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International audience; 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.

Numerous stepped spillways were built to date, namely on RCC gravity dams or on valley flanks besides embankment or rockfill dams, while some others are currently under design or construction. In some cases, slope changes may be required on stepped chutes, due to the topography, leading to reduced excavation and related economic benefits. A quite limited number of stepped spillways have been built with slope changes, whereas no systematic scientific investigation for designing such type of configuration has been conducted to date. Thus, there is a lack of information with regards to the main flow features along abrupt slope changes on stepped spillways. The present study is focussed on an experimental investigation of the air entrainment and pressure development on skimming flow on stepped spillways, in the vicinity of an abrupt slope change. Physical modelling was conducted in a relatively large scale facility, where detailed air-water flow measurements were gathered upstream and downstream of a 50˚-30˚slope change. In addition, dynamic pressure measurements were obtained on both vertical and horizontal faces of several steps in the vicinity of the slope change. The results on the air entrainment and pressure development are presented and discussed. A substantial influence was observed on the air entrainment and pressure development pattern, in comparison with typical results for constant sloping stepped spillway flows.

Physical modeling represents probably the oldest design tool in hydraulic engineering together with analytical approaches. In free surface flows, the similitude based upon a Froude similarity allows for a correct representation of the dominant forces, namely gravity and inertia. As a result fluid flow properties such as the capillary forces and the viscous forces might be incorrectly reproduced, affecting the air entrainment and transport capacity of a high-speed model flow. Small physical models operating under a Froude similitude systematically underestimate the air entrainment rate and air-water interfacial properties. To limit scale effects, minimal values of Reynolds or Weber number have to be respected. The present article summarizes the physical background of such limitations and their combination in terms of the Morton number. Based upon a literature review, the existing limits are presented and discussed, resulting in a series of more conservative recommendations in terms of air concentration scaling. For other air-water flow parameters, the selection of the criteria to assess scale effects is critical because some parameters (e.g., bubble sizes, turbulent scales) can be affected by scale effects, even in relatively large laboratory models.

Systematic physical model tests are performed on a stepped spillway equipped with a bottom aerator at the beginning of the stepped part. A deflector is used to issue a jet in order to initiate air entrainment into the flow. A horizontal slot located in the vertical face of the first step allows for air supply underneath the flow. The cavity subpressure was measured to ensure optimal aerator performance, namely atmospheric pressure conditions. The air discharge entrained below the jet is measured to derive the aerator air entrainment coefficient. The local air concentrations are spatially measured downstream of the aerator at regularly spaced profiles, allowing the investigation of air transport and detrainment as well as the average and bottom air concentrations. The present paper focuses on the resulting spatial distribution of air concentration for five deflector geometries. The chute angle, step height, approach flow Froude number and approach flow depth were kept constant, so that the differences occur mostly on the jet length and air entrainment coefficient. The flow depth and the air concentration rapidly converge towards quasi-uniform flow values downstream of the aerator.

Bottom chute aerators are installed to prevent cavitation damages and they have been studied in detail on smooth spillways. In parallel, stepped spillways became widespread in the past decades. Research has shown that stepped spillways may be endangered even more by cavitation than smooth spillways, particularly for high unit discharges. As a consequence and besides issues of energy dissipation, the unit discharge of stepped spillways is usually limited to lower values than on smooth spillways. In order to overcome that limitation, flow aeration – mainly at the beginning of the chute – is necessary. Until now only fragmentary guidelines exists for the design of such aerators. Systematic tests with bottom chute aerators on stepped spillway are performed on a physical model. A deflector is used to separate the jet from the bottom in order to produce slight negative air pressures. A horizontal slot located in the vertical face of the first step allows for air supply underneath the flow. In addition of the global air entrainment by the aerator, the local air concentrations are spatially measured downstream of the deflector. The resulting air concentration distribution allows the investigation of air transport and detrainment as well as the streamwise average and bottom air concentration. The present paper discusses a test with a typical aerator and compares it with a reference test without aerator and a test on a smooth chute with a similar aerator.

To protect spillways against cavitation damage, adding a small air concentration to the flow close to the invert is efficient. Aerator performance on smooth chutes was therefore well studied in terms of air entrainment and downstream air concentration development. Since bottom aerators are built upstream of regions exposed to cavitation, no damages have been observed on spillways. The introduction of roller compacted concrete (RCC) dams in the 1980s promoted the use of stepped spillways rarely used until then. Compared to conventional smooth spillways, they have the advantage of a higher energy dissipation rate, and of a self-aeration point located higher upstream. However, the non-aerated flow upstream of the inception point is exposed to an increased cavitation risk due to flow separation on the steps. Until recently, the uncertainty about the conditions required for cavitation inception motivated conservative unit discharges. Today, the cavitation potential on stepped spillways is better known and is significant, so that techniques are necessary to safely use stepped spillways under increased unit discharges. This research includes a physical model investigation of a deflector aerator on a stepped spillway. The key parameters influencing aerator performance and stepped spillway flow are systematically varied. The air concentration downstream of the aerator is measured at regularly spaced profiles by means of a fiber optical probe. The flow field downstream of the stepped spillway aerator can be described in three main zones: (i) thejet zone where air is entrained on the lower and upper surfaces, (ii) the spray and reattachment zone where spray is produced by the jet impact and where there are rapid variations of the average and bottom air concentrations, and (iii) the far-field zone where the flow depth as well as the average and bottom air concentrations gradually tend towards quasi-uniform conditions. The lower and upper surfaces of the jet issued by the deflector were considered to derive the effective takeoff angles. With the takeoff velocity, it allows to describe the lower and upper jet surfaces with projectile motion trajectories. The maximum jet elevation, the jet length, and the jet impact angle on the pseudo-bottom can then be determined. Similarly to smooth spillways, the air entrainment coefficient of the aerator is described as a function of the relative jet length. Besides, a relation for the air entrainment coefficient in function of the Froude number and the deflector geometry is presented. The average and bottom air concentration developments show a minimum shortly after the jet impact, followed by a maximum in the spray zone. These extrema are quantified and are related to the relative jet length. In the far-field zone, unlike smooth chutes, no continuous detrainment is observed for the bottom air concentration. Both the average and the bottom air concentrations gradually converge to quasi-uniform flow values. Tests with an increased approach flow bottom roughness showed a large increase of air entrainment due to the higher flow turbulence, but only small average and bottom air concentration differences downstream of the jet impact result. A pre-aerated approach flow leads to slightly higher average and bottom air concentrations downstream of the aerator. The design of a stepped spillway aerator is presented in the end to summarize the results obtained and their practical application.