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-   -   Problem with periodic boundary conditions (http://www.cfd-online.com/Forums/fluent/100539-problem-periodic-boundary-conditions.html)

 jm_ngs April 26, 2012 12:52

Problem with periodic boundary conditions

Hi all

I am a PhD student, and I'm new in ansys fluent.
I am trying to simulate a 2D airfoil with periodic boundary conditions. As you can see in attached picture, the left and right boundaries are periodic, and the top and bottom boundaries are defined as outflow.

What I want is to consider the output of first iteration as the input of the second iteration.

I tried to perform some simulations, with translational motion, but the results make no sense. Can you help me???
I performed some simulations with "fixed" wing airfoil, and the results went fine. I am using the same turbulence model, but I am sure that the problem is in the definition of periodic boundary conditions.

How to define the periodic boundary conditions, correctly?

The final objective is to take into account the losses due to the airfoil wake.

In addition, I attached some pictures of my previous results with periodic boundary conditions, in case it helps to identify the problem.

http://img205.imageshack.us/img205/4936/2dairfoil.png
http://img88.imageshack.us/img88/797...00pa10000i.png

 LuckyTran April 26, 2012 15:06

Quote:
 Originally Posted by jm_ngs (Post 357257) I am trying to simulate a 2D airfoil with periodic boundary conditions. As you can see in attached picture, the left and right boundaries are periodic, and the top and bottom boundaries are defined as outflow.
I am not sure how your flow loop is set up. Which direction is the flow, left to right or top to bottom or what? You have a periodic boundary condition and two outlflows, where is the flow source? Also, with periodic boundary conditions that means you have infinite space of lots of airfoils. Is this what you are trying to simulate?

At best, your boundary conditions leaves the problem underdefined. As you have outflow with no inflow. The only stable solution is no flow!

Quote:
 Originally Posted by jm_ngs (Post 357257) What I want is to consider the output of first iteration as the input of the second iteration.
Can you clarify what you mean by output of first iteration as the input of the second iteration? In the process of iterating, this is exactly what is done. The output of the first iteration is used as the input to the second iteration.

 Zigainer April 26, 2012 16:27

Could it be, that your two outflows (guess flow is from let to right) should be something like a pressure-far-field?

 jm_ngs April 26, 2012 18:34

Quote:
 Originally Posted by LuckyTran (Post 357278) I am not sure how your flow loop is set up. Which direction is the flow, left to right or top to bottom or what? You have a periodic boundary condition and two outlflows, where is the flow source? Also, with periodic boundary conditions that means you have infinite space of lots of airfoils. Is this what you are trying to simulate? At best, your boundary conditions leaves the problem underdefined. As you have outflow with no inflow. The only stable solution is no flow! Can you clarify what you mean by output of first iteration as the input of the second iteration? In the process of iterating, this is exactly what is done. The output of the first iteration is used as the input to the second iteration.

The loop of flow is from left to right. (at the periodic boundary conditions, I defined a favorable pressure gradient of 100Pa.

I was trying to simulate a helicopter rotor (or other kind of rotor). Is there any other way to simulate that kind of stuff in 2D?

In this way, I was trying to take into account the wake losses due to the interference of the previous airfoil. And have many airfoils one after anothers is exactly what I want.

When I say that "What I want is to consider the output of first iteration as the input of the second iteration" I mean that values of first iteration in right periodic boundary condition, will be the input values for second iteration in left periodic boundary conditions.

Anyway, if you can explain me other simple way to simulate a rotational motion in 2D, I will be very gratefull :D

 jm_ngs April 26, 2012 18:38

Quote:
 Originally Posted by Zigainer (Post 357291) I agree with LuckTran, so please provide some more information. Could it be, that your two outflows (guess flow is from let to right) should be something like a pressure-far-field?
I am not shure, but I read in Ansys user guide that pressure far field boundary condition is only for compressible flows, and (originally), I want to simulate incompressible flows. I used outflow, since in userguide tells that it is a zero gradient boundary condition, but correct me if I am wrong.

And you are right, I am trying to simulate the flow from left to right!

 ChenZhan April 26, 2012 22:27

hello everyone:
i have a similar problem when reading a reference about porous meida.In order to be convinent for your reading and understanding the means.This paper was appended at the end of this text.
Now i descripe my question.This paper's auther make a periodic boundary conditions to simulation the effective thermal conductivity,in which he impose a temperature drop on the periodic boundary according to his cited reference [20].While when i see the cited reference 20,finding that,i need to modify the coefficient matrix of the discreted equations,yet this is just my guess.
Can you give me some suggestion on how to carry out this boundary using the fluent?
Any help will be appreciated!
Have a nice day ,my friend!;)
oh,sorry:( because the Max Filesize limit,i cannot upload the reference on to the forum.If you want to know more detail pleas load from this URL:
Title:A numerical technique for computing effective thermal conductivity of fluid-particle mixtures

HELP ME MY FRIENDS!!!
Email :chenzhana@163.com

 LuckyTran April 27, 2012 01:19

Quote:
 Originally Posted by ChenZhan (Post 357332) hello everyone: i have a similar problem when reading a reference about porous meida.In order to be convinent for your reading and understanding the means.This paper was appended at the end of this text. Now i descripe my question.This paper's auther make a periodic boundary conditions to simulation the effective thermal conductivity,in which he impose a temperature drop on the periodic boundary according to his cited reference [20].While when i see the cited reference 20,finding that,i need to modify the coefficient matrix of the discreted equations,yet this is just my guess. Can you give me some suggestion on how to carry out this boundary using the fluent? Any help will be appreciated! Have a nice day ,my friend!;) oh,sorry:( because the Max Filesize limit,i cannot upload the reference on to the forum.If you want to know more detail pleas load from this URL: http://scholar.google.com.hk/scholar...ed=0CB0QgQMwAA Title:A numerical technique for computing effective thermal conductivity of fluid-particle mixtures HELP ME MY FRIENDS!!! Email :chenzhana@163.com
TIP: don't hijack someone else's thread when they need help if you want help yourself. especially if it's not even related

 ChenZhan April 27, 2012 02:32

Quote:
 Originally Posted by LuckyTran (Post 357340) TIP: don't hijack someone else's thread when they need help if you want help yourself.
Oh , i am sorry for that.
Write this help i only consider that the common key words could arise much more attention, and no other hostility. I appologize for that!
sorry again!

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