bluuebunny/crossref_metadata_2025_split huggingface dataset:
Combined-Mode Parametric Oscillations in Mechanical Systems
Authors: Sato Chikara 
Published: March 2021

Abstract
Parametric oscillations formulated by linear or nonlinear differential equations of order two are often encountered in the textbooks on the theory of nonlinear oscillations. Mathieu equation or its modified nonlinear version are seen in many papers as the typical examples. Parametric oscillations of the higher order than two, however, are less investigated, and the fundamental form is little known, especially the knowledge on the engineering viewpoint is little published in relation with mechanical or physical models.
This paper deals with the parametirc oscillations governed by periodic differential equations with the higher order than two, presenting the fundamental mathematical form with many physical, mechanical, and electrical examples. General nonlinear systems with parametric excitation are not easy to treat with, because they have many characters depending on the specific forms of the systems. In this paper we confine systems which have three conditions, first excitation frequency region is limited, second symmetry condition or reciprocity is kept, third no energy condition of nonlinear functions. These conditions are partly mathematical and partly physical. In order to treat with parametric oscillation we think that the causes of oscillations are only parametric instability. Without parametric excitation no significant oscillations are assumed to exist.
A Generalized Nonlinear Model for Active Magnetic Bearings
Authors: Gao Jianmin , Zhu Xiaomei , McCafferty Ray , Leighton Nigel 
Published: March 2021

Abstract
This paper describes a generalized nonlinear model of active magnetic bearings, for arbitrary number, size and position of pole-pairs, including the coordinate coupling between the vertical and horizontal directions, the nonlinearility of the power amplifier and the effect of finite permeability of the ferromagnetic bearing material. As an example, a practical magnetic bearing with four pole-pairs is examined. The predicted force from the model is quite consistent with experimental results. For the PD controller implemented for a horizontal rotor, the expectation of the equilibrium of the rotor also agrees quite well with the measurement.
Vibrations of a Cracked Rotor (3/2-Order Super-Subharmonic and 1/2-Order Subharmonic Resonances)
Authors: Ishida Yukio , Hirokawa Kazuharu , Hirose Mitsuo 
Published: March 2021

Abstract
When a transverse crack appears in a rotating horizontal shaft due to fatigue, various kinds of resonances occur. In this paper, the vibration characteristics of the 3/2-order super-subharmonic resonance and the 1/2-order subharmimic resonance are investigated. We represent the spring characteristics of a cracked shaft by a piecewise linear type nonlinearlity and approximate it by a power series. The following are clarified, (a) These two kinds of resonances have similar vibration characteristics, (b) They occur only in horizontal shafts, (c) Although they are related to the parametric excitation, they occur as the steady-state oscillations due to nonlinearlity. (d) The amplitudes of these steady-state oscillations decrease as the unbalance increases.
Occurrence of Combination Tones in a String
Authors: Yasuda Kimihiko , Kato Mitsuhiro 
Published: March 2021

Abstract
The occurrence of combination tones in a string subjected to two harmonic excitations is considered. Attention is focused on the case in which the sum or difference of the excitation frequencies is near one of the natural frequencies of the string. To analyze the problem, the method of multiple scales is used. In analyzing the problem it is taken into account that a string has its natural frequencies in the ratio of prime integers. 1 : 2 : 3 : ⋯. The analysis shows that, due to this, the combination tones with various characters can occur.
Melnikov-Based Open-Loop Control of Escape for a Class of Nonlinear Systems
Authors: Simiu Emil , Franaszek Marek 
Published: March 2021

