# Solving Poisson equation with UDS in Fluent

Hello i solve in my computer two UDS to calculate the electric potential :

1st : is the LAPLACE equation and i obtain adequate results .

2nd : is the POISSON equation and i meet that the value of source term is huge i.e sf=Irho/e0I

where e0 = 8.854E-12 F/m

and rho = 0.0008 C/m^3

Best regards

• Hi,

As long as you are taking the result you should be using in each cell and dividing by the corresponding cell volume to make sure that your units are consistent with Fluent's - you should be good. Please make sure you are doing this. When you integrate over these individual source values, you should obtain the final source term that you wish to apply (with the correct units).

Thank you.

Karthik

• Hello,

You should be able to add the necessary source term using a UDF or through Fluent Expressions. Did you try either of these methods? What is the issue you are seeing?

Karthik

• The expression of source term gives 185679602.92 which is very huge to estimate it like source term which gives in the turn huge value in result !

• These are volumetric sources. Are you converting these sources to the corresponding volumetric sources?

THank you.

Karthik

• HOW CAN I DO THIS ? (converting these sources)

• Source terms are in units of something per second per m3. So a mass source is kg/s/m3 This can mean we have a very large source value if the cell zone is very small: the usual example is a heat source for a chip, it's only a few tens of Watts but the volume is generally only a few cm3.

• THE CONSTANT IS 185679602.92 SO THE FLUENT WILL AUTOMATICALLY CONSIDER THIS VALUE LIKE [ 1/s.m3] WHERE THE VALUE WILL BE MORE LARGER

I THINK THAT WE SHOULD MULTIPLY THE VALUE OF SOURCE TERM WITH THE CELL VALUE ?

• Hi,

As long as you are taking the result you should be using in each cell and dividing by the corresponding cell volume to make sure that your units are consistent with Fluent's - you should be good. Please make sure you are doing this. When you integrate over these individual source values, you should obtain the final source term that you wish to apply (with the correct units).

Thank you.

Karthik