TAIL WATER INFLUENCED TRANSITION FROM FREE SURFACE TO PRESSURIZED FLOW IN BOTTOM OUTLET TAILRACE TUNNELS

摘要

Common design criteria for bottom outlets stipulate free surface flow in the tailrace tunnel as to avoid unpredictable conditions concerning air pressure and air discharges as well as pressure fluctuations in a possibly pressurized air-water discharge. Nevertheless, these criteria cannot always be attained for different reasons, e.g. supplementary heightening of dams. Another aspect possibly leading to flow conditions other than free surface mixture flow may be an unavoidable influence from the tail water. The consequences of such an influence were assessed at two prototype sites and a physical model. Beginning with the common design case with no influence from the tail water, the paper presents a second case in which a hydraulic jump occurs in the tailrace tunnel. Yet, in this second case still no pressurized mixture discharge occurs after the hydraulic jump. Nevertheless, significant differences in the air discharge into the tunnel and the related air pressure head in the gate chamber can be shown. The paper concludes with a third case in which after the hydraulic jump induced by a high tail water level pressurized air-water mixture discharge is imminent. Also here, consequences on the air discharge into and pressure distribution in the tailrace tunnel are discussed.