# Numerical and Experimental Analysis of the Oil Flow in a Planetary Gearbox
## 1 Introduction
#### Literature Review
The adopted MRF technique is limited to a regime solution of an unsteady problem without information on the transient behavior. Despite low computational effort, the approach lacks an accurate description of multiphase operating conditions and oil flow phenomena. More advanced numerical techniques, such as the sliding mesh approach and the dynamic remeshing, are suitable for simulating gear rotation and interaction. FZG test rigs are used for experimental studies on efficiency and lubrication [13]. [PERSON] et al. [14; 15]implemented a CFD model of a planetary gearbox studying the no-load power losses for different oil temperatures, immersion depths, and planet carrier rotational speeds. The results were validated with experimental data in terms of power losses. Further studies by [PERSON] et al. [16; 17; 18] introduced an innovative remeshing procedure in the opensource CFD code OpenFOAM(r)[19] for the simulation of the gear meshing. The strategy is based on the computation of extruded numerical grids connected through numerical interfaces to link the non-conformal nodes. When the quality of the mesh decreases, a new mesh is computed for the new gear position. The results are interpolated and the simulation proceeds. The virtual models could predict the efficiency of the system accurately. This algorithm requires the generation of a new mesh whenever bad-quality elements originate. This means that hundreds of meshes may be needed to complete the simulation. [PERSON] et al. [20] implemented a CFD model based on the overset mesh approach (also called the overlapping grids method, or chimera framework) [21; 22] of a planetary gearbox. Filling levels of 30%, 50%, and 70% of the gearbox volume were analyzed. Owing to the numerical analysis, the system design could be optimized and the thermal behavior could be determined. The authors suggest using high filling levels to increase the cooling effects despite the higher churning losses. The commercial software STAR-CCM+ [23] was used. [PERSON] et al. [2] investigated the oil distribution of the FZG internal gear test rig, which is also the object of this study, for different oil properties and planet carrier speeds in detail. The finite volume-based CFD implementation included a dynamic remeshing approach to describe the rotation and engagement of the gears. They performed multiphase simulations by applying the Volume of Fluid (VoF) method. The roller bearings of the planet gear shafts were also modeled in the numerical simulation, but their kinematics were described with a pure rotation around the central axis of the planetary gearbox and not with a roto-translation, which resulted in doubling the computational time. They performed a mesh sensitivity analysis with respect to loss torque and oil distribution to obtain an efficient yet accurate modeling approach. The high level of detail of the developed CFD model allowed for the extraction of information on the oil flow inside the gearbox. [PERSON] et al. [2] indicate that with a rather low filling level, it is possible to supply enough oil to the different machine elements. The commercial software ANSYS FLUENT [24] was used. The simulations were parallelized on 80 cores of a high-performing computer cluster. As it can be noticed from the literature review on planetary gearboxes, the experimental studies are limited, and the numerical analyses mostly lack validation, particularly concerning the validation of the oil flow, since the internal part of the gearbox could not be visibly accessed. Moreover, significant computational resources were required to run such simulations. To increase confidence in simulation results, validation by experimental investigations in terms of oil flow is fundamental. Furthermore, efficient remeshing strategies can reduce the computational effort required for finite volume-based CFD simulations. This work fits into these objectives, providing side-by-side comparisons of high-speed camera recordings and CFD results of the oil distribution.