Isotropic / anisotropic materials
Introduction
The studied materials can be isotropic or anisotropic. In other words, the thermal conductivity is:

independent of the direction of the applied temperature gradient (isotropic material)

dependent on the direction of the applied temperature gradient (anisotropic material)
These two cases are presented in the following sections.
Isotropic materials
Isotropic materials are characterized by a thermal conductivity, which is independent of the direction of the applied temperature gradient.
The and vectors are always collinear.
The dependence between and is a scalar relationship,
which is written as:
Anisotropic materials
Anisotropic materials are characterized by a thermal conductivity, which is dependent on the direction of the applied temperature gradient.
The and vectors are not collinear.
The dependence between and is a vector relationship,
which is written as:
with k conductivity tensor:
… in Flux
The model provided in Flux is a simplified model.
The vector dependence between and which is written as:
can therefore be expressed in the form of three curves:
, ,
The conductivity tensor is written: