Rotating instability, RI, is a well-known unsteady phenomenon occurring in the off-design operation of various types of turbomachinery systems. However, the generation mechanism as well as its unsteady characteristics, especially in the case of centrifugal machines, has not yet been investigated in detail. In the present paper, therefore, research attention is focused on an unsteady vortex rotating along the impeller periphery, which is considered to be the cause of the RI, and its characteristics are investigated by experiments and two kinds of CFD analyses. A significant amplitude increase within a frequency band below the blade passing frequency is found to be caused by irregular change of the vortex rotating speed.
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