[sbaffini]

Dear Abdullah,

it was really long time ago that i worked on this and i don't have anymore any of the related stuff (i mean, the actual UDF i used). This is not random, because at the time i was a beginner and it all seemed huge; now i see it more clearly for what it is, really standard stuff. Also, today Fluent is quite different and i would write the UDF quite differently.

If you can give me your email i can send some of the references i have on the source term in general. For the UDF you have to relate to the manual.

Generally speaking what you have to do is:

- write a define on demand UDF which will mark the zone where you want the source term. there are a lot of possible choices on this. At the end, we found that selecting the bounding box of the MVG as domain for the source term would finally give a model independent from the model constant for sufficiently high values of the constant. Another option is selecting just those cells being cut by the MVG, but then the model constant has to be somehow calibrated. The marking process itself is some cycle on the grid cells and when some conditions on the cell center/nodes coordinates are verified you say that, for example, a previously activated UDM is 1; otherwise it is 0. The UDM has to be activated before executing this routine.

- After this cycle is exacuted, before the calculation, you need to separete the marked cells from the rest. This involves first marking them (in the adapt part of the menu) trough the UDM you just defined and then separate the marked cell zone

- After this separation you can activate the source term just for this separated cell zone. The source term will then just be the replication of a given formula. I used the model of Bender, Anderson and Yagle. We found also important, for the convergence, the inclusion of the source term implicitly, which in Fluent requires the inclusion of the source term derivative with respect to the three velocity components.

- finally you can just run the simulation.

As inexperienced user (it was my first contact with Fluent and CFD in general) it took me 2-3 months to get it work. For an experienced user it is just a one day work (i can only imagine how frustrated was, at that time, my thesis supervisor).

For what concern the grid refinement, the basic idea of the model is that it should run without sufficient refinement, otherwise you could just insert it for real. I can suggest to work with a grid which is just sufficient for the rest of the flow without the MVG and calibrate the model constant consequently. Of course, the more you refine the grid the more the model will give you details as for the real MVG.

Finally, some works showed that for the specific model i used, MVG geometry details were unimportant. You will also notice from the model equation. This means that you only need few geometrical details of the MVG to be included in the model, like the area and the two linear dimensions.