Experimental study of large scale plunging jets

The lack of knowledge regarding air entrainment caused by large jets in hydraulic structures, such as downstream Pelton turbines, has led EDF (Electricité de France) to carry out a test series to optimize the La Coche Power plant. The data extracted from this work have been used in the frame of a research project. A first experiment, performed on a large jet impacting the free surface of an open channel, has confirmed that physical phenomena concerning the behavior of large scale plunging jets are still not well understood. The main parameters that have been measured are the penetration depth, the entrained air flow rate, the average bubble size below the free surface, and the rising slope of the bottom boundary of the bubble plume. Globally, when comparing the data collected downstream of the jet impact zone with the predictions proposed in the literature, the different relations are not scalable with the jet dimension. Moreover, it appeared that these parameters are intimately linked to the jet state upstream the impact point. Consequently, a second experimental set-up was designed to measure the dynamic pressure and the void fraction inside the jet. These results have shown that even an elementary parameter such as the breaking length is not well predicted by existing formula. Indeed, high-frequency videos have proved that in certain cases, the jet is still continuous while literature predicts a broken jet. In addition, the experimentations show that the jets are flapping during the fall and thereby impacting the free surface over an enlarged region.