What is Fluid Mechanics?

• Fluid mechanics deals with the study of all fluids under static and dynamic situations.
• Fluid mechanics is a branch of continuous mechanics which deals with a relationship between forces, motions, and statical conditions in continuous material. 
• This study area deals with many and diversified problems such as surface tension, fluid statics, flow in enclose bodies, or flow round bodies (solid or otherwise), flow stability, etc.
• In fact, almost any action a person is doing involves some kind of a fluid mechanics problem. 
• Furthermore, the boundary between the solid mechanics and fluid mechanics is some kind of gray shed and not a sharp distinction (see chart of Fluid mechanics for the complex relationships between the different branches which only part of it should be drawn in the same time.). 
• For example, glass appears as a solid material, but a closer look reveals that the glass is a liquid with a large viscosity. 
• A proof of the glass “liquidity” is the change of the glass thickness in high windows in European Churches after hundred years. 
• The bottom part of the glass is thicker than the top part. Materials like sand (some call it quick sand) and grains should be treated as liquids. It is known that these materials have the ability to drown people. 
• Even material such as aluminum just below the mushy zone also behaves as a liquid similarly to butter. 
• After it was established that the boundaries of fluid mechanics aren’t sharp, the discussion in this book is limited to simple and (mostly) Newtonian (sometimes power fluids) fluids which will be defined later.
• The fluid mechanics study involve many fields that have no clear boundary between them. 
• Researchers distinguish between orderly flow and chaotic flow as the laminar flow and the turbulent flow. 
• The fluid mechanics can also be distinguish between a single phase flow and multiphase flow (flow made more than one phase or single distinguishable material). 
• The last boundary (as all the boundaries in fluid mechanics) isn't sharp because fluid can go through a phase change  condensation or evaporation) in the middle or during the flow  and switch from a single phase flow to a multi phase flow. 
• Moreover, flow with two phases (or materials) can be treated as a single phase (for example, air with dust particle).