Abstract
The performance of certain nonlinear stochastic systems is deemed acceptable if, during a specified time interval, the systems have sufficiently low probabilities of escape from a preferred region of phase space. We propose an open-loop control method for reducing these probabilities. The method is applicable to stochastic systems whose dissipation- and excitation-free counterparts have homoclinic or heteroclinic orbits. The Melnikov relative scale factors are system properties containing information on the frequencies of the random forcing spectral components that are most effective in inducing escapes. This information is useful in practice even if the dissipation and excitation terms are relatively large. An ideal open-loop control force applied to the system would be equal to the negative of a fraction of the exciting force from which the ineffective components have been filtered out. Limitations inherent in any practical control system make it impossible to achieve such an ideal control. Nevertheless, numerical simulations show that substantial advantages can be achieved in some cases by designing control systems that take into account the information contained in the Melnikov scale factors.
Simulation of Stochastic Processes and Fields for Monte Carlo Simulation Applications: Some Recent Developments
Authors: Deodatis George , Popescu Radu , Prevost Jean H. 
Published: March 2021

Abstract
Two of the latest developments concerning the spectral representation method (used to simulate stochastic processes and fields) are presented in this paper. The first one introduces an extension of the spectral representation method to simulate non-stationary stochastic vector processes with evolutionary power. The proposed simulation formula is simple and straightforward and generates sample functions of the vector process according to a prescribed non-stationary cross-spectral density matrix. The second development introduces another extension of the spectral representation method to simulate multi-dimensional, multi-variate, non-Gaussian stochastic fields. In this case, sample functions are generated according to a prescribed cross-spectral density matrix and prescribed (non-Gaussian) probability distribution functions. Numerical examples are provided for both developments.
Use of Directional AR and ML Spectra for Detection of Misfired Engine Cylinder
Authors: Lee Chong-Won , Han Yun-Sik , Park Jong-Po 
Published: March 2021

Abstract
A powerful diagnostic method of cylinder power faults, which uses the two-sided directional power spectra of complex-valued engine vibration signals, is presented and tested with an automobile engine. As the spectral estimators, the AR, ML and FFT methods are compared and the multi-layer neural network is employed for pattern recognition. Experimental results show that the success rate for identifying the misfired cylinder is higher with the combined AR and ML estimator than the FFT method.
Reduction of the Junction Degrees of Freedom Before Assembly in Dynamic Substructuring
Authors: Brahmi Khaled , Bouhaddi Noureddine , Fillod René 
Published: March 2021

Abstract
The dynamic behavior of large complex structures is often achieved by substructuring methods where the substructures are modeled separately. One of advantages of these methods is that one component may be changed without remodeling the others. This is particularly useful when components are designed and analyzed by different organizations.
Althougth these methods use a variety of reduction techniques to couple the substructure models, the size of the final condensed system remains important due to large number of the junction degrees of freedom (Abv: dof) required for assembly.
In this paper, we propose a method using a modal projection on the Ritz basis which, when correctly chosen, reduces the number of these dof before the assembly of substructures. This method is applied to certain substructuring methods: dynamic condensation, modal synthesis using free or constrained interfaces.
Multiresolution Reproducing Kernel Particle Methods in Acoustic Problems
Authors: Liu W. K. , Chang C. T. , Chen Y. , Uras R. A. 
Published: March 2021

Abstract
In the analysis of complex phenomena of acoustic systems, the computational modeling requires special attention for a realistic representation of the physics. As a powerful tool, the finite element method has been widely used in the study of complex systems. In order to capture the important physical phenomena, p-finite elements and/or hp-finite elements are employed. The reproducing kernel particle methods (RKPM) are emerging as an effective alternative due to the elimination of a mesh, and the ability to analyze a specific frequency range. Additionally, a wavelet particle method based on the multiresolution analysis encountered in signal processing has been developed. The interpolation functions consist of spline functions with built-in window. A variation in the size of the window implies a geometrical refinement, and allows the filtering of the desired frequency range. Preliminary analysis of the wave equation shows the effectiveness of this approach. The frequency/wave number relationship of the continuum case can be closely simulated by using the reproducing kernel particle methods. A similar methodology is also developed for the Timoshenko beam.
Effects of Coupling Misalignment on the Vibration Characteristics of a Two Stage Turbine Rotor
Authors: Arumugam P. , Swarnamani S. , Prabhu B. S. 
Published: March 2021

