Validation Problems in Computational Modelling of Natural Convection

Problems of validating computational models for naturally convective flows are discussed. It is shown that even in the case of laminar flow when all property variations are included in simulations, a good agreement with experimental data can be problematic due to inaccuracy in boundary conditions. For turbulent flows, the validation becomes even more challenging as the difference between experimental and numerical results could be the effect of an inadequate turbulence or subgrid model or the use of inappropriate boundary conditions. As a result, the better agreement with experimental data might be obtained by “improving” the turbulence or subgrid model, or by obtaining “more accurate” boundary conditions, or a combination of the two. This issue is explored using the example of LES in simulating buoyancy-driven flow in a tall rectangular cavity. The challenge of unknown boundary conditions has also been explored for a reduced laboratory model of an open-ended channel heated from one side. In this case, in order to obtain agreement between the numerical and the experimental data, the disturbances needed to be introduced at the inlet of the channel. By extending a computational domain to include the laboratory it was shown that the large structures meandered around the laboratory and disrupted the flow in the channel. Similar large disturbances were also detected experimentally. Therefore, it is shown that for a reduced model only tuning is possible and validation in a classical sense cannot be achieved.