Flow induced pulsations generated in corrugated tubes

Corrugated tubes can produce a tonal noise when used for gas transport, for instance in the case of flexible risers. The whistling sound is generated by shear layer instability due to the boundary layer separation at each corrugation. This whistling is examined by investigating the frequency, amplitude and onset of the pulsations generated by 2” artificially corrugated tubes and cable feeds. Special attention is given to the influence of the geometry of the corrugations and to the influence of the boundary conditions of the tubes. Two distinct modes are measured. One high mode with a typical Strouhal number Sr=0.35 and one with a Strouhal number of Sr=0.1. The relative length scale for the corrugations to be used in the Strouhal number is a modified gap width, which is the gap width excluding the downstream edge radius. The exact Strouhal number for a corrugation is furthermore dependent on details of the corrugation. The propagation of acoustic waves in the corrugated pipes are subjected to a reduction in the effective speed of sound and an increased damping due the increased pressure drop. The effective speed of sound is reduced due to the added impedance of the corrugation volumes. The damping can be described by adding a frictional pressure drop to the visco-thermal damping. For the visco-thermal damping it was assumed that the corrugated pipe was a smooth pipe. With these assumptions the damping could be matched to the measurements within 10%