Search

Highlights • The concept of utilizing strain energy stored on a joint is explored on preventing bolt loosening. • This work opens avenues for analyzing bolt joints from the elastic strain energy stored perspective. • Using FE simulated data, a design tool is developed to output optimal Belleville washers. Abstract Despite the advantages and widespread use of bolted joints in manufacturing, one major problem is their tendency to loosen, which can ultimately cause the joint to fail. This research evaluates strain energy storage in the Belleville washer, to determine how the washer's design could be modified to counteract relaxation in the bolt, which causes loosening. Finite Element Analysis (FEA) is used to study the strain energy, bolt preload, deflections, and other parameters of various geometric configurations of Belleville washers and shows that the strain energy is not optimally utilized with conventional Belleville washers, which have constant wall thickness. Tapering the Belleville washer reduces its weight significantly, thereby increasing the washer's energy storage capacity. The FEA data are used to develop a tool for helping to design Belleville washer geometry that can prevent bolt loosening. One of the tool's inputs is the bolt deflection relaxation within the acceptable bolt preload window. This work opens further research avenues for analyzing anti-loosening joints from the perspective of the elastic energy stored in the joint and thereby predicting when the joint might fail. [ABSTRACT FROM AUTHOR]

This paper describes a general physical background that originates integro-differential problems with specific reference to aero-elastic coupling, and offers two techniques of control for this class of problems. The central result of the paper is that integro-differential equations with kernel exponential series admit an optimal solution described, in turn, by a Volterra integral equation in terms of the control. Numerical simulations show how controls prevent the flutter instability of a two-dimensional wing and a wind turbine blade. [ABSTRACT FROM AUTHOR]

In this paper the energy harvesting problem involving typical MEMS technology is reduced to an optimal control problem, where the objective function is the absorption of the maximum amount of energy in a given time interval from a vibrating environment. The interest here is to identify a physical upper bound for this energy storage. The mathematical tool is a new optimal control called Krotov’s method, that has not yet been applied to engineering problems, except in quantum dynamics. This approach leads to identify new maximum bounds to the energy harvesting performance. Novel MEMS-based device control configurations for vibration energy harvesting are proposed with particular emphasis to piezoelectric, electromagnetic and capacitive circuits. [ABSTRACT FROM AUTHOR]

The present paper reports on the application of the VFC (Variational Feedback Control) to the mitigation of the vibrational motion of continuous structures. The control method has been very recently proposed in the context of mechanical engineering to obtain suspensions characterized by better comfort performances. In this paper the same methodology is indeed applied to a simply nonlinear supported beam that is forced by a seismic excitation. The earthquake shaking, modelled as a random displacement of the beam hinges, is moderated by the presence of a controllable damper interposed between the beam and the oscillating ground, and its damping is subjected to the VFC controller. The inputs processed by the controller are the accelerations measured by two sensors, one on the ground, and the second on the beam. Numerical results are examined and some preliminary considerations are presented in which the considered method is applied to piezoelectromechanical beams. [ABSTRACT FROM AUTHOR]

Active and semi-active control of oscillating devices and structures is a challenging field and this paper proposes an original investigation based on variational controls that can be successfully applied to mechanical systems. The method produces a general class of new controllers, named VFC – Variational Feedback Controllers, that is the main theoretical contribution of the paper. The value of the theory relies on using a reformulation of the Variational Optimal Control Theory, that has in general the limit of producing control program strategies and not directly feedback control methods. The difficulties are in fact related to the intrinsic nature of the variational optimal control, that must solve initial and final boundary conditions. A special definition of the class of the considered objective functions, permits to skip this difficulty, producing a pure feedback control. The presented theory goes beyond with respect to the most acknowledged LQR variational-based techniques, in that VFC can be applied to more general nonlinear dynamical systems, even with finite time horizon. To test the effectiveness of the novel approach in real engineering problems, a deep investigation on nonlinear suspension systems treated by VFC is proposed in this paper. To this aim, VFC is systematically compared with the most recent methods available in this field and suitable to deal with nonlinear system control of car suspensions. In particular, the comparative analysis is made in terms of both comfort and handling key performance indexes, that permits to easily and significantly compare different control logics, such as the Sky-hook and Ground-hook control families, the Acceleration and Power Driven Dampers. The results of this comparison are collected in a performance plane, having comfort and handling indexes as coordinate axes, showing that VFC controllers completely cover the regions reached by the other mentioned control logics in this plane, but reveal to have access to new valuable areas of unexplored different performance combinations. [ABSTRACT FROM AUTHOR]

