AREA AND PRESSURE PROFILES FOR COLLAPSIBLE-TUBE OSCILLATIONS OF 3 TYPES

A technique for measuring the evolution of tube shape in terms of cros s-sectional area as a function of both time and axial position during the course of self-excited oscillation is described. The conductance c atheter method is used to measure instantaneous shape-independent area at incrementally adjusted positions along the length of the tube. Pre ssure at the downstream end of the tube is also recorded, and this sig nal is subsequently used to line up all the area recordings. The resul t is a surface of cross-sectional area versus position and time that s hows explicitly how disturbances propagate along the tube, the localiz ation of the oscillations to the downstream end, and the shape signatu re of a given mode of oscillation. Derived results include profiles of the oscillation maximum, minimum and amplitude along the tube, and th e time-course of movement of the area minimum (the tube throat) during the cycle. Similar procedures have been used in conjunction with a fi ne pressure-transducing catheter to obtain pressure surfaces. The tech nique is limited to strictly periodic oscillations, and to this end th e extent of periodicity of operating points representing different com binations of controlling parameters has been systematically investigat ed. Three different modes of oscillation of a single collapsible tube have been observed with these techniques. These modes have previously been shown to be distinct, with sharp transitions separating them. Two of the modes have a similar low frequency, and are distinguished by w aveform shape: the collapse is either brief relative to cycle length o r lasts for approximately half the period. The third mode has approxim ately three times the frequency of the other two. On the basis that th e higher-frequency mode is obtained by transition from the brief-colla pse mode as the appropriate parameter is varied, it was previously pos tulated that these two were more closely related dynamically, and the detailed examination of waveforms here confirms this link.