Aeration characteristics of ski jump jets

Aeration Characteristics of Ski Jump Jets
Lukas Schmocker; Michael Pfister; Willi H. Hager, F.ASCE; and Hans-Erwin Minor

Abstract: Scour downstream of ski jumps may be avoided by jet deflection to an area where the energy dissipation is accomplished. The main purpose of this experimental study was the analysis of the jet air entrainment downstream of a ski jump, both for pure water andpreaerated approach flow conditions. A systematic variation of the Froude number and the flow depth in the approach flow channel resulted in a range of discharge characteristics, whereas the geometry of the ski jump was maintained for all tests. The air concentration profile was measured at different locations downstream from the ski jump to evaluate the: (1) jet air concentration distribution; (2)location of minimum air concentration along the mixture flow jet and development of the minimum and the cross-sectional average air concentrations; (3) jet trajectories; and (4) process of air entrainment characteristics and jet disintegration. The results demonstrate the significant effect of the approach flow Froude number, the approach flow depth, and of preaeration on jet disintegration.

CEDatabase subject headings: Aeration; Energy, Hydraulics; Hydraulic structures; Jets; Scour

Introduction
Ski jumps as applied in hydraulic structures are a main element to dissipate energy from high-head dams for relatively large unit discharges. They may also be used as terminal structures of bottom outlets, but then are often curved in plan view. The following relates to the straight ski jumpand extends previous observations, based on an experimental study at Versuchsanstalt fur Wasserbau, Hydrologie und Glaziologie (VAW), of the Swiss Federal Institute of Technology ETH, Zurich.

Although ski jumps were incorporated in many hydraulic schemes over the past decades, relatively few works on their basic hydraulic features are available. Most observations are site specific, such thatthe design guidelines of ski jumps are currently incomplete. Most of these hydraulic structures are, therefore, model tested prior to the final design stage. The loss of a ski jump or damages caused by scour that were previously not accounted for may result in particular-require a detaled hydraulic consideration (Vischer and Hager 1995, 1998; Khatsuria 2005)

Resent works on ski jumps includethose of Juon and Hager (2000) with a literature review on past hydraulic studies of general character, and a preliminary investigation on the plane and spatial flow patterns of ski jump. They presented information relating to the pressure distribution on the flip bucket, the take-off angles of both the lower and the upper jet trajectories, the energy dissipation from the takeoff to the impactsections, and the choking features of the ski jump bucket. It was demonstrated that both the lower and the upper jet trajectories are of parabolic shape with the take-off angle different from the bucket angle, depending on the relative approach flow depth and the corresponding Froude number. Heller et al. (2005) also investigated scale effects, and found that water jets issued into the air require aminimum approach flow depth of the order of 40 mm.

The main purpose of the present research was to analyze the air entrainment characteristics of a plane jet downstream of a ski jump, both for pure water and preaerated approach flow conditions. Whereas Heller et al. (2005) employed point gauges to record the jet trajectories, an air-water concentration probe was used in the present work. Theresults of this study, thus, relate to the development of the air concentration of highly turbulent water, and air-water jets in the atmosphere, and therefore emphasize the jet disintegration characteristics (bin 1993; Ervine 1998) and the resulting plunge pool scour (Manson 1993; pagliara et al. 2006). The present results may also by relevant to considerations of water jets for fire fighters, yet...