Influence of jet aeration on pressures around a block embedded in a plunge pool bottom
The influence of the air entrained by water jets on the dynamic pressures applied on the bottom of a plunge pool and inside underlying fissures was analyzed with systematic experiments. The large experimental facility reproduced aerated high-velocity jets up to 22.1 m/s impinging on a pool and impacting on an instrumented cubic block embedded on the bottom. Plunging and submerged jets are compared, as well as jet impingement on the center or on the side of the block. A relationship is proposed to describe the time-averaged pressures at stagnation as a function of the relative pool depth, considering pressure measurements in this position as well as recent experimental evidence on the jet centerline velocity decay. Air bubbles influence the dynamic pressures on the rock bottom by reducing jet momentum, but also by reducing the jet dissipation rates in the water pool. These two processes are opposed. The reduction of momentum, consequence of a jet with a lower apparent density, results in lower pressures, while lower jet dissipation in the pool results in higher kinetic energy of the jet impacting the bottom and higher pressures. Finally, the spectral contents show that the resonance frequencies of aerated jets are shifted as a consequence of wave celerity reduction caused by lower mean densities inside the fissures, which is an evidence of the presence of air bubbles.