Experimental investigation of a sheared thermally unstable boundary layer*

In many natural and industrial systems heat is transported by mixed convection,  where an externally driven flow causes advection of the temperature field in addition to the fluid motion due to buoyancy of differentially heated areas.  We investigate experimentally the heat flux across a thermally unstable boundary layer subject to a shear flow. In the experiment, we use a two meter long aluminum plate that is subject to a shear flow above its heated surface. For varying Richardson and Rayleigh numbers, we measure the heat flux from the plate, as well as the vertical velocity and temperature profiles above it by using hot
wire anemometry and thermistors arrays. The fluid is air at atmospheric pressure, resulting in a Prandtl number of Pr = 0.75. Goal of this research is not only to investigate the transition from fully forced to dominantly free convection, but we also hope to better understand the difference between a shear driven turbulent boundary layer and a laminar boundary layer which is perturbed by thermal plumes. The later question is of great importance for understanding the ultimate regime of Rayleigh-B\'enard convection.