CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Periodic Boundary Conditions

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree12Likes

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   October 20, 2014, 05:55
Default Periodic Boundary Conditions
  #1
CZL
New Member
 
CZL
Join Date: Oct 2014
Posts: 7
Rep Power: 8
CZL is on a distinguished road
Hi all, I am new to fluent and this forum, so please pardon me if i am asking something too simple and straight forward.

I am investigating flow characteristics through a pipe of a certain repeated geometry and hence the use of the periodic boundary condition.

Since i am dealing with laminar, incompressible flow, I only specified the inlet mass flow rate and the temperature.

One parameter of interest is the pressure drop across this small part of the whole configuration.

After solving for 10000 iterations, the pressure at the outlet is the same as that of the inlet.

I am clueless as to why this is so.
CZL is offline   Reply With Quote

Old   October 20, 2014, 15:05
Default
  #2
Member
 
David Stanbridge
Join Date: Apr 2010
Location: Norwich, UK
Posts: 59
Rep Power: 13
swiftaircraft is on a distinguished road
Are the periodic boundaries applied axially or radially? What outlet condition have you used? Pressure outlet or outflow?
__________________
David Stanbridge

Swift TG Solutions Limited
www.swifttgsolutions.com
swiftaircraft is offline   Reply With Quote

Old   October 20, 2014, 21:21
Default
  #3
CZL
New Member
 
CZL
Join Date: Oct 2014
Posts: 7
Rep Power: 8
CZL is on a distinguished road
Tranlational (axial) periodic conditions were used. Correct me if i am wrong, with mass flow rate bring specified, the boundary condition specified is velocity inlet right? Pressure based solver used.
CZL is offline   Reply With Quote

Old   October 21, 2014, 02:16
Default
  #4
Member
 
David Stanbridge
Join Date: Apr 2010
Location: Norwich, UK
Posts: 59
Rep Power: 13
swiftaircraft is on a distinguished road
You cannot use translational periodics along the length of the pipe. If the pipe is long and straight and has periodicity radially then I would suggest to use rotational periodics to reduce the cell count and define an inlet and outlet. In this way you will be able to detect a pressure loss along its length.
chaitanyaarige likes this.
__________________
David Stanbridge

Swift TG Solutions Limited
www.swifttgsolutions.com
swiftaircraft is offline   Reply With Quote

Old   October 21, 2014, 03:00
Default
  #5
CZL
New Member
 
CZL
Join Date: Oct 2014
Posts: 7
Rep Power: 8
CZL is on a distinguished road
Isn't rotational periodics for propellers? The repetition is along the length of the pipe, and hence the choice to use translational periodic conditions. I'm sorry, confused here.
CZL is offline   Reply With Quote

Old   October 21, 2014, 03:31
Default
  #6
Senior Member
 
Cees Haringa
Join Date: May 2013
Location: Delft
Posts: 607
Rep Power: 0
CeesH is on a distinguished road
A pipe isn't translational-periodic, because you are always dealing with flow development effects at the enterance. (also, if it was, completely translationally periodic, what would be the point of simulating?)

A pipe is rotational periodic, around the central axis. If the pipe is smooth, you can even model it as a 2-d axisymmetric geometry.
CeesH is offline   Reply With Quote

Old   October 21, 2014, 04:46
Default
  #7
Member
 
David Stanbridge
Join Date: Apr 2010
Location: Norwich, UK
Posts: 59
Rep Power: 13
swiftaircraft is on a distinguished road
I have nothing more to add to the reply of CeesH. It is spot on.
__________________
David Stanbridge

Swift TG Solutions Limited
www.swifttgsolutions.com
swiftaircraft is offline   Reply With Quote

Old   October 22, 2014, 05:33
Default
  #8
CZL
New Member
 
CZL
Join Date: Oct 2014
Posts: 7
Rep Power: 8
CZL is on a distinguished road
Firstly, thank you very much for all your replies.

but hmm... i think that you guys may has misunderstood what i was trying to model.

the pipe i am trying to analyse has a sine wave like geometry, with 12 periods in the streamwise direction (direction of flow).

as the developing region is small, it can be assumed that fully developed flow will be observed in most of the the length of the pipe.

hence, instead of meshing 12 sine waves, i just want to model 1 wave with translational periodic boundary conditions.

periodic zone (inlet)
shadow zone (outlet)

what i wish to do is to check the pressure drop for 1 wave and try to extrapolate the information for the entire length.

however, after determining the mass flow rate, i get the ave pressure on the inlet = pressure on the outlet.

laminar flow, pressure based solver.

this should not be the case as there should be some pressure drop in 1 wave, and for 12 waves, the pressure drop should be 12 times more.

for the entire array of 70 channels, it should be approximately be 70 x 12 x delta P for 1 wave.

