This paper presents the application of a two-phase computational fluid dynamics (CFD) model to carry out a detailed investigation of the nonlinear wave field surrounding a pair of columns placed in the tandem arrangement in the direction of wave propagation and corresponding harmonics. The numerical analysis is conducted using the unsteady Reynolds-averaged-Navier-Stokes/volume of fluid (VOF) model based on the openfoam framework combined with the olaFlow toolbox for wave generation and absorption. For the simulations, the truncated cylinders are assumed vertical and surface piercing with a circular cross section subjected to regular, non-breaking fifth-order Stokes waves propagating with moderate steepness in deep water. Primarily, the numerical model is validated with experimental data for a single cylinder. Future, the given simulations are conducted for different center-to-center distances between the tandem large cylinders. The results show the evolution of a strong wave diffraction pattern, and consequently, high wave amplification harmonics around cylinders are apparent.