Experimental study of the penetrative convection in gases

We present an experimental study of penetrative convection in gases, which allows reaching a lower Prandtl number Pr = 0.7 than classical water experiments. A heavy gas (SF6) fills the bottom of a meter size rectangular tank. Diffusion of SF6 in air establishes a stratified vertical profile in part of the domain. Electric resistors heat the bottom plate of the tank up to a chosen temperature while water circulation fixes the top plate temperature at room temperature. After a short transient, the SF6-rich lower layer convects and progressively invades the upper stably-stratified layer while exciting propagating internal gravity waves. Density and PIV measurements are performed, allowing to follow the growth of the convective region as well as the characteristics of the internal waves and convective fields. While mixing at the interface is often modelled in classical penetrative convection with an eddy diffusivity, we find that the convective layer growths here quadratically in time, which contradicts a stationary and homogeneous turbulent diffusion. The analysis of the wave spectra in the stratified domain reveals a peak frequency at the buoyancy frequency, with a clear cut-off above. Running PIV in both layers simultaneously allows us to compute the kinetic energy transfer from the convective zone into the stratified one, which amounts to about 4%. We also define a new scaling law for the erosion rate as a function of the Froude number.