Measurements of wave speed and compliance in a collapsible tube during self-excited oscillations: a test of the choking hypothesis.

Indirect evidence links self-excited oscillation of flow through collapsed tubes with choking, defined by the cross-sectionally averaged fluid speed u reaching the local speed of small pressure waves c. This was tested by measuring both c-u and c as functions of tube cross-sectional area during self-excited oscillation, using small superimposed high-frequency wave packets. The wavespeed c was derived from the local slope of the pressure/area relationship, measured at both high and low frequency, while c-u was taken as the upstream propagation rate of the pressure disturbances. When u = 0, these were shown to agree with each other. The propagation results showed that choking did not occur at high frequency. At the low frequency of the self-excited oscillation the results were less conclusive, because of dispersion and indirect methodology, but choking appeared not to happen at the modest flow rate of the oscillation investigated. Results on the attenuation of the wave packets were successfully explained using a model of the tube throat consisting of two equal and opposite reflection sites.