We consider the time evolution of a one dimensional n-gradient continuum. Our aim is to construct and analyze discrete approximations in terms of physically realizable mechanical systems, referred to as microscopic because they are living on a smaller space scale. We validate our construction by proving a convergence theorem of the microscopic system to the given continuum, as the scale parameter goes to zero. [ABSTRACT FROM AUTHOR]

This paper considers the damping induced on a single degree-of-freedom system when it is coupled at one end of a waveguide in which waves are radiated producing an energy loss in the oscillator motion that appears as a damping effect. In general, the whole system is described by the equation of the motion of the harmonic oscillator coupled with the wave equation of the propagation field. Hiding the variable that describes the wave motion and expressing it in terms of the oscillator vibration, a new equation for the oscillator is determined. In general, a nonconventional damping effect is born from the coupling terms. This paper examines cases in which the induced damping effect is of fractional-derivative type. This point of view produces physical examples of the way that simple mechanical structural systems, familiar to engineers, can exhibit fractional damping, a concept that does not always have a clear physical interpretation. [ABSTRACT FROM AUTHOR]

Parameters identification of nonstationary systems is a very challenging topic that has only recently received critical attention from the research community. Aim of the paper is the structural health monitoring of bridge-like structures excited by a massive moving load, whose characteristics, such as the mass and speed, are unknown, in the presence of a localized damage along the structure. A novel method for the simultaneous identification of both the load characteristics and damage parameters from vibration measurements is proposed: the data processing relies on the ensemble empirical mode decomposition and the normalized Hilbert transform. Neither a priori information about the response of the undamaged structure nor the free decay of the damaged system is required, only a single-point measurement is needed. The empirical instantaneous frequency is firstly employed to estimate the load characteristics; secondly, the effect of the moving mass is filtered from the instantaneous frequency, and then, the damage position is identified. The performance of the technique are studied varying the load characteristics, damage locations, and crack depths. The effect of ambient noise is also taken into account. Numerical experiments show the identification is rather accurate, results are not very sensitive to the crack location and depth, while they are sensibly affected by the speed of the moving load. [ABSTRACT FROM AUTHOR]

In this work we present the results of a field trial with a FBG sensor array system for the real time monitoring of railway traffic and for the structural health monitoring of both the railway track and train wheels. The test campaign is performed on the 2nd line of Milan metropolitan underground, employing more than 50 FBG sensors along 1.5 km of the rail track, where the trains are tested during daily passenger rail transport, with a roughly maximum speeds of 90 km/h. The measurements were continuatively performed for over 6 months, with a sampling frequency of about 400 Hz. The large amount of data/sensors allows a rather accurate statistical treatment of the measurement data and permits, with dedicated algorithms, the estimation of rail and wheel wear, key traffic parameters such as the number of axles, the train speed and load, and, in the next future, the detection of localized imperfections. [ABSTRACT FROM AUTHOR]

The theory of vibration of engineering structures belongs to a known chapter of mechanics. Nevertheless, new emerging technologies operating at the smallest scales open a completely new landscape in the field of nanodevices, nanostructured materials as well as biological systems. Nanoscale structures are always involved and characterized by extremely high-frequency vibrations and very small energy, implying quantum effects. This approach, even for simple oscillators, implies both conceptual and mathematical difficulties. It is a matter of fact that the analysis of complex structures, as for example beams and shells, has never been considered in the physical and engineering recent literature in a complete quantum-mechanical context using directly the Schrodinger's equation. The present paper proposes an attempt in this direction, introducing a quantum Euler beam, the QUEB model at the ground state energy. The idea is to shape a new model for flexural structures that is mathematically similar to those used in classical mechanics, but mimicking the peculiarity of the quantum motion. [ABSTRACT FROM AUTHOR]

Abstract: The present paper considers the problem of realizing an effective targeted energy pumping from a linear oscillator to a set of ungrounded linear resonators attached to it. Theoretical as well as numerical results demonstrate the efficacy of using a complex attachment as a passive absorber of broadband energy injected into the primary structure. The paper unveils also the existence of an instantaneous frequency associated with the master response characterized by intermittency: a rather surprising result for a linear autonomous system. Comparison with nonlinear energy sinks demonstrates that the two systems have some analogies in this respect and that the linear complex attachment is a very efficient energy trap. [Copyright &y& Elsevier]

