The accurate prediction of turbines performance and flow fields requires the assessment of unsteady numerical simulations. This paper presents a numerical study on the interaction between a single-stage high-pressure turbine and the first vane row of a low-pressure turbine. It focuses on the simulation of the flow within the interturbine duct and the loss generated in the downstream low-pressure vane. Former experiments provided steady and unsteady measurements in the interturbine duct and after the low-pressure vane. A 3D unsteady Reynolds-averaged Navier–Stokes (URANS) approach with phase-lagged boundary conditions is used to characterize the unsteady periodic effects in the interturbine channel and downstream in the low-pressure vane. For the numerical study, two different configurations are considered: a single-stage high-pressure turbine configuration and a high-pressure rotor coupled with a low-pressure vane. For the second one, two inlet boundary conditions are implemented upstream of the rotor: a circumferentially uniform boundary condition and a circumferentially nonuniform rotating boundary condition. The resulting flow fields are compared within the intermediate duct. A harmonic Fourier analysis is carried out to underline the effects of the high-pressure rotor. An unsteady Adamczyk decomposition of the flow field within the duct gives the influence of the different components and the levels of unsteadiness. Comparisons with experimental data show a reasonable good agreement.