The right atrium (RA) combines flows from the inferior (IVC) and superior vena cava (SVC). Here RA mixing is simulated using computational fluid dynamics, comparing four modeling approaches. A patient-averaged model (11 M cells) was created from four volunteers. We compared: (1) unsteady k–ω Reynolds-averaged Navier–Stokes (URANS) (2) implicit large eddy simulation with second-order upwind convection scheme (iLES-SOU) (3) iLES with bounded-central difference convection scheme (iLES-BCD) and (4) LES with wall-adapting local eddy-viscosity (LES-WALE). A constant inlet flow rate of 6 L/min was applied with both IVC/SVC contributions ranging from 30–70%. A higher density mesh (37 M cells) was also simulated for models 2 and 4 (equal IVC/SVC flow) to assess the accuracy of models 1–4. Results from the 11 M cell LES-WALE model showed good agreement with the 37 M cell meshes. All four 11 M cell models captured the same large-scale flow structures. There were local differences in velocity, time-averaged wall shear stress, and IVC/SVC mixing when compared to LES-WALE, particularly at high SVC flow. Energy spectra and velocity animations from the LES-WALE model suggest the presence of transitional flow. For the general flow structures, all four methods provide similar results, though local quantities can vary greatly. On coarse meshes, the convection scheme and subgrid-scale (SGS) model have a significant impact on results. For RA flows, URANS should be avoided and iLES models are sensitive to convection scheme unless used on a highly resolved grid.