The response of backflow at the inlet of an inducer to flow rate fluctuation is studied by three-dimensional numerical calculations based on k-ε turbulence model to clarify its effect on cavitation instabilities. It is shown that the size of the backflow region can be correlated with the angular momentum in the upstream of inducer and the phase of the backflow significantly delays behind quasi-steady response even at very low frequency. A response function of the angular momentum in the upstream to flow rate fluctuation is derived. This clearly shows that the backflow responds to the flow rate fluctuation as a first order lag element. The effects of backflow cavitation on cavitation instabilities are discussed and it is found that the backflow vortex cavitation would destabilize low frequency disturbances due to the effects of positive mass flow gain factor but stabilize high frequency disturbances due to the effect of cavitation compliance.

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