Abstract
Misalignment between driving and driven machines is unavoidable. Flexible couplings are used to accommodate the misalignment. The flexible mechanisms in the coupling impose moments on the rotors connected. The reaction forces developed due to the imposed moments act as periodic excitation forces on the rotating shafts. In the present work the effect of these reaction forces on the vibration characteristics of a two-stage turbine rotor-bearing system has been studied considering flexible element coupling.
Magnetic Bearing Models With Flexible Supports in Rotor Stability Analyses
Authors: Brockett Theodore S. , Barrett Lloyd E. 
Published: March 2021

Abstract
A method is presented that enables collocated and noncollocated magnetic bearing models on flexible supports to be included into the transfer matrix method for rotor stability analyses. The linear magnetic bearing model is represented as an open-loop stiffness, an actuator gain, and a frequency-dependent transfer function relating the sensed rotor displacement to the perturbation currents in the coils of the bearing actuator. The open-loop stiffness is handled in the same way that an ordinary hydrodynamic bearing is handled, while the bearing actuator gain and transfer function combination is treated as a complex-valued frequency-dependent stiffness that acts between the actuator location and the sensor position. The flexible support is modeled as a lumped mass with linear principal and cross-coupled stiffness and damping coefficients. The singularities introduced into the eigenvalue search surface from representing degrees of freedom from the main line of the rotor as transfer functions are canceled. This enables eigenvalues to be found in the vicinity of singular points. The stability of a specific rotor-bearing system is studied as a function of support flexibility and damping thus demonstrating the proposed method.
Influence of Geometric Parameters on Annular Fluid Seal Characteristics
Authors: Bonneau Olivier , Lucas Victor , Frene Jean 
Published: March 2021

Abstract
The numerical prediction of the dynamical behavior of turbopumps is very important. The space technology and the field of energy products are in constant development and it is necessary to quantify the influence of each component. The dynamical characteristics of annular seals must be calculated with accuracy.
The aim of this work is to quantify the influence of geometric parameters on the dynamical behavior of the shaft.
Three parameters will be studied: the duct loss parameter (at the seal entrance), a conical seal, and a misaligned seal. The two last geometrical defects have a direct influence on the film thickness.
It is important to insist on the influence of the entrance duct loss which governs, in large part, the stiffness calculus (and then the stability). The most difficult problem is to evaluate this duct loss which depends on the seal geometry, Reynolds number and fluid characteristics...
This study shows the important rôle played by geometrical parameter of a seal. The conicity and the misalignment modify the dynamical behavior of the shaft. These effects are essentially due to the axial flow which generates a pressure field due to axial film geometry. It should be noted that in the case of predominant circumferential flow these conclusions are totally different.
Pseudo Static Excitation of Turbine Blades Under Acceleration
Authors: Vyas Nalinaksh S. 
Published: March 2021

Abstract
Coriolis forces, on a blade mounted on a disc rotating with angular acceleration, give rise to a pseudo static excitation to the blade. The resulting vibrations are simultaneously influenced by the changing stiffness of the blade under acceleration. The occurrence of this shock phenomenon has been reported in earlier studies for uniform beams. In this paper the phenomenon is investigated for a typical tapered, pretwisted, turbine lade with an asymmetric aerofoil cross section and mounted on the disc at a stagger angle. The governing equations of motion, for uniform rates of acceleration are derived employing Lagrangian equations. The vibration response patterns of the blade, obtained numerically, for typical acceleration rates are illustrated.
On the Active-Passive Hybrid Vibration Control Actions of Structures With Active Constrained Layer Treatments
Authors: Liao W. H. , Wang K. W. 
Published: March 2021

