EXPERIMENTAL FLOW STUDY WITHIN A SELF OSCILLATING COLLAPSIBLE TUBE

The flow field within a self-excited flexible tube was studied by employing flow visualization, velocity and pressure measurements. Under low positive tansmural pressures at the tube inlet (of the order of 50 mm H₂O) the tube was set to an oscillatory motion, the initiation of which was due to a flow asymmetry. Namely, although initially the flow was separated from both sides of the formed divergent nozzle close to the tube outlet, at an arbitrary instant this became attached to one side, but stayed separated in the remaining part of the nozzle. When this flow asymmetry occurred, the walls approached each other and the tube neck formed there by started oscillating streamwise, setting the tube to an oscillatory motion. During the downstream motion of the tube neck, it was shrunk, causing a twofold increase of the local velocities compared to the inlet ones, which remained almost constant. On the contrary, when moving upstream, the tube neck expanded, causing a flow reversal in this area and a flow deceleration in the remaining part of the tube. The pressure signals upstream and downstream of the tube exhibited a phase difference, the latter leading, taking an order of magnitude higher values than the first one.