# Gidaspow udf

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 April 17, 2013, 01:26 Gidaspow udf #1 New Member   Join Date: Jul 2012 Posts: 19 Rep Power: 7 hey hi friends, i have a little problem but i am unable to figure it out i am using this gidaspow drag model for my fluidised bed simulation but i am getting different results when i am incorporating this drag law through inbuilt gidaspow law and by writing a UDF for it. i have seen from fluent manual that the equations which i have used are same in the udf as given in manual. Can anybody please tell me what may be the reason behind this? Thank you in advance.

 April 17, 2013, 03:02 #2 Senior Member     Daniele Join Date: Oct 2010 Location: Italy Posts: 998 Rep Power: 18 Maybe it could be of great help if you attach your udf. Daniele

 April 18, 2013, 04:31 #4 Senior Member     Daniele Join Date: Oct 2010 Location: Italy Posts: 998 Rep Power: 18 Hi, I read the fluent user guide; when you compute fdrgs you are discriminating For the particle Reynolds number, but as you can see here (https://www.sharcnet.ca/Software/Flu...mp-cd-gidaspow) the Gidaspow model calculates fdrgs with one formula: fdrgs=(24/(void_g*reyp))*(1+0.15*(pow((void_g*reyp),0.687))) Accordingly to the user guide your calculations of Cd refer to the "symmetric" or to the "Shiller and Naumann" models. Maybe this is the "error" in your udf. Daniele

 April 18, 2013, 04:41 #5 Senior Member     Daniele Join Date: Oct 2010 Location: Italy Posts: 998 Rep Power: 18 In addition I think it is better to add a fabs: Code: abs_v = sqrt(fabs(slip_x*slip_x + slip_y*slip_y));

 April 19, 2013, 01:26 #6 New Member   Join Date: Jul 2012 Posts: 19 Rep Power: 7 hey Daniele, i have also tried in which i didnot discriminate for reynolds number...but still the results are different for cases with udf and without udf. i will do what you have suggested for using fabs() function and get back to you... thanks

 October 14, 2013, 03:27 #8 Member   mohsen Join Date: Sep 2013 Posts: 38 Rep Power: 6 Hi i want to write a udf about "gidaspow drag model with switch function" can you help me please thanks a lot in advance.

 October 15, 2013, 06:19 #9 Member   Musango Lungu Join Date: Dec 2011 Location: China Posts: 73 Rep Power: 7 @ Mohsen it's very easy to do that. Bear in mind that there are several stitching functions have been proposed for the Gidaspow model with the most commonly used being that due to Hulin and Gidaspow. The model is actually available in ANSYS FLUENT 14. Alternatively you can browse the source codes for the drag models in MFIX code and get an idea how to code it for FLUENT. If you still have trouble or doubts then feel free to contact me and I will you the specific details.

 October 15, 2013, 06:38 #10 Member   mohsen Join Date: Sep 2013 Posts: 38 Rep Power: 6 Thanks a lot Mr Musa for your reply. I will try and if I had a problem I ask you. Thanks again.

 October 15, 2013, 06:53 #11 Member   Musango Lungu Join Date: Dec 2011 Location: China Posts: 73 Rep Power: 7 @ Mohsen you are welcome. Look at this paper Leboreiro, J., Joseph, G. G., & Hrenya, C. M. Revisiting the standard drag law for bubbling, gas-fluidized beds. Powder Technol 2008, 183, 385-400. for the discussion regarding the different stitching functions to address the discontinuity of the Gidaspow model. For browsing the MFIX code you have to be a member to have access. It's easy and free to join. Membership will usually take a day or two to be activated uopn application. Though the source code is in FORTRAN it can be easily inferred for use in C.

 October 15, 2013, 07:14 #12 Member   mohsen Join Date: Sep 2013 Posts: 38 Rep Power: 6 Ok.Thanks.

 October 17, 2013, 19:23 #15 Member   Musango Lungu Join Date: Dec 2011 Location: China Posts: 73 Rep Power: 7 Try my code and let me know if you get the desired results. All the best. esmaeil likes this.

 October 18, 2013, 06:47 #16 Member   mohsen Join Date: Sep 2013 Posts: 38 Rep Power: 6 Dear Musa Thank you so much I tried with your code , but still the results are different from other authors Do you think the code has still a problem? esmaeil likes this.

 October 18, 2013, 07:52 #17 Member   Musango Lungu Join Date: Dec 2011 Location: China Posts: 73 Rep Power: 7 Hi Mohsen the code does not have a problem. I have coded it as given in several references. I also made reference to the MFIX source code of this drag model. Results might be different due to other reasons perhaps. You can go through the authors complete CFD code and see if matches with yours. Ensure that the boundary conditions and other parameters are the same. Check your Grid also. If you still don't get the same results then perhaps you can contact the authors of the work you are trying to mimic and ask them for the specifics. Good luck!

 October 18, 2013, 17:46 #19 Member   Musango Lungu Join Date: Dec 2011 Location: China Posts: 73 Rep Power: 7 Mohsen you are absolutely right the reyp has to be multiplied by void_g in the equation containing Cd but this has not been done in the code although it has been shown in the paper . Secondly there is no condition for switching from one regime to the other i.e from Ergun for the dense bed (void_g > 0.8) to WenYu for ( void_g<= 0.8). The stitching function simply ensures smooth transition from one regime to the other meaning that the condition of regime change must still be given.

 October 18, 2013, 18:20 #20 Member   mohsen Join Date: Sep 2013 Posts: 38 Rep Power: 6 Musa I tried with that code and the results were similar results of paper so do you think results of this paper are incorrect?

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