# SOS!! Periodic BC

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 October 31, 2013, 00:25 SOS!! Periodic BC #1 New Member   Prashant Thaker Join Date: Jul 2013 Location: guwahati, India Posts: 15 Rep Power: 5 hi, I am a new user to fluent, trying to model pollutant dispersion in air. my geometry consists of two lines of buildings with a street(releasing pollutant) in between. the inlet of ambient air is 100m behind the buildings. I need to specify such a boundary condition which keeps the same wind velocity profile at the inlet and at the plane containing building rooftop. I tried transalational periodic bc but got an error: 'no. of faces not the same' so 2 questions: 1. is translational periodic bc fit for my requirement? 2.how to get rid of the above mentioned error. I am using FLUENT meshing. Don't have access to other software. plz help!! Prashant

 October 31, 2013, 02:52 #2 Senior Member     Paolo Lampitella Join Date: Mar 2009 Location: Italy Posts: 598 Blog Entries: 17 Rep Power: 19 First of all, when using periodicity among any two faces, they have to be exactly the same (e.g., a square and an identical traslated/rotated square, a circle and an identical traslated/rotated circle). Second, i don't know if you can actually have both rotation and traslation among the two faces. I guess not. Third, in the basic periodicity approach as implemented in Fluent, the grids on the two faces also have to be exactly the same. To do this in Gambit (e.g., when the grid is not structured) you have to link the two faces before meshing them. In this case, you don't need to bother about setting specific boundary conditions in Gambit; just set the two faces as walls and then apply periodicity among them in Fluent trough (TUI): grid->mo->mp I don't know how the grid part is done in fluent meshing (Fluent 14.5?). Another possibility in Fluent is to use interfaces among the two faces. In this case you don't need the same grid. Refer to the manual for this case. prashantthaker208 likes this.

 October 31, 2013, 08:01 #3 New Member   Join Date: May 2013 Posts: 2 Rep Power: 0 Hi, i am using 'Match Control' when meshing. ... and the '/mesh/modify-zones/make-periodic' command in Fluent. prashantthaker208 likes this.

 November 1, 2013, 04:16 #4 Senior Member     Paolo Lampitella Join Date: Mar 2009 Location: Italy Posts: 598 Blog Entries: 17 Rep Power: 19 Dear Prashant, i'm sorry but i forgot to mention that you may have problems in doing scalar transport in Fluent with periodicity. It is allowed for the temperature, but then some specific scaling is used in Fluent among the periodic faces to avoid having a constant rise in temperature or a uniform temperature at regime. You should check the manual on this.

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