Fluid mechanics physical characterisics and kinematic

Fluids are the energy engineer's basic thermodynamic systems, whether in machines – engines of all types or refrigeration machines and heat pumps – or in heat exchange or production processes – heat exchangers, boilers, furnaces, etc. For this reason, it's important to know the properties of fluids, and particularly those specific to their flow: viscosity. When viscosity is low, it is usually neglected, and the fluid is considered perfect. On the other hand, viscosity can reflect very different behaviors from one fluid to another. From this point of view, we distinguish between Newtonian and non-Newtonian fluids. For the former, viscosity forces are proportional to deformation speeds. The relationship is more complex for non-Newtonian fluids.

The rate of deformation is also a factor to be taken into account in fluid kinematics. Whereas for a solid, motion is composed of translation and rotation, in the case of a liquid, deformation must be added. This addition is responsible for a certain complexity in fluid kinematics compared to that of undeformable solids. This complexity is further enhanced by either Lagrangian or Eulerian concepts in the treatment of fluid flow problems.

There is, however, a special case of flow in which kinematics becomes simpler, and that is in non-rotating, so-called irrotational flows. In these flows, whose mathematical treatment is simplified, fluid viscosity no longer has any effect. This makes them extremely interesting from an energy point of view.