ABSTRACT
The study of fluid dynamics starts with the concept of viscosity. The first model for the viscosity of gases is based on the kinetic theory. It assumes that gases are made up of rigid spherical molecules that move around randomly at an average speed that depends on temperature. For liquids, the mechanism of viscosity is fundamentally different from that for gases. The momentum equation expresses conservation of momentum in a fixed volume of space. Momentum can be transported into or out of the volume in two ways: by convection or by molecular forces. The transport of heat and species in fluids involves convection and molecular diffusion, and therefore obey equations that are quite similar to the momentum equation. While analytical solutions have been found for many simple laminar flow problems, turbulent flows are far too complex and must almost invariably be solved by numerical methods or computational fluid dynamics.
