The S-shaped characteristics of high-parameter pump-turbines are the main factor of its stable operation

and the fundamental way to deal with the S-region lie in hydraulic design. For an ultra-high head pump-turbine

a crucial step to do this is a deeper study of the S-region characteristics through considering its tip-gap flows. This paper presents numerical simulations of the 3-D steady flows in a prototype pump-turbine in China

using a RNG k-ε turbulence model. The pressure distributions

flow regimes and structures

and vortex distributions in the main channel and tip gaps are analyzed

and it is deviated from design operation

the pressure gradient keeps increasing sharply in both the vaneless section and tip-gap passages

until its maximum value occurs under reverse pump operation. A circumferentially distributed ring forms in the vaneless section is found

which retains a high pressure and a huge pressure difference from its surroundings

thereby causing unstable operation

vibration and noise. This is an important cause of the S-shaped characteristics

and deepens our understanding of ultra-high head pumped storage units for better hydraulic designs.