This study presents a numerical investigation through Computational Fluid Dynamics (CFD) of the behavior of the lateral liquid sloshing inside a cylindrical tank under normal gravity. The analysis considers the presence of a single baffle ring at different filling levels. A validation analysis is conducted on a cylinder with smooth walls to determine the accuracy of the methodology employed and the mesh convergence level. CFD data are then used to estimate the parameters of a pendulum Reduced Order Model (ROM), which is able to provide results in good agreement with the literature. Moreover, the results suggest the need for a modified model including an additional cubic spring parallel to the dashpot, in order to take into account non-linear damping effects generated by the vortex shedding at the tip of the anti-sloshing device.