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J. Vib. Acoust. 2017;140(1):011001-011001-9. doi:10.1115/1.4037214.

This paper presents an experimental study to find out an effective parameter which is useful to enhance the progression rate of drifting vibro-impact systems excited by a harmonic force. It is assumed that the system performance would be better if the excitation force stays in a harmonious relationship with the natural motion of the impact mass. This hypothesis has been numerically analyzed and then experimentally verified. The phase lag between the excitation force and the motion of the impact mass is used to identify the best situation, where the system progression rate is maximal. It has been found that the highest progression rate of the system can be obtained when the phase lag is around one-eighth of the excitation period.

Commentary by Dr. Valentin Fuster
J. Vib. Acoust. 2017;140(1):011002-011002-18. doi:10.1115/1.4037176.

A new global spatial discretization method (NGSDM) is developed to accurately calculate natural frequencies and dynamic responses of two-dimensional (2D) continuous systems such as membranes and Kirchhoff plates. The transverse displacement of a 2D continuous system is separated into a 2D internal term and a 2D boundary-induced term; the latter is interpolated from one-dimensional (1D) boundary functions that are further divided into 1D internal terms and 1D boundary-induced terms. The 2D and 1D internal terms are chosen to satisfy prescribed boundary conditions, and the 2D and 1D boundary-induced terms use additional degrees-of-freedom (DOFs) at boundaries to ensure satisfaction of all the boundary conditions. A general formulation of the method that can achieve uniform convergence is established for a 2D continuous system with an arbitrary domain shape and arbitrary boundary conditions, and it is elaborated in detail for a general rectangular Kirchhoff plate. An example of a rectangular Kirchhoff plate that has three simply supported boundaries and one free boundary with an attached Euler–Bernoulli beam is investigated using the developed method and results are compared with those from other global and local spatial discretization methods. Advantages of the new method over local spatial discretization methods are much fewer DOFs and much less computational effort, and those over the assumed modes method (AMM) are better numerical property, a faster calculation speed, and much higher accuracy in calculation of bending moments and transverse shearing forces that are related to high-order spatial derivatives of the displacement of the plate with an edge beam.

Commentary by Dr. Valentin Fuster
J. Vib. Acoust. 2017;140(1):011003-011003-8. doi:10.1115/1.4037142.

This paper examines an approach for determining the entropy of coupled oscillators that does not rely on the assumption of weak coupling. The results of this approach are compared to the results for a weakly coupled system. It is shown that the results from each methodology agree in the case of weak coupling, and that a correction term is required for moderate to strong coupling. The correction term is shown to be related to the mixed energy term from the coupling spring as well as the geometry and stiffness of the system. Numerical simulations are performed for a symmetric system of identical coupled oscillators and an asymmetric system of nonidentical oscillators to demonstrate these findings.

Topics: Entropy
Commentary by Dr. Valentin Fuster
J. Vib. Acoust. 2017;140(1):011004-011004-9. doi:10.1115/1.4037300.

This research presents a study of the free vibration of thin, shallow elliptical shells. The equations of motion for the elliptical shell, which are developed from Love's equations, are coupled and nonlinear. In this research, a new approach is introduced to uncouple the transverse motion of the shallow elliptical shell from the surface coordinates. Through the substitution of the strain-compatibility equation into the differential equations of motion in terms of strain, an explicit relationship between the curvilinear surface strains and transverse strain is determined. This latter relationship is then utilized to uncouple the spatial differential equation for transverse motion from that of the surface coordinates. The approach introduced provides a more explicit relationship between the surface and transverse coordinates than could be obtained through use of the Airy stress function. Angular and radial Mathieu equations are used to obtain solutions to the spatial differential equation of motion. Since the recursive relationships that are derived from the Mathieu equations lead to an infinite number of roots, not all of which are physically meaningful, the solution to the eigenvalue problem is used to determine the mode shapes and eigenfrequencies of the shallow elliptical shell. The results of examples demonstrate that the eigenfrequencies of the thin shallow elliptical shell are directly proportional to the curvature of the shell and inversely proportional to the shell's eccentricity.

Commentary by Dr. Valentin Fuster
J. Vib. Acoust. 2017;140(1):011005-011005-11. doi:10.1115/1.4037469.

Structural characteristic deflection shapes (CDSs) such as mode shapes which contain spatial knowledge of structures are highly sensitive for damage detection and localization. Nevertheless, CDSs are vulnerable to measurement noise, which degrades the accuracy of damage identification. In order to enhance CDS-based damage identification, contributions are made in three aspects. First, a robust CDS estimation approach is proposed based on common principal component analysis, which estimates the CDSs as the common diagonalizer of a set of covariance matrices by joint approximation diagonalization (JAD). Second, an adaptive gapped smoothing method (GSM) is proposed and validated to be more accurate than the traditional GSM. Third, a new damage identification index capable of localizing damage and indicating relative damage severity is defined without requiring information of healthy structures. Finally, numerical and experimental examples of beams and a frame with cracks are studied to demonstrate the advantages of the proposed damage identification method in terms of noise robustness and accuracy.

Commentary by Dr. Valentin Fuster
J. Vib. Acoust. 2017;140(1):011006-011006-13. doi:10.1115/1.4037468.

