This article presents the study of the Source Diagnostic Test fan rig of the NASA Glenn (NASA SDT). Numerical simulations are performed for the three different outlet guide vane (OGV) geometries (baseline, low count, and low noise) and three rotational speeds corresponding to approach, cutback, and sideline operating conditions, respectively. Unsteady Reynolds-averaged Navier–Stokes (URANS) approach is used. The in- and out-duct flow including the nacelle are considered in the numerical simulations, and results are compared against available measurements. Due to the blade count of the fan and OGVs (22 fan blades and either 54 or 26 blades for the OGVs), the simulation can only be reduced to half the full annulus simulation domain using periodic boundary conditions that still represents a significant cost. To alleviate this issue, a URANS with phase-lagged assumption is used. This method allows to perform unsteady simulations on multistage turbomachinery configurations including multiple frequency flows with a reduced computational domain composed of one single-blade passage for each row. The large data storage required by the phase-lagged approach is handled by a compression method based on a proper orthogonal decomposition (POD) replacing the traditional Fourier series decomposition (FSD). This compression method improves the signal spectral content especially at high frequency. Based on the numerical simulations, the flow field is described and used to assess the losses generated in the turbofan architecture based on an entropy approach. The results show different flow topologies for the fan depending on the rotational speed with a leading edge shock at high rotational speed. The fan boundary layer contributes strongly to losses with the majority of the losses being generated close to the leading edge.