Non-Newtonian fluid

Non-Newtonian fluid

In this section, we want to introduce Newtonian and non-Newtonian fluids using the concept of viscosity. We know that the viscosity of different fluids depends a lot on the type of fluid. According to the definition of viscosity, it is called the fluid’s resistance to flow. First, we will explain the definition of Newtonian fluid and non-Newtonian fluid. And next, we will examine the application of CFD in simulating some of the most important phenomena for us, in which there are non-Newtonian fluids.

Newtonian and non-Newtonian fluids

The viscosity of a fluid is one of the main parameters in the analysis and study of fluid behavior. A Newtonian fluid is a fluid whose viscosity is only a function of temperature and pressure and does not depend on the strain rate or velocity gradient. But in non-Newtonian fluids, the viscosity changes with the movement of the fluid. Newtonian and non-Newtonian fluids are two types of fluids that behave differently and show different resistances when subjected to certain forces and moved.

When subjected to a force, Newtonian fluids, such as water, their viscosity remains constant. In other words, they show the same resistance to the applied forces.

Shear stress formula in fluids

While in non-Newtonian fluids, when they are affected by force, and due to this force, a local velocity gradient is created in them, their viscosity changes, and it is even possible to behave like a show a solid and rigid body. Non-Newtonian fluids are used in some industrial applications due to their unique properties.

Non-Newtonian fluid

Newtonian and non-Newtonian fluids have wide applications in various industries, such as automotive, construction, electronics, food processing, and pharmaceuticals. Newtonian fluids are used for lubrication, cooling systems, and hydraulic systems. In contrast, non-Newtonian fluid applications are used for shock absorption, damping, vibrations, and flow rate control and to manufacture paints, adhesives, and sealants.

Non-Newtonian fluids are divided into two groups, the first group of fluids whose viscosity depends on the amount of force and the time of force application (time-dependent), the second group of fluids whose viscosity depends only on the amount of force (independent of time). The first group includes rheopectic and thixotropic fluids; the second includes dilatant, pseudoplastic, and Bingham fluids.

RHEOPECTIC FLUIDS

In rheopectic fluids, the viscosity increases with the application of force and the passage of time. An example of this fluid is cream. If we stir it with a spoon, its viscosity gradually increases and becomes harder.

 

THIXOTROPIC FLUIDS

The change in viscosity and behavior of thixotropic fluids is the opposite of rheopectic fluids. That is, their viscosity decreases with the application of force and the passage of time. A famous example of a thixotropic fluid is honey. If we stir honey with a spoon for a long time, its viscosity decreases gradually and becomes smoother.

DILATANT FLUIDS

In this type of fluid, when a strong force is applied to the fluid, its viscosity increases. In other words, the increase in shear stress is directly related to the rise in the viscosity of these fluids. In dilatant fluids, the viscosity depends only on the amount of applied force. In these fluids, the viscosity increases as the force on the fluid increases. As the tension increases, the fluid becomes stiffer and behaves like a solid, so you can even walk on it. This feature is used in entertainment.

Suspensions can exhibit such behavior. By striking with your fist, you harden this fluid so that your fist does not enter the fluid. But if you slowly put your hand into the liquid, the hand will sink into the liquid.

PSEUDOPLASTIC FLUIDS

In this group of non-Newtonian fluids, such as dilatant non-Newtonian fluids, the viscosity depends only on the amount of applied force. It is not affected by the duration of the force application. But in pseudoplastic fluid, unlike dilatant fluids, the viscosity decreases as the applied force increases, and the fluid becomes smoother. The sauce is a clear example of a Pseudoplastic non-Newtonian fluid. This sauce is hard when it is in its container, but applying pressure or impact makes it smooth and comes out of the container.

BINGHAM FLUIDS

There is another type of time-independent non-Newtonian fluid known as Bingham fluid. Bingham fluids need initial stress to flow, and the stress applied to them must reach a threshold for these types of fluids to start moving. This is while other fluids, i.e., Newtonian fluids, non-Newtonian dilatants, and pseudoplastic fluids, do not need to reach a certain stress threshold. These fluids are called plastic fluids. Bingham fluid is a type of plastic fluid whose viscosity remains constant. It should be noted that Bingham’s pseudo-plastic fluid, like Bingham’s plastic fluid, needs initial stress to flow, with the difference that its adhesion decreases with the increase of strain rate. Toothpaste, silica-polymer micro and nanocomposites, sand, mud, and bitumen are Bingham fluids.

Simulation of non-Newtonian fluids with CFD

So far, we have introduced the types of non-Newtonian fluids. In the following, we will investigate the use of CFD in simulating some of the most important phenomena for us in which there are non-Newtonian fluids.

BLOOD FLOW IN A VESSEL

One of the most famous non-Newtonian fluids is blood. On the other hand, the simulation of blood flow in a vessel helps us study the types of blood vessels. By simulating the blood flow in the vessels, we can calculate the force and shear stress applied to the vessel wall due to the blood flow. Calculating this force can be useful in predicting the place of rupture in the vessel due to various causes, such as increased blood pressure.

In this way, medical engineers can prevent more dangerous problems, such as stroke and vein rupture, before they occur by suggesting medical solutions. Therefore, viscosity models have been developed in CFD software such as Fluent to define non-Newtonian fluids. Among these non-Newtonian models, Piecewise-Linear, Polynomial, Sutherland Law, and Piecewise-Polynomial can be mentioned.

CFD Vessel

INDUSTRIAL EQUIPMENT LUBRICATION

Lubrication of industrial equipment is essential to increase the lifespan of industrial devices and parts, and it should be done repeatedly and at different times during machine operation. Therefore, checking and simulating lubrication systems can be used for better and more efficient design. The fluids used for lubrication are all types of oils, and we know that most oils show non-Newtonian behavior, so we must use non-Newtonian models developed in CFD software to simulate them.

Lubrication

RELATED PRODUCTS