The surface switching phenomenon is an irregular switching of free surface shapes of the fluid in an open cylindrical vessel driven by constant-speed rotation of a bottom disk (Suzuki et al. 2016). Unlike the polygonal flow observed in a shallow fluid layer, which rotates regularly, this phenomenon involves an irregular surface switching between non-axisymmetric shapes and an axisymmetric shape, accompanied by significant surface descending events. Special attention is paid to the influence of the disk-rim gap, the gap between the outer edge of the rotating disk and the inner lateral wall of the cylinder. Based on the findings from rotor-stator flows that increasing the disk-rim gap reduces the critical Reynolds number to generate turbulence, we use the disk-rim gap to control the internal noise generated by the flow in the gap region. We establish experimentally that a slight increase in the disk-rim gap causes significant changes in this system, that is, frequent appearance of surface descending events which involve a non-axisymmetric shape with a strong mixing flow (turbulent flow) and an axisymmetric shape with laminar flow, as well as a shift of the critical Reynolds numbers that defines the characteristic states. Furthermore, increasing the number of surface descending events enables us to perform a statistical analysis of the surface descending events. We show that descent events obey a memoryless random process, and that the event is qualitatively different from the quasi-periodic vertical oscillations observed within a non-axisymmetric state in terms of the mean surface descending velocity. These results are used to understand phenomena related to rotating flows with free surfaces.
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