Abstract
This paper is concerned with the analysis of active and passive hybrid actions in structures with active constrained damping layers (ACL). A system model is derived via Hamilton’s Principle, based on the constitutive equations of the elastic, viscoelastic, and piezoelectric materials. The model converges to a fully-active piezotronic system as the thickness of the viscoelastic material (VEM) layer approaches zero. A mixed Galerkin-GHM method is employed to discretize and analyze the model in time domain. With an LQR optimal control formulation, the effects of the active constrained layer configuration on the system vibration suppression performance and control effort requirements are investigated. Analysis illustrate that the active piezoelectric action with proper feedback controls will always enhance the damping ability of the passive constrained layer. On the other hand, the viscoelastic layer will reduce the direct control authorities from the active source to the host structure. The significance of this effect depends very much on the viscoelastic layer thickness and material properties. Therefore, with some parameter combinations, the ACL configuration could require more control effort while achieving less vibration reductions compared to a fully-active system. Through analyzing the performance and control effort indices, the conditions where this active-passive hybrid approach can outperform both the passive and active systems are quantified. Based on this study, design guidelines can be set up to effectively integrate the host structure with the piezoelectric and viscoelastic materials, such that a true active-passive hybrid control system can be achieved.
Robustness of Natural Controls of Distributed-Parameter Systems
Authors: Hwang Jai Hyuk , Kim Doo Man , Lim Kyoung Ho 
Published: March 2021

Abstract
In this paper, the effect of parameter and spatial discretization errors on the closed-loop behavior of distributed-parameter systems is analyzed for natural controls. If the control force designed on the basis of the postulated system with the parameter and discretization errors is applied to control the actual system, the closed-loop performance of the actual system will be degraded depending on the degree of the errors. The extent of deviation of the closed-loop performance from the expected one is derived and evaluated using operator techniques. It has been found that the extent of the deviation is proportional to the magnitude of the parameter and discretization errors, and that the proportional coefficient depends on the structures of the natural controls.
Vibration Control of the Piping System: A Method Using Feedback-Error-Learning
Authors: Takita Yoshihiro 
Published: March 2021

Abstract
This paper presents a vibration control method for piping systems using a feedback control system constructed with LQ-control and a neural network featuring feedback-error learning. The piping system is normally flexible, therefore, natural frequencies of the system fluctuate variably when the density of the content. This paper shows that the piping system changes dynamics according to increases or decreases of the mass effects. In order to reduce the first vibration mode of the piping system without spillover instability, the control system is designed using LQ-control with feedback-error-learning applied to an adapted nonlinear feedback controller. The effectiveness of this control method is confirmed by the neural network simulation program named NeuroLab and is experimented using data measured by the control system constructed with the digital signal processing unit.
H∞ Structural Controller Design Based on Absolute Acceleration Measurement
Authors: Yamada Nariyasu , Nishitani Akira 
Published: March 2021

Abstract
This paper discusses the H∞ structural control system design procedure for a building structure on the basis of the measurement of the top floor absolute acceleration response. The discussion covers the design procedure from its basic concept to its details. The validity of the presented H∞ controller design is experimentally demonstrated.
Updating Substructure Models With Dynamic Boundary Conditions
Authors: Link Michael , Qian Zheng 
Published: March 2021

