Visualization of longitudinal convection roll instabilities in an inclined enclosure heated from below

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Abstract
Experiments have been performed on the stability of buoyancy-driven flows of a high-Prandtl-number fluid in an inclined rectangular enclosure. Visualization of the stable planform of convection for various Rayleigh numbers and inclination angles is provided by a temperature-sensitive liquid crystal and gold-coated film heater assembly which serves as the lower surface of the enclosure. This assembly produces a nearly constant heat flux surface with a thermal conductivity of the same order as that of the test fluid. The results indicate that for large angles of inclination from the horizontal a steady transverse roll(s) structure is stable. As the angle of inclination is decreased steady longitudinal rolls replace the transverse roll(s) and for low angles a steady square-cell convection planform is observed. A region of unsteady wavy longitudinal rolls is also observed at sufficiently high Rayleigh numbers for low to moderate angles of inclination. In general the wavenumber of the longitudinal rolls increases with angle of inclination from the horizontal. Two distinct types of instability mechanisms are observed which modify the wavenumber of the longitudinal rolls: a cross-roll instability, which is a disturbance perpendicular to the original roll axis; and a pinching mechanism which combines two neighbouring longitudinal roll pairs into a longer wavelength roll pair.