Transient states during spin-up of a Rayleigh–Bénard cell

Abstract

The influence of impulsive spin-up on Rayleigh–Bénard convection in a cylindrical cell with radius-to-height ratio Γ=0.5 is investigated. Velocity and temperature fields in the thin layer adjacent to the top of the cell are measured by means of particle image velocimetry and thermochromic liquid crystal visualization. The cell, initially in steady nonrotating convection, is rapidly accelerated to steady rotation about its vertical axis with dimensionless rotation rates 4×103⩽Ω⩽8×104. Rayleigh numbers Ra vary from 5×107 to 5×108. In a large part of this domain of the parameter space, axisymmetric structures appear near the top of the cell. In most cases, a single ring forms with a radius close to 3/4 of the radius of the cell. The same experiment without heating produces typical Ekman spin-up devoid of ring structures. The ring is characterized by a local decrease in temperature and azimuthal velocity. As the flow evolves, an azimuthal velocity gradient forming across the ring causes shear instability and leads to the emergence of Kelvin–Helmholz vortices.