Transition to a Free-surface flow at end of a horizontal conduit

Observation indicates that if the discharge in a conduit exceeds a certain value, the flow springs out from the whole outlet cross section, but below this discharge, the flow detaches itself from the top of the conduit before the outlet and forms a cavity whose shape and position is stable. A reduction of the discharge allows the cavity to move upstream, at first gradually, then with further discharge reductions a rapid motion can be detected, and in the end, the whole length of the conduit is seen to return to the free-surface regime, signalling the end of the cavity-flow regime. The discharges that mark the initiation and end of the cavity flow are close, so that this phase of the transitional flow is relatively abrupt. The present study defines the transition characteristics from full conduit flow to free surface flow within a closed conduit and provides some insight into the geometrical and dynamical properties of this transition process. A potential flow model is used to define the profile of the cavity and the calculations are compared with experiment.