High Re K Epsilon vs Low Re K Epsilon

Ruonin · February 20, 2012, 09:21

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

Ruonin · February 22, 2012, 10:49

Anyone?

I'm really stuck in this matter...

julien.decharentenay · February 22, 2012, 19:34

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

Ruonin · February 23, 2012, 09:17

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

julien.decharentenay · February 23, 2012, 19:56

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.

February 20, 2012, 09:21	High Re K Epsilon vs Low Re K Epsilon	#1
Ruonin Member Join Date: Apr 2011 Posts: 56 Rep Power: 15	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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February 22, 2012, 10:49		#2
Ruonin Member Join Date: Apr 2011 Posts: 56 Rep Power: 15	Anyone? I'm really stuck in this matter...

February 22, 2012, 19:34		#3
julien.decharentenay Senior Member Julien de Charentenay Join Date: Jun 2009 Location: Australia Posts: 231 Rep Power: 17	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

February 23, 2012, 19:56		#5
julien.decharentenay Senior Member Julien de Charentenay Join Date: Jun 2009 Location: Australia Posts: 231 Rep Power: 17	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.