LES and URANS Strategies for the Computation of Natural Convection Flow within 3-D tilted Differentially Heated Cavitie

In the present study, both LES and unsteady RANS computations are presented, for turbulent natural convection of air inside differentially-heated rectangular tilted cavities using a finite volume code (Code_Saturne), for an aspect ratio of H/L= 28.6 and Rayleigh number of 0.86106. The two opposing long walls are maintained at uniform and different temperatures. Attention is focused on two angles of inclination, 15 to the horizontal with hot lower and cold upper wall, the 15 unstable case and the mirror image of this case, where the angle is the same but with a hot upper and cold lower wall, the 15 stable case. Subsequent comparisons of 3-D unsteady computations of different RANS models with LES are used in order to explore to what extent these models are able to reproduce the large-scale unsteady flow structures. The LES computations for both the stable and unstable tilted cavities returned three-dimensional time-averaged flow fields. In the case of unstably stratified enclosure, the iso-Q contours suggest that the flow is highly unsteady with coherent turbulent structures in the core of the enclosure. Time-averaged temperature, wall parallel velocity and resolved turbulence intensities resulting from LES computation show close agreement to measured data. All RANS models have been found to be able to reproduce the 3-D unsteady flow features present in the 15 unstable cavity. However, the low Reynolds number models required grids which maintain the resolution within the cavity core, as well as the near wall regions and may therefore be prohibitively expensive for many industrial applications. Flow within the 15 stable cavity also shows some 3-D features, although, its significantly less unsteady. The unsteady RANS models tested have been less successful in reproducing this flow pattern.