Fluid-Structure Interaction of Stirrers in Mixing Vessels

Mixing stirrers are subject to severe damages when the rotational speed approaches the Eigenfrequency. Because of resonant vibrations, the stirrer deflection approaches infinity in the no damping case. Damping due to fluid-structure interaction between the mixing stirrer and the fluid in the vessel has major influence on the Eigenfrequency. Coupled analysis of the flow field within a mixing vessel and the structural dynamic response of the stirrer is necessary in order to evaluate vibrational amplitudes to guarantee life time safety for the stirrer. A simplified numerical model based on Newmark's integration scheme is developed for the stirrer dynamics that is suitable to be implemented in a CFD code as a user subroutine. Results in terms of Eigenfrequencies are compared to results of analytical formulas and FEM results and show excellent agreement. The fully fluid-structure coupled analysis is also presented. As a new aspect, a rotating grid (sliding mesh) was combined with a deformable grid to simulate the impeller movement. The results are compared to experimental and analytical data and show good agreement.