Comparison of flow fields around the Joubert BB2 submarine under straight-ahead, rising, and diving conditions using RANS and PANS turbulence models

This study presents a comparative computational fluid dynamics (CFD) investigation of the Joubert BB2 submarine under straight-ahead (0 Deg.), rising (+10 Pitch), and diving (−10 Pitch) conditions, focusing on how turbulence modelling influences resistance components and stern-wake characteristics. Simulations were performed using a finite-volume solver in OpenFOAM with the k–ω SST framework, employing both a conventional Reynolds-averaged Navier–Stokes (RANS) closure and a partially averaged Navier–Stokes (PANS) formulation with locally varying resolution control. The numerical methodology was first assessed against benchmark information provided by MARIN using the total resistance level and the propeller-plane wake characteristics, providing a consistent baseline for model-to-model comparisons. Across all attitudes, the predicted total resistance coefficients from RANS and PANS were generally comparable; however, resistance decomposition revealed systematic differences, with PANS tending to yield slightly higher pressure resistance and lower viscous resistance than RANS, particularly under inclined conditions. Model-dependent discrepancies were also observed in the stern wake and propeller-plane inflow: compared with RANS, PANS produced a more pronounced and spatially extended axial-velocity deficit and maintained broader regions of elevated vorticity, indicating different preservation of rotational wake features and coherent vortical structures. Overall, the results show that, even when integrated resistance is similar, the turbulence modelling approach can lead to meaningful differences in component-level resistance and wake topology relevant to downstream propulsor inflow assessment.