Turbulent jets are commonly used in engineering applications. Systems of multiple parallel jets have an important flow structure that could accomplish rapid and efficient mixing. The mixing feature of parallel jets has several engineering applications, such as its application in Generation IV very-high-temperature nuclear reactors, where the coolants merge in the upper or lower plenum after passing through the reactor core. Computational fluid dynamics (CFD) simulations are extensively employed in the study of the mixing phenomenon of parallel jets. Therefore, the validation of various turbulence models is of great importance to the process of ensuring that the numerical results are trustworthy and that they serve as a guide for future designs. In this study, an open source CFD library, namely OpenFOAM, was utilized to conduct the numerical simulation of the twinjets. This work consists of two parts; one part focuses on steady-state simulations, and the other on transient simulations. In the first part, the Reynolds-averaged Navier-Stokes (RANS) models, such as the realizable k-epsilon and the shear stress transport (SST) k-omega, were used for the steady-state validation study. Steady-state simulations showed that with proper boundary conditions at the inlets, the mean velocity data agreed with the experimental data well within an engineering accuracy (14%). In the second part, the partially averaged Navier-Stokes (PANS) models were implemented in the code, and were utilized to conduct transient simulations. Experimental fluctuating inlet boundary conditions were employed. The results obtained from the PANS and the unsteady Reynolds-averaged Navier-Stokes (URANS) models were compared with the experimental data. The PANS model presented good agreement in terms of the merging point (4.3%). In addition, the k-epsilon PANS model was compared with the k-epsilon URANS model. Power spectrum density (PSD) analysis was performed based on the velocity at four sample locations to compare resolved frequencies between the PANS and the URANS models. It was observed that PANS model presented better capabilities in resolving higher turbulence flow frequencies compared with the URANS, based on the PSD analysis. (C) 2017 Elsevier Ltd. All rights reserved.