Ski-jump energy dissipation : design of a ski-jump to maximise energy dissipation and aeration.

Fraser, Cameron Neal (2016-03)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: One of the most effective and economical methods for the dissipation of hydraulic energy from flood waters is to project the flows into a free trajectory jet form to a location where the impact creates a plunge pool in the downstream river bed. This type of energy dissipation can be created by a ski-jump energy dissipator which has become an increasingly popular form of hydraulic energy dissipation for large dams in recent years due to its ability to safely convey high velocity flow in excess of 30m/s to the downstream river, however very limited definitive and comprehensive guidelines have been created. There is not only insufficient documentation for the conceptual and detailed design of ski-jump energy dissipators, there is also insufficient documentation for the dimensioning of the downstream plunge pools. Both are necessary to guarantee that the passage of major floods do not threaten the structural integrity of the permanent works. The origins of ski-jumps can be dated back as far as the mid-1930s where they were successfully introduced on the Dordogne hydraulic scheme in France. This revolutionary scheme designed a circular arc spillway over the power plant with the intention of conveying high velocity flow in the form of a trajectory jet over power plant and plunge down onto the riverbed at a substantially far distance away from any dam apparatuses as to mitigate potential structural damage. Due to the success of this design, it became very popular in France, Spain and Portugal. During the period 1930-1940 the first spillways of its kind were constructed under its new name, “Saut de ski”. This research includes hydraulic testing of different ski-jump buckets for a general design. The objective of this is to obtain a design that would maximise energy dissipation and enhance air entrainment as well as establishing a comprehensive guideline to the design of ski-jumps. Energy dissipation by a ski-jump may be assessed by evaluating a number of identified contributing parameters by means of a physical hydraulic model. The parameters of importance include; 1) the geometric profile of the water jet trajectory such as the trajectory distance, trajectory height, horizontal and transverse impact width; 2) dynamic impact pressure distribution; 3) maximum dynamic impact pressure head; 4) impact velocity head; and 5) air entrainment. These results demonstrate the significant effect of the Froude number, bucket angle and bucket shape. The results for the different ski-jump buckets of all mentioned parameters are presented, descripted and discussed, and concluding with the design that best dissipates energy for the ski-jump buckets tested. This design was able to improve the pressure distribution area significantly as well as decrease the maximum dynamic pressure head by up to 20m when compared to a standard 40o circular shaped flip bucket design. An increase in the aeration by up to 20% at the centreline was achieved when compared to a standard design.

AFRIKAANSE OPSOMMING: Een van die mees effektiefste en ekonomieste metodes om van hidrouliese energie van vloedwater wat oor ‘n dam-oorloop vloei te demp, is om die vloei te projekteer in die vorm van ‘n vrye straal na ‘n plonspoel stoomaf van die damwal. Hierdie tipe energie demping kan gedoen word met behulp van ‘n energie dempende ski-sprong. Oor die afgelope jare word die ski-sprong toenemend gebruik vir die oorlope van groot damme. Die rede hiervoor is dat dit die vermoë het om die water wat oor ‘n dam vloei veilig na ‘n plonspoel stroom-af van ‘n damwal te verplaas. Daar bestaan egter beperkte riglyne vir die ontwerp van ski-sprong oorloop strukture. Daar is nie net onvoldoende riglyne vir die ontwerp van ski-sprong oorlope nie, maar ook onvoldoende riglyne vir die afmetings van die gepaardgaande plonspoel stroom-af van die damwal. Albei komponente van ‘n ski-sprong oorloop is uiters noodsaaklik om te verseker dat groot vloede nie die strukturele integriteit van ‘n damwal bedreig nie. Die oorsprong van die ski-sprong kan terug gedateer word so ver as die middel 1930’s waar dit in die Dordogne hidrouliese skema in Frankryk suksesvol toegepas is. Hierdie revolusionêre skema het ‘n sirkerlvormige boog oorloop met ‘n krag stasie aan die stroom-af kant daarvan. Die oorloop is ontwerp om die hoë snelheid vloei van die dam-oorloop in die vorm van 'n geprojekteerde straal oor die kragstasie te verplaas na ‘n plons-poel in die rivierbedding. As gevolg van die sukses van hierdie ontwerp, was dit baie gewild in Frankryk, Spanje en Portugal. Gedurende die tydperk 1930-1940 is die eerstes van hierdie tipe oorloop gebou onder die nuwe naam, "Saut de ski" of ski-sprong. Hierdie navorsing sluit hidrouliese toetse van verskillende ski-sprong vorms in met die doel om ‘n vorm vir die ski-sprong te ontwikkel wat energie demping maksimeer en belugting van die geprojekteerde straal te, verbeter asook om meer omvattende ontwerp riglyne te definieer. In hierdie ondersoek word energie demping van verskillende ski-sprong vorms ge-evalueer deur middel van die meting van die belangrikste parameters in ‘n fisiese hidrouliese model. Die parameters van belang wat ondersoek is sluit in; 1) die geometriese profiel van die water straal se trajek soos, die trajek afstand, trajek hoogte en horisontale en dwars impak breedte; 2) dinamiese impak druk verspreiding op die vlak van die plonspoel; 3) die maksimum dinamiese impak druk hoogte; 4) impak snelheidshoogte; en 5) belugting van die geprojekteerde straal. Die resultate van hierdie ondersoek toon dat die Froude nommer asook die projekteringshoek en vorm van die ski-sprong ‘n beduidende invloed het. Die resultate van die ondersoek van die verskillende ski-sprong vorms in terme van bogenoemde parameters word aangebied, beskryf en bespreek. Op basis van die resultate word die ski-sprong vorm met die beste energie dempingseienskappe aangedui. Die beste ontwerp was in staat om die druk verspreiding in die plonspoel aansienlik te verbeter, sowel as die maksimum dinamiese druk hoogte met tot 20m te verlaag in vergelyking met 'n standaard 40o sirkel vormige ski-sprong ontwerp. Die beste ontwerp het ook 'n toename in die belugting van die geprojekteerde straal met tot 20% op die middellyn getoon in vergelyking met 'n standaard ontwerp.

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