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Old   September 8, 2013, 04:45
Default Problems of bad results with LES
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Hi foamers,

I have several problems with my wall-jet case for monthes. I have calculated the 2D case both with RANS models and LES models. However, the LES results are not good as I expected, especially in the viscous area, the velocity from the LES result is much lower than that from RANS result and EXP data. For I have set the same boundary conditions for both RANS and LES case, so I don't know what on earth leads to the "bad" result of LES. What's more, I don't know how to set the boundary condition for B and nuSgs in 0 folder. For my case, I just modified the B and nuSgs files with the tutorial case, and I'm not sure they are right.

Case describe:
velocity: inlet_cool: use data from exp; inlet_hot: use data from exp;
wall_down:fixed value (0 0 0); wall_up: symmetry Plane;
outlet:zerogradient;
pressure: inlet_cool: zeroGradient; inlet_hot: zeroGradient;
wall_down:zeroGradient; wall_up: symmetry Plane;
outlet:fixed value 0;
Temperature: inlet_cool: 300k; inlet_hot: 310k;
wall_down:zeroGradient; wall_up: symmetry Plane;
outlet:zerogradient;

This is the framework structure:2013-09-08 16:08:04的屏幕截图.png
This is the velocity results:U.jpg
These are my 0 and system folders of LES:0_system_les.tar.gz
These are my 0 and system folders of RANS:0_system.tar.gz

Can anybody help me please?

Best regards!
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Old   September 8, 2013, 09:44
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Hi Sunxing,

Why did you expect to get the qualitatively correct results using 2D results? In 2D case, the turbulence is not true. But 2D case is still meaningful for debugging the code.

Quote:
Originally Posted by Sunxing View Post
Hi foamers,

I have several problems with my wall-jet case for monthes. I have calculated the 2D case both with RANS models and LES models. However, the LES results are not good as I expected, especially in the viscous area, the velocity from the LES result is much lower than that from RANS result and EXP data. For I have set the same boundary conditions for both RANS and LES case, so I don't know what on earth leads to the "bad" result of LES. What's more, I don't know how to set the boundary condition for B and nuSgs in 0 folder. For my case, I just modified the B and nuSgs files with the tutorial case, and I'm not sure they are right.

Case describe:
velocity: inlet_cool: use data from exp; inlet_hot: use data from exp;
wall_down:fixed value (0 0 0); wall_up: symmetry Plane;
outlet:zerogradient;
pressure: inlet_cool: zeroGradient; inlet_hot: zeroGradient;
wall_down:zeroGradient; wall_up: symmetry Plane;
outlet:fixed value 0;
Temperature: inlet_cool: 300k; inlet_hot: 310k;
wall_down:zeroGradient; wall_up: symmetry Plane;
outlet:zerogradient;

This is the framework structure:Attachment 25159
This is the velocity results:Attachment 25160
These are my 0 and system folders of LES:Attachment 25166
These are my 0 and system folders of RANS:Attachment 25165

Can anybody help me please?

Best regards!
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Old   September 8, 2013, 20:33
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Quote:
Originally Posted by openfoammaofnepo View Post
Hi Sunxing,

Why did you expect to get the qualitatively correct results using 2D results? In 2D case, the turbulence is not true. But 2D case is still meaningful for debugging the code.
I know that it's not suitable to simulate a 2D case with LES model. However for time efficiency, I hope the 2D case can give me a "good" enough result in a short time. And I think at least the velocity result shouldn't be so lower than the exp data in the viscous area.
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Old   September 9, 2013, 01:46
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Did you check y+ values at the wall? Also, did you check the effect of inlet conditions, it seems to me that you are applying a fixed value inlet condition, which is of course not true for turbulence.

You are using LES, but what subgrid model do you use?

You are using a zero value pressure at the outlet, are you sure that is correct for these kind of boundary layer flows?

Did you try to confirm your model with normal wall-jets from literature? I am convinced that there are a few out there.
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Old   September 9, 2013, 02:05
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Quote:
Originally Posted by Bernhard View Post
Did you check y+ values at the wall? Also, did you check the effect of inlet conditions, it seems to me that you are applying a fixed value inlet condition, which is of course not true for turbulence.

You are using LES, but what subgrid model do you use?

You are using a zero value pressure at the outlet, are you sure that is correct for these kind of boundary layer flows?