Abstract: A novel technique for the identification of the tire–road grip conditions is presented. This is based on the use of strain information inside the tire, from which relevant characteristics of the tire–road contact can be extracted also through a factor named area slip ratio. This process forms the basis of a technology for grip identification that requires a new model of the tire dynamics. The model permits to determine closed form analytical relationships between the measured strain and the area slip ratio. On this basis, a procedure that can extract the contact kinematic parameter from the time history of the internal strain of the rolling tire is presented. Numerical simulations offer the chance to validate the identification algorithm. [Copyright &y& Elsevier]

Abstract: The introduction of vibration-based Nano Electro-Mechanical Transistors (NEMT) opens a new horizon for mechanics in computer science. NEMT working principle is based on an electrical charge shuttle between two electrodes operated by a vibrating conductor body. Advantages of these novel devices would be very low power dissipation, limited influence of external electromagnetic disturbances, and improved thermal resistance. The paper introduces an analytical model for such a device, in which the matching of a mechanical resonator and an electric circuit is studied: the coupling is provided by capacitance effects, electrostatic force and the quantum tunneling. The approach is quasi-classical, describing the quantum phenomena through a non-linear conductance and using a continuous variable for the charges. Through suitably introduced simplifications, the model is reduced to a set of two differential equations in terms of pillar position and charge. These equations represent the simplest model still preserving the basic phenomenology of the investigated system. Numerical simulations show different possible motion regimes, both in the single- and multiple-module configurations, the latter able to reproduce the conventional transistor functionality. This opens the way to mechanical voltage-driven switches or amplifiers. [Copyright &y& Elsevier]

Abstract: Why it is difficult to solve a vibroacoustic problem; what are the hidden pitfalls that one is called to overcome; are there methods or techniques suggested in other fields of physics that can help to formulate an efficient vibroacoustic prediction model? The authors provide personal answers to these questions and suggest also a classification of the numerous prediction methods proposed in the literature. [Copyright &y& Elsevier]

This paper presents a novel approach to the study of shock in elastic structures. An energy storage property for linear elastic non-dissipative structures is demonstrated, then extrapolated also to damped systems. This property relates locations of energy accumulation along the structure to the form of its stiffness and mass matrices. This result can be profitable used at a design stage of complex structures undergoing shock excitations. On the theoretical ground, the obtained results disclose indeed general properties of energy distribution in dynamical systems with implications on energy localization as well as on energy equipartitioning. Finally, these findings open a new point of view on energy transmission in structures, with potential application in the context of Statistical Energy Analysis. [ABSTRACT FROM AUTHOR]

Abstract: Hilbert–Huang transform (HHT) is an innovative data-processing technique for analyzing nonstationary and nonlinear signals. A novel HHT-based method for damage detection of bridge structures under a traveling load is proposed. The technique uses a single point measurement and is able to identify the presence and the location of the damage along the beam. The measured data are processed by the HHT technique, and none a priori information is needed about the response of the undamaged structure. Damage location is revealed by direct inspection of the first instantaneous frequency, which presents a sharp crest in correspondence of the damaged section. The identification capabilities of the proposed technique are studied varying the damage locations, crack depths and velocity of the moving load. The effect of ambient noise is also taken into account. Theoretical as well as numerical results show the identification is rather accurate, results are not very sensitive to the crack depth and ambient noise, while they are sensibly affected by the damage location and by the speed of the moving load as well. Theoretical analysis identifies a characteristic load velocity interval, depending both on the first natural frequency of the bridge and the damage location, within which the HHT can be successfully applied. [Copyright &y& Elsevier]

Abstract: This paper presents the theory of a novel mechanism of energy absorption and induced damping in structural systems and its application to aerospace industry. The underlying principles of the physical phenomena have been addressed in several earlier publications, which focused on prototypical systems of absorbers that consist of a set of single-degree-of-freedom resonators. This paper generalizes those theoretical developments to the case of a cluster of beams attached to a continuous primary structure, to develop predictive methods for the expected performance of this new type of absorber, with particular emphasis on its optimal design. An embodiment of the conceived device is illustrated for an aerospace structure, a satellite, with the purpose of reducing the vibration of the electronic components on board during lift-off. Experimental results illustrate the feasibility and the attractiveness of this new absorption technique. [Copyright &y& Elsevier]

