For rotordynamic calculations on multistage pumps, one must know–among other things–the hydrodynamic forces generated by a vibrating impeller. In linearized form these forces may be described by stiffness, damping, and mass matrices. Such matrices were measured with the help of a test apparatus using the “rocking arm” principle for an impeller of 0.35 m (13.8 in.) dia at various speeds. It is shown that the hydrodynamic forces generated by the vibrating impeller can be described reasonably well by the standard rotordynamic stiffness, damping, and mass matrices, except for the direct damping term which had to be supplemented by a damping term whose magnitude is proportional to displacement rather than velocity. It is further shown how the test results can directly be compared with results from tests using circular orbits.

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