The numerical simulation of low frequency pressure pulsations in the high-head Francis turbine

• Numerical technique for calculating pressure pulsations in hydro turbines is presented.
• This technique is applied for simulating unsteady turbulent flow in a hydro turbine.
• The flow structure and its influence on the intensity of non-stationary processes is analyzed.
• It is shown that the pressure pulsations are mainly caused by the precessing vortex rope.
• The comparison of simulation results with the experimental data shows reasonable agreement.

Numerical technique for calculating pressure pulsations in the high-head hydro turbines is presented in the paper. The numerical technique is based on the use of DES turbulence model and the approach of “frozen rotor”. This technique is applied for simulating unsteady turbulent flow in a flow path of a high head hydropower plant. The flow structure downstream the runner and its influence on the intensity of non-stationary processes is analyzed. It is shown that the low-frequency pressure pulsations in water turbines are mainly caused by the precessing vortex rope downstream the runner. The vortex dynamics basically determines the pulsational characteristics of turbine operation. It is found that the averaged velocity components of flow downstream the runner can serve as indicators, which characterize the intensity of transient phenomena in the draft tube of water turbine. The comparison of simulation results with the experimental data shows reasonable agreement in the integral characteristics (efficiency, flow rate, power) as well as pulsational characteristics (intensity and frequency) of the flow and turbine operation.