2004 年 32 巻 12 号 p. 721-728
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.