Journal of Theoretical and Computational Acoustics

This paper collects the state of the art and the tremendous progress that has been made in hybrid modeling of aeroacoustic sound. Hybrid modeling is defined such that flow and acoustics are modeled separate and connected by an aeroacoustic model. The contributions will be classified with respect to the aeroacoustic models being developed, covering Lighthill’s analogy, Ffowcs Williams and Hawkings, vortex sound, linearized Euler equations (LEE), and different perturbation equations modeling flow induced sound. Within each topic, specific applications, such as jet noise, aircraft noise, ground mobility, noise, fan noise and human phonation, are covered. We focus on the accomplishments and provide the authors’ contribution to aeroacoustic research. Eventually, a concise summary of the different methods and their capabilities is included.

Reverberation level (RL) is modeled in a shallow water environment with an underwater sound channel and internal solitary wave (ISW) using the coupled mode method. Numerical RL result based on one-way coupled mode shows an abnormal increase when a source is located near the channel axis and the ISW is located far from the source. The abnormal increase is analyzed by using a two-mode approach (assuming a trapped mode and a bottom interacting mode). The two-mode approach explains the relation between the RL increase and the ISW location explicitly: the ISW transfers trapped mode energy to bottom interacting mode energy, and its increasing rate is a function of its modal attenuation and ISW location from the source. The sensitivity test according to several ISW parameters is also performed.

Acoustic scattering-field reconstruction of structures with arbitrary shape is the research basis of the scattering characteristics for underwater targets. Firstly, using boundary element method (BEM) and acoustic radiation modes (ARMs) solution in fluid domain, it is proved that the scattering pressures can be expressed by ARMs. Secondly, the acoustic field distribution modes (AFDMs) are constructed by ARMs and a new acoustic transfer matrix (ATM) which is acquired by the simplification of the traditional ATM. At the same time, the scattering pressures can be expressed as the product of the AFDMs and the modal expansion coefficients. Thus, the scattering reconstruction problem is converted into the exact solution problem of the modal expansion coefficients. Aiming at the existing noise of both the pressures at measuring point and AFDMs, the total least square (TLS) algorithm is introduced to acquire the accurate solution. Further, considering the ill-conditioned AFDMs matrix, the truncated total least square (TTLS) algorithm is introduced to solve the modal expansion coefficients. Simulation results show that the capability of resisting noise contamination is limited for the reconstruction algorithm based on TLS and that the reconstruction algorithm based on TTLS has a better denoising performance than the TLS one. At the same time, for smaller wave numbers, the modal orders for reconstruction at different noise levels are approximately equal and the reconstruction errors are small. The simulation results also demonstrate that the reconstruction algorithm based on TTLS has a better denoising performance at smaller wave numbers than at higher wave numbers. For the higher wave numbers, the modal orders for reconstruction decrease and the reconstruction errors increase with the decrease of the signal-to-noise ratio (SNR). For the backward reconstruction at smaller wave numbers, the influence to reconstruction results, which arises from structure complexity ascending and evanescent waves existing, should be considered when the reconstruction surfaces are near the structures.

We propose a fast 2D full-waveform forward and inverse scattering solver to reconstruct ultrasonic speed and attenuation properties. The forward method is based on the volume integral equation, accelerated by the extended Born approximation and biconjugate-gradient fast Fourier transform method. The inverse method is based on the extended contrast source inversion. The research results show that our proposed method can fully unravel multiple scattering effects and achieve sub-wavelength resolutions. Numerical experiments indicate that the efficiency and robustness of the forward and inverse methods.

The Kirchhoff approximation (K-A) to calculate the acoustic backscattering of a complex structure can be evaluated using a discretized version of its surface (i.e, a mesh). From the computational viewpoint, the most handy approach is the one based on flat facets. However, in the high-frequency range, where the K-A provides good agreement and is therefore applicable, it requires a mesh with such a large number of facets that it turns impractical. To avoid these difficulties, a mesh of curved triangles can be used to model the scatterer’s complex structure. Previous computational implementations reported in the literature did not accomplish satisfactory results for high frequency. In this work, we propose a numerical model based upon an iterative integration using Gauss–Legendre rules. The model was validated against exact solutions and led us to achieve adequate results in the high-frequency range.