We investigate the interaction of a particle with a finite-size bath represented by a set of independent linear oscillators with frequencies that fall within a finite bandwidth. We discover that when the oscillators have particular frequency distributions, the finite-size bath behaves much as an infinite-size bath exhibiting dissipation properties and thus allowing irreversible energy absorption from a particle immersed in it. We also present a reinterpretation of the Langevin equation using a perturbation approach in which the small parameter represents the inverse of the number of oscillators in the bath, elucidating the relationship between finite-size and infinite-size bath responses. [ABSTRACT FROM AUTHOR]

Statistical vibroacoustics, also called statistical energy analysis (SEA) in the field of engineering, is born from the application of statistical physics concepts to the study of random vibration in mechanical and acoustical systems. This article is a discussion on the thermodynamic foundation for that approach with particular emphasis devoted to the meaning of entropy, a concept missing in SEA. The theory focuses on vibration confined to the audio frequency range. In this frequency band, heat is defined as random vibration that is disordered vibration and temperature is the vibration energy per mode. Always in this frequency band, the concept of entropy is introduced and its meaning and role in vibroacoustics are enlightened, together with the related evolutionary equation. It is shown that statistical vibroacoustics is non-equilibrium thermodynamics applied to the audio range. [ABSTRACT FROM AUTHOR]

Studies on prototypical systems that consist of a set of complex attachments, coupled to a primary structure characterized by a single degree of freedom system, have shown that vibratory energy can be transported away from the primary through use of complex undamped resonators. Properties and use of these subsystems as by energy absorbers have also been proposed, particularly using attachments that consist of a large set of resonators. These ideas have been originally developed for linear systems and they provided insight into energy sharing phenomenon in large structures like ships, airplanes, and cars, where interior substructures interact with a master structure, e.g., the hull, the fuselage, or the car body. This paper examines the effects of nonlinearities that develop in the attachments, making them even more complex. Specifically, two different nonlinearities are considered: (1) Those generated by impacts that develop among the attached resonators, and (2) parametric effects produced by time-varying stiffness of the resonators. Both the impacts and the parametric effects improve the results obtained using linear oscillators in terms of inhibiting transported energy from returning to the primary structure. The results are indeed comparable with those obtained using linear oscillators but with special frequency distributions, as in the findings of some recent papers by the same authors. Numerically obtained results show how energy is confined among the attached oscillators. [ABSTRACT FROM AUTHOR]

As reported in several recent publications, an undamped simple oscillator with a complex attachment that consists of a set of undamped parallel resonators can exhibit unusual energy sharing properties. The conservative set of oscillators of the attachment can absorb nearly all the impulsive energy applied to the primary oscillator to which it is connected. The key factor in the ability of the attachment to absorb energy with near irreversibility correlates with the natural frequency distribution of the resonators within it. The reported results also show that a family of optimal frequency distributions can be determined on the basis of a variational approach, minimizing a certain functional related to the system response. The present paper establishes a link between these optimal frequency distributions and the energy equipartition principle: optimal frequency distributions are those that spread the injected energy as uniformly as possible over the degrees of freedom or over the modes of the system. Theoretical as well as numerical results presented support this point of view. [ABSTRACT FROM AUTHOR]

Abstract: The question of energy sharing among complex engineering vibrating systems is still an open problem. On the basis of some recent investigations, this paper is addressed to the prediction of the equilibrium energies of interacting conservative resonators and to a better understanding of the principle of energy equipartition for large undamped systems. The analysis explores the field of linear and nonlinear vibrations, the effects of inhomogeneity, the weak or the strong coupling as well as the effect of the initial energy distribution among the subsystems. The principle of energy equipartition is shown to react in a different fashion to these factors. Some general arguments, supported by the results of numerical experiments, enlightening promoting or inhibiting factors to the reaching of energy equipartition conditions, are given. [Copyright &y& Elsevier]

