Due to the rapid growth of the power density in data centers, the thermal management of the data center has become more and more important. Computational Fluid Dynamics has been proven to be one of the most effective tools in the design and analysis in data centers due to its ease of use and fast prediction in data centers. Due to the large size of a data center facility, one of the most challenging tasks in the data center CFD simulation is to establish simplified models to provide a fast but accurate prediction for both the temperature and flow field in data centers. In the past few years, people have proposed many tile models specifically to simulate the jet effect of the perforated tile. However, other aspects of the simulation were still in the early stage, e.g. rack models and the effect of buoyancy forces. This paper mainly studied the effect of the three rack models, which were Black Box Model, Modified Black Box Model, and Volumetric Heat Model. The effects of the buoyancy force were also studied. The result showed that buoyancy force had a huge impact on the simulation result, while the three rack models investigated had little difference.