Bénard-Marangoni thermal oscillators: An experimental study

An experimental study of time-dependent Bénard-Marangoni convection in small aspect ratio containers and high Prandtl number fluids is presented. Two similar systems with the same aspect ratio, depth, and type of fluid are compared: one with adiabatic lateral boundaries and the other with open boundaries. In spite of the fact that very different plan forms and flows appear, both systems exhibit an oscillating plan form and a discrete narrow-band frequency spectrum in the temperature signal, having the same threshold and similar frequencies. Coincidence in temporal behavior between physical variables ~temperature or velocity fields! and signals obtained from a distance representative of plan-form evolution or surface curvature is shown in the interval of the control parameter studied. The origin of a time-dependent regime arising from a boundary layer instability is discussed. Plan-form, temperature, and velocity measurements confirm this assumption, showing that this mechanism, first proposed in Rayleigh-Bénard convection, also appears under the conditions of these experiments, in time-dependent behavior of Bénard-Marangoni convection. @S1063-651X~97!04303-1#