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A problem of incorrect fluid flow field using Openfoam

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Old   June 7, 2017, 01:08
Default A problem of incorrect fluid flow field using Openfoam
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Maria
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Dear all,

I'm a new Foamer, and currently learning how to use it. And now I have a very confusing problem.

My model is simple, just a planar jet flow. Which is showed in figure 1, the gas inlet flow velocity is higher in the middle, and lower in the sides. But I can only get this correct flow field using icoFoam, when I use other solvers, like pimpleFoam, pisoFoam, reactingparcelfoam, etc., the flow field will be wavering, as is shown in Figure 2. The gravity is zero, and the flow field is wavering from up and down, not just one side.

Can anybody tell me why? and how to fix this? I can't just use icoFoam, because it's only for laminar flow, and cant do anything else.

Many thanks.

Maria
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Old   June 7, 2017, 03:25
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This is likely due to an interaction of the jet and the walls. If you want to simulate a free jet, consider making the domain wider. You could also compare the results of pimpleFoam, when using a RAS-type turbulence model and an LES-type turbulence model.
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Old   June 7, 2017, 10:28
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The second result seems to be unplausible, but it is realistic. If a wall comes near a flow, the wall creates some kind of suctions which moves the flow toward the wall. A tiny irregularity starts that process.
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Old   June 7, 2017, 23:21
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Quote:
Originally Posted by GerhardHolzinger View Post
This is likely due to an interaction of the jet and the walls. If you want to simulate a free jet, consider making the domain wider. You could also compare the results of pimpleFoam, when using a RAS-type turbulence model and an LES-type turbulence model.
Thanks, I will try to make the domain wider.
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Old   June 7, 2017, 23:22
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Quote:
Originally Posted by piu58 View Post
The second result seems to be unplausible, but it is realistic. If a wall comes near a flow, the wall creates some kind of suctions which moves the flow toward the wall. A tiny irregularity starts that process.
Thanks for your explanation, it makes sense to me. But the result from Fluent doesn't have this phenomenon, do you know why?

Maria
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Old   June 7, 2017, 23:51
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Quote:
Originally Posted by marialhm View Post
Thanks for your explanation, it makes sense to me. But the result from Fluent doesn't have this phenomenon, do you know why?

Maria

What pressure interpolation you have for Fluent?
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Old   June 7, 2017, 23:57
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Quote:
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What pressure interpolation you have for Fluent?
Hi, I chose PISO.
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Old   June 8, 2017, 01:46
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Hi, I chose PISO.

PISO is not pressure interpolation. It seems you did not make any changes to it, so that means you have standard pressure interpolation if it was old version of fluent and second order central if it is new fluent.

The pressure interpolation differences could cause what you are seeing.
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Old   June 8, 2017, 05:30
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Quote:
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PISO is not pressure interpolation. It seems you did not make any changes to it, so that means you have standard pressure interpolation if it was old version of fluent and second order central if it is new fluent.

The pressure interpolation differences could cause what you are seeing.
Oh, is that the reason? but I think in Openfoam, I also use the default setup, and the second order.
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Old   June 8, 2017, 08:46
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Hi all,

@marialhm

I think it would be MUCH easier if you disclose your geometry, mesh, boundary conditions, schemes, linear solvers, convergence criteria used. Right now, people are trying to explain why the second picture is possible, while simpler answer could be: "your simulation does not converge". And for icoFoam it could be just lucky coincidence.
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Old   June 8, 2017, 09:23
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Quote:
Originally Posted by alexeym View Post
Hi all,

@marialhm

I think it would be MUCH easier if you disclose your geometry, mesh, boundary conditions, schemes, linear solvers, convergence criteria used. Right now, people are trying to explain why the second picture is possible, while simpler answer could be: "your simulation does not converge". And for icoFoam it could be just lucky coincidence.
Please see figure 3 for the model, and the files of fvScheme and fvSolution are in figure 4 and 5. Please give your suggestions, thanks!

Regards,

Maria
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Old   June 8, 2017, 09:25
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Quote:
Originally Posted by alexeym View Post
Hi all,

@marialhm

I think it would be MUCH easier if you disclose your geometry, mesh, boundary conditions, schemes, linear solvers, convergence criteria used. Right now, people are trying to explain why the second picture is possible, while simpler answer could be: "your simulation does not converge". And for icoFoam it could be just lucky coincidence.
following is figure 5.
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Old   June 8, 2017, 09:57
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You have hidden the most interesting thing: PIMPLE dictionary in fvSolutions (can't you just post gzipped files?). Also boundary conditions are missing or should we assume, you use "standard" ones?.
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Old   June 8, 2017, 10:05
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Quote:
Originally Posted by alexeym View Post
You have hidden the most interesting thing: PIMPLE dictionary in fvSolutions (can't you just post gzipped files?). Also boundary conditions are missing or should we assume, you use "standard" ones?.
Pimple dictionary is,

nNonOrthogonalCorrectors 0;
nCorrectors 2;

Boundary conditions for U is in figure 3. For P, it's fixed value in outlet, and others are zeroGradient.

Acturally I tried several cases, different cases may have small differences. but almost all of them have wavering flow field.

Thanks again.

Maria
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Old   June 8, 2017, 10:13
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So, basically, you do not check convergence, use GAMG solver and limitedLinear scheme. Do you adjust time step? What is your Courant number? Did you try with PCG instead of GAMG? Did you try to check for convergence (using residualControl)? Did you try other discretisation schemes?
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Old   June 8, 2017, 10:21
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Quote:
Originally Posted by alexeym View Post
So, basically, you do not check convergence, use GAMG solver and limitedLinear scheme. Do you adjust time step? What is your Courant number? Did you try with PCG instead of GAMG? Did you try to check for convergence (using residualControl)? Did you try other discretisation schemes?
The maximum Co is 0.5~1, I changed limitedlinear scheme into upwind, but didnt change GAMG, maybe I should.
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Old   June 8, 2017, 10:31
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Maybe you could also post domain sizes? So people can at least try to reproduce your behaviour.

I do not believe single GAMG -> PCG change will solve your problem. Though it can postpone start of the oscillations (like it does in case of von Karman vortex street).
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Old   June 8, 2017, 10:34
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Quote:
Originally Posted by alexeym View Post
Maybe you could also post domain sizes? So people can at least try to reproduce your behaviour.

I do not believe single GAMG -> PCG change will solve your problem. Though it can postpone start of the oscillations (like it does in case of von Karman vortex street).
domain size:
x length: 2m
y:0.45m
injection width: 0.01m

Thanks.

Last edited by marialhm; June 8, 2017 at 10:34. Reason: wrong information
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Old   June 8, 2017, 23:54
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Hi all,

I think it's not the problem of settings, but it's the reality. @Uwe Pilz is right.

But the problem is, how can I set up to avoid this phenomenon? I tried outlet boundaries for the bottom and top, but the results are still not satisfactory.

Thanks in advance.

Maria
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Old   June 9, 2017, 06:24
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You could try with symmetry boundary condition.
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