It is widely known that there arises high frequency pressure oscillation in the water conduits of pumped storage power plant as one of the transient phenomena of pump-turbines, of which propagation along pipe lines has not been theoretically identified. With regard to the rock-bored pressure tunnels adopted for most pumped-storage plants, the studies by M. Fanelli and L. Suo, specifying that the wave speed and its attenuation depend on exciting frequency, is applicable to clarify above-mentioned mechanisms. Measured data of pressure oscillation and its propagation at the Okukiyotsu No. 2 pumped storage plant are introduced to examine the theory.

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To analyze transient free-surface-pressurized flows with entrapped air in a drainage system, a virtual slot model with ceiling have been proposed. Using this model, calculations for a reduced model and the actual size model are carried out to examine a scale effect on the transient flow. The results show that up-scaling the system causes the increase in compressibility of entrapped air so that the rate of pressure rise decreases. It is also confirmed that the partial differential equations for the modified slot model have real eigenvalues and then the initial value problem is well-posed when the velocity difference between air and water is not so large. The increase in the velocity difference yields a system having complex eigenvalues, however the well behaved numerical solutions can be obtained since the frictional term in the defferential equations suppress the numerical instability.

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