## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

#### charge density in MHD module

• Emine Goktepe
Subscriber

Hello,

I am utilizing Ansys Fluent MHD module to track the charged particles in a laminar fluid flow.

To define the amount of charge, there is a field in the interface of the inert material properties named as "charge density". In the fluent theory guide its unit is stated as C/m3.(please see the picture below). In general, I do not see this equation given for a volume. It is generally written per particle in other sources. So, i have a difficulty to understand the meaning of the m3 in this equation.

Should I calculate this charge density value assuming:

the number of particles can fit in a cube with 1m dimension [n/m3] x number of charge per particles [C/n] (in this case depending on the diameter of the particles, number of particles will change)

or,

the total charge on a particle[C]/particle volume[m3]

Both assumptions seems doesnt work since I tried cases using both methods. Can somebody please explain what does this volumetric particle charge density means?

• Ulrich
Ansys Employee

Hi Emine,

where have you found this paragraph with the equation exactly in the Fluent Theory Guide and of which version?

Kind Regards, Ulrich S.

• Emine Goktepe
Subscriber

Hello Ulrich,

https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v232/en/flu_th/flu_mhd_sec_electric_potential.html

It is the MHD module part of the theory guide.

Which way do you thin is the best to define charge density for particle tracking?

Best regards,

• Ulrich
Ansys Employee

Thanks for the exact reference. Now I have found it.

You asked for the meaning of the volume specific (m3) in the equation. This is due to the fact that the Lorentz force is included in other conservation equations as source terms, and source terms are always volume specific in Ansys Fluent.

Moreover, you have asked for the meaning of the volumetric charge density. The volumetric charge density is the amount of electric charge per volume. For Fluent the charge density is a material property which needs to be known. For guidance on how to determine this property I would recommend doing some background reading on electromagnetism or doing a web search for “charge density”.

• Emine Goktepe
Subscriber

Hello Ulrich,

Thank you for explanation. Actually I searched many resources including ansys short lectures about electro magnetism and electric field. The problem I am facing here is that, when we talk about charge on a particle its unit is in [C]. If we talk about surface charge density we divide it to surface, and if we say volumetric charge density I understand i should divide the amount of charge assigned to particle to volume of particle.

But this parameter is in the material properties section, not in the DPM properties. So, I understand from here it should be something material specific as you said. In other words, it should be a parameter doesnt depend on particle diameter etc.  These two doesnt seem parallel assumptions.

I also seach for the material parameter "material charge density", "volumetric charge density" terms. In each source it is explained as charge/particle volume. Not a material specific property.

Can you please share an example material that its charge density is defined in the literature. Unfortunately all I found is case specific calculations.

And if possible, can you please also share a tutorial/example of fluent with usage of particle volumetric charge density?

kind regards,