Turbulence and internal waves in side-heated convection

We present an experimental study of the convective motion of gas contained in a cubic cell heated from the side. The Rayleigh number (Ra) is varied from 4×105 to 1×1011 by changing the pressure of the gas. Using local temperature probes and shadowgraph visualization, we observe two distinct types of motion coexistent in the cell: turbulent flow and waves. A turbulent large scale circulation around the periphery of the cell, with side eddies along each plate, is observed for Ra>3×107. The turbulent fluctuations are confined to the regions near the hot and cold plates, while the bulk of the cell is stably stratified. We measure the thermal boundary layer thickness; its scaling with Ra has an exponent close to 2/7, as measured in Rayleigh-Bénard convection. In the central part of the cell, we observe internal waves, with a frequency corresponding to the Brunt- Väisälä frequency of the mean vertical temperature gradient. This system provides a laboratory environment for the study of fluctuation-generated gravitational waves in stratified gases.