•    In unsteady flow phenomenon, the change in velocity occurs so rapidly and elastic forces are important.

•    As a result of the elasticity of the fluid and also lack of perfect rigidity of solid pipe boundaries, changes pressure do not take place instantaneously throughout the fluid, but it involves the transmission of pressure in waves or surges.


•    A change of velocity at a particular point in a fluid always gives rise to a change of pressure e.g. water hammer in pipeline.

•    Shows the transmission of pressure wave in pipe when the valve at end of pipe is suddenly closed.

•    Shows that water is flowing with a uniform velocity just before closing the valve.

•    If the value is suddenly closed, the fluid immediately next to the valve is stopped.

•    As a result of this, the fluid next to the value is compressed slightly, its pressure is increased
and the pipe expand slightly as a result of the rise in pressure.

•    Each element of fluid column thus stops the element until all the fluid element in pipe is brought to rest.

•    At any instant after closing of the valve but before all the fluid has stopped, there is a discontinuity of condition of pipe.

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