How to... define cylindrical anisotropic conductivity in Ansys Fluent (cyl-orthotropic)

Many of the problems that are modeled in CFD today concern multi-layer objects. It often happens that these  models have large sizes and thin layers. Due to this specificity, we have certain conduction properties in various directions of solid body. Drawing such models in the 1: 1 scale causes many problems, primarily related to the generation of a very large number of finite elements. Sometimes the computer hardware parameters do not allow us to perform such an analysis.

Cyl-orthotropic in Ansys Fluent 

A good way out of this situation is to model simplified geometric models with defined anisotropy. The Fluent program offers many definitions of this type of conduction property. Today we will discuss defining the anisotropy of cylindrical elements with layers.

In thermal processes, we often deal with heating of roller elements consisting of thin layers. Then the conductivity in different directions of the material varies significantly. Often, such elements have a radial conductivity that is less than 10% of the total axial conductivity. However, because they are elements consisting of folded layers, the total conductivity (100%) will occur in the other two directions (axial and tangential). Cyl-orthotropic (C-O) seems to be an ideal function for modeling this type of rollers.

In C-O we define the conductivity in three directions (A). In the axial direction, i.e. heat conduction in the direction of the vector perpendicular to the flat surface of the roller. In the radial direction, i.e. the direction in which we define the conductivity between successive layers of the rolled material. The last direction is tagential, i.e. it is heat conduction "along" layers (B).

First, enter the Materials option in the Fluent tree. We choose the material on which we will define the anisotropy. We edit the material (in our case aluminum) by developing the Thermal conductivity option. We chose the definition of cyl-orthotropic (next photo).

How to define anisotropic conductivity in Ansys Fluent 

If our object is perfectly in the center of the main Axis Origin coordinate system, we leave it with the default values (0, 0, 0 - red frame). In Axis Direction (Green frame) we need to set the axis of rotation of our object (in my case the axis of rotation was in X). In the next three options (orange, blue and gray frames) we define the conductivity values in the appropriate directions that were described earlier. Of course, as with the standard option of defining conductivity, we have the option of defining values in relation to temperature and defining your own user function. By defining UDF, we can, for example, model the increasing resistance between successive layers along with the distance from the axis of rotation.

Cyl-orthotropic options in Ansys Fluent material properties

Thanks to such a path towards simplifying models, we are able to solve complex thermal systems. Our mesh of finite elements will not require much work and will not consist of many elements. The partial equations will "behave better" while the solver is running.

In the next posts, I will try to present other models of anisotropic conductivity and how to define porous materials.