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-   -   Solving a Laplace equation for a scalar quantity (https://www.cfd-online.com/Forums/fluent/90524-solving-laplace-equation-scalar-quantity.html)

 ali hemmati July 13, 2011 04:40

Solving a Laplace equation for a scalar quantity

Hello,

I am trying to solve , i.e. homogeneous Laplace equation, in which is a scalar quantity and is described in a zone(2D or 3D). We know boundary values of in borders of the zone. This problem is very analogous to head conduction at steady state, yet is the electric potential here, not temperature.

I am wondering how to implement this problem in FLUENT. I think I have to mesh the zone and use a udf or something like that.

 Amir July 13, 2011 07:40

Hi Ali,
you can easily do that by implementing a UDS, user defined scalar; in it's definition, ignore unsteady and convective terms.

Amir

 Clarence91 October 2, 2014 11:05

Hi Amir,

Can you elaborate your statement? How can we justify that unsteady and convective terms can be ignored for solving the laplace equation? (i need to find the change in electric field in my geometry)
I think you know how my geometry looks like :P , I have to find the electric field in the rotor domain.
If possible can you share the UDF with me?

Thank you.
-Mark

 Amir October 2, 2014 13:34

Quote:
 Originally Posted by Clarence91 (Post 512708) Hi Amir, Can you elaborate your statement? How can we justify that unsteady and convective terms can be ignored for solving the laplace equation? (i need to find the change in electric field in my geometry) I think you know how my geometry looks like :P , I have to find the electric field in the rotor domain. If possible can you share the UDF with me? Thank you. -Mark
Dear Mark,

"UDS" is "User-Defined Scalar" equation and is available in FLUENT, something like energy equation and totally different from "UDF"! In the process of enabling a UDS, you can manage the unsteady and convective terms. For more info, please refer to the manual.

Bests,

 Tharanga February 11, 2015 20:56

Hi,
Can we solve defined scalar in x, y, z direction separately (for 3D geometry) or how can we see the scalar values in x y z directions using post processing?
Regards
Tharanga

 Amir February 12, 2015 05:02

Quote:
 Originally Posted by Tharanga (Post 531503) Hi, Can we solve defined scalar in x, y, z direction separately (for 3D geometry) or how can we see the scalar values in x y z directions using post processing? Regards Tharanga
Hi,
For sure you cannot solve it separately, but if you want to see the scalar values in a specified direction, you can simply draw a line and plot the scalar values over it...

Bests,

 Tharanga February 12, 2015 06:08

Hey, Thank you for the reply. I have solved the laplace eq (3D) and I need to get surface integration over x y z direction (separately) . Is there any method for that? cheers

 Tharanga February 12, 2015 06:12

If I elaborate more, I cant choose from the drop down menu x-scalar values etc. Therefore, I'm struggling to find x y z values over a surface. Thank you.

 pakk February 12, 2015 06:24

With a user defined scalar, you calculate a scalar, not a vector.
A scalar does not have a x-component... So your question does not make any sense.

If you solved an electrostatic problem, your scalar is probably the electric potential. Which component do you want? The x-component of the electric field? You should know the relation between those two.

 Tharanga February 12, 2015 06:36

It is a homogenized flow problem. Ultimately, I need to solve laplace eqn (vector) with BC's . Yes, you are right but there is no other way (I think) than UDS method in fluent to solve Laplace. Please suggest me an alternative method. Thank you

 pakk February 12, 2015 07:00

You don't make clear why you need another way. What is wrong with the UDS method?

And are you sure you want a vector laplace equation??? That is very unusual. I don't know why a "homogenized flow problem" would lead to a vector variant of the laplace equation.

 Tharanga February 16, 2015 21:42

Hi, Thank you very much for your kind replies. I solved it as three different problems in fluent. I got the results. :D

Regards,
Tharanga

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