Conference paper

Dynamic Pressures around a Confined Block Impacted by Plunging Aerated High-Velocity Jets

The impingement of high-velocity water jets on unlined plunge pools causes scour on the rock foundations. The scour process is the result of complex physical phenomena that happen consecutively. Turbulence and dynamic pressures play a major role, especially in the jet development during air travel, in the diffusion process in the pool, in the pressure fluctuations in the water-rock interface and in the pressure propagation inside rock fissures. Air entrainment significantly influences the whole process. Air bubbles in the plunge pool influence energy dissipation by counter-acting the flow. The bubbles may also enter rock fissures, where they will change properties of pressure wave propagation and amplification. Air is entrained in the jet during the air trajectory and also in the plunge pool at impact. These phenomena cannot be reproduced in Froudebased reduced-scale models without important scale effects. The present research aims at assessing the influence of air entrainment on dynamic pressure fluctuations caused by impinging high-velocity jets. Near-prototype jet velocities and pressures were reproduced experimentally. A cubic metallic block representing an element of the fissured rock was conceived to measure dynamic pressures at 12 different positions. Plunge pool water depths varied from 0 to 80 cm, while jet velocities varied from 2.5 to 22.1 m/s. Jet aeration was provided by 6 openings in the jet nozzle. The results confirm that mean pressures and pressure fluctuations are reduced with air entrainment. The conclusions provide relevant elements for future research on the influence of air entrainment on high-velocity jets for scour assessment.

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