Momentum Transport Under Various Flow Conditions

The linear momentum of a moving fluid can be transported, like thermal energy or solute concentration, simultaneously by diffusion and convection: an important difference in these latter cases is that momentum is a vector quantity, whereas temperature and concentration are scalars. This chapter gives a simplified microscopic model of the viscosity which is the proportionality coefficient for momentum transfer and then compares the relative effectiveness of convection and diffusion mechanisms, which results in a definition of the Reynolds number and of similar dimensionless combinations for thermal and mass transport. Illustrations of the variation of flow properties with the Reynolds number for the flow in a tube and around obstacles are presented.