The flow of a fluid is transient or unsteady if its flow parameters (i.e. velocity and pressure) are dependent not only on the position in the coordinate system used to describe the field of flow, but also on time. A distinction is made between three different types of transient flow phenomena.
Transient flow phenomena
- Stochastically irregular phenomena, e. g. turbulent fluctuations (see Fluid mechanics)
- Run-up or run-down phenomena, e. g. when starting or stopping a centrifugal pump (see Start-up process and Starting torque)
- Periodic phenomena, e. g. pulsations caused by surge pressures in piping, unstable H/Q curves of centrifugal pumps or the influence of the rotating impeller
As a result of velocity changes at a fixed point in space, local accelerations or decelerations create additional mass forces in a transient flow which cause corresponding changes in pressure. These can manifest themselves as marked short-term pressure increases resulting from the sudden closing of a shut-off element in a long, fluid-filled pipe, or as increased pressure losses caused by pulsating flow.
Periodically transient flow phenomena can, given a sufficiently low frequency of changes, often be treated as quasi-steady phenomena. On an averaged basis, they are at any given point in time subject to the same flow conditions as steady flow.
The flow through a rotating set of vanes (see Impeller) is, strictly speaking, always a transient flow if viewed from a stationary coordinate system (see Absolute velocity). At fixed point in space, both velocity and pressure change periodically as the vanes pass.
However, the flow in an impeller and its immediate vicinity can be considered steady as long as it is described using a system of coordinates which rotates with the impeller (see Relative velocity). The centrifugal and Coriolis forces arising in this relative system must be taken into account.