Experimental study of large scale plunging jets

Abstract : The lack of knowledge regarding the air entrainment caused by large jets in hydraulic structures, especially downstream Pelton turbines, has led EDF (Electricité de France) to carry out a dedicated experiment to the enhancement of the La Coche Power plant. The data extracted from this work have been used to provide a more precise analyze to follow a research purpose. The first part of this study, is detailed in this article, it has been confirmed that physical phenomena concerning the behavior of large scale plunging jets are still not well understood. The available data measured downstream the jet impact with the first experimental apparatus have been analyzed and compared to data from the literature. The main parameters which have been studied are: the penetration depth, the entrained air flow rate, the average bubble size under the free surface and the accent slope of the bubble plume bottom. Globally the different relations are not scalable with jet scale. Moreover it appeared that the variables are intimately linked to the jet state upstream the impact point. Consequently a second experimental set was designed to measure the dynamic pressure and the void rate inside the jet. The experimental results have shown that a quite simple parameter as the breaking length is not well forecasted by existing formula. The high frequency videos have proven in certain cases that a jet is still continuous when the main formulae predict a broken jet. In addition to that, the experimentations show that the jets are flapping during the fall what is too difficult to understand with the classical way of investigations (pressure sensor and optical probe) which are not synchronized with the jet motion.