Abstract: Scale model tests and theoretical analysis are used in this study to examine the pressure characteristics and critical hydraulic conditions of a new jet-rotary joint cascade inner energy disspator that combines a submerged jet with a horizontal rotary flow. The results show that when the submergence above the crown of the jet nozzle exit is no less than 20% of its diameter, a bi-stable flow regime is formed ? a stable submerged jet and a stable horizontal rotary flow. In this case, the wall pressure features with obviously staged variations along the tunnel in the jet section and horizontal rotary flow section, with the maximum relative pressure drops of 0.75 and 0.72 across the two sections respectively. Pressure at the jet exit is increased linearly with the increasing water depth in the shaft. Relative pressure at the rotation generator exit depends on the Froude numbers at this exit and the rotation blocking contractor exit and also on the water level drop from the vertical shaft to the tunnel tail water; it is decreased linearly with an increase in any one of these three factors. We give the critical hydraulic conditions for the dissipator to operate in the bi-stable regime: the Froude number at the jet exit as an upstream condition, and the pressure at the rotation generator exit as a downstream condition. The results would help the design and optimization of jet-rotary joint cascade inner energy dissipators and their engineering application.

Key words: cascade inner energy dissipator, model test, critical hydraulic conditions, jacking pressure, wall pressure