Dynamic pressure distribution around a fixed confined block impacted by plunging and aerated water jets

The scour process created by the impingement of high-velocity water jets on fissured rock is the result of complex physical phenomena. Of great importance are the jet development during the trajectory through the atmosphere, the diffusion process inside the plunge pool, the pressure fluctuations at the water-rock interface and the propagation of pressure waves inside rock fissures. Air entrainment also plays an important role. Air can be entrained during the trajectory in the atmosphere and at the point of impact into the plunge pool. Particular characteristics of air bubbles, such as compressibility and buoyancy, will influence jet dissipation in the plunge pool. Furthermore, air bubbles are present at the water-rock interface, from where they can enter rock fissures and thus change properties of pressure waves propagation and amplification. These phenomena cannot be reproduced in Froude-scale models without important scale effects. Experiments were carried out with the objective of investigating the influence of jet aeration on pressures at the water-rock interface of a plunge pool and inside rock fissures. The experimental set-up generates prototype jet velocities up to 22.1 m/s. Air was provided at the nozzle by a pumped aeration system. For each test scenario, a similar non-aerated water jet was tested for comparison. The so formed air-water jets impinge into an 80cm deep plunge pool. Additionally, water depths varied from 50 to 80 cm for non-aerated tests. Corresponding non-dimensional ratios between pool depth and jet diameter varied from 6.9 to 11.1, resulting in core and developed jet impact on the bottom. The fractured rock media was represented by a confined cubic metallic block of 200 mm side placed on the bottom of the pool, equipped with 12 micro pressure transducers evenly distributed on the water-rock interface and through the represented fissures. The transient pressures were analyzed by means of non-dimensional pressure coefficients and the spectral contents of the pressure signals. Results confirm that mean pressures and oscillations are affected by the aeration of the jet, both on the water-rock interface as inside the fissures. Moreover, evidence is shown that part of the air content in the plunge pool is able to enter the fissures and influence resonance effects. This study gives useful insight on the influence of jet aeration on the rock scour process