Geared turbofan (GTF) technology is promising for reducing specific fuel consumption, jet noise, and engine emissions, making gearbox efficiency crucial. Power losses in gearboxes are either load-dependent (mechanical) or load-independent (fluid-dynamic), with the latter needing better modeling. This thesis investigates fluid-dynamic losses in gear lubrication for GTF engines, where lubrication is achieved through an oil jet impacting the gear teeth. In the literature, similar problems are studied through experiments and numerical simulations, primarily using Eulerian CFD solvers for their precision in describing physical behaviour at a microscopic level. Simulating an oil jet lubrication problem involves managing a multiphase system in a moving domain, which is computationally demanding with Eulerian CFD using the VOF method and moving mesh techniques.