Norman T. Huff

Factors affecting molecular dynamics simulated vitreous silica structures

To obtain accurate results in molecular dynamic (MD) simulations of glasses, it is essential to chose proper force fields (FF), proper length of the simulation cell, and proper cooling cycle to generate 300 K structures from liquid. Herein we establish guidelines for these choices. We find that the MS-Q force field (FF) and the Molecular Simulations Incorporated (MSI) glass FF both lead to agreement with the radial distribution function (RDF) from small angle neutron scattering (SANS) experiments. We find that the simulation cell should contain about 3000 atoms to obtain run to run density variation less than 1% and consistency in the first two RDF peaks of 0.001 nm. A cell of 648 atoms gives run to run density variation of up to 5% and consistency in the first three RDF peaks of 0.001 nm. We find that a good compromise between accuracy and reproducibility of results and simulation time is to start with NVT dynamics at 8000 K followed by cooling to room temperature at a rate of 100 K/2 ps.

Acoustic attenuation of hybrid silencers

The acoustic attenuation of a single-pass, perforated concentric silencer filled with continuous strand fibers is investigated first theoretically and experimentally. The study is then extended to a specific type of hybrid silencer that consists of two single-pass perforated filling chambers combined with a Helmholtz resonator. One-dimensional analytical and three-dimensional boundary element methods (BEM) are employed for the predictions of the acoustic attenuation in the absence of mean flow. To account for the wave propagation in absorbing fiber, the complex-valued characteristic impedance and wave number are measured. The perforation impedance facing the fiber is also presented in terms of complex-valued characteristic impedance and wave number. The effects of outer chamber diameter and the fiber density are examined. Comparisons of predictions with the experiments illustrate the need for multi-dimensional analysis at higher frequencies, while the one-dimensional treatment provides a reasonable accuracy at lower frequencies, as expected. The study also shows a significant improvement in the acoustic attenuation of the silencer due to fiber absorption. Multi-dimensional BEM predictions of a hybrid silencer demonstrate that a reactive component such as a Helmholtz resonator can improve transmission loss at low frequencies and a higher duct porosity may be effective at higher frequencies.

Design of a hybrid exhaust silencing system for a production engine

A prototype hybrid exhaust silencing system consisting of dissipative and reactive components is designed based on the boundary element method (BEM) with a specific emphasis on its acoustic performance as evaluated relative to a production system. The outer dimensions of the prototype system are comparable to its production counterpart, which has two silencers connected by a pipe. The predicted transmission loss by BEM for the prototype is compared with the experimental results in an impedance tube for both the prototype and production hardware, providing a design guidance for the former. The sound pressure levels measured at the tailpipe exit during the engine ramp-up experiments in a dynamometer laboratory are presented to compare the two systems, providing the final assessment. The acoustic effect of the pipe connecting the two prototype silencers is also examined computationally, along with a discussion of the measured flow performance and the surface temperatures of silencers in both systems.

Acoustic Characteristics of Coupled Dissipative and Reactive Silencers

The acoustic characteristics of a hybrid silencer consisting of two dissipative chambers and a Helmholtz resonator are investigated first computationally and experimentally. Complex wave number and characteristic impedance are used for the dissipative chambers to account for the wave propagation through absorbing material. Three-dimensional boundary element method (BEM) is employed to predict the transmission loss in the absence of mean flow and the predictions are compared with the experimental results obtained from an impedance tube setup. Noting that the long connecting tube between acoustic elements may reduce the transmission loss near the resonance frequency, two alternative hybrid silencers with short connecting tubes are also investigated by BEM. the present study shows the effectiveness of hybrid silencers over a wide frequency range and demonstrates the importance of understanding each acoustic element, as well as their interaction in designing silencers.

The Impact of Fiber Filling Density Variation on the Acoustic Performance of Silencers

Fibrous materials are often used to fill chambers in automotive silencers to achieve acoustic attenuation of air borne noise (especially above 200 Hz) in exhaust systems. It is clear that one of the major determinants of the acoustic performance of an absorptive silencer is the amount of fibrous material in the silencer [1]. However, there is little published data as to the impact upon acoustic performance of fiber packing density variations [2] within a silencer chamber In this study, cylindrical silencers with a straight through perforated tube have been used to experimentally study the impact of large density variations and voids, in both the radial and axial directions, upon the acoustic performance of a silencer. The acoustic transmission loss in a no-flow apparatus was the test measurement employed to determine acoustic performance.