Oscillations of the large scale wind in turbulent thermal convection

Abstract

The large scale "wind of turbulence" of thermally driven flow is analyzed for very large Rayleigh numbers between 4-1011 and 7-1011 and Prandtl number of 0.71 (air at 40°C) and aspect ratios order of one. The wind direction near the upper plate is found to horizontally oscillate with a typical time scale very similar to the large eddy turnover time. The temporal autocorrelation of the wind direction reveals an extremely long memory of the system for the direction. We then apply and extend the dynamical model of Gledzer, Dolzhansky, and Obukhov to the flow, which is based on the Boussinesq equations in the bulk and which can be solved analytically in the inviscid and unforced limit, but which completely ignores the boundary layer and plume dynamics. Nevertheless, the model correctly reproduces both the oscillations of the horizontal wind direction and its very long memory. It is therefore concluded that the boundary layers and the plumes are not necessary to account for the oscillations of the wind direction. The oscillations rather occur as intrinsic precession of the bulk flow. © 2006 American Institute of Physics.