This paper is concerned with emergence of novel effects in wave propagation in one-dimensional waveguides, when integrated with periodic nonlocalities. The nonlocalities are introduced by a connectivity superimposed to a conventional waveguide and depicted as a graph with trees and leaves, each with its own periodicity. Merging nonlocality and periodicity notions induces a distinction between homogenous and non-homogenous periodic configurations. Specifically, various unconventional phenomena linked to the presence of nonlocalities result in disruption of the energy transmission in such systems, disclosing new opportunities for vibration isolation applications. To demonstrate these effects, simple models of propagation of plane extension/compression waves in a uniform infinite rod equipped with co-axial spring-like elements is used. The homogenous case is analysed by a direct double, space and time, Fourier transform, leading to discussion of unusual dispersion effects, including vanishing and negative group velocity. In the non-homogeneous case, the canonical Floquet theory is used to identify stopbands and control their positions in the frequency domain. The results are compared with eigenfrequency analysis of unit periodicity cells and finite structures. Next, the forcing problem is considered and the insertion losses in a semi-infinite rod with nonlocal spring effects are computed to corroborate predictions of Floquet theory, providing physical explanations of the obtained results. Finally, possibilities to employ the non-local interaction forces in an active control format to generate stopbands at arbitrarily low frequencies are highlighted. [ABSTRACT FROM AUTHOR]

Two statistical techniques are developed to predict the statistical moments of the horizontal motion of a floating moored dock, known as catenary anchor leg mooring (CALM), loaded by hydrodynamic random forces. The dock is represented by a lumped mass, the mooring cables by equivalent nonlinear springs and the hydrodynamic forces are modelled by a modified Morison equation. The model of the floating dock leads to a nonlinear ordinary differential equation. Although the problem could be approached by a direct numerical integration, e.g. by Monte-Carlo simulations, because of the stochastic nature of the excitation, this would imply a large number of runs to produce results of some statistical significance. In the present paper an alternative solution is based on the development of two more efficient techniques to predict the relevant statistical moments of the dock response. The first method, called CPSP (conventional perturbation–statistical perturbation), is based on the application of two subsequent perturbation techniques, the first relying on a classical perturbation method, the second on a statistical perturbation approach. The second method, called SLSP (statistical linearization–statistical perturbation), combines indeed a statistical linearization approach together with a statistical perturbation approach. The procedures allow the linearization of the cables restoring forces as well as of the hydrodynamic load and they can be easily generalized to be applied to different dock configurations or to systems of different physical nature. The results, compared with those obtained by a Monte-Carlo simulations, show, in terms of statistical moments of the dock response, a satisfactory agreement. [Copyright &y& Elsevier]

Pseudo-damping is a counter-intuitive phenomenon observed in a special class of linear structures that exhibit an impulse response characterized by a decaying amplitude, even in the absence of any dissipation mechanism. The conserved energy remains within but designated parts of the system. Pseudo-damping develops when the natural frequency distribution of the system includes condensation points. The recently formulated theoretical foundation of this phenomenon, based on mathematical properties of special trigonometric series, makes it possible to describe a class of mechanical systems capable of displaying pseudo-damping characteristics. They include systems with discrete oscillators and one-dimensional continuous beamlike structures already reported by the authors in recent studies. This paper examines development of pseudo-damping phenomenon in two-dimensional structures, using plates and shells as examples, and shows how a preloaded plate on an elastic foundation can lead to pseudo-damping. Moreover, in the case of curved shell elements examined here, pseudo-damping can result due to the curvature of the structure, which naturally introduces condensation points in the modal density. [ABSTRACT FROM AUTHOR]

The complex envelope displacement analysis (CEDA) is a procedure to solve high frequency vibration and vibro-acoustic problems, providing the envelope of the physical solution. CEDA is based on a variable transformation mapping the high frequency oscillations into signals of low frequency content and has been successfully applied to one-dimensional systems. However, the extension to plates and vibro-acoustic fields met serious difficulties so that a general revision of the theory was carried out, leading finally to a new method, the complex envelope vectorization (CEV). In this paper the CEV method is described, underlying merits and limits of the procedure, and a set of applications to vibration and vibro-acoustic problems of increasing complexity are presented. [ABSTRACT FROM AUTHOR]

