变顶高尾水洞这种方式的特点是下游水位与洞顶的某一处相衔接,将尾水洞分成有压满流段和无压明流段。当下游处于低水位时,下游水位与洞顶较低处相衔接,此时有压满流段短,而无压明流段长,尾水管进口的真空度不会超过规范的要求。随着下游水位的升高,有压满流段的长度逐渐增长,无压明流段的长度逐渐减小,发生过渡过程时尾水管中负水击压力随之增大,但此时,下游水位升高引起淹没深度逐渐加大,两种压力叠加,尾水管进口的真空度仍不会超过规范的要求。 The top-tailing cave is characterized by a convergence of the downstream water level and the roof somewhere, and the tailwater cave is divided into a pressure-filled flow section and a pressureless flow section. When the downstream is in the low water level, the downstream water level is connected with the lower part of the tunnel roof. At this time, the pressure full flow section is short, and the pressureless flow section is long and the vacuum degree of the draft tube inlet will not exceed the requirements of the code. With the increase of downstream water level, the length of the pressure-filled flow segment gradually increases, and the length of the unpressurized flow segment decreases gradually. As a result, the negative water hammer pressure in the draft tube increases during the transition, but at this time, Submerged depth gradually increased due to water level rise, superposition of two kinds of pressure, draft tube inlet vacuum still will not exceed the normative requirements.