The flow‐field downstream of a collapsible tube during oscillation onset

The flow‐field immediately downstream of a collapsible tube during oscillation onset starting from the collapsed state was measured using two‐dimensional high‐speed particle image velocimetry. Both tube and fluid were chosen to produce oscillation at the lowest possible Reynolds number, of just over 300. The flow was examined in the plane formed by the tube axis extended into the downstream pipe and the major axis of the tube collapse cross‐section. The resulting time‐series of spatial fields of 2D velocity vectors was analysed by frequency content and by proper orthogonal decomposition. Areas of the flow where oscillation initially occurs were identified. Flow disturbances centred at various frequencies were identified, some associated with the growing oscillation arising from the instability of the fluid–structure interaction between the main flow and the tube and others associated with the instability of the confined twin jets emanating from the collapsed‐tube throat.