Von Karman Vortex Induced Vibration in a Francis Turbine: a reduced numerical model for estimating the shedding frequency

The present paper presents a numerical analysis of the von Karman vortex shedding at the blades trailing edge of a recently installed small hydro Francis turbine manifesting very loud and high frequency acoustic pulsation when operating close to the rated discharge. A reduced single passage numerical model is developed to reduce the computational effort of the simulation while ensuring high accuracy in the assessment of fluid flow. The potentials of the numerical model are investigated by comparing the frequency spectrum of the experimentally acquired acoustic frequency and the numerical pressure signals allowing to confirm the nature of the vibrations affecting the machine. Thanks to the so demonstrated ability in the evaluation of the fluid flow, the proposed numerical model represents a valid alternative to the traditional correlation-bases approach for the evaluation of the von Karman shedding frequency with a less computational effort compared with a transient simulation of the entire machine.