Flow-induced noise in corrugated pipes: Why does liquid reduce whistling?

Sound production can occur in corrugated pipes subject to an internal dry gas flow (whistling). The sound originates from an interaction between the shear layers spanning the axisymmetric cavities along the pipe and an acoustic resonance of the pipe system. Mitigation of whistling in these pipes can be obtained by the addition of small fractions of liquid to the incoming flow. Experiments are carried out to investigate the effect of droplets in the pipe and liquid accumulation within the cavities along the pipe. Shadowgraphy was combined with an interferometric technique to obtain droplet size distributions in the corrugated pipe. The additional acoustic damping associated with the obtained droplet distributions could not explain the mitigation of whistling.
Measurements of the amount of liquid accumulating within the cavities using laser-induced fluorescence show a strong correlation between the filling of the cavities and the reduction of acoustic amplitude for different gas flow rates. A dry gas flow through newly fabricated corrugated pipes with a geometry based on the measured liquid filling profiles did not exhibit any whistling within the studied parameter range. A reduction of only the depth or only the length of the cavities did not lead to the same mitigation of whistling. It therefore seems that the geometric alteration of the corrugation geometry by the presence of liquid is the most important mechanism leading to mitigation of whistling in corrugated pipes.