Abstract
Stabilizing and destabilizing effects of damping in non-conservative systems: some new results
Previous work has amply demonstrated that non-conservative systems can be made unstable by the application of damping. Systems with two neutrally stable damping levels, whereby the system initially gains stability but later loses stability as the level of damping is increased, have also been observed. The phenomenon of three damping-induced stability transitions has not been reported in the literature. Here we show that the addition of damping can cause non-conservative systems to become stable, then unstable and stable again at the same value of the non-conservative forcing variable. This combination of stability transitions is found to exist for several example systems, including linkages with follower end forces and fluid-conveying pipes. Another interesting observation is that a given system can exhibit different forms of stability transitions in different regions of its parameter space. In a particular example, the neutral stability curves corresponding to two different modes are observed to intersect, such that the boundary separating the stable and unstable regions is piecewise continuous. This observation requires that the accepted definitions of ‘stabilizing’ and ‘destabilizing’ roles of damping be revised. All of these stability transition behaviours were found by applying the Routh–Hurwitz procedure, whereby the traditional procedure is first applied to the characteristic polynomial of the system and then again to guarantee the existence of a second-order auxiliary polynomial in the Routh array. This procedure is developed in the context of examples, each of which concerns a classical apparatus, the dynamics of which are more interesting than previously believed.
Fourier and wavelet analyses of intermittent and resonant pressure components in a slot burner
In laboratory-scale burner, it has been observed that the acoustic excitations change the flame topology inducing asymmetry and oscillations. Hence, an acoustic and aeroacoustic study in non-reactive condition is of primary importance during the design stage of a new burner in order to avoid the development of standing waves which can force the flame. So wall pressure fluctuations inside and outside of a novel slot burner have been studied experimentally and numerically for a broad range of geometrical parameters and mass flow rates. Wall pressure fluctuations have been measured through cavity-mounted microphones, providing uni- and multivariate pressure statistics in both the time and frequency domains. Furthermore, since the onset of combustion-driven oscillations is always presaged by intermittent bursts of high amplitude, a wavelet-based conditional sampling procedure was applied to the database in order to detect coherent signatures embedded in the pressure time signals. Since for a particular case the coherent structures identified have a multiscale signature, a wavelet-based decomposition technique was proposed as well to separate the contribution of the large- and small-scale flow structures to the pressure fluctuation field. As a main outcome of the activity, no coupling between standing waves and velocity fluctuations was observed, but only well-localized pressure signatures with shape strongly affected by the neighbouring flow physics.
On the non-proportionality between wheel/rail contact forces and speed during wheelset passage over specific welds
This study investigates the influence on the wheel–rail contact forces of the running speed and the shape and position of weld defects along the track. For this purpose, a vertical dynamic model in the space domain is used. The model is obtained from the transformation between the domains of frequency and space using a rational fraction polynomial (RFP) method, which is modified with multiobjective genetic algorithms in order to improve the fitting of track receptance and to assist integration during simulations. This produces a precise model with short calculation times, which is essential to this study. The wheel–rail contact is modelled using a non-linear Hertz spring. The contact forces are studied for several types of characteristic welds. The way in which forces vary as a function of weld position and running speed is studied for each type of weld. This article studies some of the factors that affect the maximum forces when the vehicle moves over a rail weld, such as weld geometry, parametric excitation and contact stiffness. It is found that the maximum force in the wheel–rail contact when the vehicle moves over a weld is not always proportional to the running speed. This article explains why it is not proportional in specific welds.
