2D RANS and LES Simulations of Turbulent Flow inside a Differentially Heated Square Cavity

A series of analytical and experimental studies have been conducted at the Paul Scherrer Institut (PSI) to investigate the interactions between turbulent flow and particles inside a model containment consisting of a differentially heated cubical cavity (DHC). Analytical efforts have involved 3D Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES) of the flow at Rayleigh number (Ra) of 109 using spectral and spectral element codes, respectively. While providing deep insights of the flow dynamics, these 3D simulations are very time-consuming, and their potential extension to higher Ra numbers typical of reactor conditions is not possible with current CPU resources. In this study therefore, we employ RANS and LES in 2D using the commercial code ANSYS Fluent and compare the fluid flow results against the previous 2D DNS database. We show that the RANS predictions deviate substantially from the DNS data, missing the locations of the recirculation regions along the hot/cold walls. More importantly, the turbulence parameters (turbulent kinetic energy, Reynolds stresses) are not well captured by the RANS models despite the rather simple geometry of the problem. On the other hand, the 2D LES compares well with the 2D DNS database for the mean flow as well as turbulent statistics. This comes at the expense of a much larger computational effort compared to RANS. However, the 2D LES methodology can still be extended with the current CPU resources to tackle higher Ra numbers, and hopefully approach turbulence levels that are comparable to those of nuclear containment application