Contribución al estudio y caracterización de la generación de ruido de flujo en el sistema de escape.

Abstract

The studies of the vehicle noise emissions has been acquired a great relevance over the last few years for two reasons: (1) the increased social sensitivity to environmental contamination phenomena among which acoustic contamination is found, and (2) to the highest market requirements (higher "capability"vehicles at the same time as more comfortable ones: acoustic comfort).Within the complexity of the vehicle acoustic generation phenomena in this doctoral thesis, we will focus on one of the engine contributions: the exhaust noise. This component of noise is the result of two different phenomena: pulsating noise originated in the gas exchange process in the cylinders, and the flow noise induced by the turbulence phenomena developed by the mean flow that goes through the exhaust system. From these two phenomena the core of our study is flow noise and its prediction, under the motivation that as well as the pulsating noise being extensively studied and there being some prediction methods in it, the flow noise is unknown to a great extent. To this we should add, that while the pulsating noise contribution, main source, has been reduced the flow noise contribution has become more and more important in exhaust noise. The relevance of flow noise contribution is associated with its interaction with the acoustics in the system (it could amplify the main source contribution) and the existence of mufflers that work as flow noise generation sources.For the flow noise contribution study, the present doctoral thesis has been structured in six well-defined chapters.In the first chapter there is a summary of the different contributions that can be distinguished in the overall vehicle emitted noise. The importance of the flow noise study is justified from the present state of knowledge in this subject, and also the methodology bases used to perform it.In the second chapter, an experimental procedure for the estimation of the volume velocity fluctuation at the open end of the exhaust tailpipe of an internal combustion engine is presented. The different physical mechanisms involved in the emission process are analysed through the introduction of a realistic emission model. The possibility to consider that the pulsating component of the noise and the flow noise are decoupled to a certain extent is established, which means that this last component is generated by a mean "steady" flow superimposed to a pulsating flow. 8Once the possibility of studying the contribution of flow noise generated during the exhaust process as a steady flow effect has been justified. In the third chapter an experimental technique, sound intensity from in-duct measurements, is presented to analyse it. This technique allows the study of the flow noise generation by the individual component of an exhaust system (useful in the automotive industry) on a cold flow test bench. The proposed technique also allows to separate and to know the contributions in the final radiated noise of the internal and external sources of the muffler.Once the experimental pr...

Experiments on turbocharger compressor acoustics

“…This appendix to chapter 3 shows a short step by step guide to compute the desired X + and X − decomposed spectra required in equation 3.3.2, using a LCMV (Linearly Restricted Minimum Covariance) strategy as presented by Verdú [104]. Recall that decomposed signals for each frequency f k are obtained by weighting the transformed measurement matrix…”