Experimental study of convective structures in rotating fluids

We describe a series of laboratory experiments to study convective structures in rotating fluids (distilled water) in ranges of Rayleigh flux number Ra f from 10 6 to 2 × 10 11 and of Taylor number Ta from 10 6 to 10 12. An intermediate quasi-stationary ring pattern of convection was found to arise from the interaction of the onset of convection with the fluid spin-up, for which we determined the times of origin and destruction, the distances between the rings, and the diameter of the central ring in terms of Ra f and Ta. The ring structure evolves into a vortex grid which can be regular or irregular. In terms of Ra f and Ta the regular grid exists in the linear regime, when the number of vortices N is in accord with the linear theory, when N h^-2 Ta^1{3} ^2{3}, or in the nonlinear regime when N ∝ h −2 Ta ½ Ra f −⅙ ∝ where Ω is the angular velocity and h is the fluid depth. In the irregular regime we always have N ∝ Ω. The transition from the regular regime to the irregular one is rather gradual and is determined by the value of the ordinary Rayleigh number, which we found to be greater than the first critical number Ra ∝ Ta 2/3 by a factor about 25–40. In the transition region vortex interactions are observed, which start with rotation of two adjacent vortices around a common axis, then the vortices come closer and rotation accelerates, following which the vortices form a double helix and then coalesce into one stronger vortex. Some other qualitative experiments show that if the rotating vessel with the convective fluid is inclined to the horizontal, the vortex grid is formed along the rotation axis in accordance with the Proudman–Taylor theorem.