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This study focuses on the mechanism of flow-acoustic coupling in a T-junction, which has a transitional section entailing an expansion of pipe diameter. Six cases with varying length of the transitional section have been tested. It is shown that T-junctions with transitional sections can be a strong source of acoustic excitation and therefore can cause powerful acoustic resonance in the associated piping systems. Elimination of the transitional section reduces the excitation level drastically. The length of the transitional section with enlarged diameter is found to be the fundamental length scale determining the Strouhal number of flow excitation in the T-junction. To investigate the mechanism further, a series of experiments were conducted in a rectangular pipe system, which was considered as the 2D case of the T-junction. The results showed that the vortices formed in the outer and inner sides of the T-junction are both acoustic sources. They appear to form independently without interaction with each other, but are synchronized by the acoustic field. Flow visualization was carried out to gain images of the flow pattern when the resonance occurs. The images from flow visualization provided support to the measurements of the rectangular pipe system. Numerical simulations of the mean flow and acoustic field in the rectangular T-junction were performed. A simplied analysis of acoustic energy generation was also carried out. / Thesis / Master of Applied Science (MASc)

A theoretical study has been carried out on different methods available to analyze complex mufflers. Segmentation methods have been discussed in detail. The latest two port segmentation method has been discussed and employed for a few common muffler configurations, describing its implications and limitations. A new transfer matrix based method has been developed in view of the lacunae of the available approaches. This Integrated Transfer Matrix (ITM) method has been developed particularly to analyze complex mufflers. An Integrated transfer matrix relates the state variables across the entire cross-section of the muffler shell, as one moves along the axis of the muffler, and can be partitioned appropriately in order to relate the state variables of different tubes constituting the cross-section. The method presents a 1-D approach, using transfer matrices of simple acoustic elements which are available in the literature. Results from the present approach have been validated through comparisons with the available experimental and three dimensional FEM based results. The total pressure drop across perforated muffler elements has been measured experimentally and generalized expressions have been developed for the pressure loss across cross-flow expansion, cross-flow contraction elements, etc. These have then been used to derive empirical expressions for flow-acoustic resistance for use in the Integrated Transfer Matrix Method in order to predict the flow-acoustic performance of commercial mufflers. A flow resistance model has been developed to analytically determine the flow distribution and thereby pressure drop of mufflers. Generalized expressions for resistance across the perforated elements have been derived by means of flow experiments as mentioned above. The derived expressions have been implemented in a flow resistance network that has been developed to determine the pressure drop across any given complex muffler. The results have been validated with experimental data.