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The aim of this paper is to contribute to the understanding and to model theprocesses controlling the amplitude of the wind of Rayleigh-Benard convection.We analyze results from direct simulation of an L/H = 4 aspect-ratio domainwith periodic sidewalls at Ra = 1e5; 1e6; 1e7; 1e8 and at Pr = 1 by decomposingindependent realizations into wind and fluctuations. It is shown that deepinside the thermal boundary layer, horizontal heat-fuxes exceed the averagevertical heat-fux by a factor 3 due to the interaction between the wind and themean temperature field. These large horizontal heat-fluxes are responsible forspatial temperature differences that drive the wind by creating pressuregradients. The wall fluxes and turbulent mixing in the bulk provide damping.Using the DNS results to parameterise the unclosed terms, a simple modelcapturing the essential processes governing the wind structure is derived. Themodel consists of two coupled differential equations for wind velocity andtemperature amplitude. The equations indicate that the formation of a windstructure is inevitable due to the positive feedback resulting from theinteraction between the wind and temperature field. Furthermore, the windvelocity is largely determined by the turbulence in the bulk rather than by thewall-shear stress. The model reproduces the Ra dependence of wind Reynoldsnumber and temperature amplitude.

Wind and boundary layers in Rayleigh-Benard convection. I: analysis and modeling