numerically, and the effect of detuning the oscillators from each other is also presented.
Abstract: In this paper we present findings from an investigation of synchronization of linearly diffusively coupled van der Pol oscillators. The stability boundary of the in-phase mode of two identical oscillators in terms of the two coupling parameters is determined numerically. We show that in addition to the out-of-phase and in-phase motions of the oscillators there exist two other phase-locked motions and behavior that appears chaotic. The effect of detuning the oscillators from each other is also presented. Finally, the analytical and numerical results from an investigation of the in-phase mode system of n coupled oscillators is presented.
TL;DR: In this article, the effect of external damping on the stability of rotor-hydrodynamic bearing systems has been investigated for both vertical and horizontal configurations of the Jeffcott rotor.
Abstract: The instability of rotor-hydrodynamic bearing systems has been a major concern in highspeed rotating machine design. The threshold speed is usually considered to be a barrier that cannot he driven through. However, if some kind of external damping is introduced, the situation may be different. The study is first conducted on the Jeffcott rotor with external damping for both vertical and horizontal configurations. For the vertical system, it has been shown by both analytical and numerical study that there is a threshold speed at which the rotor will get into an upper stable region from the previous unstable region. For the horizontal system, an extended Lund Stability Method is presented which proves that the system may have two threshold speeds. This results in a stability pattern which has one region of instability sandwiched between two regions of stability. Then, the study is expanded to the extended Jeffcott rotor. With the introduction of equivalent journal mass, another threshold speed appears which forms a top and final boundary to the stable operation of the rotor. The results of a numerical study show that with the external damping increased, the first several unstable regions can be reduced or eradicated. The external damping has little effect on the last top threshold which is mainly dependent on the value of journal mass. It has also been shown that a certain value of external damping and small journal mass would be very beneficial to improve the stability characteristics of the system over the entire speed range.
TL;DR: In this paper, the authors summarized the development during the last 50 years of tilting pad journal bearing analysis and design, and the major impetus of this development was a landmark paper published by Jorgen Lund in 1964, Spring and Damping Coefficients for the Tilting-Pad Journal Bearing.
Abstract: This paper summarizes the development during the last 50 years of tilting pad journal bearing analysis and design. The major impetus of this development was a landmark paper published by Jorgen Lund in 1964, Spring and Damping Coefficients for the Tilting-Pad Journal Bearing. His paper contained the first widely published dynamic coefficients for tilting pad bearings along with his pad assembly method equations. In the 38 years since Lund's publication, many other authors have written tilting pad journal bearing codes, the first of which were based on Lund's assembly method. These assembly method codes were utilized for many years to analyze and design tilting pad bearings for improved rotordynamic performance. During this time, some key design tools were developed utilizing Lund's method. Other authors have written newer codes which solve the energy and elasticity equations iteratively with the pressure equation, including pad degrees of freedom. With the simple addition of a turbulence correction and heat balance, many designers continue to utilize Lund's method, shunning the more modern codes.
TL;DR: In this article, a flex circuit is modeled as a laminate of polyimide substrate, adhesive, and copper conductors and the model involves displacements of the flex circuit in the directions tangent and normal to the local equilibrium shape, and those motions couple with the arm's dynamics.
Abstract: A flex circuit connects the stationary electronic components in a hard disk drive to the rotating arm that carries the read/write heads and positions them above data tracks on the disk. Flex circuits are conventionally formed as a laminate of polyimide substrate, adhesive, and copper conductors. Deformation of a flex circuit is discussed in the context of the following stages: the initial unstressed shape, configurations in which stresses set and relax in response to elevated temperature, equilibrium, and small amplitude vibration. The model involves displacements of the flex circuit in the directions tangent and normal to the local equilibrium shape, and those motions couple with the arm's dynamics. Nonlinearity associated with finite curvature, partial elastic springback, and the arm 's geometry and inertia properties are incorporated within the vibration model to predict system-level natural frequencies, mode shapes, and coupling factors between the circuit and the arm. laboratory measurements using noncontact laser interferometry validate the model with respect to the circuit's shape, stiffness, restoring moment, and natural frequencies. The primary degrees of freedom for optimizing flex circuit design are the thicknesses of the individual layers within the circuit, free length, and the locations and slopes of the circuit's attachment points to the arm and electronics block. The model's predictions and trends developed from a case study in free length are discussed with a view toward reducing coupling between the circuit and arm in certain vibration modes.
TL;DR: In this article, a model for the in-plane oscillations of a thin rotating disk has been derived using a nonlinear strain measure to calculate the disk energy, which accounts for the stiffening of the disk due to the radial expansion resulting from its rotation.
Abstract: A model for the in-plane oscillations of a thin rotating disk has been derived using a nonlinear strain measure to calculate the disk energy. This accounts for the stiffening of the disk due to the radial expansion resulting from its rotation. The corresponding nondimensionalized natural frequencies are seen to depend only on the nondimensionalized rotation speed and have been calculated. The radially expanded disk configuration is linearly stable over the range of rotation speeds studied here. The sine and cosine modes for all nodal diameters couple to each other at all non-zero rotation speeds and the strength of this coupling increases with rotation speed. This coupling causes the reported frequencies of the stationary disk to split. The zero, one and two nodal diameter in-plane modes do not have a critical speed corresponding to the vanishing of the backward travelling wave frequency. The use of a linear strain measure in earlier work incorrectly predicts instability of the rotating equilibrium and the existence of critical speeds in these modes.
