Hybrid modelling and development of a long upper chamber in a surge tank

Chamber surge tanks in hydropower plants basically consist of a lower chamber and an upper chamber designed as tunnels. The main shaft is situated at the end of the lower chamber and connects to the upper chamber. Due to column separation at the transition from the main shaft to the upper chamber a waterfall occurs at a certain operation condition. This happens as the mass oscillation flows in reverse direction to the reservoir and the capacity of the
upper chamber is too small to continue a pressurized flow into the main shaft. The waterfall entrains air bubbles in a significant amount into the main shaft and following into the lower chamber. Forcing the flow through the lower chamber allows the air-water flow to degas. Additionally the upper chamber flow faces a hydraulic bore at filling.
At the laboratory of the Institute of Hydraulic Engineering and Water Resources Management at Graz University of Technology a surge tank design with a direct vertical connection of the main shaft to the upper chamber is being evaluated. This includes a structural device of minimizing the intrusion depth of a high discharge waterfall.
A combined hydraulic investigation of the upper chamber surge behavior as well as evaluating the maximum waterfall flow has been carried out. The surge tank design has been developed in terms of 1D-numerical simulations.
The upper chamber as well as the water fall is evaluated by 3D-numerical simulations regarding the results from the 1D-simulations as boundary condition. The optimized surge tank design is being verified by a physical model test.