Transient Behavior of Water Hammer in a Two-Pipe System

We experimentally investigated the propagation behavior of pressure waves in fluid coupled with elastic stress waves in tubes across a straight junction of two different tubes with different diameters and materials, named discontinuity. We measured wave speeds of water hammers upstream and downstream of the discontinuity and the transmission and reflection ratios of water hammer pressures at the discontinuity. To obtain the ratios, strain gages were attached at constant intervals in the hoop direction and the axial direction. Strain signals were converted into pressure histories and used to obtain the ratios. It is confirmed that propagation speeds upstream and downstream of the discontinuity can be estimated by the Korteweg wave speed. Furthermore, the impact responses of tubes in the hoop direction can be evaluated by characteristic impedance with an area change. However, we found that the axial strains due to the area change from the larger diameter tube to the smaller diameter tube were clearly different from values evaluated by the Poisson coupling. It is confirmed that when the tube cross-sectional area suddenly decreases by half at the straight junction of two tubes, the reflected pressure loading in the axial direction can cause the axial peak strain that is 0.6 times as large as the hoop peak strain.