Hydrostatics is the study of fluids at rest.
Hydrodynamics is concerned with the study and description of fluids in motion. The main emphasis is the teaching of the conservation laws of mass, energy and momentum.
The flow from a tank can be regarded as both steady and transient. In the steady case the fill level, and thus the width of the jet, remains constant (e.g. discharge under a weir).
When the tank is emptying during the discharge process, it is in what is referred to as the transient state.
When flowing through a pipe the friction causes the pressure energy of the fluid to fall and the internal energy of the fluid to increase.
In turbomachines the energy transfers between fluid and machine by means of flow forces.
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.