# Séminaire de Mécanique d'Orsay

## Le Jeudi 22 juin à 14h00 - Salle de conférences du LIMSI

### Surface switching statistics in a rotating fluid : effects of system internal noise

## Yuji Tasaka

Laboratory for Flow Control, Hokkaido University, Japan

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.

Reference

T. Suzuki, M. Iima and Y. Hayase, Phys. Fluids, Vol. 18 (2006) 101701

Y. Tasaka and M. Iima, J. Fluid Mech., Vol. 636 (2009) pp.475-484.

M. Iima and Y. Tasaka, J. Fluid Mech., Vol. 789 (2016) pp.402-424.

Y. Tasaka and M. Iima, Phys. Rev. E (accepted)