Effect of thermal stresses on frequency band structures of elastic metamaterial plates
We investigate the effect of thermal stresses on the band structure of elastic metamaterial plates by developing a useful finite element–based method. The thermal field is assumed to be uniform throughout the whole plate. Specifically, we find that the stiffness matrix of plate element is composed of elastic and thermal stresses parts, which can be regarded as a linear function of temperature difference. We additionally demonstrate that the relative magnitudes between elastic properties and thermal stresses will lead to non-linear effects on frequency band structures based on two different types of metamaterial plates made of single and double inclusions of square plates, respectively. Then, we validate the proposed approach by comparing the band structures with the frequency response curves obtained in finite periodic structures. We conduct sensitivity analysis and discuss in depth the sensitivities of band structures with respect to temperature difference to quantitatively investigate the effect of thermal stresses on each band. In addition, the coupled effects of thermal stresses and temperature-dependent material properties on the band structure of aluminium/silicone rubber plate have also been discussed. The proposed method and new findings in this article extends the ability of existing metamaterial plates by enabling tunability over a wide range of frequencies in thermal environments.
Pile driving acoustics made simple: damped cylindrical spreading model
Sound produced by marine pile driving activities poses a possible risk to marine life. The assessment and mitigation of this risk require a precise prediction of the expected levels. An analytical approach to estimate the radiated sound exposure levels is presented, based on the axial symmetry of the problem, resulting in damped cylindrical spreading. The approach is verified against numerical results from the recently held COMPILE benchmark workshop and validated with data from three different wind farm construction sites in the North Sea. In addition, it was found to yield more accurate estimates of the sound exposure level than an empirical decay formula sometimes used to evaluate the impact of marine pile driving.
Age effects on perceptual restoration of degraded interrupted sentences
Adult cochlear implant (CI) users show small or non-existent perceptual restoration effects when listening to interrupted speech. Perceptual restoration is believed to be a top-down mechanism that enhances speech perception in adverse listening conditions and appears to be particularly utilized by older normal-hearing participants. Whether older normal-hearing participants can derive any restoration benefits from degraded speech (as would be presented through a CI speech processor) is the focus of this study. Two groups of normal-hearing participants (younger: age ⩽ 30 years; older: age ⩾ 60 years) were tested for perceptual restoration effects in the context of interrupted sentences. Speech signal degradations were controlled by manipulating parameters of a noise vocoder and used to analyse the effects of spectral resolution and noise burst spectral content on perceptual restoration. Older normal-hearing participants generally showed larger and more consistent perceptual restoration benefits for vocoded speech than did younger normal-hearing participants, even in the lowest spectral resolution conditions. Reduced restoration in CI users thus may be caused by factors like noise reduction strategies or small dynamic ranges rather than an interaction of ageing effects and low spectral resolution.
Phonological feature–based speech recognition system for pronunciation training in non-native language learning
The authors address the question of whether phonological features can be used effectively in an automatic speech recognition (ASR) system for pronunciation training in non-native language (L2) learning. Computer-aided pronunciation training consists of two essential tasks – detecting mispronunciations and providing corrective feedback, usually on the basis of either full words or phonemes. Phonemes, however, can further be disassembled into phonological features, which in turn define groups of phonemes. A phonological feature–based ASR system allows the authors to perform a sub-phonemic analysis at feature level, providing a more effective feedback to reach the acoustic goal and perceptual constancy. Furthermore, phonological features provide a structured way for analysing the types of errors a learner makes and can readily convey which pronunciations need improvement. This article presents the authors’ implementation of such an ASR system using deep neural networks as an acoustic model and its use for detecting mispronunciations, analysing errors and rendering corrective feedback. Quantitative as well as qualitative evaluations are carried out for German and Italian learners of English. In addition to achieving high accuracy of mispronunciation detection, the system also provides accurate diagnosis of errors.
Paradigmatic variation of vowels in expressive speech: acoustic description and dimensional analysis
Acoustic variation in expressive speech at the syllable level is studied. As emotions or attitudes can be conveyed by short spoken words, analysis of paradigmatic variations in vowels is an important issue to characterize the expressive content of such speech segments. The corpus contains 160 sentences produced under seven expressive conditions (Neutral, Anger, Fear, Surprise, Sensuality, Joy, Sadness) acted by a French female speaker (a total of 1120 sentences, 13,140 vowels). In total, 11 base acoustic parameters are selected for voice source– and vocal tract–related feature analysis. An acoustic description of the expressions is drawn, using the dimensions of melodic range, intensity, noise, spectral tilt, vocalic space and dynamic features. The first three functions of a discriminant analysis explain 95% of the variance in the data. These statistical dimensions are consistently associated with acoustic dimensions. Covariation of intensity and F0 explains over 80% of the variance, followed by noise features (8%), covariation of spectral tilt and F0 (7%). On the basis of isolated vowels alone, expressions are classified with a mean accuracy of 78%.
