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The fundamental characteristics of rotordynamic fluid force moment on the backshroud of a Francis turbine runner under whirling motion were studied by model tests and computations. The backshroud of the runner was modeled by a disk set close to a casing with a small radial clearance at outer periphery. The moment was caused by an inward leakage flow which was produced by an external pump in the model test. The criteria for the moment to encourage the whirling motion through mechanical mode coupling were derived. The experiments were designed to measure the rotordynamic fluid force moments caused by various leakage flow rates with various pre-swirl and various axial clearances between the backshroud and casing. Computation was carried out based on a bulk flow model. Experimental and computational results of the fluid force moments were compared to discuss the rotordynamic stability. It was found that the fluid force moments have a destabilizing effect in wide regions except for a small region with positive whirling speed ratios.

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