Highly Resolved LES and URANS of Turbulent Buoyancy-Driven Flow Within Inclined Differentially-Heated Enclosures

Abstract

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 0.86×106. 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. In accordance with recent experimental data, the LES computations for both the stable and unstable tilted cavities returned three-dimensional time-averaged flow fields. In the case of the unstably stratified enclosure, the flow is highly unsteady with coherent turbulent structures in the core of the enclosure. Time-averaged temperature, velocity and resolved turbulence intensities resulting from LES computations show close agreement to measured data. Subsequent comparisons of different URANS schemes with LES are used in order to explore to what extent these models are able to reproduce the large-scale unsteady flow structures. All URANS schemes 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 model tested, as well as requiring a high resolution near-wall grid, also needed a finer grid in the core region than the high-Reynolds-number models, thus making it computationally very expensive. Flow within the 15° stable cavity also shows some 3-D features, although it is significantly less unsteady, and the URANS models tested here have been less successful in reproducing this flow pattern. The overall heat transfer is presented here for both differentially heated enclosures.