This chapter describes the basic physical models that Ansys Fluent provides for fluid flow and the commands for defining and using them. Models for flows in moving zones (including sliding and dynamic meshes) are explained in Modeling Flows with Moving Reference Frames, models for turbulence are described in Modeling Turbulence, and models for heat transfer (including radiation) are presented in Modeling Thermal Energy. An overview of modeling species transport and reacting flows is provided in Modeling Species Transport and Finite-Rate Chemistry, details about models for species transport and reacting flows are described in   Modeling Species Transport and Finite-Rate Chemistry – Modeling a Composition PDF Transport Problem, and models for pollutant formation are presented in Modeling Pollutant Formation. The discrete phase model is described in Modeling Discrete Phase, general multiphase models are described in Modeling Multiphase Flows, and the melting and solidification model is described in Modeling Solidification and Melting. For information on modeling porous media, porous jumps, and lumped parameter fans and radiators, see Cell Zone and Boundary Conditions.