BROADENING OF CONVECTIVE CELLS

The turbulent convection in a dry Boussinesq fluid above a heated surf ace is studied in a domain of large aspect ratio and long duration. La rge-eddy simulations are used to investigate the influence of the dyna mical character of the lower boundary and the effect of cooling at the top on the growth of convective cells. The aim is to identify physica l mechanisms that lead to the broadening of convective cells even unde r conditions where moisture processes play no role. Cooling at the top produces a vertically uniform heat-flux, and it acts to enhance the t otal kinetic energy of the whole now. Essentially, there are two compe titive effects: first, the prescribed heat-flux at the upper (lower) b oundary produces very warm (very cool) fluid that is forced to rise (s ink). The resulting large temperature-fluctuations are transported by large-scale motions and cause broad temperature-variances independent of height. On the other hand, the strong turbulent mixing (mainly near the boundaries where the turbulent kinetic energy is maximum) tries t o homogenize the now structure in such a way that the resulting temper ature-distribution is uniform. Except directly close to the walls, hor izontal and vertical temperature-gradients are reduced. Additionally, a trend to form an organized large-scale horizontal drift in one direc tion near the bottom and in the opposite direction near the top was fo und for runs without surface friction. This horizontal streaming motio n has two effects: firstly, the turbulent mixing is enhanced due to la rger shear and, secondly, separated thermals approach faster and are a ble to merge more easily, forming gradually growing cells. An adiabati c upper boundary condition leads to a heat-nux profile that is linear and decreasing with height, and to a moderate temporal growth of therm al structures. A considerable scale-reduction of the temperature struc tures occurs because of friction at the lower surface.