Turbulent natural convection in a horizontal fluid layer with time dependent volumetric energy sources.

The nature of developing and decaying turbulent convection in an internally heated fluid layer following a step change in volumetric energy generation is studied analytically. The layer is bounded from below by a rigid, zero heat flux surface and from above by a rigid, isothermal surface. Initially, the layer is in steady turbulent thermal convection. With a step change in Rayleigh number, transition to a new steady state occurs. The transient behaviour of the layer is determined by two different approaches based, respectively, on a simple boundary layer analysis and a simple eddy heat transport model. The time needed for the layer to reach its final equilibrium state is obtained. This is found to be quite different in the developing process from that in the decaying process. Some transient turbulent heat transfer quantities are calculated and discussed. Comparison is made with experimental data for the case in which the volumetric heat generation of the layer drops suddenly to zero. (A)