Abstract: As high dams are built on the high-altitude plateau of Western China, the issue of high-speed jet nappes under low atmospheric pressure emerges as a new challenge to dam construction. In this study, an experiment is conducted to examine the influence of ambient air pressure on the trajectory distance of water jets and their dynamic pressure produced at the plunge pool bottom. The study shows that a drop in atmospheric pressure increases the dynamic pressure of the plunging jet. An air pressure drop of 10 kPa causes a dynamic pressure rise of up to 3.9% and a pulsating pressure rise of up to 5.5%. In low-pressure air, the trajectory distance of a jet nappe is affected by the aeration and diffusion of the nappe flow in the air. By the similarity of model and prototype, the influence of air resistance on the trajectory distance of a jet is different between model and prototype. With air pressure dropping, air density and air resistance are decreased and the air volume entrained by a jet nappe in the air will also be reduced. This means that the diffusivity of the nappe will be reduced, and a low-pressure jet nappe will be more concentrated.

Key words: low atmospheric pressure, air drag, time-average pressure, fluctuation pressure, trajectory distance of jet