Dynamics of a shrouded cantilevered pipe subjected to internal and annular flows

The system considered consists of a hanging cantilevered pipe, surrounded at its upper part by a concentric rigid cylindrical tube, mounted in a tank full of water. Water enters the pipe at its upper and is discharged into the tank at its free end. This generates an upwards flow in the annulus between the pipe and the rigid tube, exiting the system at its upper end. The reverse flow configuration is also considered in which water enters the tank via the annulus and is aspirated by the pipe, exiting at its upper, clamped end. The free end of the discharging pipe could be plain or fitted with an end-piece which allows straight-through axial flow or forces the fluid to exit/enter radially. The system under study may be considered to be a highly idealized version of the operation of salt-mined caverns used for storage and subsequent retrieval of liquid and gaseous hydrocarbons. The problem is studied analytically, and the derivation of the pertinent linear equations of motion of the pipe is outlined. Extensive experiments conducted in a bench-top apparatus are also presented and compared with theoretical predictions. Experimental results and theoretical predictions show flutter for the pipe discharging fluid axially. Annular flow proves to have a strong destabilizing effect on the discharging pipe. The theoretical model predicts stability for the discharging pipe with radial exit flow and this is demonstrated experimentally for light-weight end-pieces. In the reverse flow configuration, flutter is observed in the experiments, while the theory predicts static divergence for low flow velocities and flutter for higher flow velocities.