In this paper, a flexible pin-on-disk system is used to simulate how squeal noise can be generated in frictional contact. As the research object, the modeling process and transient simulation method of the flexible pin-on-disk system are introduced. By means of numerical simulation, the time-varying frictional squeal reappears by introducing periodic frictional coefficient generated from rotation. Afterward, the features of time-varying squeal are studied including time-domain features, frequency-domain features, transient deformation features of the disk and the pin on the occurrence of squeal, as well as energy features. Finally, the conception and mathematical expressions of modal contribution factor are defined, and the transient modal contribution factor features of every mode are studied to make clear the function of every mode. The relationship between mode contribution factors and the vibration is revealed. It reveals that modal contribution factors between squeal and not are quite different from each other. On no occurrence of squeal, the modal contribution factors of sine and cosine modes of the disk fluctuate in the way similar to harmonic wave, and the phase difference between the contribution factors of sine and cosine mode with the same nodal circle and the same nodal diameter is 90 deg. During squeal, the coupling mode may play the most important role but not all the time. At any time, the low-frequency modes play the leading role.

Commentary by Dr. Valentin Fuster
J. Vib. Acoust. 2017;140(1):011007-011007-8. doi:10.1115/1.4037470.

In this paper, identification of energy dissipation in the joints of a lab-scale structure is accomplished. The identification is carried out by means of an energy flow analysis and experimental data. The devised procedure enables to formulate an energy balance in the vicinity of the joints to obtain local energy dissipation. In this paper, a damping matrix based on the locally identified damping coefficients is formulated. The formulated damping matrix is later used in a five-degrees-of-freedom (5DOF) system for validation. The results obtained with the proposed method are in good agreement with the experimental data, especially in the low frequency range.

Commentary by Dr. Valentin Fuster
J. Vib. Acoust. 2017;140(1):011008-011008-10. doi:10.1115/1.4037471.

Inverse patch transfer functions (iPTF) method has been developed to reconstruct the sound field of irregularly shaped sources in a noisy environment. The iPTF method, which uses classic regularization methods to solve the ill-posed problems generally, would incur some sidelobes ghosting in the process of identifying sparse sources. In view of the fact that the algorithm in wideband holography (WBH) can promote sparsity of results, a technique combining iPTF method with WBH algorithm is proposed to identify sparsely distributed sources in the present work. In the proposed technique, double layer pressure measurements are used to replace the measurements of the pressure and normal velocity which uses costly p-u probes. A gradient descent algorithm and a filtering process are applied to solve the minimization problem of identifying the normal velocities of target sources, which can suppress ghosting sources rapidly by an iterative process. In simulations, the field reconstruction results of two antiphase square piston sources show good sparsity and accuracy by employing the technique, nearly without ghosting sources. At different distances and frequencies of the two sources, the technique still performs well. Experimental validations at 200 Hz and 400 Hz are carried out in the end. The results of experiments are also coinciding with those of simulations.

Commentary by Dr. Valentin Fuster
J. Vib. Acoust. 2017;140(1):011009-011009-6. doi:10.1115/1.4037511.

An analysis is presented of the motion of a thin fiber, supported on each end, due to a sound wave that propagates in the direction perpendicular to its long axis. Predicted and measured results indicate that when fibers or hairs having a diameter measurably less than 1 μm are subjected to air-borne acoustic excitation, their motion can be a very reasonable approximation to that of the acoustic particle motion at frequencies spanning the audible range. For much of the audible range of frequencies resonant behavior due to reflections from the supports tends to be heavily damped so that the details of the boundary conditions do not play a significant role in determining the overall system response. Thin fibers are thus constrained to simply move with the surrounding medium. These results suggest that if the diameter or radius is chosen to be sufficiently small, incorporating a suitable transduction scheme to convert its mechanical motion into an electronic signal could lead to a sound sensor that very closely depicts the acoustic particle motion over a wide range of frequencies.

Commentary by Dr. Valentin Fuster
J. Vib. Acoust. 2017;140(1):011010-011010-14. doi:10.1115/1.4037521.

Due to its long-span structure and large flexibility, an electrified railway catenary is very sensitive to environmental wind load, especially the time-varying stochastic wind, which may lead to a strong forced vibration of contact line and deteriorate the current collection quality of the pantograph–catenary system. In this paper, in order to study the wind-induced vibration behavior of railway catenary, a nonlinear finite element procedure is implemented to construct the model of catenary, which can properly describe the large nonlinear deformation and the nonsmooth nonlinearity of dropper. The spatial stochastic wind field is developed considering the fluctuating winds in along-wind, vertical-wind, and cross-wind directions. Using the empirical spectra suggested by Kaimal, Panofsky, and Tieleman, the fluctuating wind velocities in three directions are generated considering the temporal and spatial correlations. Based on fluid-induced vibration theory, the model of fluctuating forces acting on catenary are developed considering the spatial characteristics of catenary. The time- and frequency-domain analyses are conducted to study the wind-induced vibration behavior with different angles of wind deflection, different angles of attack, as well as different geometries of catenary. The effect of spatial wind load on contact force of pantograph–catenary system is also investigated.

Commentary by Dr. Valentin Fuster

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