URANS prediction of flow fluctuations in rod bundle with split-type spacer grid

Turbulent flow in a rod bundle with split-type spacer grid has been studied using Unsteady Reynolds-Averaged Navier–Stokes (URANS) approach. In the previous studies of turbulent flow in rod bundles URANS (as well as steady-state RANS) simulations predicted mean velocity profiles fairly well. However, they severely underpredicted velocity fluctuations, which is investigated in the present study. Our simulations were performed with the Shear Stress Transport (SST) turbulence model and automatic wall-treatment using OpenFOAM, an open-source CFD code. Results of URANS simulations are compared with the measurements of the MATiS-H experiment, which was performed at Korean Atomic Energy Research Institute (KAERI) in 2011–2012.The URANS predictions of velocity fluctuations have been improved by appropriately summing up fluctuations resolved by the basic URANS model and non-resolved fluctuations, which were modelled with the turbulence model. This treatment of turbulent fluctuations, which are directly measured in high-quality experiments, allows more detailed evaluation of various URANS turbulence models. It was found out that the best agreement is achieved when resolved and modelled fluctuations are assumed to be uncorrelated, which indicates that the large-scale structures in this particular flow are distinct in the spectral space from the rest of turbulence. Turbulent flow in the MATiS-H experiment was reproduced by numerous authors using different approaches and our results are among the most accurate.