ABSTRACT: hermoacoustic heat engines (TAHEs) are external combustion engines primarily designed to convert thermal power into acoustic power and, eventually, into mechanical, electric or other forms of high grade power.

TAHEs rely on the presence of a porous core, often referred to as “stack”. A temperature gradient is established along the porous core and quasi-adiabatic heat exchanges occur between the solid walls of the pores and the surrounding gaseous medium undergoing pressure fluctuations. The internal geometry of the stack has tremendous impact on the efficiency of thermal-to-acoustic power conversion.

In this study, the selective laser melting (SLM) has been used to produce stacks. The SLM is an additive manufacturing (AM) technique designed for 3D metal printing. It is based on high power- density laser which melts and fuses metallic powders together. Three sets of stacks, provided with different hydraulic radii and internal geometries, have been produced.

Each set is constituted by two stacks with similar hydraulic radii, one with internal parallel plates and one with internal oblique pin array. The SLM provides precise control of the features of the printed object, allowing to explore geometries which are difficult to manufacture with conventional technologies but, possibly, more effective in the heat exchange process. This is the case of pin (and oblique pin) array geometries, which provide a reduced amount of viscous losses when the working fluid has Prandtl number Pr < 1, e.g. air.

The printed stacks have been tested in a small scale standing-wave TAHE set up in Tallinn University of Technology (TalTech). Temperatures are monitored in proximity of the hot and cold heat exchangers, as well as the sound pressure within the engine resonator. The measured quantities are shown in time and frequency domain to analyze the onset and the stability of the thermoacoustic phenomenon.

ABSTRACT: Suspended glass panels are monolithic or laminated frameless windows sustained by a number of holders, typically located in the vicinity of the edges. These panels can be used, among other purposes, as noise barriers. The vibro-acoustic behaviour of glass windows is critical at low frequencies, where the problem is often tackled by increasing the thickness, thus the mass, of the panels. As a consequence, solutions which preserve low mass are greatly sought by industries. In this study, the vibro-acoustic behaviour of different suspended glass panels is addressed. An optimization procedure is implemented, aiming at finding the position of the holders which maximizes the acoustic transmission loss (TL) averaged at low- and very low- frequency ranges. First, an iterative procedure, based on comparison of experimental and numerical modal data, has been implemented to extract the material properties (Young’s modulus and Poisson’s ratio) of the panels. Second, these properties have been used in an optimization procedure based on finite difference approximation of the objective function, the averaged transmission loss. The vibro-acoustic analyses, required by the optimization procedure, have performed by means of hybrid finite element method/statistical energy analysis (FEM/ SEA). 16 different design cases have been considered in the optimizations, i.e. 2 different frequency ranges (20-300 Hz and 20-1000 Hz), 2 panel geometries (square 1m x 1m and rectangular 2.5m x 0.8m), 2 constitutive material properties (monolithic tempered glass and laminated tempered glass) and 2 mounting solutions (4 and 6 holders). The transmission losses of the optimized and the standard configurations, where the holders are placed close to the edges, are compared.

ABSTRACT: The acoustic performance of a new type of fibre-less sound absorber, the Micro-Grooved Element (MGE), is studied in this paper. The transfer impedance and the absorption coe cient of micro-grooved and Micro-Perforated Elements (MPEs) are measured, modelled and compared.

A MGE is a double layer element that involves inlet/outlet slots and facing micro-channels engraved onto the mating surface of the layers. The main advantage of these elements is that, by means of a simple technological process, thin micro-channels with depth of less then 100 µm can be easily engraved. This allows the MGEs exhibiting higher absorption coefficients compared to traditional MPEs with 300-700 µm diameter of perforations. Moreover, due to the presence of surfaces surrounding the micro-channels, the MGEs show reduced resistance when exposed to high level of sound excitation. In this perspective, the performance of a MGE is more stable than the one of a MPE provided with the same porosity.

