A study on the correlation between fluctuating velocity in a muffler and air flow noise.

This paper describes and experimental study on the relationship between fluctuating velocity in mufflers and flow induced noise for the expansion cavity type muffler and the perforated straight-through pipe type muffler. The visualization of internal flow and the measurement of fluctuating velocity in a muffler were performed using the smoke-wire method and a hot-wire velocimeter. From these experiments, the flow pattern and the distribution of the fluctuating velocity were able to be clearly shown for each type of muffler. The correlation between the fluctuating velocity at each point in the muffler and the flow induced noise which radiated from the open end was also studied by calculating the coherence function between them. It was seen that the flow induced noise was closely correlated with the fluctuating velocity of the internal flow, especially in the ranges of tail pipe and/or cavity resonance frequencies.