This paper discusses a class of unexpected irreversible phenomena that can develop in linear conservative systems and provides a theoretical foundation that explains the underlying principles. Recent studies have shown that energy can be introduced to a linear system with near irreversibility, or energy within a system can migrate to a subsystem nearly irreversibly, even in the absence of dissipation, provided that the system has a particular natural frequency distribution. The present work introduces a general theory that provides a mathematical foundation and a physical explanation for the near irreversibility phenomena observed and reported in previous publications. Inspired by the properties of probability distribution functions, the general formulation developed here is based on particular properties of harmonic series, which form the common basis of linear dynamic system models. The results demonstrate the existence of a special class of linear nondissipative dynamic systems that exhibit nearly irreversible energy exchange and possess a decaying impulse response. In addition to uncovering a new class of dynamic system properties, the results have far-reaching implications in engineering applications where classical vibration damping or absorption techniques may not be effective. Furthermore, the results also support the notion of nearly irreversible energy transfer in conservative linear systems, which until now has been a concept associated exclusively with nonlinear systems. [ABSTRACT FROM AUTHOR]

Through two complementary approaches, using modal response and wave propagation, the analyses presented here show the conditions under which a decaying impulse response, or a nearly irreversible energy trapping, takes place in a linear conservative continuous system. The results show that the basic foundation of near-irreversibility or apparent damping rests upon the presence of singularity points in the modal density of dynamic systems or, analogously, in the wave-stopping properties associated with these singularities. To illustrate the concept of apparent damping in detail, a simple undamped beam is modified to introduce a singularity point in its modal density distribution. Simulations show that a suitable application of a compressive axial force to an undamped beam placed on an elastic foundation attenuates its impulse response with time and develops the characteristics of a nearly irreversible energy trap. [ABSTRACT FROM AUTHOR]

Abstract: This paper attempts to introduce a new point of view on energy analysis in structural dynamics with particular emphasis to its link with uncertainty and complexity. A linear, elastic system undergoing free vibrations, is considered. The system is subdivided into two subsystems and their respective energies together with the shared energy flow are analysed. First, the ensemble energy average of the two subsystems, assuming uncertain the natural frequencies, is investigated. It is shown how the energy averages follow a simple law when observing the long-term response of the system, obtained by a suitable asymptotic expansion. The second part of the analysis shows how the ensemble energy average of a set of random samples is representative even of the single case if the system is complex enough. The two previous points, combined, produce a result that applies to the energy sharing between two subsystems even independently of uncertainty: for complex systems, a simple energy sharing law is indeed stated. Moreover, in the case of absence of damping, a nonlinear relation between the energy flow and the energy (weighted) difference between the two subsystems is derived; on the other hand, when damping is present, this relationship becomes linear, including two terms: one is proportional to the energy (weighted) difference between the two subsystems, the other being proportional to its time derivative. Therefore, the approach suggests a way for deriving a general approach to energy sharing in vibration with results that, in some cases, are reminiscent of those met in Statistical Energy Analysis. Finally, computational experiments, performed on systems of increasing complexity, validate the theoretical results. [Copyright &y& Elsevier]

This paper describes a new concept referred to here as “energy sinks” as an alternative to conventional methods of vibration absorption and damping. A prototypical energy sink envisioned here consists of a set of oscillators attached to, or an integral part of, a vibrating structure. The oscillators that make up an energy sink absorb vibratory energy from a structure and retain it in their phase space. In principle, energy sinks do not dissipate vibratory energy as heat in the classical sense. The absorbed energy remains in an energy sink permanently (or for sufficiently long durations) so that the flow of energy from the primary structure appears to it as damping. This paper demonstrates that a set of linear oscillators can collectively absorb and retain vibratory energy with near irreversibility when they have a particular distribution of natural frequencies. The approach to obtain such a frequency distribution is based on an optimization that minimizes the energy retained by the structure as a function of frequency distribution of the oscillators in the set. The paper offers verification of such optimal frequency spectra with numerical simulations and physical demonstrations. [ABSTRACT FROM AUTHOR]

Abstract: The problem of an elastic lifting hydrofoil in a randomly perturbed flow is considered. It appears that in these conditions the phenomenon of hydroelastic-induced vibrations is controlled by a stochastic differential operator. By using the theory of stochastic perturbation, a technique of solution for this class of problems is proposed, leading to an effective numerical solution. The fundamentals of the method are given and it is applied to the general problem of hydroelastic vibrations. A numerical application to the case of an elastic control surface for a prototype-high- speed marine vehicle is presented. Comparisons between the results obtained by the Stochastic Perturbation Method (SPM) and those provided by standard Monte Carlo simulations (MCS) show the accuracy of the proposed method and a useful saving in computational time. A method is given for comparing the computational time required by the two methods, for a given statistical accuracy. [Copyright &y& Elsevier]

