Turbulent onset in moderately large convecting layers

We present long-time thermal data on turbulent evolution in Rayleigh-Bénard convection for cylindrical containers of aspect ratios $\Gamma =6.22\mathrm{} \mathrm{and} 7.87$ and Prandtl numbers near 0.6. The first time dependence observed was more complex than the intermittent flows reported by Ahlers and Walden for $\Gamma =4.72\mathrm{}$, although the periodicity they observed was reproduced for $\Gamma =7.87\mathrm{}$. The turbulent onset for $\Gamma =6.22\mathrm{}$, showing substantial regimes of periodicity, was quite different from that for $\Gamma =7.87\mathrm{} or 4.72$. We conclude that changes of order unity in $\Gamma$ strongly affect turbulent onset, even for moderately large aspect ratios.