ADVANCED STUDIES ON SERIES IMPEDANCE IN WAVEGUIDES WITH AN EMPHASIS ON SOURCE AND TRANSFER IMPEDANCE

Series impedances, including source and transfer impedances, are commonly used to model a variety of noise sources and noise treatment elements in duct systems. Particle velocity is assumed to be constant on the plane where the series impedances are defined. The research reported herein details investigations into measuring source and transfer impedance. Especially, the measurement and prediction of the transfer impedance of micro-perforated panel (MPP) absorbers is considered. A wave decomposition method for measuring source impedance and source strength was developed that was purely based on acoustic concepts instead of the equivalent circuit analysis. The method developed is a two-load method. However, it is not necessary to know the impedances of either load a priori. The selection of proper loads was investigated via an error analysis, and the results suggested that it was best to choose one resistive and one reactive load. In addition, a novel type of perforated element was investigated. MPP absorbers are metal or plastic panels with sub-millimeter size holes or slits. In the past, Maa\u27s equation has been used to characterize their performance. However, Maa\u27s equation is only valid for circular perforations. In this research, an inverse method using a nonlinear least square data fitting algorithm was developed to estimate effective parameters that could be used in Maa\u27s theory. This inverse approach was also used to aid in understanding the effect of dust and fluid contamination on the performance of MPP absorbers. In addition, an approach to enhance the attenuation of MPP absorbers by partitioning the backing cavity was investigated experimentally and numerically. Results indicated that partitioning improved the attenuating of grazing sound waves. The effect of modifying both the source and transfer impedances on the system response was also studied using the Moebius transformation. It was demonstrated that the Moebius transformation is a mathematical tool that can be employed to aid in determining and understanding the impact of acoustic impedance modifications on a vibro-acoustic system

Acoustic investigation of perforated liners in gas turbine combustors

Modern combustion systems in industrial applications, from the gas turbine to aero or rocket engines, have become more critical during the last few years due to an exponential increase in commercial air traffic, resulting in an elevated level of atmospheric pollution in the form of exhaust smoke. To develop an efficient combustion system under variable load conditions, bias flow has been introduced progressively in the flame tube to decrease the temperature of the combustor liner in a consistent manner. Additionally, it is introduced as a passive damping device to increase the acoustic energy absorption from the system.This thesis amalgamates gas turbine combustor liner acoustic and static pressure measurements, along with their predictions. The primary objective of this investigation is to identify the passive damper maximum acoustic energy absorption properties. It will also collect information for designers to develop a cylindrical combustor liner geometry, along with flow factor, thermodynamic property and acoustic factors. A series of experiments was conducted, and the outcome of the investigation was compared with prior research, simulated data, and predictions to validate how this examination can be fundamental in advancing modern combustion systems.The results suggest that non-zero bias flow can greatly improve energy absorption and shift the peak frequency; the system operates as a Helmholtz resonator. Static pressure measurements suggest that as the mass flow rate changes, so too does pressure ratio, which creates a nonlinear absorption property of the combustor. The liner with the lowest porosity creates the pressure curve for double layer combustors. This could prove useful in assisting architects to utilize the combustor as a damper, metering liner or, indeed, a combination of both. A semi-empirical hybrid model is developed based on experimental data

Advances in Architectural Acoustics

Satisfactory acoustics is crucial for the ability of spaces such as auditoriums and lecture rooms to perform their primary function. The acoustics of dwellings and offices greatly affects the quality of our life, since we are all consciously or subconsciously aware of the sounds to which we are daily subjected. Architectural acoustics, which encompasses room and building acoustics, is the scientific field that deals with these topics and can be defined as the study of generation, propagation, and effects of sound in enclosures. Modeling techniques, as well as related acoustic theories for accurately calculating the sound field, have been the center of many major new developments. In addition, the image conveyed by a purely physical description of sound would be incomplete without regarding human perception; hence, the interrelation between objective stimuli and subjective sensations is a field of important investigations. A holistic approach in terms of research and practice is the optimum way for solving the perplexing problems which arise in the design or refurbishment of spaces, since current trends in contemporary architecture, such as transparency, openness, and preference for bare sound-reflecting surfaces are continuing pushing the very limits of functional acoustics. All the advances in architectural acoustics gathered in this Special Issue, we hope that inspire researchers and acousticians to explore new directions in this age of scientific convergence

Acoustic metamaterials comprised of dead-end pores and black hole effect for low frequency sound absorption in linear and nonlinear regimes

