Empirical estimation of peak pressure level from sound exposure level. Part II: Offshore impact pile driving noise

Numerical models of underwater sound propagation predict the energy of impulsive signals and its decay with range with a better accuracy than the peak pressure. A semi-empirical formula is suggested to predict the peak pressure of man-made impulsive signals based on numerical predictions of their energy. The approach discussed by Galindo-Romero, Lippert, and Gavrilov [J. Acoust. Soc. Am. 138, in press (2015)] for airgun signals is modified to predict the peak pressure from offshore pile driving, which accounts for impact and pile parameters. It is shown that using the modified empirical formula provides more accurate predictions of the peak pressure than direct numerical simulations of the signal waveform

An efficient multi-time step FEM–SFEM iterative coupling procedure for elastic–acoustic interaction problems

An iterative coupling methodology between the Finite Element Method (FEM) and the Spectral Finite Element Method (SFEM) for the modeling of coupled elastic-acoustic problems in the time domain is presented here. Since the iterative coupling procedure allows the use of a nonconforming mesh at the interface between the subdomains, the difference in the element sizes concerning the FEM and SFEM is handled in a straightforward and efficient manner, thereby retaining all the advantages of the SFEM. By means of the HHT time integration method, controllable numerical damping can be introduced in one of the subdomains, increasing the robustness of the method and improving the accuracy of the results; besides, independent time-step sizes can be considered within each subdomain, resulting in a more efficient algorithm. In this work, a modification in the subcycling procedure is proposed, ensuring not only an efficient and accurate methodology but also avoiding the computation of a relaxation parameter. Numerical simulations are presented in order to illustrate the accuracy and potential of the proposed methodology.CAPES, UFJF, UFSJ, FAPEMIG and CNP

Zur Berechnung der dynamischen Wechselwirkung zwischen Bauwerken und ihrer Umgebung mittels zeitabhaengiger Randintegralgleichungen

Project C1TIB: RA 3043 (86-10) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Investigations of blistering in the sealing zone of radial lip seals dependent on the operating factors and the power loss

The causation of damage to radial lip seals (RLS) is largely unexplained, especially with regard to blistering. Initial tests indicate a dependency of blistering on the circumferential speed and the sump temperature, whereat solely a high sump temperature does not result in blisters. It is assumed that a specific combination of operating factors leads to a tribological state in the sealing zone, which favours damage. Though, the key drivers leading to blisters are still unknown especially as influencing parameters interact. The power loss in the sealing zone is a measure for the thermal energy generated in the sealing zone. The RLS Tribometer offers the possibility to control the power loss via different operating parameters such as sump temperature, speed and line load. Consequently, it is possible to investigate the influence of one operating parameter independent from the power loss. This aims at identifying the tribological conditions leading to blistering. In this work, initial tests controlling the power loss are presented and associated with their influence on the tribological state of the sealing zone

Prediction of flow-induced noise using the expansion about incompressible flow approach

In the present paper an Expansion about Incompressible Flow approach, as suggested by Shen and Sørensen1, will be used and discussed for the simulation of flow-induced sound at low Mach numbers. The method consists of two parts: First solving for the incompressible, viscous flow field, and second computing the fluctuating acoustic field. As a result of this splitting approach, one can choose optimized numerical methods and grid sizes for each of the two parts. In the current contribution an explicit finite difference scheme is employed to solve the nonlinear acoustic equations, and the propagation and scattering of the sound are computed simultaneously. The acoustic results are presented for the sound generated by a circular cylinder in a flow at the Reynolds number of 150. In particular, the influence of the discretization parameters and the implementation of numerical filters are discussed and the accuracy of this hybrid method is compared to other approaches

An efficient analytical model for baffled, multi-celled membrane-type acoustic metamaterial panels

