Masters Degrees (Civil Engineering)

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Browsing Masters Degrees (Civil Engineering) by Subject "Aeration, Artificial"

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    Artificial aeration on stepped spillways with piers and flares to mitigate cavitation damage
    (Stellenbosch : Stellenbosch University, 2017-12) Koen, Jaco; Basson, G. R.; Bosman, D. E.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: Stepped spillways have been used for approximately 3500 years and, with the recent technical advances in Roller Compacted Concrete construction, these spillways have received a renewed interest over the past few decades. However, because of the possibility of cavitation damage to the spillway chute at higher discharges, the maximum discharge that these spillways can safely handle has been limited. A pre-emptive measure to combat cavitation damage is to introduce flow aeration at the pseudo-bottom. In order to aerate the flow, various crest pier aeration structures were investigated to ultimately increase the maximum safe unit discharge capacity of stepped spillways. Different aeration structures were investigated, on two types of spillways (Type A and Type B), with the aid of two physical hydraulic models. The Type A spillway was a 1:15 scale, USBR stepped spillway with transitional crest steps and a constant step height of 1.5 m. The spillway performance of each aeration structure was determined by measuring the air concentration at the pseudo-bottom and the minimum pressure at the step riser. Experiments on the Type A spillway were carried out at a prototype unit discharge of 30 m²/s for the investigation of different pier configurations near the spillway crest. The crest pier configurations comprised two pier nose shapes, two pier lengths and the addition of a flare to the pier. The pier configuration results were compared with the performance of an unaerated stepped spillway. The maximum safe unit discharge capacity of an unaerated stepped spillway had previously been determined by Calitz (2015) to be 25 m²/s. The implementation of the bullnose, short pier, increased the maximum safe unit discharge capacity to 30 m²/s, by eliminating the risk of cavitation damage in the vicinity of the natural aeration inception point. The Type B spillway was a WES stepped spillway, with a smooth ogee crest and constant step height of 1 m, which was used to evaluate the Chinese developed Flaring Gate Pier (FGP) design. The model scale for this spillway was 1:50. The design of the model was based on the Dachaoshan Dam (China), which has a design unit discharge of 165 m²/s. The FGP designs consisted of an X-Shape and a Y-Shape FGP, together with a slit-type flip bucket. The performance of these aerators was compared to an unaerated stepped spillway for prototype unit discharges of 50 m²/s to 200 m²/s. The most notable improvement was the increase in the maximum safe unit discharge capacity to 50 m²/s in the case of the X-Shape FGP. In summary, the addition of a short, bullnose crest pier on low head/velocity stepped spillways increased the maximum safe unit discharge capacity to 30 m²/s. In the case of a high head/velocity stepped spillway, while the X-Shape FGP improved the maximum safe discharge capacity to 50 m²/s.