The aim of this work is to design and test acoustic metamaterial absorbers particularly for mitigation of low frequency sound of high intensity. The absorbers designed are formed of a series of plates with a central perforation, separated by air cavities. Two types of structures are investigated: the first has a central perforation with a constant radius (pancake structure), while in the second the pore radius gradually decreases along the thickness of the absorber (profile structures). In the structures of the first type, the wave speed reduction is abrupt, while in the second a gradual impedance matching with air is achieved. The structures developed are tested in a range of various experimental set-ups. This includes performing measurements in a conventional impedance tube (linear regime), in a specially modified impedance tube that allows pressures (RMS) up to 250 Pa using sine wave excitation (weakly nonlinear regime for continuous sound) and in a shock tube (nonlinear regime for pulsed sound of amplitude of up to 100 kPa). The models developed allow the predictions of the metamaterial structure performance at low and high sound pressure levels. In order to test the models, the absorbers of various dimensions are built, tested and the results of the measurements are compared with the model predictions. The analytical model for the pancake absorber is used to derive simple formulae for the frequency and the peak value of the absorption coefficient at the lowest frequency resonance in the linear regime, depending on the geometrical parameters. This model is complemented by a Transfer Matrix Model (TMM) and Finite Element Model (FEM) for both pancake and profile structures. The latter accounts for the influence of the structural resonances and tortuosity effect of the plates on the absorber performance.To investigate the nonlinear regime, flow resistivity measurements are performed on the samples to directly measure Forchheimer’s nonlinearity parameter. Flow resistivity measurements at low flow rates show that the periodic set of cavities does not modify resistivity significantly when compared to a simple perforated cylinder with same thickness. As flow rate increases, the flow resistivity grows linearly according to Forchheimer’s law and has a significant dependence on the absorber thickness. A nonlinear numerical model is developed accounting for the growth of flow resistivity with particle velocity amplitude in the central perforation and compared with the measurements at high amplitudes of the continuous incident sound. It is confirmed by measurements, that the peak absorption coefficient values for both types of absorbers decrease as the sound amplitude grows (irrespective of dimensions of pore radius and value of open surface area ratio). Where the peak values of absorption coefficient for the pancake absorbers are shown to be significantly reduced compared to the profile structures as amplitude strength grows to nonlinear regime. The resonance frequencies, however, remain close to their measured values independent of amplitude strength and is advantageous for both structures. Measurements in a shock tube are performed in both rigid backing and transmission set-ups, in time domain. Fast Fourier Transform (FFT) is later performed to investigate the signals. It is demonstrated that the profile absorber design is advantageous for the absorption of high amplitude pulsed sound

Industrial noise control manual

"Basic information on understanding, measuring, and controlling industrial noise is presented, along with descriptions of 61 industrial noise control projects. Noise problem analysis, basic methods of noise control, acoustical materials, and the choice of a consultant are discussed. A partially annotated bibliography of books and articles on relevant topics is provided."- NIOSHTIC-2Contract no. 210-76-0149Cover title.Also available via the World Wide Web

Microfluidics and Nanofluidics Handbook

The Microfluidics and Nanofluidics Handbook: Two-Volume Set comprehensively captures the cross-disciplinary breadth of the fields of micro- and nanofluidics, which encompass the biological sciences, chemistry, physics and engineering applications. To fill the knowledge gap between engineering and the basic sciences, the editors pulled together key individuals, well known in their respective areas, to author chapters that help graduate students, scientists, and practicing engineers understand the overall area of microfluidics and nanofluidics. Topics covered include Finite Volume Method for Numerical Simulation Lattice Boltzmann Method and Its Applications in Microfluidics Microparticle and Nanoparticle Manipulation Methane Solubility Enhancement in Water Confined to Nanoscale Pores Volume Two: Fabrication, Implementation, and Applications focuses on topics related to experimental and numerical methods. It also covers fabrication and applications in a variety of areas, from aerospace to biological systems. Reflecting the inherent nature of microfluidics and nanofluidics, the book includes as much interdisciplinary knowledge as possible. It provides the fundamental science background for newcomers and advanced techniques and concepts for experienced researchers and professionals

Aeronautical engineerng: A special bibliography with indexes, supplement 36

This special bibliography lists 628 reports, articles, and other documents introduced into the NASA scientific and technical information system in September 1973

Spacelab Science Results Study

Beginning with OSTA-1 in November 1981 and ending with Neurolab in March 1998, a total of 36 Shuttle missions carried various Spacelab components such as the Spacelab module, pallet, instrument pointing system, or mission peculiar experiment support structure. The experiments carried out during these flights included astrophysics, solar physics, plasma physics, atmospheric science, Earth observations, and a wide range of microgravity experiments in life sciences, biotechnology, materials science, and fluid physics which includes combustion and critical point phenomena. In all, some 764 experiments were conducted by investigators from the U.S., Europe, and Japan. The purpose of this Spacelab Science Results Study is to document the contributions made in each of the major research areas by giving a brief synopsis of the more significant experiments and an extensive list of the publications that were produced. We have also endeavored to show how these results impacted the existing body of knowledge, where they have spawned new fields, and if appropriate, where the knowledge they produced has been applied