A new analytical model for the oblique incidence sound transmission loss prediction of baffled panels with multiple subwavelength sized membrane-type acoustic metamaterial (MAM) unit cells is proposed. The model employs a novel approach via the concept of the effective surface mass density and approximates the unit cell vibrations in the form of piston-like displacements. This yields a coupled system of linear equations that can be solved efficiently using well-known solution procedures. A comparison with results from finite element model simulations for both normal and diffuse field incidence shows that the analytical model delivers accurate results as long as the edge length of the MAM unit cells is smaller than half the acoustic wavelength. The computation times for the analytical calculations are 100 times smaller than for the numerical simulations. In addition to that, the effect of flexible MAM unit cell edges compared to the fixed edges assumed in the analytical model is studied numerically. It is shown that the compliance of the edges has only a small impact on the transmission loss of the panel, except at very low frequencies in the stiffness-controlled regime. The proposed analytical model is applied to investigate the effect of variations of the membrane prestress, added mass, and mass eccentricity on the diffuse transmission loss of a MAM panel with 120 unit cells. Unlike most previous investigations of MAMs, these results provide a better understanding of the acoustic performance of MAMs under more realistic conditions. For example, it is shown that by varying these parameters deliberately in a checkerboard pattern, a new anti-resonance with large transmission loss values can be introduced. A random variation of these parameters, on the other hand, is shown to have only little influence on the diffuse transmission loss, as long as the standard deviation is not too large. For very large random variations, it is shown that the peak transmission loss value can be greatly diminished.</p

Analytical model for low-frequency transmission loss calculation of membranes loaded with arbitrarily shaped masses

An analytical model for the transmission loss calculation of thin rectangular and circular membranes loaded with rigid masses of arbitrary shape, the so-called membrane-type acoustic metamaterials, is presented. The coupling between the membrane and the added masses is introduced by approximating the continuous interaction force with a set of discrete point forces. This results in a generalized linear eigenvalue problem that is solved for the eigenfrequencies and eigenvectors of the coupled system. The concept of the effective surface mass density is employed to calculate the low-frequency transmission loss using the obtained eigenpairs. The proposed model is verified using numerical data from a finite element model and the convergence behavior of the point matching approach is investigated using Richardson extrapolation. Finally, a method based upon the grid convergence index for estimating the error that is introduced due to the point matching approach is presented.</p

Computational prediction of near and far field noise due to pile driving for offshore wind farms

One major long-term goal of the German government is to decrease the green- house gas emissions by 40 %. This results in a key role of offshore wind farms regarding the turnaround in energy policy. In most cases, offshore wind turbines are erected by pile driving leading to a significant noise impact. In consequence, limiting values for emitted underwater noise have been prescribed to avoid a negative influence on marine mammals. To fulfill these requirements, different sound damping systems are currently developed or under investigation. Thereby, the numerical prediction of the resulting sound pressure level is an important tool to prevent cost-intensive offshore tests. As a general approach different numerical modeling techniques are used to study the generated pressure wave, taking into account the near and far field propagation separately. To model the area near the pile of the wind turbine, a detailed finite element approach is used. For the far field propagation, numerically highly effective methods are needed to predict the sound pressure level at large distances of several kilometers from the pile. In a combined model, results of the area close to the pile are transferred to a separate model using wavenumber integration to compute the sound pressure in the far field of the pile. Detailed investigations of the far field model and the setup of the combined near field-far field model can be found in corresponding publications of the authors [1]-[4]. The focus of this contribution is on the transformation of the near field model from the time domain to a formulation in the frequency domain to be able to consider frequency-dependent effects, like, e.g., the damping characteristics of bubble curtains

A membrane-type acoustic metamaterial with adjustable acoustic properties

A new realization of a membrane-type acoustic metamaterial (MAM) with adjustable sound transmission properties is presented. The proposed design distinguishes itself from other realizations by a stacked arrangement of two MAMs which is inflated using pressurized air. The static pressurization leads to large nonlinear deformations and, consequently, geometrical stiffening of the MAMs which is exploited to adjust the eigenmodes and sound transmission loss of the structure. A theoretical analysis of the proposed inflatable MAM design using numerical and analytical models is performed in order to identify two important mechanisms, namely the shifting of the eigenfrequencies and modal residuals due to the pressurization, responsible for the transmission loss adjustment. Analytical formulas are provided for predicting the eigenmode shifting and normal incidence sound transmission loss of inflated single and double MAMs using the concept of effective mass. The investigations are concluded with results from a test sample measurement inside an impedance tube, which confirm the theoretical predictions.</p

Perforated membrane-type acoustic metamaterials

This letter introduces a modified design of membrane-type acoustic metamaterials (MAMs) with a ring mass and a perforation so that an airflow through the membrane is enabled. Simplified analytical investigations of the perforated MAM (PMAM) indicate that the perforation introduces a second anti-resonance, where the effective surface mass density of the PMAM is much higher than the static value. The theoretical results are validated using impedance tube measurements, indicating good agreement between the theoretical predictions and the measured data. The anti-resonances yield high low-frequency sound transmission loss values with peak values over 25 dB higher than the corresponding mass-law.</p