Experimental studies on acoustics of corrugated tubes

Corrugated tubes offer greater global flexibility combined with local rigidity. They are used in numerous engineering applications such as HVAC ducts and offshore gas flexible riser pipelines. In recent years, there are renewed research interests to understand a tone generation (whistling) when air flow through a corrugated tube. The whistling is interesting because airflow through a smooth tube of similar dimensions will not generate whistling. Study of whistling in corrugated tubes is important as it not only causes an undesirable noise but also will result in significant structural vibrations due to flow-acoustic-structure interaction. In this study, the experimental work carried out on the corrugated tubes to measure the whistling frequencies, sound pressure levels and input acoustic impedances are presented. Influences of various geometrical parameters such as total length of the tube, number of corrugations, and inlet and exit conditions on the whistling are studied. The effect of rigid and flexible wall on the whistling is also studied. It is observed that the measured whistling frequency values are in good agreement with the values obtained from theoretical models. It is noted that the measured input acoustic impedance values of corrugated tubes are about 7% to 10% less than the corresponding straight tubes.