Abstract
In recent years procedures for updating analytical model parameters have been developed by minimizing differences between analytical and preferably experimental modal analysis results. Provided that the initial analysis model contains parameters capable of describing possible damage these techniques could also be used for damage detection. In this case the parameters are updated using test data before and after the damage.
Looking at complex structures with hundreds of parameters one generally has to measure the modal data at many locations and try to reduce the number of unknown parameters by some kind of localization technique because the measurement information is generally not sufficient to identify all the parameters equally distributed all over the structure.
Another way of reducing the number of parameters shall be presented here. This method is based on the idea of measuring only a part of the structure and replacing the residual structure by dynamic boundary conditions which describe the dynamic stiffness at the interfaces between the measured main structure and the remaining unmeasured residual structure. This approach has some advantage since testing could be concentrated on critical areas where structural modifications are expected either due to damage or due to intended design changes.
The dynamic boundary conditions are expressed in Craig-Bampton (CB) format by transforming the mass and stiffness matrices of the unmeasured residual structure to the interface degrees of freedom (DOF) and to the modal DOFs of the residual structure fixed at the interface. The dynamic boundary stiffness concentrates all physical parameters of the residual structure in only a few parameters which are open for updating. In this approach damage or modelling errors within the unmeasured residual structure are taken into account only in a global sense whereas the measured main structure is parametrized locally as usual by factoring mass and stiffness submatrices defining the type and the location of the physical parameters to be identified. The procedure was applied to identify the design parameters of a beam type frame structure with bolted joints using experimental modal data.
Higher Physical Activity Is Related to Lower Neck Adiposity in Young Men, but to Higher Neck Adiposity in Young Women: An Exploratory Study
Authors: Arias-Tellez Maria Jose , Acosta Francisco M. , Migueles Jairo H. , Pascual-Gamarra Jose M. , Merchan-Ramirez Elisa , de Lucena Martins Clarice M. , Llamas-Elvira Jose M. , Martinez-Tellez Borja , Ruiz Jonatan R. 
Published: March 2021

The role of lifestyle behaviors on neck adipose tissue (NAT), a fat depot that appears to be involved in the pathogenesis of different cardiometabolic diseases and in inflammatory status, is unknown. In this cross-sectional and exploratory study, the authors examined the relationship between sedentary time and physical activity (PA) with neck adiposity in young adults. A total of 134 subjects (69% women, 23 ± 2 years) were enrolled. The time spent in sedentary behavior and PA of different intensity were objectively measured for 7 consecutive days (24 hr/day), using a wrist (nondominant)-worn accelerometer. The NAT volume was assessed using computed tomography, and the compartmental (subcutaneous, intermuscular, and perivertebral) and total NAT volumes were determined at the level of vertebra C5. Anthropometric indicators and body composition (by dual-energy X-ray absorptiometry) were determined. The time spent in light physical activity and moderate physical activity (MPA) and the overall PA were inversely associated with the intermuscular NAT volume in men, as were the MPA and overall PA with total NAT volume (all ps ≤ .04). Sedentary time was directly related to the total NAT volume (p = .04). An opposite trend was observed in women, finding a direct relationship of MPA with the subcutaneous NAT; of light physical activity, MPA, and overall PA with the perivertebral NAT; and of light physical activity with total NAT volumes (all ps ≤ .05). The observed associations were weak, and after adjusting for multiplicity, the results became nonsignificant (p > .05). These findings suggest that the specific characteristics of PA (time and intensity) might have sex-dependent implications in the accumulation of NAT.
Fewer butterflies seen by community scientists across the warming and drying landscapes of the American West
Authors: Forister M. L. , Halsch C. A. , Nice C. C. , Fordyce J. A. , Dilts T. E. , Oliver J. C. , Prudic K. L. , Shapiro A. M. , Wilson J. K. , Glassberg J. 
Published: March 2021

Warming autumns, fewer butterfliesMany recent studies have revealed sweeping declines in insects over the past few decades. Butterflies are no exception. Foristeret al.used three different datasets, collected by both experts and community scientists, and found that the number of butterflies has declined over the past 40 years. Although the drivers of decline are complex, the authors found that climate change—in particular, warmer months in the autumn—explain a large portion, even as warming summers actually lead to increases. This work shows that climate change impacts may be insidious and unexpected in their effects.Science, this issue p.1042
Force Evolutionary Billiards and Billiard Equivalence of the Euler and Lagrange Cases
Authors: Vedyushkina V. V. , Fomenko A. T. 
Published: March 2021

Abstract
A class of force evolutionary billiards is discovered that realizes important integrable Hamiltonian systems on all regular isoenergy 3-surfaces simultaneously, i.e., on the phase 4-space. It is proved that the well-known Euler and Lagrange integrable systems are billiard equivalent, although the degrees of their integrals are different (two and one).