The aim of this paper is to provide a theoretical and experimental analysis of the response of an elastic system carried on board a wedge-shaped body impacting the water surface. The wedge entering the water has a sudden deceleration with shock characteristics, resulting in a short-time duration and a sharp peak value. On the other hand, the carried system undergoes an oscillatory motion induced by the inertial load generated by the impact. The study of this problem reveals the occurrence of special conditions in the response of the on-board oscillator, depending on the parameters associated with the water entry problem, that lead to large elastic forces. The experiments show that when varying the characteristic natural frequency of the on-board oscillator, critical impact conditions occur characterized by very large amplitudes of the structural response that confirm the theoretical predictions. The analysis, based on the water shock spectral response developed by the authors in some previous papers, is generalized here and validated experimentally. [Copyright &y& Elsevier]

In this paper we examine the conditions that influence the return time, the time it takes before energy returns from a set of satellite oscillators attached to a primary structure. Two methods are presented to estimate the return time. One estimate is based on an analysis of the reaction force on a rigid base by a finite number of oscillators as compared with an infinite number of continuously distributed oscillators. The result gives a lower-bound estimate for the return time. A more accurate estimation results from considering the dynamic behavior of a set of oscillators as waves in a waveguide. Such an analogy explains energy flow between a primary structure and the oscillators in terms of pseudowaves and shows that a nonlinear frequency distribution of the oscillators leads to pseudodispersive waves. The resulting approximate expressions show the influence of the natural frequency distribution within the set of oscillators, and of their number, on the return time as compared with the asymptotic case of a continuous set with infinite oscillators. In the paper we also introduce a new method based on a Hilbert envelope to estimate the apparent damping loss factor of the primary structure during the return time considering transient energy flow from the primary structure before any energy reflects back from the attached oscillators. The expressions developed for return time and damping factor show close agreement with direct numerical simulations. The paper concludes with a discussion of the return time and its relation to apparent damping and optimum frequency distribution within a set of oscillators that maximize these quantities. [ABSTRACT FROM AUTHOR]

Plane wave interference and space averages play a significant role in the derivation of some vibration conductivity equations that are becoming more and more popular in modelling vibroacoustic problems. Particularly, the thermal approach and the modified vibration conductivity equations are here considered with the aim of establishing similarities and/or differences between them and stating relevant consequences. It is shown by formal developments that the thermal equation is obtained under the assumption of performing appropriate space averages, while the modified vibration conductivity equation does not need, in some cases, this condition. It is discussed, however, that in practical applications the conditions of validity of both approaches are quite similar. [Copyright &y& Elsevier]

Several energy-based methods to approach noise and vibro-acoustic problems are actually under development. These techniques provide a chance of describing the vibro-acoustic behaviour of complex systems by the energies of a limited number of sub-components. This process is the base of one of the most acknowledged methods in this field, i.e. the statistical energy analysis (SEA). However, SEA invokes only the first law of thermodynamics, i.e. the energy conservation principle. On the contrary, it seems that the formulation of a complete theory of energy transmission among oscillators would claim also for the second principle of thermodynamics. Such direction of investigation, via the entropy concept, is developed in this paper leading to a theoretical energy flow analysis to predict the energy exchanged among complex systems. Some classical SEA results are obtained as a special case of a more general approach. [Copyright &y& Elsevier]

The present paper investigates a new paradigm to control a swarm of moving individual vehicles, based on the introduction of a few random long-range communications in a queue dominated by short-range car-following dynamics. The theoretical approach adapts the small-world theory, originally proposed in social sciences, to the investigation of these networks. It is shown that the controlled system exhibits properties of higher synchronization and robustness with respect to communication failures. The considered application to a vehicle swarm shows how safety and security of the related traffic dynamics are strongly increased, diminishing the collision probability even in the presence of a hacker attack to some connectivity channels. [ABSTRACT FROM AUTHOR]

This paper considers a new problem of control related to underwater vehicles. Due to the presence of a free fluid surface and to the vortex shedding of the vehicle lifting surfaces, the mathematical model is represented by an integral-differential equation. A new optimal control technique is investigated for this model, and its results compared with the use of a standard LQR method. [ABSTRACT FROM AUTHOR]