Boundary Layer thickness estimation

moonser · March 6, 2012, 09:47

Hi guys and ladies.
I have gotten to grips with the meshing in Ansys finally and just need to be able to justify that the mesh is fine enough around the boundary layer. I am doing a VERY simple 2-d circle. I am just not sure how to work out the thickness. I have been told that I can use whatever velocity I want.

Can anyone give me a quick crash course in how to calculate/justify this.

Thanks in advance for any help.

mettler · March 6, 2012, 10:12

you can calculate the thickness based on your Re. Any fluids book will show you how.

good luck

RolandP · March 6, 2012, 10:26

I'm having a similar issue, and was just wondering if it was ok, as an approximation, to use the Blasius solution for an airfoil? Or is that too inaccurate?

mettler · March 6, 2012, 10:32

you just need a good estimation in order to make your grid fine enough to capture it in your CFD analysis.

RolandP · March 6, 2012, 10:33

Tidy, I've been going around in circles :s

I'll give it a crack thanks

moonser · March 6, 2012, 10:38

Cheers very much mate. Wasn't sure that you were aloud do that I thought I was missing a geometry factor somewhere.

pete · March 6, 2012, 15:26

Check out http://www.cfd-online.com/Tools/yplus.php

panospap · March 7, 2012, 04:51

Depending on your Re (flow material, characteristic length, velocity), you will have to use a specific thickness for your first cell to achieve a desired y+ value. For high Re you have to define very small thickness for the first cell. If you have been told you can use any velocity you want, then I would advise to run the calculation for low velocity, where you can have a good result for not so fine mesh.
The desired value of y+ depends on the turbulence model you are using. For example, for SST model y+ value should be around 1-2.
The tool given above gives you an easy way to determine all these. The only thing is that in this tool you also have to specify the boundary layer length, which is not easy to be determined in all applications.
I hope that helps.

Panos

cdf_user · March 7, 2012, 05:35

Your yplus value has to be less than 2 in order to accuratly represent the boundary layer. If it is more than two, refine your mesh.

rikio · May 30, 2012, 02:02

I am also confused about the boundary layer length. Is there anyone could show me the way to determine it?

cdf_user · May 30, 2012, 11:03

Quote:

Originally Posted by rikio

I am also confused about the boundary layer length. Is there anyone could show me the way to determine it?

Boundary Layer length is function of Reynolds number. The exact for formula is in introductory fluid mechanics books. It is something like a*(Re^b).

March 6, 2012, 09:47	Boundary Layer thickness estimation	#1
moonser New Member Stephen Join Date: Feb 2012 Location: London Posts: 6 Rep Power: 14	Hi guys and ladies. I have gotten to grips with the meshing in Ansys finally and just need to be able to justify that the mesh is fine enough around the boundary layer. I am doing a VERY simple 2-d circle. I am just not sure how to work out the thickness. I have been told that I can use whatever velocity I want. Can anyone give me a quick crash course in how to calculate/justify this. Thanks in advance for any help.

March 6, 2012, 15:26		#7
pete Administrator Peter Jones Join Date: Jan 2009 Posts: 682 Rep Power: 10	Check out http://www.cfd-online.com/Tools/yplus.php TommyM likes this.

March 7, 2012, 04:51		#8
panospap New Member Join Date: Mar 2012 Posts: 4 Rep Power: 14	Depending on your Re (flow material, characteristic length, velocity), you will have to use a specific thickness for your first cell to achieve a desired y+ value. For high Re you have to define very small thickness for the first cell. If you have been told you can use any velocity you want, then I would advise to run the calculation for low velocity, where you can have a good result for not so fine mesh. The desired value of y+ depends on the turbulence model you are using. For example, for SST model y+ value should be around 1-2. The tool given above gives you an easy way to determine all these. The only thing is that in this tool you also have to specify the boundary layer length, which is not easy to be determined in all applications. I hope that helps. Panos Neshat likes this.

March 7, 2012, 05:35	Check your yplus value	#9
cdf_user Member Join Date: Mar 2011 Posts: 50 Rep Power: 15	Your yplus value has to be less than 2 in order to accuratly represent the boundary layer. If it is more than two, refine your mesh.

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March 6, 2012, 10:12		#2
mettler Senior Member Andrew Join Date: Mar 2009 Location: Washington, DC Posts: 209 Rep Power: 18	you can calculate the thickness based on your Re. Any fluids book will show you how. good luck

March 6, 2012, 10:26		#3
RolandP New Member Roland Peters Join Date: Feb 2012 Posts: 3 Rep Power: 14	I'm having a similar issue, and was just wondering if it was ok, as an approximation, to use the Blasius solution for an airfoil? Or is that too inaccurate?

March 6, 2012, 10:32		#4
mettler Senior Member Andrew Join Date: Mar 2009 Location: Washington, DC Posts: 209 Rep Power: 18	you just need a good estimation in order to make your grid fine enough to capture it in your CFD analysis.

March 6, 2012, 10:33		#5
RolandP New Member Roland Peters Join Date: Feb 2012 Posts: 3 Rep Power: 14	Tidy, I've been going around in circles :s I'll give it a crack thanks

March 6, 2012, 10:38		#6
moonser New Member Stephen Join Date: Feb 2012 Location: London Posts: 6 Rep Power: 14	Cheers very much mate. Wasn't sure that you were aloud do that I thought I was missing a geometry factor somewhere.

May 30, 2012, 02:02		#10
rikio Senior Member Rikio Join Date: Mar 2009 Location: SH, China Posts: 182 Blog Entries: 1 Rep Power: 17	I am also confused about the boundary layer length. Is there anyone could show me the way to determine it?