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Hydraulic model study of the blowback behaviour of hte bottom outlet of the Berg River Dam, South Africa

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Gerrit Basson
added 2 research items
SYNOPSIS The Berg River Dam is equipped with the first multi level draw off environmental flood release outlet in South African and can release flows of up to 200 m 3 /s. The outlet is controlled by a radial gate and is protected by a vertical emergency gate. Commissioning tests of the emergency gate in 2008 found that large volumes of air were expelled from the air supply shaft designed to reduce expected negative pressures in the conduit during emergency gate closure. In 2009 Stellenbosch University was first commissioned by the WRC to investigate this phenomenon. The 2009 study, comprising of tests on a 1:40 scale physical model and a two-dimensional numerical computational fluid dynamics (CFD) analysis, was inconclusive on the cause of the large air releases. This report (Volume II) covers subsequent study using three-dimensional CFD analyses. The accompanying Volume I covers subsequent study using a 1:14.066 scale physical model. A three-dimensional CFD model was successfully created and results of steady-state simulations of various fixed gate opening were studied in detail and compared to physical model simulations (see Volume I). The flow patterns shown in the CFD model were very similar to those seen in the physical model and the graphical representation of flows provided by the CFD analysis helped give a clear understanding of the flow, although the discharge through the CFD model was found to be greater than that in the physical model. The model was then successfully adapted to perform a closing gate transient simulation. However, the excessive computational time required by the solver meant that only a single simulation of a very fast gate closure could be completed. Further, comparison of pressure results with those from the physical model showed that energy losses are significantly underestimated by the CFD model. The CFD model therefore could not aid in explaining the airshaft expulsions experienced in the commissioning tests.
Adele Bosman
added a research item
The Berg River Dam is equipped with the first multi-level draw-off environmental flood release outlet in South Africa and can release flows up to about 200 m3/s. The outlet is controlled by a radial gate at the outlet end, and is protected by a vertical emergency gate near the inlet end. Commissioning tests of the emergency gate in 2008 found that large volumes of air were expelled, instead of the expected air entrainment into the air vent, designed to reduce expected negative pressures in the conduit during emergency gate closure. This paper describes the testing of a 1:14 physical model representing the outlet works of the Berg River Dam to determine the reasons for the unexpected release of air from the outlet work's air vent, as observed in the field during the commissioning tests of the emergency gate in the outlet conduit. Simulations of continuous gate closure on the as-built physical model of the Berg River Dam outlet showed predominant inflow of air into the air vent during emergency gate closure, with intermittent short duration high-speed air releases during the stages of emergency gate openings between 37% and 25% open. The problem was determined to be one of intermittent air blowback from the outlet conduit via the air vent during the latter stage, rather than continuous air release for all stages of the gate opening operation. The cause of the blowback was found to be the constriction of flow due to a reduction in the conduit cross-section at the radial gate chamber located at the downstream end of the outlet conduit.