[by1704116]

Hello, my dears.
Here is my problem in detail.I have a set of values,and each of it corresponds to its specific cell. I want to assign each of them to the corresponding cell center, and calculate the gradient of these value to form a mass source in the right hand side of the continuity equation.
In one paper,it says that one could save data in a txt file,and use DEFINE_ON_DEMOND to read it, assign values to the cell center ,store into an array,and use a second order reconstruction technique to calculate the gradient.
However, the Fluent udf manual just mentions the inherent variables,such as C_U(c,t),C_R(c,t), to use Gradient or Reconstruction Gradient macro.
I could store data into an array, but I don't know how to assign it to specific cell, and how could I use udf marco to calculate gradient?
Could someone give me some suggestions on my problem?
Thanks a lot for reading my post.

[AlexanderZ]

to assign value to specific cell you should decide how you are going to recognize if that is a cell you are looking for.

usually people are using coordinates to do that.

to apply specific values you may use both UDF or just profile.

as far as I understand, to get a gradient you should apply you values as a source of user defined scalar (C_UDSI). set proper boundary condition for it (usually specific value, for instance 0), turn on that scalar equation and calculate.
Also you will need to change solver settings:

In order to retain the
gradient data (when you want to set up user-defined scalar transport equations, for example), you can
prevent the solver from freeing up memory by issuing the text command solve/set/expert and
then answering yes to the question, “Keep temporary solver memory from being freed?”

for more information look into Ansys Fluent Customization manual

macro to get gradient of scalar is C_UDSI_G(c,t,i)

[by1704116]

Originally Posted by AlexanderZ

to assign value to specific cell you should decide how you are going to recognize if that is a cell you are looking for.

usually people are using coordinates to do that.

to apply specific values you may use both UDF or just profile.

as far as I understand, to get a gradient you should apply you values as a source of user defined scalar (C_UDSI). set proper boundary condition for it (usually specific value, for instance 0), turn on that scalar equation and calculate.
Also you will need to change solver settings:

In order to retain the
gradient data (when you want to set up user-defined scalar transport equations, for example), you can
prevent the solver from freeing up memory by issuing the text command solve/set/expert and
then answering yes to the question, “Keep temporary solver memory from being freed?”

for more information look into Ansys Fluent Customization manual

macro to get gradient of scalar is C_UDSI_G(c,t,i)

Dear Alexander,
Thanks for your quick reply. I've read your suggestions, but I still have some questions about how to realize it.
1、use coordinates to assign values to cells.
I may have not expressed exactlly. There is a zone full of cells, and I want to assign these valus to this zone. If use coordinates, I could only thinkof these steps. First output cell center coordinates from mesh, and then make a txt file with these coordinates and corresponding value that I've had. If I read the txt file to an array, how could I recognize which cell corresponds to its specific value? Loop over all the cells,and use 'if' statement to decide whether one cell coordinate matches with the array or not? Could you give me one example code on this kind of problem if it is possible?
2、use UDS
According to the udf manual, as if when I need to calculate an equation formed with the usteady, flux or diffusivity of a variable,then I use it?
Don't know why I can't upload pictures. I can just describe my problem in words. I just need to sole ordinary Euler equations with mass source in the right hand side of continuity equation. and values I've mentioned all through this post is a set of constant data and its gradient is constant too.So, what do you think about my new description of the question. Should I still use UDS or other method?

Look foward to your reply, Alexander.


I could attach a picture to describe the equations I need to deal with, please read the post below.

[by1704116]

Fortunately, I could attach a picture. there is a set of Euler equations with source terms on the right hand of them. The variable 'b' is what I want to assign to the cell center, and the one to be solved graident.