Abstract
Air entrainment at the intake of a bottom outlet often gives rise to air pockets in its conduit and formation of geysers. The outlet in question comprises a bulkhead gate, gate shaft, horizontal conduit, and exit. Operations show that it suffers from appreciable flow fluctuations and blowouts in the tailwater, which leads to gate operation restrictions. For the purpose of understanding the hydraulic phenomenon, both prototype discharge tests and three-dimensional computational fluid dynamics (CFD) modeling of two-phase flows are performed. The operational focus of the facility are small and large gate openings. The CFD results reveal that, with air entrained in the gate shaft, continual breakup and coalescence of air bubbles in the conduit typify the flow. At small openings below 1 meter, the air–water flow is characterized by either distinct blowouts of regular frequency or continuous air release. In terms of geyser behaviors inclusive of frequency, the agreement is good between field and numerical studies. At large openings, the gate becomes fully submerged, and the flow is discharged without air entrainment and blowouts. The paper showcases the air–water flow features in a typical bottom outlet layout in Sweden, which is intended to serve as an illustration of the study procedure for other similar outlets.
Highlights
A plunging jet often occurs in civil facilities like hydropower, irrigation, water treatment, and urban sewerage
The purposes of the study are (1) to evaluate the extent of air entrainment through prototype tests, (2) to see whether computational fluid dynamics (CFD) modeling quantitatively reproduces the hydraulic phenomenon observed in the field tests, (3) to identify major flow scenarios in terms of gate openings, (4) to estimate the volume of the trapped air as it is otherwise by other means difficult to quantify, and (5) to provide a basis for countermeasures and structural changes
It is up to CFD modeling to demonstrate the extent of air entrainment and geysers at larger gate openings
Summary
A plunging jet often occurs in civil facilities like hydropower, irrigation, water treatment, and urban sewerage. The air pocket pressure is significantly higher than the hydrostatic value, resulting in the rapid acceleration of air and water and the jetting out of air–water mixture They show that geysers are more likely to occur for small risers and large air volumes. The purposes of the study are (1) to evaluate the extent of air entrainment through prototype tests, (2) to see whether CFD modeling quantitatively reproduces the hydraulic phenomenon observed in the field tests, (3) to identify major flow scenarios in terms of gate openings, (4) to estimate the volume of the trapped air as it is otherwise by other means difficult to quantify, and (5) to provide a basis for countermeasures and structural changes. The study aims to provide a reference for understanding air entrainment in similar outlet layouts
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