CFD (computational fluid dynamics) calculations have been performed for turbulent flow inside a plenum model that resembles a section of the lower plenum in a typical prismatic VHTR (Very High Temperature Reactor). Different RANS (Reynolds Averaged Navier-Stokes) based turbulence models are employed to investigate the capability in capturing unsteady large scale coherent structures due to vortex shedding at two different Reynolds numbers. A grid convergence study is conducted with those models which were able to capture unsteady vortex shedding. The non-linear interaction of mesh quality, turbulence model and numerical scheme lead to flow regime changes with significantly different unsteady behavior. This makes it difficult to assess numerical and modeling uncertainty using the procedure that is recommended for Journal of Fluids Engineering (JFE). Some remedies to overcome this difficulty are recommended. The numerical uncertainty in the local values of velocity components at selected locations inside the plenum, as well the uncertainty associated with derived quantities such as wall shear stress at critical locations are calculated and reported. Since there are no experiments corresponding to the present cases simulated, the current analysis can be considered as a blind application of the proposed uncertainty estimation procedure.

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