To restore flow downstream of dams, siphon systems controlled by valves can be installed. Opening or closing these valves controls the flow rate at the outlet. Before installing these siphons, a study of the priming phase may be necessary to ensure its stability. This is followed by a study of the siphon’s flow rate to ensure that its geometrical characteristics will enable the valve opening and hydraulic load to restore the nominal flow rate.
This study has two distinct objectives:
3D modeling enables us to study the stability of the flow during priming, i.e. to check that there is no break in the hydraulic head during this transitional phase, which would stop the water being drawn upstream. The use of VoF models (Volume of Fluid) enables us to visualize the evolution of the air and water phases during valve opening, and thus: observe the establishment of the water flow, identify the establishment of a continuous air column between upstream and downstream of the siphon that could create the hydraulic head break.
A second phase of steady-state calculations is used to determine the flow rate as a function of the input parameters (the valve opening, and therefore the flow cross-section, and the initial hydraulic load). The different flow rates obtained by varying the hydraulic load lead to an opening-dependent flow coefficient, thus defining the flow law for a given opening. Ultimately, this correlation can be used for flow control by a controller.
Here’s a video of CFD modeling to calculate the flow rate of a dam siphon.
CFD simulations can be used to characterize the various discharge devices used in dams. We can model the flow rate of labyrinth, piano key or circular weirs, as well as bottom gates and flap gates, in order to determine the flow vs. load laws as accurately as possible.
