Numerical analysis of viscous flow in narrow gaps between rotating disks gives several solutions for the same value of the Reynolds number. The non-uniqueness of solution can be considered as indication of the flow instability and close laminar-turbulent transition. Although the obtained solutions have the same boundary values of velocity components and their first derivatives, these solutions differ by the second derivative of the axial velocity. This second derivative is generally proportional to the axial component of the boundary acceleration. Consequently, the mentioned difference can be associated with disk oscillation/vibration and can exhibit the minimum stimulating acceleration as function of Reynolds number. Although the flow bifurcation can be associated with the laminar-turbulent transition, such analysis of viscous flow gives no information on principal flow modes that may appear due to this stimulation. These principal modes can be found in the framework of potential theory. According to many experimental and numerical researches, the flow between rotating disks is composed from a quite thin boundary layer and the practically inviscid core. The Reynolds-dependent time-averaged circumferential velocity in this core is smaller than the disk rotation speed. Possible modes of flow oscillations and their frequencies depend on the ratio of this circumferential velocity to the rotation speed.

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