86,519 research outputs found
A new energy approach to the analysis of complex and uncertain system
No full textA new approach for predicting the response of a complex and uncertain structural-acoustic system was developed, namely the Time Asymptotic ensemble Energy Average (TAE). This approach belongs to the class of the energetic methods, since the system dynamic is described in terms of global parameters (the energies of a subset of the system) and a statistical approach is developed by introducing random natural frequencies, whose variability is due to stochastic perturbations of physical and geometrical parameters of the system. The developed method allows the evaluation of the energy sharing among two or more subsystems, for both weak and strong coupling. The originality of this method lies in the development of an asymptotic expansion technique, which permits to evaluate the energy distribution among the subcomponents of a system in both transient and steady state conditions in terms of only few modes of the system and the related marginal probabilities, determined with a low computational cost. The proposed method has been experimentally validated for two different configurations: a twoplate and a three-plate assemblies. The typical experiment consists in a transient excitation on a subcomponent of the structure and a measure of the dynamical responses of the plates at different locations upon all the subsystems, in order to derive a space-average value of the vibrational energies
Insights into the energy equipartition principle in large undamped structures
No full textThe 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 its 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. Sonic general arguments, supported by the results of numerical experiments, enlightening promoting or inhibiting factors to the reaching of energy equipartition conditions, are given. (C) 2008 Elsevier Ltd. All rights reserved
Time domain energy response of uncertain structures
No full textThe dynamic response characterization of a complex system comprises of multiple heterogeneous substructures remains one of the most challenging problems in structural dynamics. Inherent uncertainties and the high dimensionality of the problem led to develop approaches imported also from other areas of physics and engineering.
In this paper a new approach, namely the Time Asymptotic ensemble Energy average (TAE), a statistical method for the prediction of the transient energy sharing among two or more subsystems, for both weak and strong coupling is developed.
The originality of this method relies on an asymptotic expansion technique, able to evaluate the energy distribution among the subcomponents of a system in transient and steady state conditions, based on the low order modes response, i.e. at a low computational cost.
The method is experimentally validated for and a good agreement with theoretical prediction is found
Investigation and modelling of the turbulent wall pressure fluctuations on the bulbous bow of a ship
No full textFor the effective operation of sonar systems mounted inside the bulb of fast ships, it is important to reduce all the possible noise and vibration sources that radiate noise and interfere with sonar sensor response. In particular, pressure fluctuations induced by turbulent boundary layers on the sonar dome surface represent the major source of self-noise for on-board sensors. Reliable calculations of structural vibrations and noise radiated inside the dome require valid statistical descriptions of wall pressure fluctuations beneath the turbulent boundary layer. Previous research about wall pressure fluctuations deals with equilibrium turbulent boundary layers on flat plates in zero pressure gradient flow, for which scaling laws for power spectral densities and empirical models for the cross spectral densities are well established. On the contrary, turbulent boundary layers on bulbous bow exhibit the combined effects of three-dimensionality, streamline and spanwise curvatures and pressure gradients. In order to collect information about realistic configurations, wall pressure fluctuations were measured in an experimental campaign performed in a towing tank; data were collected at two different locations along a large scale model of a ship bulb and their spectral characteristics were investigated in terms of auto and cross spectral densities. Mean flow parameters of the boundary layer, required in the analysis, were obtained by a finite volume code that solves the Reynolds Averaged Navier Stokes Equations. The applicability of classical scaling laws for pressure spectra on zero pressure gradient flat plate was investigated, together with the spatial characterization of the wall pressure fluctuations in the space-frequency domain; parameters of some semi-empirical models available in the scientific literature were tuned to fit the measured pressure field
AN INVESTIGATION ON THE ENERGY EQUIPARTITION PRINCIPLE IN STRUCTURAL DYNAMICS OF UNCERTAIN SYSTEMS
No full textThe 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 long term equilibrium energies of interacting conservative resonators. More specifically, the goal would rely in a better understanding of the principle of energy equipartition, that still presents many questionable points. The analysis tries to explore both the field of linear as well as nonlinear vibrations, being the principle of equipartion obeyed in a different fashion in the two cases. Although the present work is a preliminary step in the analysis of this complex subject, some conclusions are stated and supported by the results of numerical experiments
Experimental evaluation of the pressure fluctuations induced by the turbulent boundary layer on the hull of a fast ship
No full textIn last years, due to the increase of the ship performances in terms of maximum speed a lot of interest have arisen for the analysis of the noise produced by hydrodynamic sources and transmitted on board. Among the others the turbulent boundary layer attached to the hull of the vessel is recognised to be one of the most important. The random travelling pressure field associated to the turbulent boundary layer induces high frequency vibrations of the hull's elastic parts that radiate noise on board the vessel. Thus the problem of the characterisation of the pressure field is similar to that related to noise transmission in the aeroplane fuselages but, due to presence of the free water surface, depends on the Froude number as well as on the Reynolds number. An experimental campaign was carried out on a model of a fast ship in a towing tank to investigate the problem. Laser doppler velocimetry and numerical simulations are used to evaluate the mean flow velocity parameters. An array of pressure transducers located along the hull to separate the effects due to Re and to Fr are used to measure the pressure fluctuations. The statistical characteristics of the pressure spectra are obtained from the analysis of the experimental data and the validity of different scaling laws is investigated
