# Velocity field on periodic faces

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March 28, 2016, 02:04
Velocity field on periodic faces
#1
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Tarkes Dora
Join Date: Jun 2014
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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.
Attached Images
 Section.jpg (184.8 KB, 15 views) periodicBC.png (21.2 KB, 12 views)

 March 28, 2016, 04:30 #2 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,737 Rep Power: 71 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
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Tarkes Dora
Join Date: Jun 2014
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Quote:
 Originally Posted by FMDenaro yes, all variables (scalars or vectors) must be exactly the same... your plot shows that the periodicity is not satisfied
Thank you for the response. Could you take a look into the image of the equations ? Is the equations for Perioidic BC is correct ?

Also could you suggest what could be going wrong?

 March 28, 2016, 07:01 #4 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,737 Rep Power: 71 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
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Tarkes Dora
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Quote:
 Originally Posted by FMDenaro 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
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:
 Originally Posted by tarkesdora 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 ?

I am not sure how "rotational periodic" BC.s is related to standard periodicity

March 29, 2016, 07:19
Calculations on periodic symmetris
#7
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Tarkes Dora
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Quote:
 Originally Posted by FMDenaro I am not sure how "rotational periodic" BC.s is related to standard periodicity
I just found few slides and help manual of fluent talking about perioidcity.

http://www.slideshare.net/khalid_nit...ambit-workshop

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:

• zero normal velocity at a symmetry plane
• zero normal gradients of all variables at a symmetry plane
As stated above, these conditions determine a zero flux across the symmetry plane, which is required by the definition of symmetry. Since the shear stress is zero at a symmetry boundary, it can also be interpreted as a "slip'' wall when used in viscous flow calculations.

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