Zonal flow excited by circular oscillations of inner solid core in a rotating spherical cavity is experimentally studied. The ratio of the radii of the core and the cavity is equal to 0.5. The core oscillates in the equatorial plane around the cavity axis, with the rotation frequency, under the action of an external static field (gravity field), which is directed perpendicularly to the rotation axis. Differential rotation of the core is absent; for this, one of the core poles is fastened to the nearest cavity pole by fishing line. It is found that the oscillating core excites in the cavity an averaged azimuthal axisymmetric fluid flow. The flow consists of a system of nested coaxial cylindrical surfaces rotating with different angular velocities. Maximums of the lagging differential rotation of the fluid are at the distances of 0.15, 0.50 and 0.90R_2 from the axis, where R_2 is the cavity radius. It is shown that the averaged differential rotation is generated in the oscillatory boundary layers near the boundaries of the core and the cavity. The intensity of the flow increases by the square law with the amplitude of the core oscillations. With increase of the amplitude of the core oscillations the axisymmetric flow becomes unstable: the excitation of 2D vortex system near the rotation axis takes place. Another type of instability is associated with the appearance of an azimuthal wave on the outer shear boundary, which develops independently of the vortex system. The instability of one another type manifests itself in the excitation of 2D system of rolls.
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