This paper presents preliminary results of a computational study conducted to analyze the turbulent flow in the lower plenum of an advanced next generation gas-cooled reactor. The turbulence models used in the current simulations are the Detached Eddy Simulation (DES) model and the transient RANS (Reynolds Averaged Navier Stokes) model. The current study is limited to flow in a row of confined cylinders designed to mimic a model of a prismatic gas-cooled reactor lower plenum design. The experimental configuration consists of a finite array of short graphite posts supporting the reactor core. Five cylinders, which represent vertical support posts in the lower plenum of an advanced reactor concept, are emplaced on the cross-stream centerline. In the current work, an idealized model was used to model a region of the lower plenum for a simplified set of conditions that enabled the flow to be treated as an isothermal, incompressible fluid with constant properties. The simulated results are compared with available experimental data, which were obtained using three-dimensional Particle Image Velocimetry (PIV). The two-equation realizable k-ε model is used as the baseline model for both the Unsteady Reynolds Averaged Navier Stokes Equations (URANS) as well as the DES simulations. The flow unsteadiness accounts for the fluctuations due to unsteady vortex shedding. The DES simulations predicted the flow unsteadiness more accurately than the URANS simulations. The simulated time-averaged quantities were also compared with the experimental data. The RANS simulations and the DES simulations provide almost same predictions for the time averaged quantities. The predicted results show discrepancies with the experimental results.

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