That is my understanding.

The strange thing is that when i started with a coarse grid, there is pressure drop observed. as i refined the grid, it disappeared some how. with that, there is no way i can do my grid convergence test.
anijg05 likes this.
CZL is offline   Reply With Quote

Old   October 22, 2014, 06:50
Default
  #9
Member
 
David Stanbridge
Join Date: Apr 2010
Location: Norwich, UK
Posts: 59
Rep Power: 13
swiftaircraft is on a distinguished road
You cannot use translational periodics for this problem. Even with translational periodics then for your problem you still need a defined inlet and a defined outlet as well as the translation periodic boundaries. You will have to model the 12 waves. However if the waves are in one plane then you could split the model in half and have a symmetry plane. This will reduce the cell count.
ghost82, CeesH and chaitanyaarige like this.
__________________
David Stanbridge

Swift TG Solutions Limited
www.swifttgsolutions.com
swiftaircraft is offline   Reply With Quote

Old   October 22, 2014, 07:33
Default
  #10
Senior Member
 
ghost82's Avatar
 
Daniele
Join Date: Oct 2010
Location: Italy
Posts: 1,016
Rep Power: 23
ghost82 will become famous soon enough
Quote:
Originally Posted by swiftaircraft View Post
You cannot use translational periodics for this problem. Even with translational periodics then for your problem you still need a defined inlet and a defined outlet as well as the translation periodic boundaries. You will have to model the 12 waves. However if the waves are in one plane then you could split the model in half and have a symmetry plane. This will reduce the cell count.
Hi!
In my opinion this is true from a geometrical point of view, but there could be secondary flows inside the pipe, which can determine pressure drop: I think you'd better to simulate the whole geometry.
ghost82 is offline   Reply With Quote

Old   October 22, 2014, 07:38
Default
  #11
CZL
New Member
 
CZL
Join Date: Oct 2014
Posts: 7
Rep Power: 8
CZL is on a distinguished road
noted with great thanks.

will investigate further.
CZL is offline   Reply With Quote

Old   October 24, 2014, 04:44
Default
  #12
Senior Member
 
RodriguezFatz's Avatar
 
Philipp
Join Date: Jun 2011
Location: Germany
Posts: 1,297
Rep Power: 24
RodriguezFatz will become famous soon enough
I don't see why this would not work. Maybe I don't get you right. Can you post a small picture of the "1-sine" domain, with some captions?
__________________
The skeleton ran out of shampoo in the shower.
RodriguezFatz is offline   Reply With Quote

Old   October 24, 2014, 05:05
Default
  #13
Senior Member
 
ghost82's Avatar
 
Daniele
Join Date: Oct 2010
Location: Italy
Posts: 1,016
Rep Power: 23
ghost82 will become famous soon enough
Yes, it is possible to simulate a translational periodic bc; you need to specify the mass flow across the periodic boundary (which is a known value in this case) and a "first guess" for pressure gradient.
The translational periodic is valid only for fully developed flow: if this is the case then it can be simulated.
ghost82 is offline   Reply With Quote

Old   October 24, 2014, 05:18
Default
  #14
Senior Member
 
RodriguezFatz's Avatar
 
Philipp
Join Date: Jun 2011
Location: Germany
Posts: 1,297
Rep Power: 24
RodriguezFatz will become famous soon enough
Yes, that's what I think, too. And since his approximation is:
"as the developing region is small, it can be assumed that fully developed flow will be observed in most of the the length of the pipe."
this is a valid approach...
ghost82 likes this.
__________________
The skeleton ran out of shampoo in the shower.
RodriguezFatz is offline   Reply With Quote

Old   October 24, 2014, 08:56
Default
  #15
CZL
New Member
 
CZL
Join Date: Oct 2014
Posts: 7
Rep Power: 8
CZL is on a distinguished road
Thank you all once again for your replies.

It seems that there isnt a problem with the assumption which i have made, rather i think that the problem lies with the possibility that my result may not have converged.

I have attached images of the stream lines and the plots of the residuals of 2 different grid resolution.

