Hydrodynamic forces on the impeller of a waterjet propulsion system

In a waterjet system, a mixed-flow pump is used for ship propulsion. The suction flow to this pump is highly non-uniform due to the boundary layer under the hull of the ship and the shape of the inlet duct. Even at normal operating conditions, variations in axial velocity can be as high as 50 percent of the average value. This paper presents an experimental investigation of the resulting hydrodynamic forces on the impeller. Measurements are performed on a scale model of a commercial waterjet pump. A newly designed dynamometer is used to measure the instantaneous forces and bending moments. This dynamometer is built between the impeller and the shaft of the pump and co-rotates with the impelier. Various suction flow distributions to the pump are achieved by means of a device situated in the suction pipe. Results of measurements show the influence of the suction flow and blade interaction on forces. Among the most important observations are a backward whirling motion of the rotor system and a considerable steady radial force with an upward component.