Lorentz Force Bug - Bug CFX 16.2
 

Hey,
I would like to simulate an electric Arc with CFX.
I am using Ansys 16.2 with beta features activated.
Based on my simulation results I have the presumption that CFX calculates the Lorentz force not correctly. So I created a test file to check my guess.
The structure is a simple copper cable. One end is defined as inlet with a flux in of 1 A/m^2, the opposite end is on ground level. Every surface is implemented as a wall. The cylinder wall is defined with zero flux. Furthermore, an external magnetic field of 1 T is imposed.
I expect an electron shift caused by the Lorentz force and a hall voltage.
If I simulate a steady state or a transient simulation, the results always show a homogeneous current density and no hall voltage.
That is the first problem.



The second problem is, if I check the results and visualize the current density with a surface and rotate the structure wildly, I can see an electron shift as long as I hold the left mouse key to rotate the object. The object can stand still, but I need to push the mouse key. If I don’t hold the mouse key anymore, so that the object is static, the current density is homogeneous again. That is the second problem.


Please check the attached pictures of the homogeneous and in homogeneous current density.

Has anybody perhaps an idea how to solve these problems?

Thank in advance for your response



Best Greetings
Lukas

Dear Lukas,

Would you mind explaining your setup for the electromagnetic models ? If I understood, you setup an explicit magnetic induction vector which I assume is aligned perpendicular to the expected current density.

Since you are modeling a conducting material, you selected Electric Potential model (conservation of electric current). Your boundary conditions are clear, and no additional information is needed (I think)

There is no mention of Particle model setup for the electron you mentioned.

From the ANSYS CFX theory, I have difficulty to understand how B influences J (or E for that matters) for a steady state model. Are expecting the influence to come from the electron traveling through the domain under the effects of the Lorentz force, and the presence of the charge of distorting the electric field ? Is the simplified implementation removing how the Hall effect is created ?

The ANSYS CFX implementation for electrostatics (Gauss Law) is beta, and there is no account for volumetric electric charge (which the electron represents). Would this be the source of the problem ?

Dear Opaque,
You understood my setup correctly. Do you need further information on the electromagnetic model?
I did activate Electric Potential model, the Electric potential and the Magnetic Vector Potential. Please check the attached image.
Ok, I was not sure, if there is a particle model setup for electrons.
I also simulated a transient model and there is also no electron shift which should result in a change of the current density.
In general B always influences moved electrons, in this case J. I am not that familiar with the CFX Solver theory, that I can predict how CFX handles this. This setup will lead to the hall effect in reality and I don’t think the setup is too simplified.
Yes, that could be the Problem, I will verify that. Do you know if the implementation for electrostatics is still a beta feature in the newest Software (I think, it is 17.1)?

In addition to my second problem:
I checked the output file and I am 99% sure that this is just a display error of CFX. Perhaps I will upload a Youtube video showing how I created this bug. I will also contact the support soon.

A quick search for Hall effect, and Generalized Ohm's Law and a look at the ANSYS CFX documentation will show the implementation is not meant for modeling the Hall effect, i.e. it is a limitation not a defect.

For the Hall effect to be included, the Hall Coefficient must be known and the appropriate term added to the definition of the current density as well as to the Lorentz force.

The model in ANSYS CFX is for non-charged fluids/species as far as I can read from the documentation.

If you can write the mathematical model required for the Hall effect to be included, you may be able to evaluate if it can be added via source terms on the proper equations.