Mitigating the propagation of low frequency noise sources in ducted flows represents a challenging task since wall treatments have often a limited area and thickness. Loading the periphery of a duct with a periodic distribution of side-branch Helmholtz resonators broadens the bandwidth of the noise attenuated with respect to a single resonator and generates stop bands that inhibit wave propagation. However, significant flow pressure drop may occur along the duct axis that could be reduced using micro-perforated patches at the duct-neck junctions. In this study, a transfer matrix formulation is derived to determine the sound attenuation properties of a periodic distribution of MPPs backed by Helmholtz resonators along the walls of a duct in the plane wave regime. In the no-flow case, it is shown that an optimal choice of the MPP parameters and resonators separation distance lowers the frequencies of maximal attenuation while maintaining broad stopping bands. As observed in the no-flow and low-speed flow cases, these frequencies can be further decreased by coiling the acoustic path length in the resonators cavity, albeit at the expense of narrower bands of low pressure transmission. The achieved effective wall impedances are compared against Cremer optimal impedance at the first attenuation peak.

Acoustic metamaterials are engineered materials with properties hard or impossible to find in natural materials (e.g. negative effective density and/or negative bulk modulus). Therefore, a myriad of novel applications could be imagined and some of them have already been theoretically and/or experimentally demonstrated. Gradient index acoustic lenses, acoustic cloaks or acoustic absorbing panels are some common examples. Here, we review the coordinate transformation approach (transformation acoustics) which provides the material parameters needed to precisely control the acoustic wave propagation. Then, we use this technique to design an acoustic black hole and a 3D acoustic ground cloak. We use numerical simulations to explore the practical feasibility of the material parameters required by these applications and design non-resonant, highly sub-wavelength unit cells that will implement them in practice.

Noise propagation mechanisms in presence of a rotational flow are currently receiving some attention from the aircraft industry. Different methods are used in order to compute the acoustic wave propagation in sheared flows in terms of pressure perturbations (e.g. Linearized Euler Equations (LEE), Lilley’s and Galbrun’s equations). Nevertheless, they have drawbacks in terms of computational performance (high number of DOFs per node, inadequacies of classical numerical schemes like standard FE). In contrast with other studies, in this work, the fluctuating total enthalpy is selected as the main variable in order to describe the acoustic field, which obeys to a convected wave equation obtained by linearization of momentum (Crocco’s form), energy and continuity equations and with coefficients depending on flow variables. The resulting 3D convected wave operator is an extension of the Möhring acoustic analogy which is able to predict the sound propagation through rotational flows in the subsonic regime and is well adapted to FE discretization. A 2D convected wave equation is generated from the previous operator. This is followed by a numerical solution based on FEM with two types of boundary conditions: non reflecting BC and incident plane wave excitation. The numerical results are used to estimate the reflection coefficient generated by the shear flow. The new acoustic wave operator is compared to well-known theories of flow acoustics (Pridmore-Brown wave operator) and shows promising results. Finally additional development steps are presented so further improvements on the new operator can be carried out.

Inside micro cavities, specific dissipative mechanisms influencing acoustic wave propagation occur due to viscous and heat-conducting nature of the fluid. This work focuses on a possible extension of the so called “Low Reduced Frequency” model for acoustic wave propagation in a thermoviscous fluid. This extension is built starting from geometrical and physical assumptions (boundary layer theory, straight waveguides) and consists in the incorporation of a stationary laminar and subsonic mean flow. The resulting equivalent fluid model provides a new damping coefficient which depends on the Mach number, the shear and thermal wave numbers and the cross-sectional profiles of axial velocity and temperature. The main application area is the study of acoustic attenuation within automotive catalytic converters or also thin fluid layers like cooling systems in small electronic devices. This formulation has been implemented for a simple one dimensional thin tube. Convergence to the original model in the absence of mean flow has been reached and comparisons with variational solutions given by Peat show good agreements.

The operation of many industries, such as power plants or many piping systems, demands knowledge of generated pressure pulsations. The effect of acoustic wave amplification in piping systems can be detrimental to the integrity and life of whole plant. Therefore, understanding of the nature of acoustic wave propagation in water filled piping systems needs to be established based on fundamental experiments and analysis. Chatoorgoon et al. [1] and Rzentkowski et al. [2], compared their no flow experiments with theoretical calculation, and realized that the resonant frequency shifts increased linearly, with resonant frequency increasing. This paper presents an experimental study showing that linear wave theory, based on a transmission matrix method does predict well the acoustic resonance frequency from 50 to 500 Hz. and the resonant frequency shifts were negligible. Study of tube wall thickness, material (stainless steel and Aluminum) and some equal branch configurations for “Closed-end” tubes are discussed.

Although tire/road noise and tire vibration phenomena have been studied for decades, there are still some missing links in the process of accurately predicting and controlling the overall tire/road noise and vibration. An important missing link is represented by the effect of rolling on the dynamic behavior of a tire. Consequently, inside the European seventh framework program, an industry-academia partnership project, named TIRE-DYN, has been founded between KU Leuven, Goodyear and LMS International. By means of experimental and numerical analyses, the effects of rolling on the tire dynamic behavior are quantified. This paper presents the results of vibration measurements on a rotating tire with an embedded accelerometer. Modal parameters of the rolling tire are estimated from an operational modal analysis. In addition, the dispersion curves, which give detailed insight in the wave propagation behavior of a structure, are analyzed for the rolling tire. The goal of these analyses is to deepen the understanding on the influence of rolling on the tire dynamic behavior.

The circular cylinder pipe is extensively used for the supplement of the gas. The leakage of this gas induces the catastrophic problem when it leases into open area in the city without any monitoring. A correlation method has been mostly used for the detection of the leakage. It is needed a good coherence and an efficient energy transmission to the external sensors for the reliable estimation of the correlation. This paper investigated theoretically the propagation of the acoustic wave of the circular cylinder pipe containing the gas in a pipe for the development of the leakage monitoring system. The acoustic wave is propagated through the waveguide of the circular pipe with the characteristics acoustically coupled by the gas contained in a cylinder and the shell. However, as a special case, the acoustic waves in a metal pipe containing gas are corresponded closely to the uncoupled in-vacuo shell waves and to the rigid-wall duct fluid waves. In this case, the dominant acoustic energy can be estimated at the frequencies in which coincidence between the shell modes and the acoustic modes occurs. In the paper, the characteristics of the dominant waves are theoretically investigated and analyzed experimenttally with a long steel pipe. The measured data is clearly analyzed by the continuous wavelet transform and by spectral density analysis.

This paper proposed an approach to construct the acoustic cloak by a network of subwavelength Helmholtz resonators. Based on transmission line model to describe the acoustic wave propagation inside such effective anisotropic medium, we derived the acoustic parameters such as effective density and compressibility. Our simulation demonstrates the propagation of acoustic waves can be bent and excluded from an object inside the cloak with no perturbation of exterior field, which may have great potential application in ultrasound noise control.