On the onset of horizontal convection

The convective flow driven by a differential heating along a horizontal boundary, commonly referred to as horizontal convection, is investigated. In particular, the inception of the convection cell following impulsive application of heating, i.e. the onset of horizontal convection, is studied experimentally in a rectangular box filled with water. Piecewise boundary conditions of uniform/constant heat flux and temperature are imposed along the box length at its bottom wall. The convective heat transfer coefficient on the heated half of the base is evaluated using the heated thin foil sensor modified to account for the unsteady term of the energy balance. The experiments are carried out over a range of Rayleigh numbers (based on heat flux input, box length and initial fluid properties) from 1.6 × 1011 to 1.3 × 1012 and for a Prandtl number (based on the initial water temperature) of approximately 6. Flow visualizations with the shadowgraph technique are also performed to complement heat transfer measurements. Thermocouple data are acquired inside the domain for validation purposes. A scaling for the characteristic time of the transient is proposed and verified. In the range of the investigated Rayleigh and Prandtl numbers, three subsequent phases in the onset process are identified: pure heat conduction through the fluid layer, Rayleigh-Bènard convection with transition of the boundary layer; onset and time evolution of longitudinal rolls. The presence of longitudinal rolls is justified via an analogy with Görtler vortices theory and results show a Nusselt number enhancement on the heated side of the order of 200% with respect to that on the cold one.