Transient flow: the velocity of a fluid particle changes with the time and the position. Transient flows occur during discharge processes, during startup and shutdown processes of turbomachines or in the case of fluid oscillations and water hammer processes.
Transient drainage processes are taken into consideration e.g.:
- when deciding on the dimensions of storage reservoirs
- in the design of bottom outlets in dams
- when water flows out of a basin into a pipe
- when two fluids with different speeds meet each othere.
Water hammer is caused by the inertia effect of the flowing fluid being subjected to an abrupt changes in velocity, e.g. when rapidly closing a valve. There is a distinction to be made between rapid pressure changes that propagate with the sound velocity (water hammer) and slow pressure changes caused by mass oscillations.