Instability of forced flow in a rotating cylindrical pool with a differentially rotating disk on the free surface

The instability of forced flow in a rotating cylindrical pool with a differentially rotating disk on the free surface is investigated through a series of unsteady three-dimensional numerical simulations. The results show that the basic flow state of this system is axisymmetric and steady, but has rich structures at the meridian plane. However, when the rotation Reynolds number exceeds a critical value, the flow will undergo a transition to three-dimensional oscillatory flow, characterized by the velocity fluctuation waves traveling in the azimuthal direction. The main characteristics of the flow patterns are presented, including the propagating direction, velocity, amplitude and wave number, which depend on the rotation rates and directions of the disk and the cylindrical pool, and the critical conditions for the onset of oscillatory flow are also determined. For the case of disk-only rotation, the centrifugal instability is responsible for the flow transition, and when the disk iso- and counter-rotates with the cylindrical pool, the mechanisms for the transition are elliptic and of circular shear instabilities, respectively.