Apparent mass torque coefficients for fluctuations of flow rate and angular velocity are determined experimentally for two-dimensional centrifugal impellers. Nearly sinusoidal fluctuations of flow rate and angular velocity are produced by using crank mechanisms, and the resulting unsteady torque on the impeller is measured. The torque is divided into components in-phase and out-of-phase with the displacements. The in-phase components are used to determine the apparent mass coefficients. Drag torque coefficients are defined and used to represent the out-of-phase components. The tests are conducted under various frequencies and amplitudes of the fluctuations with zero mean flow rate and rotational velocity. The apparent mass torque coefficients are compared with theoretical values obtained under the assumption of a two-dimensional potential flow. The experimental values are 5 to 20 percent larger than the theoretical ones and no appreciable effects of the frequency and the amplitude are observed within the range of the experiments.

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