Identification of the effective control parameter to enhance the progression rate of vibro-impact devices with drift
This article 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 analysed 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.
A new global spatial discretization method for calculating dynamic responses of two-dimensional continuous systems with application to a rectangular Kirchhoff plate
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 the 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 a much higher accuracy in the 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.
Entropy for strongly coupled oscillators
This article 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 with 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 non-identical oscillators to demonstrate these findings.
Free vibration of thin shallow elliptical shells
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 non-linear. 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. The 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.
Parametric excitation of vibrations in printing machines
In this article, the parametric excitation of vibrations in printing machines is analysed both experimentally and theoretically. Initially, selected results from experimental studies of these drive belt–induced torsional vibrations are discussed. For this, 12 different drive belts are used during experiments at a test rig and at a sheet-fed offset printing machine. The measured vibration amplitudes from machine and test rig correlate very well. Depending on the chosen drive belt, large vibration amplitudes occur during printing in the vicinity of a critical production speed, where the first drive belt order coincides with the machine’s first eigenfrequency. This results in a clearly visible rhythm during printing and is therefore unacceptable. It is demonstrated experimentally that active vibration control can be used successfully to reduce these parametrically excited torsional vibrations in sheet-fed offset printing machines. Furthermore, the machine is modelled as a dynamical multi-degree-of-freedom system with time-periodic coefficients, and the parametric excitation of torsional vibrations is studied numerically. First, a numerical stability analysis is carried out employing Floquet theory. The system’s damping parameters are derived from the measured machine data. Accordingly, a simulation model with drive control results in parametrically excited torsional vibration amplitudes that match very well with the measured amplitudes.
Robust inverse dynamic control of a manoeuvring smart flexible satellite with piezoelectric layers
In this article, a satellite with two flexible appendages and a central hub is considered. The piezoelectric layers are attached to both sides of the appendages and used as actuators. The governing equations of motion are derived based on Lagrange method. Using Rayleigh–Ritz technique, ordinary differential equations of motion are obtained. A robust inverse dynamic control is applied to the system to not only control the three-axis manoeuvre of the satellite but also suppress the vibrations of the flexible appendages. Finally, the system is simulated and the simulation results show good performance of this controller.
Non-linear thermally induced vibrations of non-homogeneous rectangular plate of linearly varying thickness in the presence of external force
An analysis with numerical results is presented for non-linear thermally induced forced vibrations of rectangular plate of variable thickness on the basis of classical plate theory. The thickness of the plate is considered as linearly varying in the x-direction. Approximate formulae are proposed for estimating the maximum deflection of a rectangular plate subject to a uniformly distributed harmonic lateral load. The effect of structural parameters such as thermal constant and taper constant with different aspect ratios on the vibration of simply supported-free-simply supported-free plate for maximum deflection for the different values of the fundamental frequency of vibration is studied. Results are presented in tabular form.
Experimental study on the electromechanical hysteresis property of macro-fibre composite actuator
The hysteresis characteristic in macro-fibre composite (MFC) actuator is intimately related to its application in vibration control system. In this article, the electromechanical hysteresis property of MFC actuator is studied. First, an experimental study on the electromechanical behaviours under different voltages and frequencies is carried out, and the hysteretic property of the MFC actuator is investigated. A digital signal processing (DSP) system is used to control input voltage and a digital image correlation (DIC) system as a non-contact setup is used to obtain the output strain of the MFC actuator in the experiment. The experimental results indicate that the relationship of voltage and strain displays hysteresis with non-local memory. Second, the Preisach model is used to describe the hysteresis characteristic of the MFC actuator. In order to improve the accuracy of the model, the modifications are made, in which the experimental data under the quasi-static frequency range and the congruency property are used to establish the modified Preisach model. Finally, the hysteresis characteristics of the MFC actuator predicted from the proposed model are compared with those obtained from the classical Preisach model. The results indicate that the proposed model gives better accuracy than the classical Preisach model, and it is suggested that this study on the hysteresis model of the MFC actuator can be used in active vibration control.
