A model for the prediction of direct and indirect (flanking) sound transmissions is presented. It can be applied to geometries with extrusion symmetry. The structures are modelled with spectral finite elements. The acoustic domains are described by means of a modal expansion of the pressure field and must be cuboid-shaped. These reasonable simplifications in the geometry allow the use of more efficient numerical methods. Consequently the coupled vibroacoustic problem in structures such as junctions is efficiently solved.
The vibration reduction index of T-junctions with acoustic excitation and with point force excitation is compared. The differences due to the excitation type obey quite general trends that could be taken into account by prediction formulas. However, they are smaller than other uncertainties not considered in practice. The model is also used to check if the sound transmissions of a fully vibroacoustic problem involving several flanking paths can be reproduced by superposition of independent paths. There exist some differences caused by the interaction between paths, which are more important at low frequencies.
In this paper, the effect of the rotation on the natural frequencies and mode shapes of a submerged-confined disk inside a casing with water is studied analytically, numerically and experimentally.; To analyze the disk behavior, an analytical model is developed. The model assumes the thin-plate theory for the disk vibration and the Laplace Equation for the velocity potential of the flow on the upper and lower parts of the disk, considering a constant rotating speed of water for each part.; A CED simulation of the flow inside the tank has been performed in order to determine the averaged rotating speed of the water on the upper and lower parts of the disk for different velocities. The averaged rotating speed is introduced in the analytical model and in a FEM numerical model of the test rig.; For the experimental investigation a test rig has been developed. It consists of a disk rotating inside a casing filled with water; the rotating speed can be varied from 0 to 8 Hz. The disk is made of stainless steel having a diameter of 400 mm and a thickness of 8 mm. The radial gap between the disk and the casing is of 7 mm, and the axial gap between the disk surface and the upper cover is 10 mm. For the excitation, four piezoelectric patches attached on the disk have been used. In order to measure the response miniature accelerometers are placed on the disk surface at several locations. Signals are transmitted from the rotating to the stationary system through a slip ring located at the tip of the shaft. Natural frequencies and mode shapes of the rotating disk are obtained experimentally for several rotating speeds.; These results are discussed in detail and compared with those ones obtained with the analytical model and numerical simulation. The influence of the rotation of the surrounding water with respect to the disk is determined in this paper. (C) 2014 Elsevier Ltd. All rights reserved.
A study on the optimal procedure for obtaining SEA (statistical energy analysis) coupling loss factors (CLF) numerically is presented. The energies of an SEA system with two subsystems (one excited, the other one unexcited) are obtained from deterministic numerical simulations. Three different ways of isolating the CLF are explored: from the power balance of the excited subsystem (first approach) or the unexcited subsystem (second approach) and from the power transmitted through the connection (third approach). An error propagation analysis shows that the first approach is unreliable and that the second approach is the best option. As application examples, the CLF between some typical building structures is computed. These examples illustrate the potential of the estimated CLFs to solve larger problems with SEA and show the influence of the type of excitation on the coupling loss factor estimation. Finally, a simplified technique to account for the effect of studs in double walls with SEA is presented.
The transmission of sound through slits and openings between cuboid-shaped rooms is analysed. A deterministic model that describes the pressure fields inside the rooms in terms of eigenfunctions and uses the Dirichlet-to-Neumann technique in order to reproduce the slit effect is presented. An efficient formulation of the problem is obtained thanks to the splitting of the original domain into three domains: sending room, slit, receiving room. The geometry and boundary conditions of the problem can be modelled in detail like in an element-based
numerical technique (such as the finite element method) but with smaller computational costs. The model is compared with numerical solutions, existent models and published experimental data. Afterwards it is used to analyse some aspects such as the influence of slit dimensions, opening position, room properties (dimensions and absorption) that cannot be taken into account with the available models. These usually suppose that the slit or opening connects two unbounded acoustic domains.
