[saif]

hi everyone

actually, i m tryng to implement a low turbulence abid model that i get it from some site web in FLUENT, with application of flow around a square cylinder at very low reynolds =(250) , the BCs for K and epsilon were estimated from formulas given in Fluent user guide's in the inlet domain, (K=2.4e-09 and Epsilon=2.75e-13 ) , and velocity inlet for top and down domain with 0 gradient for both turbulence quantities, for the exsit domain i used the outflow BC,
the compilation of the model were successifully done, but when running the simulation after a fews itterations, the code crashes and show ' a floting point error' : invalid number.
i m really upset because i spent a lot of time to solve my broblem, i will verry glad if some one can help me to solve my issue ,
i can attach the UDF code if some one have time to look around


thanks

[saif]

anay suggestions ???


did the problem comes from the set of BCs ???


please help

[AlexanderZ]

switch off your UDf, try to simulate case with standard k-e turbulence model first using same BC.
If the the problem still exists, than the cause is:
-BC
-mesh quality
-other
If simulation will be completed successfully, than the problem is in your UDF (most likely)

best regards

[saif]

thanks a lot Alexander for ur suggestions, it's really helpful,
yes i tried this methode before, i lunched the simulation with a standard k-E and i get successfully a converged solution,
but we should remember that a types of BCs in this case, isn't the same in the case of the use of a UDF , because we should set BCs for a 2 UDS (K nd E), either by fixed value or flux value for both quantities, when in the satndard k epsilon model we have in Fluent, for instance ,the choice to use Intensity and Hydraulic diameter,
thanks again once, and hope that u can give me another suggestions,

[saif]

in attachement the UDF for Abid model

[AlexanderZ]

may be the problem comes from BCs. Actually how your domain looks like? I guess it has one inlet, one outlet, and some kind of channel between them.
from your first message:

the BCs for K and epsilon were estimated from formulas given in Fluent user guide's in the inlet domain, (K=2.4e-09 and Epsilon=2.75e-13 ) , and velocity inlet for top and down domain with 0 gradient for both turbulence quantities, for the exsit domain i used the outflow BC

inside the code I can see, that you initialize K=2.4e-09 and Epsilon=2.75e-13 everywhere in domain. But it is not BCs.
BCs for UDS we should put manually for each face in domain. By default, BC for UDS is "specified flux" -> UDS boundary value 0. It means 0 gradient, as you've mentioned.
However, I believe at inlet as well as outlet , you should put there other condition (second option) "specified value" -> UDS boundary value K=2.4e-09 and Epsilon=2.75e-13
I'm not about correct values.

I hope it will help

Best regards

[saif]

thank u Alexander, to ur peek in the UDF and for ur reply


i set the BCs as u can see in the picture:


did these BCs correct??, what do u think?


may be should i add some statements or macro in the UDF?


i would be very glad help me solve my issue


best regards

[saif]

in attachement the picture explaining my BCs

[AlexanderZ]

Unfortunately, I have no experience in implementation of turbulence models. I may only guess.
So I feel, that there should be some turbulence at outflow BC, so flux = 0 is incorrect, from my point of view. I'm not sure about top and bottom inlets, try to change outflow first, and later may be apply same BC at TOP and Bottom as at the left inlet.

best regards

[saif]

thank u Alexander, i really apreciate ur attention to my issue,


till now i could'nt solve my problem, i tried a severals simulation with differents BCs and the same message error appear('floating point error'),
i will wait for other suggestions


best regards