TL;DR: In this article, an active monitoring method for damage detection applied to composite structures is studied, which is shown to be effective, reliable and straightforward for the specimens considered in the present study, which are composed of different materials and suffer various levels of damage.
Abstract: This study focuses on an active monitoring method for damage detection applied to composite structures. Honeycomb sandwich and carbon fiber composite structures are studied. Two kinds of damage are considered: delamination and impact damage. Wavelet analysis methods are adopted to postprocess the raw monitored signal. A new damage signature is introduced to determine the presence and extent of damage in composites, while eliminating the influence of different distances between the active actuator and active monitoring elements. The proposed method is shown to be effective, reliable, and straightforward for the specimens considered in the present study, which are composed of different materials and suffer various levels of damage. An online real-time active monitoring system for damage detection is described that is based on this research.
TL;DR: In this article, the acoustic response of curved polymeric active diaphragms is investigated. And the authors conclude that the acoustic output is governed by the structural ring resonance, which can be designed such that the most efficient acoustic radiation is within the particular frequency range of operation.
Abstract: Piezoelectric active diaphragms hold promise as an alternative to using passive diaphragms driven by voice coils for sound generation and noise cancellation applications. This paper presents an in-depth investigation of the acoustic response for curved polymeric piezoelectric active diaphragms. Simple analytical models were derived and experimentally validated to predict the structural dynamic and acoustic responses for generic polyvinylidene fluoride (PVdF) constant curvature active diaphragms with variable geometric parameters (width, radius, subtended angle, thickness). These models are useful for design purposes and for capturing the overall behavioral trends. To analyze the acoustic response mechanisms further, three-dimensional numerical models were also developed and experimentally validated. Parametric studies based upon these models reveal the potential of high acoustic outputs (over 100 dB in the far field) from optimized geometric configurations with subtended angles differing from the conventionally utilized flat, semicircular or circular configurations. These studies, corroborated by experiments on a variety of active diaphragm prototypes, conclude that the acoustic output is governed by the structural ring resonance, which can be designed such that the most efficient acoustic radiation is within the particular frequency range of operation.
TL;DR: In this article, the authors describe the dynamic properties of an industrial Squeeze-Film Damper (SFD) bearing design using the well-known perturbation approach, where the reaction forces induced by small movements away from the position of equilibrium are expanded into a Taylor series in terms of displacement, velocity and acceleration.
Abstract: The dynamic properties of an industrial Squeeze-Film Damper (SFD) bearing design are described using the well-known perturbation approach, where the reaction forces induced by small movements away from the position of equilibrium are expanded into a Taylor series in terms of displacement, velocity, and acceleration. Although generally negligible, the acceleration term can become significant in SFD bearings when inertia effects in the damper lands are enhanced by the flow in a central circumferential oil supply groove. By using a bulk flow approximation in the oil supply groove an explicit expression is derived for the acceleration term. Experimental results confirm the significance of the oil supply groove geometry and appear to validate the bulk flow approximation.
TL;DR: In this paper, a parametric identification procedure based on recursive evaluation of response harmonic amplitude series is presented for a rotor-bearing system supported in rolling element bearings and compared with analytical values and experimental results of previous works.
Abstract: Volterra series provides a structured analytical platform for modeling and identification of nonlinear systems. The series has been widely used in nonparametric identification through higher order frequency response functions or FRFs. A parametric identification procedure based on recursive evaluation of response harmonic amplitude series is presented here. The procedure is experimentally investigated for a rotor-bearing system supported in rolling element bearings. The estimates of nonlinear bearing stiffness obtained from experimentation have been compared with analytical values and experimental results of previous works.
TL;DR: In this article, the authors investigated the noise transmission behavior of an advanced grid-stiffened (AGS) composite structure by combining numerical and experimental methods, and found that acoustic resonances played an important role for noise transmission through the structure.
Abstract: The noise transmission behavior of an advanced grid-stiffened (AGS) composite structure has been investigated by combining numerical and experimental methods. Structural-acoustic coupling was found to be light, permitting separate analysis of the structure and acoustic cavity. Finite element analysis permitted the resonant frequencies of acoustic cavity and structure to be calculated, which play an important role for noise transmission through the structure. Acoustic mode shapes permitted internal coincidence frequencies to be estimated and provided insight into modal pressure distributions, when considering payload location. Experimental structural and acoustic modal analysis permitted the resonant frequencies and damping ratios for the structure and cavity to be determined, which in turn were used to corroborate the FEA model. Finally, direct measurement of the noise transmission was performed based on noise reduction spectrum (NRS), which is calculated front spatial averages of the RMS acoustic pressures inside and outside of the shell. It was found theft the NRS was dominated by acoustic resonances, which were marked by sharp dips in the NRS curve. Internal coincidence of the axial wavenumbers was also found to be a significant mechanism for noise transmission. External coincidence and ring frequencies were found to provide less of an impact on the overall NRS for the structure.