The finite layer method (FLM) is presented as a discretisation technique for the computation of noise transmission through double walls. It combines a finite element method (FEM) discretisation in the direction perpendicular to the wall with trigonometric functions in the two in-plane directions. It is used for solving the Helmholtz equation at the cavity inside the double wall, while the wall leaves are modelled with the thin plate equation and solved with modal analysis. Other approaches to this problem are described here (and adapted where needed) in order to compare them with the FLM. They range from impedance models of the double wall behaviour to different numerical methods for solving the Helmholtz equation in the cavity. For the examples simulated in this work (impact noise and airborne sound transmission), the former are less accurate than the latter at low frequencies. The main advantage of FLM over the other discretisation techniques is the possibility of extending it to multilayered structures without changing the interpolation functions and with an affordable computational cost. This potential is illustrated with a calculation of the noise transmission through a multilayered structure: a double wall partially filled with absorbing material.
Magnetorheological (MR) dampers are a promising alternative to structural active actuators as they provide adjustable damping over a wide range of frequencies without large power requirements. However, the complex dynamics that characterizes these devices makes it difficult to formulate control laws based on the MR damper model. Instead, many semiactive control strategies proposed in the literature have been based on the idea of “clipping” the voltage signal so that the MR damper force “tracks” a desired active control force which is computed on-line. With this idea many algorithms have been proposed using, among others, techniques such as optimal control, H8H8 control, sliding mode control, backstepping and QFT. This work presents a semiactive control strategy based on the same idea of “clipping” the voltage signal but using a simpler PI design. The proportional and integral gains of the controller are calculated so that the controller guarantees stability, minimization of the closed loop response and robustness against modeling errors. Effectiveness of the control strategy is compared to some others techniques and passive cases as well. Simulation results shows that this simple strategy can effectively improve the structural responses and achieve performance index comparable to that of more complex algorithms.
Díaz-Cereceda, C.; Hetherington, J.; Poblet-Puig, J.; Rodriguez-Ferran, A. Journal of sound and vibration Vol. 330, num. 12, p. 2801-2817 DOI: 10.1016/j.jsv.2010.12.019 Data de publicació: 2011-01-17 Article en revista
Vibration transmission through structural connections is modelled in a deterministic way by means of modal analysis. This model is used first to study the effect of elastic joints across the floor in the transmission of impact noise. They are an effective means of reducing impact noise propagation, and can almost eliminate it for small values of the joint stiffness. The method is also used to study the acoustic relevance of studs in lightweight floor transmission. Different ways of modelling the studs are presented and compared. For the examples developed, the best option is to use springs for modelling the studs rather than more complex models involving springs and beams. Also the different behaviour of point and line connections is verified, as well as the influence of the position of the studs.
The partial quadratic eigenvalue assignment problem (PQEVAP) concerns the reassignment of a small number of undesirable eigenvalues of a quadratic matrix pencil, while leaving the remaining large number of eigenvalues and the corresponding Seismic isolation can protect delicate equipment housed in structures under earthquake attacks. One of the common approaches to isolate equipments is by using isolated secondary raised floors on which the equipments are mounted. This paper presents a new rolling-based seismic isolation bearing, referred to as roll-n-cage (RNC) isolator, for motion-sensitive equipment protection using the raised-floor approach. The RNC isolator is described, modeled and characterized. The effectiveness of the RNC isolator is numerically assessed considering the case of equipment housed in upper floors of a building, where the accelerations are amplified and the motion contains strong components at long periods. The numerical results reveal that the proposed RNC isolator device can attenuate seismic responses effectively under different ground motion excitations while exhibiting robust performance for a wide range of structure–equipment systems.
Models of physical non-linear systems are prone to different kinds of uncertainties. This paper presents a backstepping-based adaptive control designed for a class of one degree-of-freedom uncertain non-linear systems. The true system does not need to be known for the control design. A functional description is assumed with uncertain coefficients and an uncertain residual function. These uncertainties are bounded and lump the discrepancies between the adopted description and the real behaviour. The adaptive controller is able to handle these uncertainties and make the closed loop globally uniformly ultimately bounded when the system is subject to an unknown excitation from which a bound is known. One goal is that the transient and asymptotic performances depend explicitly on the design parameters. This feature of the control scheme establishes the main difference with other control methods used to control non-linear systems, in particular chaotic systems. The efficiency of the approach is tested by numerical simulations on Duffing oscillators and systems with non-linear and hysteretic stiffness under external loads.