# Transient flow

Transient flow, is flow where the flow velocity and pressure are changing
with time. When changes occur to a fluid systems such as the starting or stopping
of a pump, closing or opening a valve, or changes in tank levels, then transient
flow conditions exist: otherwise the system is steady state. Often transient
flow conditions persist as oscillating pressure and velocity waves for some
time after the initial event that caused it.

Transient flow is usually referred to as surge or water hammer, and the terms
are commonly interchanged. But strictly speaking surge is where the mass oscillation
of the fluid is the dominating force and the compressibility of fluid is not
significant, for example two connecting reservoirs oscillating up and down.
Water hammer on the other hand is where the compressibility of the fluid has
the dominant effect such as the sudden closure of a valve.

Transient flow can result in significant transient pressures that may exceed
the design limit of the pipes and fittings. Or it may be important to maintain
a steady flow for example in the process industry. Therefore a detailed understanding
of transient flow can be very important.

Transient flow problems are commonly solved using a finite difference method
known as the method of characteristics. The equations generated by this method
can only readily be solved with a computer. There are a number of commercial
software tools such as Flowmaster and Wanda, which can solve transient flow
problems. However to use the tools requires a detailed understanding of the
flow problem particularly the stiffness of the fluid, the pipe and the pipe
supports as these effect the wave speed which has a dominant effect during
water hammer.

At Fluid Mechanics we have many years experience in solving transient flow
problems. We can provide advice on cost effective ways of minimising transient
flow effects