MS23.1 - Vibro-Acoustics

Statistical Energy Analysis (SEA) techniques have been largely employed to model vibroacoustic systems as it simplifies the equations of motion of the system optimising the computing time, a useful feature to analyse random ensembles. However, the approach is limited to linear systems. Modelling vibro-acoustic systems with nonlinear characteristics is a challenging problem as there rarely exist analytical solutions for the dynamic response. An example of a system that includes nonlinear features in the transmission path is the suspension system of a vehicle, where vibrations that are result of the interaction between the wheels and the road ultimately arise the sound pressure levels in the car cabin, which is affected by such nonlinearities making it difficult to accurately estimate. A nonlinear interface between the excitation point and a statistical structure has been included in an experimental setup that represents the suspension system with the aim of exploring key features, or otherwise, of the effect that a transmission path with nonlinear stiffness has on the structural response of a randomised statistical system to a random input. A numerical model has been developed to simulate the dynamic response of the structure by adopting the infinite plate assumption to model the statistical structure as an SEA dissipative mechanism, hence the equations of motion are largely simplified as they result in a single-degreeof- freedom second order differential equation. Numerical simulations of the model here developed agree remarkably well with the experimental data, where the averaged dynamic response and the generation of high-order harmonics are accurately estimated. Additionally, the measured loss of coherence at the frequencies where harmonics are present is also predicted by the model.

The dynamic response of thin-walled slender tubes is dominated by noisy flexural modes. One topic of interest in research today is the use of acoustic metamaterials to reduce noise emissions from structures. These aspects are combined in the present work in which a solution for reducing the noise emission from thin-walled tubes are sought. The Wave Finite Element Method (WFEM) is used to estimate the dynamic response of propagating waves in the tubes. Extrudable rings of thermoplastic material are added to the inside of the steel tubes as local resonators for changing the response. The rings are added in sections to create a periodic waveguide which introduces stop-bands for the flexural waves. Additionally local bending modes of the rings couple with the global axial, torsional and bending modes to reduce global vibrations at the resonant frequencies of the local modes. The results from the dispersion diagram obtained through WFEM is compared to the response of the structure in simple linear harmonic analyses conducted in the standard FE software ANSYS. This is done in different loading scenarios to determine the impact of adding the resonator rings to different excitations and frequencies.

Ground motion induced by human activities related to construction has been a classical topic of great interest. Damage to nearby structures due to settlement, disturbance of occupants, jeopardy of serviceability and structural integrity are common effects of excessive vibrations. Pile driving is one of the main activities that involves the preceding risks and a number of studies has been conducted with a view to understand and minimize those risks. Currently, the environmental impact of vibrations induced by anthropogenic activities is drawing further attention. Infrastructure projects in urban areas have been traditionally the focus of such studies. However, at present the disturbance induced by dynamic sources in other habitats gains significant interest, with a view to endangerment of various species. A major activity in the field of offshore wind, that pertains to this case, is the installation of offshore foundations, i.e. monopiles. To estimate the level of environmental vibrations induced by such activities and assess the necessity of mitigation measures, improvement of the available predictive tools is necessary. In this paper, a numerical study is performed to investigate the effect of a non-linear pile-soil interface on the resulting driving-induced soil motion. In particular, a non-linear 3-D axisymmetric pile-soil interaction model that has been developed for vibratory installation analysis is employed, that is capable of describing the pile penetration process and has been benchmarked against field data. The model is comprised by a thin cylindrical shell (pile), a linear elastic layered half-space (soil) and a history-dependent frictional interface. The wave motion in the soil is captured accurately, by modelling the layered soil half-space via the Thin-Layer Method (TLM) coupled with Perfectly Matched Layers (PMLs). The numerical solution of the coupled problem is sought via a hybrid frequency-time scheme based on sequential application of the Harmonic Balance Method (HBM). The numerical results are compared with the respective ones from a linear model that neglects the non-linear pile-soil contact. Comparisons are drawn between the two approaches in terms of ground motion magnitude and frequency content, showcasing the effect of pile penetration inclusion on the resulting wavefield in the soil medium.

One of the objectives in the study of periodic structures is the development of passive control techniques dedicated to application in light structures, to reduce the transmission of noise and vibration. Periodic structures have been gaining notoriety in the scientific community for their unique dynamic properties, such as the Bragg scattering effect. The Bragg phenomenon results in ranges of frequencies where waves do not propagate, known as bandgaps. Due to this phenomenon, the study of such structures has excelled in reducing vibration within the desired frequency range. In this context, this work deals with the study of the dynamic behavior of periodic structures composed of rods with non-uniform cross-section areas, in search of the formation of bandgaps in finite mono-coupled structures about the propagation of longitudinal waves. The work proposes the modeling and analysis of finite periodic structures with exponentially varying cross-section areas, with conically varying cross-section areas, and with catenoid varying cross-section areas. Structures are modeled using dynamic stiffness and receptance matrices. The transfer matrix method is used to determine the properties of the entire structure from the structural properties of a single cell. Analytical expressions representing the transmissibility of structures composed of a single cell are also calculated. An analysis regarding the geometry of the structure is performed on the use of asymmetric and asymmetric cells. The importance of the orientation of asymmetric cells in relation to the excitation force is highlighted. To validate the methodology, experimental tests were carried out with a structure composed of rods with conically varying cross-section areas. The results show that in relation to bandwidth and minimum transmissibility, structures composed of rods with exponentially varying cross-section areas, with conically varying cross-section areas and with catenoid cross-section areas present similar responses. In relation to cell orientation, asymmetrical cells present better performance than symmetrical cells, however, the frequency band occurs in a higher frequency range.