Strujanje u umirujućim bazenima stepenastih brzotoka

Abstract

Kod preliva visokih brana, umirujući bazeni predstavljaju najčešće korišćeno rešenje za disipaciju mehaničke energije vode pre njenog ispuštanja u nizvodno rečno korito. Zbog značaja i visoke efikasnosti, umirujući bazeni detaljno su izučavani, prvenstveno u sprezi sa glatkim brzotocima. Disipacione sposobnosti umirujućih bazena mogu se poboljšati upotrebom različitih stabilizacionih elemenata, čiji efekti omogućavaju smanjenje dužine hidrauličkog skoka, odnosno gabarita bazena. Slični efekti mogu se postići i promenom podužnog nagiba dna bazena. Razvoj tehnologije valjanog betona pospešio je primenu stepenastih brzotoka, koje (u odnosu na glatke) odlikuje bolja disipacija mehaničke energije. Međutim, uslovi strujanja koji pritom nastaju – intenzivnije mešanje vode i vazduha, praćeno povećanjem dubine – bitno se razlikuju od onih koji su poslužili pri razvoju preporuka za tipske bazene sa stabilizacionim elementima. Iako tipski bazeni i u slučaju stepenastih brzotoka mogu pojačati disipaciju energije, njihovi efekti su slabiji od očekivanih. Zbog toga je od posebnog značaja procena uticaja gabarita i položaja stabilizacionih elemenata tipskih bazena koji su namenjeni za glatke brzotoke, na disipaciju energije u bazenima stepenastih brzotoka. Ovo istraživanje posvećeno je ispitivanju uticaja koji stepenasta kontura brzotoka ima na efikasnost disipacije mehaničke energije u umirujućem bazenu. Pri tome, ispitani su bazeni sa horizontalnim dnom i bazeni sa dnom u negativnom podužnom nagibu i to, sa i bez, dodatnih stabilizacionih elemenata. Ocena kvaliteta disipacije mehaničke energije sprovedena je prvenstveno na osnovu rezultata merenja dubina i brzina tečenja, za šta su korišćene savremene metode beskontaktnog merenja, bazirane na tehnikama obrade slika iz video zapisa eksperimenta.

Stilling basins are the most commonly used structures for the large dam spillways energy dissipation. Due to their high efficiency, stilling basins (with the hydraulic jumps) are well investigated, mainly for smooth spillways. Energy dissipating capabilities of the stilling basins can be improved with the use of different types of stabilizing elements, which allow for the reduction of the hydraulic jump length and consequently for the size of the stilling basin. Similar effect can be achieved by changing the basin bed slope. Development of the roller compacted concrete technology has encouraged the use of the stepped spillways which (relative to the smooth spillways) have higher energy dissipation rate. However, the resulting flow conditions (more intense air–water mixing, coupled with higher flow depths) differ from those for which the standardized basins with stabilizing elements were developed. Consequently, the use of standardized basins can increase energy dissipation, but the effects are lower than expected. Hence, the assessment of the effects of the dimensions and position of the standardized-basins stabilizing elements for smooth chutes, on the efficiency of energy dissipation of stepped-spillway basins, is of special interest. This research concerns the impact that stepped spillway can produce on the energy dissipation efficiency of the stilling basin. Hence, horizontal and negative slope stilling basins were investigated, with and without the use of the stabilizing elements. Assessment of the energy dissipation efficiency is based mainly on the results of depth and velocity measurements, which were performed using contactless methods, based on image processing approach. Results of this investigation indicate that the energy dissipation quality of stepped spillway stilling basins can be improved using the stabilizing elements for smooth-chute basins, but with modified dimensions and position, as well as using the negative basin bed slopes. Therefore, the estimation of the main flow parameters for the basins with stabilizing elements, and negative–sloped basins is presented. The significance of the research is also shown through the use and development of state-of-the-art contactless measurement techniques: (1) Laser Particle Image Velocimetry (Laser PIV) technique was successfully applied for the estimation of flow velocities downstream of the hydraulic jump roller, and (2) a novel method for the estimation of the flow depths is presented, based on the image processing approach. Ultimately, the analysis of the obtained data provided design recommendations for the stepped-spillway stilling-basin design.