Transient flow
Transient flow is such a flow where the velocity and pressure changes over time. Transient flows usually occur during the starting or stopping of a pump, the opening or closing of a tank, or simple changes in tank levels. Transient flow usually refers to surge or water hammer. The main reason transient flow can be a problem is it can cause pressure that would exceed the limits of pipes, fittings, etc. Transient flow is shit.