Modal analysis of the wake past a marine propeller
No full textModal decomposition techniques are used to analyse the wake field past a marine propeller achieved by previous numerical simulations (Muscari et al. Comput. Fluids, vol. 73, 2013, pp. 65-79). In particular, proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) are used to identify the most energetic modes and those that play a dominant role in the inception of the destabilization mechanisms. Two different operating conditions, representative of light and high loading conditions, are considered. The analysis shows a strong dependence of temporal and spatial scales of the process on the propeller loading and correlates the spatial shape of the modes and the temporal scales with the evolution and destabilization mechanisms of the wake past the propeller. At light loading condition, due to the stable evolution of the wake, both POD and DMD describe the flow field by the non-interacting evolution of the tip and hub vortex. The flow is mainly associated with the ordered convection of the tip vortex and the corresponding dominant modes, identified by both decompositions, are characterized by spatial wavelengths and frequencies related to the blade passing frequency and its multiples, whereas the dynamic of the hub vortex has a negligible contribution. At high loading condition, POD and DMD identify a marked separation of the flow field close to the propeller and in the far field, as a consequence of wake breakdown. The tonal modes are prevalent only near to the propeller, where the flow is stable; on the contrary, in the transition region a number of spatial and temporal scales appear. In particular, the phenomenon of destabilization of the wake, originated by the coupling of consecutive tip vortices, and the mechanisms of hub-tip vortex interaction and wake meandering are identified by both POD and DMD
Spectral characteristics of wall pressure fluctuations for surface ships
No full textWall pressure fluctuations generated by the turbulent boundary layer (TBL) which extends on the wall of moving vehicles, play an important role in the mechanism of noise generation through fluid-structure coupling. In the case of marine applications, the calculation and the suppression of the radiated noise is crucial to ensure sonar efficiency as well as to improve the confort level on board new high speed vessels for passenger transportation. The studies performed in this field, most of them devoted to aeronautical applications and based on the analysis of experimental data, deal with equilibrium flow for which scaling laws for the power spectral density and theoretical models for the cross spectral density are available. The problem in the case of marine vehicles presents some peculiarities because of pressure gradients due to both free surface and hull curvature. Their effects on wall pressure spectra are in this work analyzed comparing pressure experimental data measured in different locations along the hull where favorable, adverse and zero pressure gradients are present. These particular zone are identified by analyzing the mean velocity and pressure field obtained by RANS numerical simulations. The results of this analysis provides interesting information on the characteristics of wall pressure spectra and on the validity of the scaling laws for the power spectral density in the presence of pressure gradients
Hydrodynamic and hydroelastic analyses of a plate excited by the turbulent boundary layer
No full textRecent studies have demonstrated that the characterisation of wall-pressure fluctuations for surface ships is of great interest not only for military applications but also for civil marine vehicles. A ship model towed in a towing tank is used to perform pressure and structural measurements at high Reynolds numbers. This facility provides ideal flow conditions because background turbulence and noise are almost absent. Free surface effects are naturally included in the analysis, although in the particular section chosen for the present study do not have significant consequences on pressure spectra. Scaling laws for the power spectral density are identified providing the possibility to estimate pressure spectra for different flow conditions and in particular for full-scale applications. The range of validity of some theoretical models for the cross-spectral density representation is analysed by direct comparison with experimental data of wall-pressure fluctuations measured in streamwise and spanwise direction. In a second phase, an indirect validation is performed by comparing the measured vibrational response of an elastic plate inserted in the catamaran hull with that obtained
numerically using, as a forcing function, the modelled pressure load. In general, marine structures are able to accept energy mainly from the sub-convective components of the pressure field because the typical bending wavenumber values are usually lower than the convective one; thus, a model that gives an accurate description of the phenomenon at low wavenumbers is needed. In this work, it is shown that the use of the Chase model for the description of the pressure field provides a satisfactory agreement between the numerical and the experimental response of the hull plate. These experimental data, although acquired at model scale, represent a significant test case also for the real ship problem
Analysis of the scaling laws for the turbulence driven panel responses
No full textThe high computational costs, associated to the numerical solution of the fluctuating pressure field generated at the wall by the turbulent boundary layer and of the induced structural response, push for the exploration of alternative methodologies of analysis. Wall pressure fluctuations spectra are often modeled using semi-empirical expressions based on the experimental evidence and on the identification of universal scaling laws. In this work the possibility to adopt a dimensionless representation, able to provide a universal expression for the structural response of plates under turbulent boundary layer excitations, is investigated with the help of pressure fluctuations and acceleration experimental data sets. The test article is a plane thin plate wetted by a fluid over one face, the boundary layer is fully developed and pressure gradient effects are negligible. The attention is devoted to the investigation and the definition of a normalization of the required axes: the excitation frequency and the power spectral density of the structural response. The analysis is initially based on analytical models for the structural response under turbulent boundary layer excitations. The proposed scaling laws are successively and successfully applied to four data sets measured in different conditions both in wind tunnels and in a towing tank
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