Pipes with a corrugated inner surface, as used in flexible pipes for gas production and transport, can generate a high amplitude tonal sound (singing). Small quantities of liquid can result in a significant amplitude reduction or total mitigation of this sound production. To evaluate different potential mechanisms, liquid injection tests were done in both a horizontal and vertical small scale (49 mm) setup including high speed camera recordings using a transparent corrugated section.

The singing amplitude decreased linearly with the liquid injection rate for both orientations, although the effect in the vertical setup was even faster. Liquid injection resulted also in higher onset velocities.

The video recordings showed partially filled corrugations. For horizontal corrugations, liquid crept upward in the corrugations. In the vertical test, liquid accumulated at the upward edge with intermitted liquid spill over to downstream corrugations. The liquid fill up did not change significantly with higher liquid loads. Taking the fill-up grade and additional damping into account, a match could be made between the measured singing amplitude and a predicted singing amplitude as function of liquid rate. However, no model has been found to predict the liquid fill-up yet.

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