Rotordynamic force and moment can cause shaft vibrations in pumps and water turbines. The rotordynamic fluid force which acts on back shrouds of the impeller and runner generates the rotordynamic fluid moment, and it can cause whirling motions of overhung rotors. A test facility in which the back shroud was modeled by a rotating disc was made and the whirling motion of the rotor due to the rotordynamic fluid moment could be observed. It was found in computations that a cause of the rotordynamic fluid moment can be explained by an inertia of a radial clearance flow in ranges of higher whirling angular velocity ratios in which the flow has stronger unsteadiness and a pressure loss in the seal at the periphery of the disc in all ranges of whirling angular velocity ratios.
