Abstract: After the construction of the side inlet/outlet of a pumped storage power station is completed, usually the adjacent cofferdams cannot be dismantled completely, leaving the cofferdam residues in situ. At low operating water levels, adverse circulations are likely to form in the surface layer of water flows in front of the inlet/outlet under the influence of residual cofferdams and topography, and tend to cause large water surface fluctuations in outflow mode or induce harmful vortices above the inlet/outlet in inflow mode. This paper explores effective measures to improve flow patterns around a side inlet/outlet and cofferdam residues, using the computational fluid dynamics method in a case study of the Weifang pumped storage power station. To avoid the circulation, we modify and optimize the cofferdam slope and its bottom edge location, and compare and verify the effect of different schemes against the measurements on a physical model. The results show that when residual cofferdams exist in front of a side inlet/outlet, flow patterns at the water surface in the reservoir area are usually stable in inflow mode, but an adverse circulation is very likely to form around the cofferdams in outflow mode and it cannot be avoided by adopting a symmetrical design of the cofferdams. When the cofferdam’s slope bottom edge is along the direction of the diffusion flow in the open channel section, the flow spreads smoothly and evenly into the reservoir area, greatly improving the flow conditions.

Key words: pumped storage power station, side inlet/outlet, cofferdam, circulation, numerical simulation