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High Re K Epsilon vs Low Re K Epsilon

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Old   February 20, 2012, 10:21
Question High Re K Epsilon vs Low Re K Epsilon
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R. Silva
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Hi folks,

I am somewhat new to CFD (only been doing for some months ), and I have a doubt which I hope some can clear...

I am trying to implement a Low Re K-Epsilon turbulence model to a backstep and compare it to a High Re K-Epsilon turbulence model, to show that, in this particular case, the Low Re K-Epsilon turbulence model represents the situation better.

However, I get a pressure drop that is higher for the Low Re K-Epsilon turbulence model than the pressure drop for the High Re K-Epsilon turbulence model...

Is this correct? shouldn't it be the opposite?
Or perhaps I made a mistake in the modelling step?

Hope someone can help!

Best Regards,

Rui Silva
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Old   February 22, 2012, 11:49
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R. Silva
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Anyone?

I'm really stuck in this matter...
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Old   February 22, 2012, 20:34
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Julien de Charentenay
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Hi Rui,

In this specific case, you will need to ensure that the mesh is appropriate to the model employed (and ensure that the wall treatment is correct).

Low-Reynolds k-epsilon turbulence model:
- The first cell y+ should be around 1 (or less);
- the wall treatment is based on first cell being the viscous sub-layer.

High-Reynolds k-epsilon turbulence model:
- the first cell y+ should be around 15-30;
- A high-Reynolds wall treatment is to be used.

From your post, I am unsure that you are using two different mesh that are consistent with the employed model.

Kind regards,
Julien
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Old   February 23, 2012, 10:17
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R. Silva
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Hello Julien,

thank you for your reply.

What I am doing is modifying the standard high Reynolds k-epsilon (according to Wilcox) that comes with a cfd software (comsol) so that it becomes the
Launder-Sharma Low Re k-epsilon Turbulence model.

So I modified the equations to incorporate the new set of damping functions and boundary conditions near the wall.

After reading some posts and papers I have refined my mesh.
But since I am using a modified standard k-epsilon model, I cannot see y+!

I have attached pics of my mesh near the wall and some size info!
Hope the pics help...

Is there a way to evaluate y+ for a low re k-epsilon model??

Best Regards,

Rui
Attached Images
File Type: png Mesh1.png (43.0 KB, 18 views)
File Type: png Mesh2.png (37.1 KB, 24 views)
File Type: jpg Mesh3.jpg (6.8 KB, 15 views)
File Type: png Mesh4.PNG (13.3 KB, 21 views)
File Type: png Mesh5.PNG (13.8 KB, 17 views)
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Old   February 23, 2012, 20:56
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Julien de Charentenay
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Have a look at: http://www.cfd-online.com/Wiki/Dimen..._wall_distance for the definition of y+ - which can be evaluated for a low Reynold k-epsilon model.

There is also a simple y+ estimator on the web page.

Also, you could use the other calculations to estimate the friction velocity (under the assumption that the other calculations friction velocity is accurate).

The rule y+ around 1 is a guidelines - do not get stuck with it.
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