4. Rigid Water Column Theory and Applications

Transient flows in hydropower stations are given each time when hydraulic machines, such as pumps and turbines, are started, stopped or regulated. For computing such transient flows, basically, both the rigid and the elastic water column theory are applicable. Against the elastic water column theory which is generally applicable for all types of transient flows, the rigid water column theory ignores the compressibility of the fluid and elasticity of the walls of the pipeline. Thus, it basically has its application restrictions to “short pipes”. It is, furthermore, unable to resolve high-frequency shock pressure fluctuations in the flow. However, the method may have its favorable applications when the low-frequency flow oscillation in a hydraulic system is in focus. Such a low-frequency flow oscillation is quite comparable to the flow oscillation in an open U-tube. In hydropower stations, it is often necessary to know the damped flow oscillations, for instance, between the lake and the surge tank after each load regulation (Fig. 3.1), especially, after an undesired load rejection. This often includes evaluations of both the stability performance of the system and the capacity of the used surge tank. Reliable computations with appropriate computational algorithms must always be completed.