Did you try to confirm your model with normal wall-jets from literature? I am convinced that there are a few out there.
Hi Bernhard,

For my case the y+ is lower than 1 at the wall. Yes, I have applied a fixed value from exp data with a fluctuationScale (0.03 0.01 0.01), I think this can be enough for turbulent income, am I right? I have used smagorinsky(with damping function), dynOneEqEddy and dynamic smagorinsky models. All of them give lower velocity in the viscous area. I think a zero value pressure at the outlet is right, because for the RANS model, I also set this condition and it turns out well.

Regards!
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Old   September 9, 2013, 02:10
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How can you impose a velocity fluctuation in the z-direction?

Anyhow, you will have to perform a 3D simulation at least once to prove that your approach is correct.

By the way, are you using your RANS mesh for the LES?
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Old   September 9, 2013, 02:11
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Quote:
Originally Posted by Bernhard View Post
Did you check y+ values at the wall? Also, did you check the effect of inlet conditions, it seems to me that you are applying a fixed value inlet condition, which is of course not true for turbulence.

You are using LES, but what subgrid model do you use?

You are using a zero value pressure at the outlet, are you sure that is correct for these kind of boundary layer flows?

Did you try to confirm your model with normal wall-jets from literature? I am convinced that there are a few out there.
I used a incompressible solver by modified it with adding temperature in it, so I think using a fixed value pressure at the outlet is ok.
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Old   September 9, 2013, 02:18
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Quote:
Originally Posted by Bernhard View Post
How can you impose a velocity fluctuation in the z-direction?

Anyhow, you will have to perform a 3D simulation at least once to prove that your approach is correct.

By the way, are you using your RANS mesh for the LES?
Maybe you are right that imposing a velocity fluctuation in the z-direction is wrong, but I set it by referring the tutorial in the incomprssible/pisoFoam/les/pitzDaily/0/U file.
I'm performing a 3D simulation for the case for more than 10 days, but it's so time costly and I think it still need more than one month to get the result. The mesh for LES is more finer than that for RANS.
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Old   December 24, 2013, 00:08
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Quote:
Originally Posted by Sunxing View Post
Hi foamers,

I have several problems with my wall-jet case for monthes. I have calculated the 2D case both with RANS models and LES models. However, the LES results are not good as I expected, especially in the viscous area, the velocity from the LES result is much lower than that from RANS result and EXP data. For I have set the same boundary conditions for both RANS and LES case, so I don't know what on earth leads to the "bad" result of LES. What's more, I don't know how to set the boundary condition for B and nuSgs in 0 folder. For my case, I just modified the B and nuSgs files with the tutorial case, and I'm not sure they are right.

Case describe:
velocity: inlet_cool: use data from exp; inlet_hot: use data from exp;
wall_down:fixed value (0 0 0); wall_up: symmetry Plane;
outlet:zerogradient;
pressure: inlet_cool: zeroGradient; inlet_hot: zeroGradient;
wall_down:zeroGradient; wall_up: symmetry Plane;
outlet:fixed value 0;
Temperature: inlet_cool: 300k; inlet_hot: 310k;
wall_down:zeroGradient; wall_up: symmetry Plane;
outlet:zerogradient;

This is the framework structure:Attachment 25159
This is the velocity results:Attachment 25160
These are my 0 and system folders of LES:Attachment 25166
These are my 0 and system folders of RANS:Attachment 25165

Can anybody help me please?

Best regards!
Hi Sunxing

what solver did you used for wall jet simulation? I am also simulating wall jet with temperature variation. And I changed standard buoyantBoussinesqPimpleFoam to capable with LES. But the results is too bad, even worse than RANS. So how did you make it?
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Old   December 24, 2013, 01:57
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Quote:
Originally Posted by simonsg View Post
Hi Sunxing

what solver did you used for wall jet simulation? I am also simulating wall jet with temperature variation. And I changed standard buoyantBoussinesqPimpleFoam to capable with LES. But the results is too bad, even worse than RANS. So how did you make it?
Hi simon,


I used pisoFoam and added temperature function

Best regards

Sunxing
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Old   March 31, 2014, 05:11
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Hi Sunxing,

I am new Foamer and I implemented a very simple LES model as shown by a tutorial pisoFoam/pitzDaily.

I am searching to use the dynamic Smagorinsky model for incompressible non-homogeneous turbulent flow inside an injector. This model is suitable for my case?

May you give more details of following files:

- constant/LESProperties

- 0/nuSgs, 0/k and 0/epsilon

Thanks in advance,

vut
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