THE ACOUSTICS OF AUTOMOTIVE MUFFLERS: 1D-3D BEM ANALYSES, EXPERIMENTAL VALIDATION AND OPTIMIZATION
Auriemma, Fabio (2011) THE ACOUSTICS OF AUTOMOTIVE MUFFLERS: 1D-3D BEM ANALYSES, EXPERIMENTAL VALIDATION AND OPTIMIZATION. [Tesi di dottorato] (Inedito)
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A number of numerical and experimental studies, performed in the field of technical flow duct acoustics, are presented in this thesis. The analyses have been implemented on the automotive mufflers, and the fluid-dynamic aspects have been taken into account too. The noise attenuation characteristics of a three pass perforated muffler with final end resonator have been investigated. Acoustic performances have been quantified by the Transmission Loss (TL) parameter, and different numerical methods have been adopted. Firstly, a non-linear one-dimensional (1D) time domain approach is utilized to predict the TL profile in a low frequency range. Secondly, a linear three dimensional (3D) boundary element method (BEM), in the frequency domain, has been utilized to obtain more accurate results at higher frequencies. The effects on muffler performance of different mean flow velocities and gas temperatures have been investigated too. Advantages and disadvantages of the above mentioned numerical approaches have been highlighted by analyzing a two-tube cross-flow commercial muffler. Besides the obvious differences due to the non linear/linear and 1D/3D formulations, the 1D method has been successfully used in simulations where a mean flow was present. On the other hand, after importing the results of a previously modal analysis, a complex BEM fluid-structure interaction analysis has been performed. Interesting results about the structure excitation, at certain frequencies, have been carried out. Moreover, the radiated noise has been assessed, once imposed a certain excitation at inlet. An experimental campaign has been carried out in the acoustic laboratory of Tallinn University of Technologies (TTU), Estonia, aimed to validate the above numerical results. Testes have put in evidence the strong effects, on muffler performances, of the constructive features related to the manufacturing process, such as the coupling of adjacent surfaces and the actual shape of components. Hence, although numerical analyses are usually performed on ideal geometries (perfectly matched and shaped), when the behaviour of a real muffler must be simulated, a more detailed schematization must be used. To this aim, a number of tricks are suggested. A brand new acoustic optimization procedure, based on a 1D non-linear solver coupled to a genetic algorithm through a number of external routines, has been developed, aimed to maximize the acoustical attenuation or to find the best tradeoff between acoustical and fluid-dynamic performances. The proposed approach constitutes a valid instrument to improve the muffler design process, providing a consistent enhancement of TL and a contemporary reduction of back pressure for the examined case.
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