Abstract: Understanding the complex thermal-hydraulic behaviors in nuclear reactor cores requires precise modeling of flow transition within rod bundle geometries. In this study, numerical simulations were performed to investigate the fluid flow characteristics inside a 37-pin polymethyl methacrylate (PMMA) rod bundle, which was previously employed in physical pressure distribution measurements. Three distinct numerical approaches were evaluated: the laminar flow model, the SST k-ω turbulence model coupled with the γ-Re_θ transition model (SST-GRT), and the SST k-ω model with low-Reynolds-number correction (SST-LOWRE). The simulation results indicate that the laminar model can accurately predict the pressure drop within the rod bundle when the bundle Reynolds number ( Re_\textb ) is less than 830. However, a deviation was observed under very low-flow conditions. At Re_b=53, the friction factor of the rod bundle deviates from the classic linear Darcy’s law by more than 10%, highlighting the presence of localized, microscopic turbulent flow structures in the tightly packed rod bundle even in a deep laminar state. As the flow velocity increased past Re_b＞950, turbulence began to develop and dominate the flow. Consequently, the pressure loss and friction factor calculated by the laminar model started to fall significantly below the experimental data, owing to the model’s inability to account for turbulent momentum diffusion and Reynolds stresses. Consequently, the pressure drop and friction factor calculated by the laminar model start to underpredict the experimental data, owing to the inability of the laminar model to account for turbulent momentum diffusion and Reynolds stresses. Furthermore, the transitional performance of the two advanced models was analyzed. The SST-LOWRE model captured the development of the laminar-to-turbulent transition process. However, the predicted critical transition Reynolds number was higher than the experimentally observed value. Most notably, the SST-GRT model failed to predict the transition, as it could not effectively activate the turbulent kinetic energy. As a result, its transport equations mathematically degenerated into standard molecular viscous transport equations, yielding results that completely coincided with those of the laminar model. These findings provide valuable references for the selection of turbulence models targeting flow transition within complex fuel assembly geometries.