Deflector-jets affected by pre-aerated approach flow

Abstract

Jets generated at chute ends move a potential scour zone away from the dam, and dissipate flow energy to reduce the scour. Usually, so-called flip buckets or ski jumps are provided to generate jets, yet they are rarely issued by deflectors. The application of a theoretical mass point parabola is limited to predict the jet features, because effects related to the take-off angle and the disintegration process affect the jet air transport and trajectories. This research discusses the effect of pre-aeration of the approach chute flow on the jet features. A comparison with a former study indicates that mainly the upper jet portion is affected, that the black water jet core becomes shorter, and that the air transport increases. Although the lower trajectory is less affected and thus remains more coherent, the small effect of pre-aeration is sufficient to reduce the total jet throw length, so that pre-aerated jets tend to be shorter as compared with black-water approach flow jets, in agreement with prototype observations.

Hydraulic and structural design

“…As shown by Erpicum et al (2013), the prototype flow tends to separate from the crest if a certain discharge is exceeded, whereas the model flow then still clings on the structure (teapot effect). This affects the jet trajectories as well as the related disintegration and air entrainment Hager 2012, Pfister andChanson 2012). The flow separation depends on the crest pressures, which are different in models as compared to prototypes.…”

Trajectories and air flow features of ski jump-generated jets

“…For the latter, aerators for cavitation prevention are installed or self-aeration of the flow occurs, so that the approach flow to the ski jump is pre-aerated. Then, the upper trajectory is typically lifted and the lower reduced (Pfister and Hager 2012), and the take-off velocity increased due to friction reduction (Minor 1987, Wood 1991. To summarize, a careful analysis of a prototype flow and a thorough engineering judgment are required when designing ski jumps.…”

“…(3)], whereas the falling trajectory portion deviates due to spray and wind." Pfister and Hager (2012) add: "If the jet is sufficiently long and issued with a large velocity, so that primary jet disintegration is fully established and the secondary becomes dominant, then a 'rupture' of the upper jet trajectory occurs." Primary jet disintegration is the disintegration of the jet itself under the influence of turbulence and gravity, whereas secondary disintegration describes the disintegration of water packages separated from the jet under aerodynamic interaction.…”

“…Thus, tests with W o < 80 and R o < 1.3 × 10 5 were ignored herein (test numbers in parentheses in Table 1). Pfister and Hager (2012) discussed the similarity of primary and secondary jet disintegration, concluding that particularly the secondary disintegration is affected as the stable drop size seems constant. This effect mainly concerns the part beyond the 90% air concentrations, which is irrelevant to define the jet surface.…”

“…Besides, they described the jet trajectories with the take-off point and its maximum elevation, to avoid the criticized projectile approach. Pfister and Hager (2012) demonstrated that pre-aeration of the approach flow upstream of a jet-generating deflector influences the jet air features, as the jet black-water core length is reduced. The chute upstream of a ski jump has a bottom angle φ relative to the horizontal (Fig.…”