An investigation of the stability and control of a combined thermocapillary-buoyancy driven flow

The study of flows driven by thermocapillary forces is a relatively new area within the subject of fluid mechanics with an increasing number of technical applications. As the name implies, thermocapillary flows are those in which the fluid's thermal field interacts with surface tension forces at a liquid-gas interface to drive or modify the velocity field in the bulk of the fluid. For most liquids, surface tension is a decreasing function of temperature, thus surface-temperature gradients result in an effective applied shear stress which drives the surface flow in the direction opposite to the surface-temperature gradient. Historically, flows where thermocapillary forces are important are those with small dimensions such that buoyancy forces do not overshadow thermocapillary effects.; In the present work, combined thermocapillary-buoyancy driven convection in a thin horizontal slot was investigated experimentally, with an emphasis on the generation of hydrothermal-wave instabilities. Hydrothermal-wave instabilities are a convective instability in the form of oblique traveling thermal waves. For sufficiently thin layers (depth {dollar}leq{dollar} 1.25 mm), pure hydrothermal waves were experimentally observed, and were found to be oblique as predicted by a previous theoretical analysis. For thicker layers, both a steady-multicell state and an oscillatory state were found to exist, but the oscillatory state was not in the form of a pure hydrothermal wave. A modified linear-stability analysis was performed in which buoyancy forces were included and the results compared favorably with the experimental findings.; The final stage of the experimental work was an investigation into the suppression of the hydrothermal-wave instability. An open-loop control experiment was developed which used selective heating of the liquid's free surface to suppress the traveling hydrothermal waves. Emission from a CO{dollar}sb2{dollar} laser at 10.6 {dollar}mu{dollar}m was formed into a sheet which was then directed down onto the free surface of the liquid, thus creating a line of applied surface heating. The timing of the heating was directed to coincide with the passing of the cool region of the hydrothermal wave, thus interrupting the instability. Timing and magnitude of the surface heating was accomplished by sensing the temperature signal of the passing hydrothermal waves, and passing a phase-and gain-modified signal to a controller for the CO{dollar}sb2{dollar} laser. Suppression of the hydrothermal waves was demonstrated visually through infrared imaging of the liquid's free surface, providing proof of concept of the control scheme.