Oscillatory motion in Rayleigh-Bénard convection confined in a thin rectangular container

Rayleigh-Bénard convections (RBCs) generated in a laterally thin fluid layer with a moderate thickness has been investigated recently from purposes to clarify relaxation of two-dimensional (2D), Hele-Shaw confinement and quasi-2D restriction of fully three-dimensional (3D) motion. We performed laboratory experiments using different thickener solutions adjusting Prandtl number Pr to investigate flow transition of the RBC in quasi-2D rectangular containers. The experimental setup was designed to modify the thickness of fluid layer by inserting acrylic plates, which also provides good thermal insulation. Rayleigh number Ra examined is distributed from 1 ⨯ 10⁵ to 4 ⨯ 10⁶ corresponding to the range in between quasi-Hele-Shaw regime and 3D convections according to the criteria recently proposed by Letelier et al. (2019) using permeability ε and Rayleigh-Darcy number, Ra_D, ε² Ra_D << 1. Velocity vector fields obtained by PIV represented the flow transition from elongated quasi-2D plumes to emergence of oscillatory corner rolls. The transition occurs around ε²Ra_D = 1 independent of Pr.