The bottom parameters of the deep ocean are difficult to obtain through in situ measurement. These parameters demonstrate a significant physical meaning for predicting sound field accurately. Thus, geoacoustic inversion is required. An acoustic experiment was performed on a reliable acoustic path (RAP) in the Philippine Sea in 2013. A single bottom-moored hydrophone was deployed as the receiver, and the explosive charges were chosen as the sources. The experimental bottom loss (BL) versus angle was obtained with the water depth above 5000m for bottom parameter inversion. The inversion sediment parameters show a clay-silt feature. The marginal probability distributions (MPDs) represent that the inversion results have a high credibility. This method provides a feasible solution for the inversion of the bottom parameters in the deep ocean.

An acoustic metasurface (AM) is proposed to achieve a sound tunnel with switchable functionalities of asymmetric transmission and bidirectional sound isolation, which is composed of an array of steel containers filled with two inert gases, argon and xenon. Acoustic asymmetric transmission can be observed when the AM is placed vertically. The functionality of the tunnel can be switched to bidirectional sound isolation by simply rotating the AM without changing the physical structure of the device. The proposed switchable tunnel has broadband multi-functional response with a simple adjustment method, showing potential for bidirectional sound isolation and sound sensing.

The source locations of axisymmetric modes of screech tones are numerically investigated. Fourth-order optimized compact scheme and fourth-order Runge–Kutta method are used to solve the 2-D axisymmetric Euler equations. The screech tone is successfully reproduced, and the change in wavelength with respect to jet Mach number shows good agreement with the experimental data. At various low supersonic jet Mach numbers, the time-averaged contours of Mach number and root-mean-square pressure are investigated to identify the location of maximum interaction between shock cell structures and vortices. The source locations of two axisymmetric modes, A1 and A2 modes, are distinctly visualized and identified; the screech tones of A1 mode are generated at the apex of fifth shock cell, and the screech tones of A2 mode are generated at the apex of fourth shock cell. Based on the observation, a simple formula for the prediction of axisymmetric modes of screech tones is proposed. The formula is derived based on a form of Rossiter equation, with the assumption of different convection speeds along the jet mixing layer. The proposed formula successfully estimates the frequency of two axisymmetric modes of screech tones, which verifies that the identified source locations of the axisymmetric screech tones are reasonable.

For various types of finite acoustic sources placed on an infinite cylindrical baffle, the pressure solution in cylindrical coordinates can be given by an infinite series of Inverse Fourier Integrals involving a singular quotient of Hankel functions. A hybrid method is introduced addressing these integrals’ singularity analytically and truncating their infinite integration range with predictable error. Maximum number of significant terms to be taken into account is discussed and determined. Results are obtained for a wide range of dimensionless frequency values ($\mathrm{\text{ka}}=0.001$–100) and observation point distances ranging from 3 to 100 radii of the cylindrical baffle. As an application, the baffle diffraction step of the infinite cylindrical baffle is evaluated for the on-axis pressure of a uniformly-vibrating piston.

Pile driving is used for constructing foundation supports for offshore structures. Underwater noise, induced by in-water pile driving, could adversely impact marine life near the piling location. Many studies have computed this noise in close ranges by using semi-analytical models and Finite Element Method (FEM) models. This work presents a Spectral Element Method (SEM) wave simulator as an alternative simulation tool to obtain close-range underwater piling noise in complex, fully three-dimensional, axially-asymmetric settings in the time domain for impacting force signals with high-frequency contents (e.g., frequencies greater than 1000Hz).

The presented numerical results show that the flexibility of SEM can accommodate the axially-asymmetric geometry of a model, its heterogeneity, and fluid-solid coupling. We showed that there are multiple Mach Cones of different angles in fluid and sediment caused by the difference in wave speeds in fluid, a pile, and sediment. The angles of Mach Cones in our numerical results match those that are theoretically evaluated. A previous work¹⁸ had shown that Mach Cone waves lead to intense amplitudes of underwater piling noise via a FEM simulation in an axis-symmetric setting. Since it modeled sediment as fluid with a larger wave speed than that of water, we examined if our SEM simulation, using solid sediment–fluid coupling, leads to additional Mach Cones. Because this work computes the shear wave in sediment and the downward-propagating shear wave in a pile, we present six Mach Cones in fluid and sediment induced by downward-propagating P- and S-waves in a pile in lieu of two previously-reported Mach Cones in fluid and sediment (modeled as fluid) induced by a downward-propagating P-wave in a pile. We also showed that the amplitudes of the close-range underwater noise are dependent on the cross-sectional geometry of a pile. In addition, when a pile is surrounded by a solid of an axially-asymmetric geometry, waves are reflected from the surface of the surrounding solid back to the fluid so that constructive and destructive interferences of waves take place in the fluid and affect the amplitude of the underwater piling noise.