Flow generated by colliding laminar natural convection boundary layers

This paper describes an experimental study of the transient development of flow generated by the collision of two natural convection boundary layers in a rectangular enclosure for Rayleigh numbers (defined by the height of the enclosure and overall temperature difference) in the range ∼5x105 to ∼3x108. The enclosure contained either Silicone oil or water as the working fluid and one vertical wall comprised a heated plate on the bottom half and a cooled plate on the top half. Measurements are presented of the transient velocity field as determined using particle tracking velocimetry and of fluid temperature near the boundary layer collision zone. These results indicate that there are three phases of development of the flow field. Firstly, the laminar boundary layers develop, resulting in a steady jet of fluid emanating from the collision zone. Secondly, a fingering instability arises initially, it would appear, as a result of the arrival of the boundary layer leading edge flow at the collision zone. Thirdly, this instability decays in the case of low Rayleigh number or develops into a quasi-oscillatory flow in the alternative.