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Experiments on Thermally Driven Convection

Summary: Experiments on Thermally
Driven Convection
Guenter Ahlers
Experimental results for thermally driven convection in a thin horizontal
layer of a nematic liquid crystal heated from below or above and in a mag-
netic field are reviewed and, when possible, compared with theoretical cal-
For the case of planar alignment in a horizontal field, convection occurs
only when heating is from below. The threshold is lowered dramatically by a
heat-focusing mechanism. There is excellent agreement between experiment
and theory for those properties which depend only on the linear terms in
the equations of motion, namely for the primary bifurcation line and the
wavevector as a function of magnetic field. Predictions for the location of
and behavior near two Lifshitz points are quantitatively confirmed by the
experiments. For the nonlinear properties, agreement with weakly nonlinear
theory is not so satisfying. The predicted tricritical points have not been
found in the measurements, and at modest fields the primary bifurcation
appears to be supercritical and to lead to a state of spatio-temporal chaos
instead of being subcritical. The experiments reveal an interesting param-


Source: Ahlers, Guenter - Department of Physics, University of California at Santa Barbara


Collections: Physics