Duke Paper Describes Measurements Onset of Convection near a Critical Point

A.B. Kogan and H. Meyer at Duke University have studied convection in 3He near the critical point, where fluids become very compressible and the Prandtl number Pr characterizing the flow becomes large. Their paper "Heat Transfer and Convection Onset in a Compressible Fluid: 3He near the Critical Point" on this subject has been accepted for publication in the Physical Review E. In their experiments, the temperature change deltaT across a thin horizontal fluid layer is measured as a function of the vertical heat current, and the onset of the convection is observed. The crossover from the regime governed by the usual "Rayleigh" criterion (for incompressible fluids) to that of the asymptotic "Schwarzschild" criterion, (for very compressible fluids), has been observed in their experiment and the results agree quantitatively with predictions. The convective currents above the onset have been studied for various values of the Prandtl number, and some unexpected results were obtained at the highest values of Pr (~ 590). Also, the time transients of deltaT (t) upon turning the heat flow on and off are found to change quite dramatically as the compressibility diverges. These results are of interest to two groups of theorists presently doing numerical simulation work, and with whom the Duke group is collaborating.