Breakdown of wind in turbulent thermal convection

We report experiments on turbulent Rayleigh-B\'enard convection of air in a cylindrical large-scale facility with a diameter of 70.3em0exmeters and Rayleigh numbers up to Ra≈10¹². The facility is used to explore the structure of the large-scale circulation for continuously varying aspect ratios between Γ≈1 and Γ≈10. Using autocorrelation functions derived from high-resolution time series of temperature and velocity near the cooling plate we demonstrate that the well-known single-roll structure (often called ``wind'') breaks down when the aspect ratio increases beyond the critical value Γ₁=1.68±0.22. We further show that at Γ₂=3.66±0.46 a second transition from an oscillatory two-roll structure to an unstable multi-roll structure takes place. The value of Γ₂ represents a lower bound for the aspect ratio that is necessary to reach homogeneous convection---a turbulent state that is free from the influence of lateral walls.