A number of different MGEs with varied internal geometries has been tested at different excitation levels. The acoustics of the micro-channels is treated in details, the impedance end corrections are determined and the non-linear effects are accounted for. The linear behaviour of MGEs is described by adapting the models for slit-shaped perforated elements (SSEs). The quasi- and non- linear behaviours are expressed as a function of the Shear number and Strouhal number by curve fitting the experimental results.

ABSTRACT: In this paper the propagation of acoustic plane waves in turbulent, fully developed flow is studied by means of an experimental investigation carried out in a straight, smooth-walled duct.

The presence of a coherent perturbation, such as an acoustic wave in a turbulent confined flow, generates the oscillation of the wall shear stress. In this circumstance a shear wave is excited and superimposed on the sound wave. The turbulent shear stress is modulated by the shear wave and the wall shear stress is strongly affected by the turbulence. From the experimental point of view, it results in a measured damping strictly connected to the ratio between the thickness of the acoustic sublayer, which is frequency dependent, and the thickness of the viscous sublayer of the turbulent mean flow, the last one being dependent on the Mach number. By reducing the turbulence, the viscous sublayer thickness increases and the wave propagation is mainly dominated by convective effects.

In the present work, the damping and wall impedance have been extracted from the measured complex wavenumber, which represents the most important parameter used to characterize the wave propagation. An experimental approach, referred to as iterative plane wave decomposition, has been used in order to obtain the results. The investigations have been carried out at low Mach number turbulent flows, low Helmholtz numbers and low shear wavenumbers. The aim is to overcome a certain lack of experimental results found by the authors of the most recent models for the plane wave propagation in turbulent flows, such as Knutsson et al. [15] and Weng et al. [16].

ABSTRACT: The goal of this paper is to present a novel type of advanced acoustic material - micro grooved element (MGE) - which is designed for noise control in a wide range of applications. MGEs have been proved to offer a respectable alternative for the existing micro-perforated elements (MPEs), while being cost effective and causing low pressure loss. These elements have been found to be suitable for substitution of fibrous materials, typically present in silencer units. Currently, the cost of the MPEs is relatively high due to the technological complexity of manufacturing process. On the other hand, cheaper solutions of MPEs, based on irregularly shaped micro-apertures, potentially cause higher pressure loss due to surface roughness. The key concept of the MGEs is the use of micro-grooves forming acoustic channels, instead of the micro-holes of MPEs, which the sound wave has to pass. This allows to replace the laser perforation process, used to manufacture the MPEs with circular cross section, with less time consuming and more cost effective alternatives. The acoustical performance of the MGEs has been modeled by adapting the theoretical models provided by Allard and Maa for rectangular ducts, circular duct and non-linearities. The transfer impedance and the absorption coefficient of a number of MGEs and of several types of MPEs have been experimentally measured and compared by using the classical two-port method. Additionally the non-linear behavior of such elements has been experimentally investigated by varying the excitation sound pressure level during the tests.

ABSTRACT: In this work, the acoustic and fluid-dynamic performances of a commercial three-chamber perforated muffler were simulated with a three-dimensional boundary element method and also a one-dimensional approach. The inner insulating material (wool) was taken into account in the performed analyses, together with the presence of a mean flow across the muffler in order to predict both the transmission loss and the pressure drop Δp. Three-dimensional analyses were experimentally validated in a wide frequency range and in the absence of mean flow and were utilized to build a more precise one-dimensional representation of the device. In this way, better agreement between the one-dimensional results and the experimental data was realized, at least in the frequency range characterized by planar wave propagation (below 800 Hz). Once validated, the one-dimensional model was coupled to an external optimizer to perform acoustic and fluid-dynamic optimizations of the considered muffler. Initially, a genetic algorithm was employed to modify the internal muffler geometry and to improve the transmission loss, in the absence of mean flow, in the 100–800 Hz frequency range. A second optimization was also performed to identify the trade-off between the acoustic performance and the fluid-dynamic performance, in terms of the transmission loss and Δp, in the 100–400 Hz frequency range.