The fundamental characteristics of rotordynamic fluid force moment on the backshroud of a Francis turbine runner in precession motion were studied by model tests and the computations based on a bulk flow model. The backshroud of the runner was modeled by a disk set close to a casing with a small radial clearance at outer periphery. An inward leakage flow was produced by an external pump in the model test. The effects of the leakage flow rate, the pre-swirl velocity at the inlet of the radial clearance, and the axial clearance between the backshroud and casing on the fluid force moment were examined. It was found that the fluid force moment encourages the precession motion at small forward precession angular velocity ratios and the region encouraging the precession motion is affected by the pre-swirl velocity. Through the comparisons of the fluid force moment, unsteady pressure and velocity distributions in the clearance under the precession motions with and without the rotation of the disk, it was found that the normal moment generated by the precession motion without the rotation of the disk did not have the effect of encouraging the precession motion. The swirl flow due to the rotation of the disk was found to be responsible for encouragement of the precession motion.
