Hydrodynamic Added Mass and Damping of the Kaplan Turbine

Kaplan turbine runner rotates in water flow inside an enclosed discharge ring. The vibratory runner motion in the fluid flow induces pressure forces onto the wet runner surfaces with inertia effects conveniently described by the so-called hydrodynamic added mass and damping. These inertia effects influence the wet natural frequencies and the amplitudes. The role of the hydrodynamic added mass and damping in the Kaplan turbine shaft rotor dynamics has not been sufficiently well understood. This paper focuses on comprehensive understanding of these phenomena across the Kaplan design range. The results are based on a method derived from Theodorsen’s unsteady thin airfoil theory and on the Finite Element Method (FEM). The former method includes the water flow, the runner rotation and the circulatory effects, which makes it possible to calculate the added damping and evaluate the accuracy of FEM. The most critical vibration modes and shaft line configurations have been identified with inherent weaknesses in typical shaft line models. The added damping has been quantified. The numerical results have been compared to the experimental results.