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March 28, 2016, 02:04 
Velocity field on periodic faces

#1 
New Member

Hi,
I am trying to solve a 3D transient gas flow in a cylindrical shaped domain. The domain has two rotational periodic faces. The periodic BC was implemented in fluent and the problem was solved satisfying the convergence criteria. I have taken a section from the 3D domain and now I am looking at the velocity vectors (at a particular time) on the periodic faces as shown in the attached figure. The vectors are plotted inplane(due to 3D complex flow direction) Kindly could you clarify/help me on the following: 1. Does the magnitude of the velocity on the periodic faces would remain same ? Is it compulsory that it would be same? I mean to say does it depends on the flow field at the central region ? 2. The direction of the velocity magnitude would be same? If mathematically I want to write the BC the attached picture perioidcBC is correct ? or a negative sign would appear in the velocity term ? Kindly excuse if the question is simple, as my background is on solid mechanics and I am trying explore my problem which can solved by fluid mechanics. I would appreciate your kind help and suggestion on this. 

March 28, 2016, 04:30 

#2 
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Filippo Maria Denaro
Join Date: Jul 2010
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yes, all variables (scalars or vectors) must be exactly the same...
your plot shows that the periodicity is not satisfied 

March 28, 2016, 06:18 

#3  
New Member

Quote:
Also could you suggest what could be going wrong? 

March 28, 2016, 07:01 

#4 
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Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,737
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you must set the condition in such a way to get all the velocity components equal... if you fix only the velocity magnitude then the angle is not set properly


March 28, 2016, 07:28 

#5 
New Member

I have used Fluent software to solve the problem. As such I have used the rotational perioidic BC using the TUI options in Fluent. I am not sure whether it makes the components equal automatically ? What could be other way I set this condition in fluent ?


March 28, 2016, 09:35 

#6  
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Filippo Maria Denaro
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Quote:
I am not sure how "rotational periodic" BC.s is related to standard periodicity 

March 29, 2016, 07:19 
Calculations on periodic symmetris

#7  
New Member

Quote:
http://www.slideshare.net/khalid_nit...ambitworkshop (Please look at slide 44) https://www.sharcnet.ca/Software/Ans..._periodic.html According to the help manual sldies across the perioidic faces pressure drop is zero. That means P(face2)=P(face1). But no details about the velocity is mentioned. In fluent help manual i could find the following on symmetric planes: ANSYS FLUENT assumes a zero flux of all quantities across a symmetry boundary. There is no convective flux across a symmetry plane: the normal velocity component at the symmetry plane is thus zero. There is no diffusion flux across a symmetry plane: the normal gradients of all flow variables are thus zero at the symmetry plane. The symmetry boundary condition can therefore be summarized as follows:
Link: http://www.afs.enea.it/project/neptu...ug/node251.htm Can any body help me in understanding this ? Is the calculation procedure on symmetric planes applicable to rotational periodicity ? 

March 29, 2016, 13:11 

#8 
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,737
Rep Power: 71 
symmetric BC.s are different from periodical BC.s...
see the link below, I suppose that fluent automatically apply a rotational periodicity bc.s to the vector direction http://www.afs.enea.it/project/neptu...ug/node252.htm 

Tags 
cylinder domain, flow 3d, periodic bc, velocity 
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