ABSTRACT
Aiming at unstable operation and vibration of the multiple Francis turbines supplied water by one bifurcated penstock under off-design condition, the RNG k-ε turbulence model with embedded large eddy simulation is presented in this paper. Firstly, this approach is compared to other turbulence models and verified by the experimental data. Then the transient flow is simulated, where RNG k-ε turbulence model is applied to the whole flow passage, while in the key parts like runner and draft tube, RNG k-ε turbulence model is frozen and replaced by embedded LES model. Next, the vortices group in the runner and draft tube are clearly captured. The impact inflow at the inlet of blades causes the interblade and blade-edge vortices in runner, and the flow structure in draft tube appears to be the vortex ropes. Finally, time-frequency analysis of pressure pulsation shows that the hydraulic interference between two turbines and the low frequency pressure pulsation in draft tube are the main reasons for vibration.
