Investigation of the sudden air release up the airshaft of the Berg river dam bottom outlet structure during emergency gate closure using numerical modelling methods

Thesis

Pulle, Doreen (2011-12)

Thesis (MScEng)--Stellenbosch University, 2011.

ENGLISH ABSTRACT: The design of the Berg River Dam bottom outlet structure with multitude draw offs was based on various hydraulic model tests on a 1:40 model that was used for original design and a 1 in 20 physical model which was used to produce the final design. These tests indicated no foreseeable malfunction and showed that the 1.8 m² air vent would provide sufficient air flow to minimize the negative pressures that would develop behind the emergency gate during its closure or opening. However, during the first trial commissioning of the dam outlet structure, air was unexpectedly expelled through the air vent at a velocity so high that the recta-grids covering the shaft were blown to a height of over 3m while the gate was closing at a rate of approximately 0.0035 m/s. The air flow velocity up the air vent was approximately 45m/s and occurred when the gate was approximately 78% closed. A brief report on the test indicated that the source of air may have been a vortex formation in the vertical intake tower upstream of the emergency gate entraining air which was drawn through the gate and released up the air vent. The purpose of this research was to utilize 3-dimensional numerical modelling employing Computational Fluid Dynamics (CFD) to carry out numerical simulations to investigate the above mentioned malfunction and thereby establishing whether the given hypotheses for the malfunction were valid. For purposes of validating the CFD modelling, a 1:14.066 physical model was constructed at the University of Stellenbosch hydraulics laboratory. The 3-dimensional CFD model was used to investigate the said incident, using steady state simulations that were run for various openings of the emergency gate. The intenetion was to establish whether there was an emergency gate opening which would reproduce the air release phenomenon. The results obtained from the numerical model showed a similar trend to those of the physical model although there were differences in values. Neither model, showed a sudden release of air through the vent. It was concluded that the unsteady air-water flow out of the air vent may have been caused by the variation of the discharge with time causing unbalanced negative pressures in the outlet structure. Therefore, it was recommended that further CFD transient simulations should be undertaken incorporating a moving emergency gate.

AFRIKAANSE OPSOMMING: Die ontwerp van die bodemuitlaat van die Bergrivierdam met multivlakuitlate is gebaseer op verskeie hidrouliese modeltoetse op a 1:40 fisiese model wat vir die oorspronklike ontwerp gebruik is, asook „n 1 tot 20 fisisiese model wat gebruik is om die finale ontwerp te lewer in 2003. Hierdie toetse het geen beduidende afwykings aangedui nie en het bewys dat die 1.8mª lugskag voldoende lugvloei sal toevoer om die negatiewe drukking wat stroomaf van die noodsluis ontstaan gedurende die sluitingsproses, sal minimaliseer. Gedurende die inlywingtoets in die veld in 2008 van die noodsluis, is lug onverwags teen 'n hoë snelheid deur die lugskag opwaarts uitgelaat, wat die rooster wat die skag beskerm teen 'n hoogte van oor 3m geblaas het terwyl die sluis teen 'n tempo van ongeveer 0.0035 m/s toegemaak het. Die lugvloeisnelheid in die lugskag was ongeveer 45m/s en het plaasgevind toe die sluis ongeveer 78% toe was. 'n Kort verslag oor die veldtoets dui aan dat die bron van die lug dalk werwelvorming in die vertikale inlaattoring stroomop van die noodsluis was, met lug wat deur die sluis getrek was en opwaarts in die lugskag vrygelaat is. Die doel van die navorsing was om drie-dimensionele numeriese modellering met rekenaar vloeidinamika (RVD) te benut om numeriese similasies uit te voer om die bogenoemde abnormale werking van die lugskag te ondersoek en daarmee vas te stel of die gegewe aannames van krag is. Vir die doel om die RVD modellering te verifieer is 'n 1:14.066 fisiese model gebou by die Universiteit van Stellenbosch se waterlaboratorium. Die 3-dimensionele RVD model is gebruik om die genoemde probleem te ondersoek, deur stasionêre simulasies wat vir verskillende openinge van die noodsluis geloop is te gebruik. Die doel was om vas te stel of daar 'n spesifieke noodsluisopening is wat die vrylating van die lug veroorsaak het. Die uitslag verkry deur die numeriese model het dieselfde windrigting soos die van die fisiese model gewys, alhoewel daar verskille in die waardes was. Nie een van die modelle het .n skielike vrystelling van lug deur die lugskag gewys nie. 'n Afleiding is gemaak dat die nie stasionêre lug-water vloei uit die lugskag moontlik veroorsaak was deur die verandering van die vloei met tyd veroorsaak deur ongebalanseerde negatiewe druk in die uitlaatstruktuur. Daarom is daar voorgestel dat verdere RVD nie stasionêre simulasies gedoen word met 'n bewegende noodsluis.

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