We report temperature cross correlation and velocity profile measurements in the aspect-ratio-one convection cell filled with water. A sharp transition from a random chaotic state to a correlated turbulent state of finite coherence time is found when the Rayleigh number becomes larger than a critical value ${\mathrm{Ra}}_c\simeq 5\times {10}^7$. The experiment reveals a unique mechanism for the onset of coherent oscillations in turbulent Rayleigh-Bénard convection.

• Received 2 May 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.094501