Journal of Theoretical and Computational Acoustics

The paper summarizes the research carried out at the University of Crete and the Foundation for Research and Technology-HELLAS aiming at the statistical characterization of underwater acoustic signals and their subsequent use for geoacoustic inversions and applications in ocean acoustic tomography. In these applications, an acoustic signal recorded in the marine environment due to some source is used as the carrier of information on the physical parameters of the environment. Statistical characterization of acoustic signals is a pre-processing technique aiming at the definition of signal observables, to be used as input data of appropriately defined inverse problems aiming at the estimation of critical parameters of the marine environment. The statistical characterization scheme was introduced as a way to define signal observables especially in cases that typical observables such as ray arrivals or modal arrivals cannot be identified in the recorded signals. Moreover, the setting of the associated inverse problem requires just a single recording device, which renders its application practically and relatively cheap in comparison with signal inversion methods requiring reception at an array of hydrophones. The characterization scheme is based on a wavelet transform of the signal at various levels, followed by the statistical description of the wavelet sub-band coefficients. It is shown that A-stable symmetric distributions are capable of defining the statistics of these coefficients, the characteristic parameters of which are the observables of the signal to be exploited for the inversions. As the inverse problems associated with the sought applications are formulated as optimization processes, an objective function to be used as a similarity measure is defined, which in the case of the statistical characterization method is the Kullback–Leibrer Divergence (KLD), capable of comparing probability density functions. The inversion processes are performed by means of neural networks or genetic algorithms, and the performance of the combined signal characterization and inversion method has been tested with simulated and real data. It is shown, that the method works well especially with noise-free or denoised signals.

Elastic wave in the seabed caused by low frequency noise radiated from ship is called ship seismic wave and can be used to identify ship target. In order to obtain the propagation features of ship seismic wave in shallow sea with thick sediment, this paper introduces an algorithm for synthetic seismogram aroused by low frequency point sound source in shallow sea based on Biot’s wave theory for saturated porous media. Numerical calculation of synthetic seismogram at seafloor was carried out at a typical shallow sea environment with thick sediment. According to the results of numerical examples, the time series of seismic wave at seafloor is mostly composed of interface wave and leaky modes. The interface wave can propagate to far distance with small attenuation when the source frequency is very low. When the source frequency increases, the interface wave can no longer propagate to far distance like the leaky modes because the attenuation of sediment increases rapidly with frequency. The porosity and permeability of sediment in shallow sea have some influence on the dispersion characteristic of seismic wave at seafloor.

An analytical model is developed to investigate the sound transmission loss from orthogonally rib-stiffened plate structure under diffuse acoustic field excitation. The validity and feasibility of the model are verified by comparing the present theoretical predictions with the numerical results published previously. The influences of structure geometrical parameters on sound transmission loss are subsequently presented. The optimization algorithm is used to search for the optimal structural parameters with the objective to maximize the sound transmission loss over a frequency band. Furthermore, the sensitivity of structural parameters on the overall vibration and acoustic performance of the stiffened plates structure is also analyzed.

For flow noise simulations, the nonreflecting boundary condition (NRBC) is significant to confine the computational domain to a small domain. Lattice Boltzmann method (LBM) has advantages for noise because of its low dissipation, but is limited to the uniform grid. In this paper, an absorbing boundary condition (ABC) based on perfectly matched layer (PML) technique is introduced to LBM. Then PML stability is analyzed and a new strategy is developed to achieve robustness. Invoking the decoupling time integration, the underlying equation for streaming is solved with the nodal discontinuous Galerkin method. Benchmark acoustic problems were used to demonstrate the PML absorption. Moreover, PML parameters, long time behavior and inhomogeneous pseudo mean flow are discussed. The methodology appears to work very well and would be hoped for practical flow noise computation.

