Non-Newtonian flow

What are Newtonian and non-Newtonian flows?

A Newtonian Fluid is one where there is a linear relationship between stress and strain rate: the ratio of stress to strain rate is the fluid’s viscosity. A Hookean solid is one where there is a linear relationship between stress and strain: the ratio of stress to strain is the modulus of the solid. Many materials have intermediate properties between a Newtonian fluid and a Hookean solid. If the properties are predominantly solid-like, the materials are called non-Hookean, which are described as viscoelastic. If they are predominantly fluid-like, they are called non-Newtonian, and the materials are defined as elastic viscous.

A non-Newtonian fluid has, therefore, a simultaneously elastic and viscous nature. All fluids are non-Newtonian on an appropriate time scale, though the time scale is concise for many common fluids such as air and water. When the time-scale of a flow is much less than the relaxation time  Of a viscous elastic material, elastic effects dominate. This typically happens when there are abrupt changes in flow geometry. When on the other hand is much greater than  Elastic effects relax sufficiently for viscous effects to dominate. This typically happens when there are no abrupt changes in flow geometry. The ratio of to  Is a dimensionless number of particular significance in the study of non-Newtonian fluids. This number is called the Deborah Number or the Weissenberg number. If the Deborah or Weissenberg number is small, elastic effects can be neglected and the non-Newtonian fluid treated as a purely viscous material, albeit with a non-constant viscosity. Non-Newtonian fluids are divided into time-dependent and time-independent, or purely viscous.

Time-Independent Fluids are those fluids for which the shear rate at a given point is only dependent upon the instantaneous shear stress at that point. These materials are sometimes referred to as “non-Newtonian viscous fluids” or “purely viscous fluids.”

Non-Newtonian flow

In some fluids, the viscosity varies with time when the shear rate is constant. Two different types of behavior can be distinguished, thixotropic and rheopectic. Viscosity decreases with increasing time; for a rheopectic fluid, viscosity increases with increasing time. The variation of viscosity with time is associated with changes in the fluid structure. When straining the fluid breaks down that structure, for example, by destroying local linkages within the fluid, viscosity decreases with increasing time, and the fluid is thixotropic. A similar argument can be advanced for the decrease in viscosity with increasing shear rate for a pseudoplastic fluid.

In contrast, when straining builds up the structure, for example, by causing local alignment in the fluid, viscosity increases with increasing time, and the fluid is rheopectic. Again, a similar argument can be advanced for increasing viscosity with a dilatant fluid’s increasing shear rate. Therefore, an analogy exists between thixotropy and pseudo-plasticity on the one hand and between rheopexy and dilatancy on the other. Nevertheless, variation of viscosity with time is different from, and should not be confused with, variation with shear rate.

Non-Newtonian flow

MR-CFD, an expert in the field of CFD simulations

Having several years of experience in simulating a wide range of problems in various CFD fields using Fluent software, the MR-CFD team is ready to offer extensive modeling, meshing, and simulation services based on the need of our customers. Our services are not limited to the mentioned subject, and the MR-CFD team is ready to undertake different and challenging projects. You can consult with our experts freely and without charge at first, and then order your project by sending the problem details to us using the following address.

[email protected]

By entrusting your project to the MR-CFD team, you will not only receive the related project’s files (Geometry, Mesh, Fluent files). Also, you will be provided with an extensive tutorial video demonstrating how you can create the geometry, mesh, and define the needed settings in the Fluent software all by yourself. And these all come with post-technical support from the MR-CFD team.

MR-CFD experts are ready to fulfill every Computational Fluid Dynamic (CFD) needs. Our service includes industrial and academic purposes considering a wide range of CFD problems. MR-CFD serves in three main categories: ANSYS Fluent Consultation, ANSYS Fluent Training, and ANSYS Fluent Project Simulation. MR-CFD company has gathered experts from various engineering fields to ensure the quality of CFD services. Your CFD project would be done in the shortest time, with the highest quality and reasonable cost.

Time-Independent Fluids are those fluids for which the shear rate at a given point is only dependent upon the instantaneous shear stress at that point. These materials are sometimes referred to as “non-Newtonian viscous fluids” or “purely viscous fluids.”

Back To Top
Search

Refund Reason

Whatsapp Call On WhatsApp