Large scale plunging jets : Impact, energy dissipation and air entrainment – JetPlume

Liquid jets impacting a pool entrain air beneath the free surface when their velocity is large enough. Large-scale plunging jets encountered in hydroelectric infrastructures play an important role on the performance and safety of dams. Furthermore, they play a relevant role on the ecosystem balance in rivers and basins, through generation of huge bubble clouds that can change the chemical balance on it. Despite the industrial and environmental relevance of these flows, our modelling capacities are so weak that we cannot answer even very fundamental questions. For instance, predicting the flowrate and bubble sizes of air entrained in a pool by a plunging jet is still an unresolved problem. A thorough understanding of the evolution during free fall, impact against a liquid surface and consequences of plunging jets (i.e. bubbly flow generated within the reservoir) is then key to increase our capabilities of modelling. It would also enable efficient, safe and environmentally friendly energy production.

We propose a joint numerical-experimental investigation on the air entrainment and impact dynamics of large-scale jets under the influence of gravity. A combination of small and large-scale experiments and numerical simulations, will allow us to explore a vast range of the parameter space, that includes the jet fall height, its initial velocity, the nozzle size and geometry, among others. It involves two institutions with experience in the experimental study and modelling of these flows (LEGI and LMFA). LEGI will also have access to a large-scale facility, unique in the world for fundamental studies of impact jets: the ‘JetHigh’ experimental platform. It allows to generate plunging jets with injected velocities from 2 to 35 m/s with an injection diameter between 26 mm to 213 mm, along a 10-meter free fall height impacting a 23 m meter depth plunge pool. We will therefore be able to test and adapt our models and numerical simulations to realistic conditions. The team is completed by CORIA, that will use state-of-the-art numerical tools. For instance, they have developed the ARCHER code, that allows to quantify both the properties of the water-air interface and the evolution of the jet even at large scales. EDF will be part of both the experimental and numerical analysis, as it will contribute with its expertise on large-scale experiments and their expertise and capabilities for numerical simulations.

As stated, the key measured parameters will be the jet aeration (entrained flow rate and void fraction), surface corrugation sizes, jet dynamics, as well as the bubble cloud size and recirculation dynamics. Other important parameters are the size distribution of the bubbles generated and the spatial distribution of local quantities of interest (such as concentrations, phase velocities, flux, etc.). In particular, their local evolution with depth is crucial. These parameters will be first measured on a common round jet configuration (benchmark case) by all partners, then at the large scale on JetHigh (LEGI) and at the lab scale in other geometries, e.g. 2D sheet, array of jets (LMFA). A practical objective of our project is also to develop new numerical tools and sensors at lab scales which will subsequently be tested in JetHigh and subsequently in hydroelectric facilities.

The strength of our project resides in the wide range of scales we can cover with our setups and simulations; the fact that experimental partners are recognised specialists of fragmentation, two-phase flows , turbulence and more generally flow stability ; complementarity with CORIA which is a renowned specialist on two-phase flow simulations ; and finally the partnership with EDF, which will help us remain relevant to the conditions of applications, and which will bring a complementary expertise in hydraulic engineering.