The “Ultimate State” in turbulent Rayleigh-Benard convection

Speaker:

Guenter Ahlers - UCSB

Many geophysical and astrophysical phenomena involve turbulent Rayleigh-Benard convection (RBC) at Rayleigh numbers (dimensionless temperature differences) Ra > 10^20. Laboratory studies of RBC for the most part have been limited to Ra < 10^12. It is expected theoretically that the basic physical processes involved in this phenomenon will change fundamentally for Ra near 10^14. Thus extrapolation from laboratory results to the parameter values characterizing many phenomena in nature at best is highly uncertain. This talk will report on efforts to extend the experimentally explored Ra range to Ra ~ 10^15.

The experiments were carried out in a new facility for the study of RBC at the Max Planck Institute for Dynamics and Self-Organization in Gottingen, Germany. Measurements of the heat transport in the dimensionless form of the Nusselt number Nu, and of the turbulent fluctuations as expressed by a Reynolds number Re, were made over the Rayleigh-number range 10^12<Ra<10^15. We used gaseous SF6 as the fluid and pressures up to 19 bars for a cylindrical sample of height L = 2.2 m and diameter D = 1.1 m in a new High-Pressure Convection Facility (HPCF). The data reveal a transition in the dependence of Nu and Re on Ra over the range 10^13 <Ra < 5 × 10^14. For larger Ra Nu(Ra) and Re(Ra) were found to be consistent with the predictions of Kraichnan and Grossmann and Lohse for an ultimate (asymptotic) state of RBC.