In general, matter can be classified by the physical forms in which Presents itself. These forms, known as phases, are the solid, the liquid and the of gas or steam. The fluids comprise the liquid and gas (or vapor) phases Of the mattery. In general, we are all familiar with the characteristics distinctive of those phases, compared with the solid phase. Nevertheless, we also know that liquids and gases have aspects completely different; therefore, we must look for a common characteristic that allows us to classify them simply as fluids. When discussing fluid dynamics, we are interested in the behavior of fluids in movement and the way in which that this behavior is related to the moments and forces applied. Both liquids and gases and vapors have in common a form different from reacting when subjected to tangential efforts, which explains its "fluidity" and provides the basic key to developing the principles of fluid dynamics.
A fluid will deform continuously under (tangential) shear stresses, It does not matter how small these are. As we will discuss later, The magnitude of the effort depends on the speed of angular deformation. A solid, on the other hand, will deform proportionally to the applied force, after which the static equilibrium will be reached; and in this case, the magnitude The tangential stress depends on the magnitude of the angular deformation.
Not all fluids show exactly the same relationship between effort and the speed of deformation. A fluid is called Newtonian, if the effort Tangential is directly proportional to the angular deformation speed, starting from zero effort and zero deformation. In these cases, the constant of proportionality is defined as the absolute or dynamic viscosity. Thus, Newtonian fluids have the property of having a viscosity dynamic independent of the movement to which the fluid is subjected. The Most common fluids, such as air and water, are Newtonian. There is an analogy between Newtonian fluids that have a constant viscosity which relates to the effort with the speed of deformation and the solids that obey. To Hooke's law, with a constant modulus of elasticity, which relates to the effort with the magnitude of the deformation.
Newtonian fluid
It is a fluid whose viscosity can be considered constant over time. The curve that shows the relationship between stress or shear versus its deformation rate is linear. It was named by Isaac Newton since he described it as a viscous flow. In this type of fluid, the viscosity can be considered constant in time and only depends on the temperature.
The Newtonian fluid lacks elastic properties, is incompressible, isotropic and unreal; although many real fluids offer behavior similar to the Newtonian within a range of gradients. Comply with Newton's law of viscosity, therefore, the relationship between shear stress and strain rate is linear.
The fluids that show a variable proportionality between effort and rapidity of deformation are known as non-Newtonian. In such cases, proportionality may depend on the time interval during which the fluid is subject to effort; as well as the magnitude of it. A great number of fluids, or unusual use, but which are extremely important, They are non-Newtonian. Some substances, especially some of the plastics, have a creeping effort, below which they behave as a solid, but beyond this, they behave like a fluid. The rheology deals with plastics and non-Newtonian fluids applied to engineering.
Non-Newtonian fluid
A non-Newtonian fluid is that fluid whose viscosity varies with time and the shear stress applied to it. As a result, a non-Newtonian fluid does not have a defined and constant viscosity value, unlike a Newtonian fluid. Although the concept of viscosity is commonly used to characterize a material, it may be inadequate to describe the mechanical behavior of some substances, specifically, non-Newtonian fluids. These fluids can be better characterized by other rheological properties, properties that have to do with the relationship between stress and tensors under different flow conditions, such as shear stress conditions.
