Buoyant-thermocapillary instabilities in extended liquid layers subjected to a horizontal temperature gradient

We report experiments on buoyant-thermocapillary instabilities in different ially heated liquid layers. The results are obtained for a fluid of Prandtl number 10 in a rectangular geometry with different aspect ratios. Dependin g on the height of liquid and on the aspect ratios, the two-dimensional bas ic flow destabilizes into oblique traveling waves or longitudinal stationar y rolls, respectively, for small and large fluid heights. Temperature measu rements and space-time recordings reveal the waves to correspond to the hyd rothermal waves predicted by the linear stability analysis of Smith and Dav is [J. Fluid Mech. 132, 119 (1983)]. Moreover, the transition between trave ling and stationary modes agrees with the work by Mercier and Normand [Phys . Fluids 8, 1433 (1996)] even if the exact characteristics of longitudinal rolls differ from theoretical predictions. A discussion about the relevant nondimensional parameters is included. In the stability domain of the waves , two types of sources have been evidenced. For larger heights, the source is a line and generally evolves towards one end of the container leaving a single wave whereas for smaller heights, the source looks like a point and emits a circular wave which becomes almost planar farther from the source i n both directions. (C) 2001 American Institute of Physics.