Many other phenomena affect fluid flow. These include temperature differences which lead to heat transfer and density differences which give rise to buoyancy. They, and differences in concentration of solutes, may affect flows significantly or, even be the sole cause of the flow. Phase changes (boiling, condensation, melting and freezing), when they occur, always lead to important modifications of the flow and give rise to multi-phase flow. Variation of other properties such as viscosity, surface tension etc. may also play important role in determining the nature of the flow. With only a few exceptions,these effects will not be considered in this book.