Dynamic characteristics of the clearance flow between a rotating and axially oscillating disk and a static disk were examined by experiments and numerical simulations based on a bulk flow model. In the case without pressure fluctuations at the inlet and outlet of the clearance, parallel and contracting flow paths had an effect of stabilizing the axial oscillation of the rotating disk. The enlarged flow path had an effect of destabilizing the axial oscillation due to the negative damping and stiffness for outward and inward flows, respectively. It was shown that the fluid force can be decomposed into the component caused by the inlet or outlet pressure fluctuation without the axial oscillation and that due to the axial oscillation without the inlet or outlet pressure fluctuation. A method to predict the stiffness and damping coefficients is proposed for general cases when the clearance is combined with an arbitrary flow system.

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