High resolution is about 8x that of the low res one.

https://www.dropbox.com/sh/pgdjf1ivi...WnZpguz7a?dl=0
for the high resolution one, the mass flow rate at the outlet does not match the one which i have specified in the boundary condition.

any suggestions as to how i could reduce the iteration count?
CZL is offline   Reply With Quote

Old   October 24, 2014, 09:08
Default
  #16
Senior Member
 
RodriguezFatz's Avatar
 
Philipp
Join Date: Jun 2011
Location: Germany
Posts: 1,297
Rep Power: 24
RodriguezFatz will become famous soon enough
Hey CZL, why the energy equation? If the temperatur changes along the pipe, there is no way this can be periodic. Or am I wrong?
ghost82 likes this.
__________________
The skeleton ran out of shampoo in the shower.
RodriguezFatz is offline   Reply With Quote

Old   October 24, 2014, 10:31
Default
  #17
Super Moderator
 
flotus1's Avatar
 
Alex
Join Date: Jun 2012
Location: Germany
Posts: 2,791
Rep Power: 39
flotus1 has a spectacular aura aboutflotus1 has a spectacular aura about
Some thoughts on the statements in this thread:

1) Translational periodic boundary conditions can be used even for straight pipe flows
This simply implies that the flow is fully-developed, an assumption that may be valid at least for some part of the geometry we are dealing with here.
And of course we have to assume that the fluid we are dealing with is incompressible.

2) Fluent is able to deal with temperature variations even for straight pipes with translational periodic boundary conditions applied. The temperature is rescaled at the "inlet" based on the temperature value you provide at the periodic interface. Again, this only works with constant-density fluids.
Basically the same procedure as with a pressure gradient along a translational periodic flow.
Periodic flow with heat transfer is even one of the basic tutorials for Fluent.

Since the energy equation does not seem to converge very well in your cases, try decreasing the under-relaxation factor for this equation (->Solution controls)
The other equations seem to converge perfectly at least on the coarse grid.
If convergence is too slow on the finer grid, try interpolating the solution from the coarse grid as an initial condition for the fine grid. (File -> Interpolate)
teguhtf, RodriguezFatz and Rashed like this.

Last edited by flotus1; October 24, 2014 at 15:55.
flotus1 is offline   Reply With Quote

Old   October 24, 2014, 13:20
Default
  #18
CZL
New Member
 
CZL
Join Date: Oct 2014
Posts: 7
Rep Power: 8
CZL is on a distinguished road
thanks for the inputs!

will try to do as suggested.

I used energy equation to track how this passive geometry is better in terms of transferring heat rather that the straight case as the curvature kind of induces fluid mixing, enhancing heat transfer.

That means that the absolute values of the solution is not of concern, rather delta T will be taken note of.

My understanding is that if delta T is better than the conventional straight geometry, it will definitely also be better in the developing region due to the flow profile and hence, this be concluded to be overall more efficient...

does it sound logical? i mean is this method applicable?
CZL is offline   Reply With Quote

Old   October 24, 2014, 15:59
Default
  #19
Super Moderator
 
flotus1's Avatar
 
Alex
Join Date: Jun 2012
Location: Germany
Posts: 2,791
Rep Power: 39
flotus1 has a spectacular aura aboutflotus1 has a spectacular aura about
Sounds reasonable.
But keep in mind that the curved geometry not only enhances heat transfer compared to a straight channel, it also increases pressure loss and might be quite difficult to manufacture.
So if pressure loss is a relevant factor check that your method is better than simply a longer straight channel.
flotus1 is offline   Reply With Quote

Old   October 27, 2014, 03:44
Default
  #20
Senior Member
 
RodriguezFatz's Avatar
 
Philipp
Join Date: Jun 2011
Location: Germany
Posts: 1,297
Rep Power: 24
RodriguezFatz will become famous soon enough
Quote:
Originally Posted by flotus1 View Post
2) Fluent is able to deal with temperature variations even for straight pipes with translational periodic boundary conditions applied. The temperature is rescaled at the "inlet" based on the temperature value you provide at the periodic interface. Again, this only works with constant-density fluids.
Basically the same procedure as with a pressure gradient along a translational periodic flow.
Periodic flow with heat transfer is even one of the basic tutorials for Fluent.
Thanks flotus, good to know.
__________________
The skeleton ran out of shampoo in the shower.
RodriguezFatz is offline   Reply With Quote

Reply

Tags
boundary condition, laminar, periodic, pressure drop

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
PEMFC module + multiple periodic boundary conditions vkrastev FLUENT 2 December 22, 2014 04:15
Overflow Error in Multiphase Modelling with Two Continuous Fluids ashtonJ CFX 6 August 11, 2014 14:32
Error finding variable "THERMX" sunilpatil CFX 8 April 26, 2013 07:00
periodic boundary conditions fro pressure Salem Main CFD Forum 21 April 10, 2013 00:44
periodic boundary conditions mranji1 Main CFD Forum 4 August 24, 2009 23:45


All times are GMT -4. The time now is 18:19.