A major purpose of hydraulic tanks is the segregation of air, which can be supported by different design measures. To improve these measures CFD multiphase simulation can be used, as it is capable to assess the air flow within the oil. The different possibilities of CFD simulation are presented. Here single-phase flow models, simplified multiphase models as well as full multiphase flow models are discussed and evaluated. An example of each presented method is given and the results are compared. Last the capabilities of validating the simulations on a test bench are briefly discussed.