On the construction of multiscale surrogates for design optimization of acoustical materials
This article is concerned with the use of polynomial metamodels for the design of acoustical materials, considered as equivalent fluids. Polynomial series in microstructural parameters are considered and allow us to approximate the multiscale solution map in some well-defined sense. The relevance of the framework is illustrated by considering the prediction of the sound absorption coefficient. In accordance with the theoretical results provided elsewhere in the literature, it is shown that the surrogate model can accurately approximate the solution map at a reasonable computational cost, depending on the dimension of the input parameter space. Microstructural and process optimization by design represents two envisioned applications.
Underwater assessment of anthropogenic noise sources using a field recording method
Concern about underwater noise has been increasing due to the high number of projects needing environmental impact assessment to know how the underwater environment could be affected by pollutant noise, especially when living beings are involved. Since in countries like Chile there is no current legislation about anthropogenic underwater noise, the main objective of this work was to face this topic in Chile. To achieve it, noise sources present in rivers from Valdivia City – located in south centre Chile – were evaluated. Underwater and airborne noise emission measurement, coming from a high number of anthropogenic noise sources, both mobile and stationary, was carried out using a field method. It uses a digital recorder through which the noise emitted from the evaluated source is recorded. Subsequently, using a software, the desired noise descriptors are obtained. Measurements were carried out in both summer and winter seasons, between December 2015 and December 2016. To have a database of sources as those assessed in this study, measured in field conditions and with low natural background noise, takes a relevant value when working with mathematical models of acoustic prediction. The information we have to introduce into them must be as accurate and similar as possible to those acoustic characteristics of the noise sources considered. It is important to state how necessary standardizing procedures used to measure and present results of underwater measurements appears now. There exists a wide variety of literature showing divergences regarding this. This work tried to bring into the area of underwater measurements techniques and considerations used in air measurements, since, strictly speaking, the change is only in the involved fluids.
Active acoustic cloaking spherical shells
The scheme of efficient and practical directional active acoustic cloaking is proposed in contrast to the attractive though hard-to-utilize concept of passive acoustic cloaking which is based on composite acoustic metamaterials or negative index materials. In this article, the scattering cancellation is planned to occur in a desired direction such as backscattering in far field, with great interests in sound navigation and ranging (SONAR) applications. The invisibility of the target object/region occurs due to a surrounding spherical cloaking shell composed of a casing layer stimulated by a radially polarized piezoelectric bonded layer. With concerns regarding the feasibility of the proposed configuration, the electrical excitation is limited only to its monopole mode (breathing mode). An illustrative numerical example is presented in order to investigate the practicability of the configuration especially with emphasis on the required voltage and power supply.
Numerical simulation of target strength measurements from near to far field of fish using the method of fundamental solutions
We propose the application of a numerical model based on the method of fundamental solutions (MFS) to the estimation of the target strength (TS) of single fish, a crucial parameter for evaluating fish abundance when using active acoustical techniques. We consider a realistic beam and the influence of the evolution of the scattered field with distance maintaining the target in the far field of the ultrasonic transducer. The cases of a fluid sphere and a prolate spheroid model for the swimbladder are used to verify the application of MFS against other analytical and numerical methods. In addition, a model considering only the swimbladder and the fishbone is presented to evaluate the differences between dorsal and ventral aspect measurements and to understand previous experimental works performed with Atlantic salmon (Salmo salar) in floating cages.