A semi-empirical steam excitation force model is presented for freestanding unshrouded blades. The model is resting on a superposition of the classical Thomas-Alford cross forces and additional fluid derived forces. The additional fluid derived forces are caused by static pressure modifications in the blade tip region and large-scale redistributions of the inflow velocity caused by the varying tip gap width due to eccentricity. The empirical model parameters are obtained by means of computational fluid dynamics (CFD) and experimental turbine cascade results. The Thomas-Alford cross force contribution is formulated by means of an analytical relation for the effective discharge coefficient. It is found, that the Thomas-Alford coefficient for the excitation cross-force increases if the blade tip gap decreases. That effect is caused by taking viscous flow effects into account, and it is in good agreement with available literature data. The direct force contribution results mainly from local pressure variation in the blade tip region, and it depends also on the mean tip gap width. A further effect is caused by an upstream redistribution of the velocity due to the varying tip gap width. This effect was discovered by Song and Martinez-Sanchez first, and it is confirmed experimentally by cascade measurements in the present paper.

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