Thermo-acoustic instabilities in high power density gas turbine engines have to be predicted in order to avoid unexpected shutdown events. To predict these instabilities, the acoustics behavior of the combustion system needs to be analyzed. The work presented in this paper on combustor-turbine interaction is focused on reflection coefficient analysis. The study is based on a simplified two-dimensional (2D) geometry representing the vane section and another geometry corresponding to a real engine alike combustor/turbine design. Compressible Large Eddy Simulation (LES) is applied based on the open source Computational Fluid Dynamics package OpenFOAM. A forced response approach is used imposing a sound wave excitation at the inlet of the combustion chamber. The applied Non-Reflecting Boundary Conditions (NRBC) are verified for correct behavior and plausibility of the acoustic set up. Multi-harmonic excitation with small amplitudes is used to preserve linearity. The numerical results are compared to analytical formulae in order to test the validity of both approaches for the chosen geometries.