Unsteady natural convection with summer boundary conditions in a habitat at high Rayleigh number and at high time

•Thermal and dynamic fields of a habitat are numerically explored.•The study is done under summer conditions.•Convection had a late start.•The system became very unstable at high time once the convection developed.•There is a rapid change on the flow when the active and the cold walls are close. A numerical study of the thermoconvective instabilities at high time in a habitat filled with Newtonian fluid is conducted. The gable roof of the habitat is subjected to a heat flux of constant density, and its side walls and floor are, respectively, adiabatic and isothermal. Based on the Boussinesq assumptions, the summer thermal and dynamic conditions are numerically studied using unsteady natural convection equations formulated with vorticity and stream-function variables. The finite volume method is used to generate the set of equations, which are solved by the iterative under-relaxation line-by-line method of Gauss–Seidel. Sudden changes in the average Nusselt number and in the extreme values of the stream functions at Ra=1×108 show that the initially unicellular flow in a pseudo-conductive regime becomes a multicellular flow with the emergence and disappearance of cells. In time and space, the change of the flow behaviour is observed more rapidly if the active walls are close to the cold horizontal wall.