This work concerns with improving the transmission loss offered by a noise barrier separating two acoustic spaces in the low-frequency range. A novel concept of local mitigation of the transmitted noise at a target receiver location is presented by controlling the directivity of the transmitted noise through a point mass attachment on the barrier surface. Mass attachment at an arbitrary location is shown to increase the noise transmission. Optimal locations of the mass to minimize the sound transmission in the normal, oblique and tangential directions to the barrier are obtained. Optimal placement of the mass results in a major transmission reduction around the target location, much more than achievable by a uniform distribution of the mass over the barrier. Total transmitted pressure is also reduced; however, local reduction around the target location is much higher. The effects of variation in the excitation frequency and magnitude of the attached mass are investigated.

The present work investigates the flow and the sound field generated by high subsonic jets in crossflow (JICF). The problem arises when a jet is exhausted perpendicularly into a moving medium. Although being characterized as a very complex flow, the JICF has a well-known fluid dynamics, but a sound field yet to be more explored. Therefore, a hybrid methodology of low computational cost aeroacoustic prediction tool is applied in this work for the complete investigation of this problem. A single jet operating at Mach number 0.75 in a crossflow regime with effective velocity ratios of 4 and 8 is studied herein. The fluid dynamics is solved by the Reynolds Average Navier–Stokes (RANS) equations, and the noise calculations are performed using a statistical method known as the Lighthill Ray-Tracing (LRT) method. The numerical results for the acoustic and flow fields were in reasonable agreement with the experimental data available showing good applicability of this kind of methodology for solving JICF.

Single-scattering operators are used to extend the seismo-acoustic parabolic equation to problems involving transitions between areas with and without ice cover, which are common in the marginal ice zone. Gradual transitions are handled with single-scattering operators for sloping fluid–solid interfaces. Sudden transitions, which may occur when the ice fractures and drifts, are handled with a single-scattering operator that conserves normal displacement and tangential stress across the vertical interfaces between the range-independent regions that are used to approximate a range-dependent environment. The approach is tested by making comparisons with a finite-element model for problems involving range-dependent features in the ice cover and in a sediment that supports shear waves.

To acquire a stable reverberation signal from an irregular ocean bottom, we derive the analytic surface integral of a scattered signal using Stokes’ theorem while approximating the bottom using a combination of polygon facets. In this approach, the delay difference in the elemental scattering area is considered, while the representative delay is used for the elemental scattering area in the standard reverberation model. Two different reverberation models are applied to a randomly generated rough bottom, which is composed of triangular facets. Their results are compared, and the scheme using analytic integration shows a converged reverberation signal, even with a large elemental scattering area, at the cost of an additional computational burden caused by a higher order approximation in the surface integral of the scattered signals.

A numerical model for wave propagation simulation in damaged isotropic and anisotropic solids is proposed in this paper. The wave propagation problem is formulated using two coupled first-order differential equations for both isotropic and anisotropic solids, and a wavenumber corrector-based pseudo-spectral method is used to obtain the time-domain solution using the forward and inverse Fourier transformation. Following this, explicit modeling of crack-like damage commonly seen in engineering materials is investigated. Parametric and convergence study is performed to investigate the proposed simulation algorithms. Numerical examples are used to verify the proposed methodology by comparing the results from analytical solutions and classical finite element methods. Discussions and conclusions are drawn based on the proposed study.

Aerial blast wave propagation is affected by numerous physical parameters such as the source energy, the topography, the ground nature, or the atmospheric conditions. Direct high performance computing simulations is a valuable tool, but remains extremely costly in complex environment which limits the number of simulations. It is best used locally or to build reference solutions useful for the evaluation of simplified fast running models. Indeed, as the blast wave propagates, its strength weakens and after some distance, linear acoustic theory may be valuable. Here, we present the results of a one-way coupling procedure between a nonlinear Euler direct simulation code for the near field, including the detonation phase, and a parabolic linear acoustic model for the far field. Both models take into account for topography and atmospheric conditions. We show that, compared to the full Euler simulations, the coupling procedure provides accurate results at a reduced computational cost for blast wave propagation in the vicinity of an actual pyrotechnic site. The quality of the signals is also improved in comparison with classical parabolic simulations started with an analytical solution.