Mixing Stirrers are subjected to severe damages [Strohmeier (1996), Strohmeier and Ho¨lzl (1998)] when the rotational speed approaches the critical rotational speed nkrit (eigenfrequency). Because of resonant vibrations, the stirrer deflection approaches infinity (no damping case). The possibilities of an accurate design of mixing stirrers with analytical calculations [Fischer and Strohmeier (2000)] are often very unsatisfactional, due to the complex effects of the fluid medium on the structure (impeller), and consequently its critical speed and its vibrational amplitudes cannot be readily defined. To consider transient fluid effects and out-of-balance forces, it is necessary to implement a coupled analysis of flow field and structural dynamic response of the stirrer in the CFD code as a user subroutine. 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.

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