# How to decide the number of grid lines in boundary layer

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 January 20, 2013, 17:21 How to decide the number of grid lines in boundary layer #1 Senior Member     Meimei Wang Join Date: Jul 2012 Posts: 494 Rep Power: 8 Hi, May I ask how to decide how many grid lines(at the direction proportional to the wall) do I need(to resolve boundary layer well)? Of course grid independence study is always good practice. But I always need a first guess of this number to have my first grids before the grid independence study. Thank you very much. immortality likes this. __________________ Best regards, Meimei

 January 21, 2013, 03:52 #2 Senior Member   cfdnewbie Join Date: Mar 2010 Posts: 557 Rep Power: 12 That depends on the order of your scheme and on the type of flow you are considering. Impossible to answer in general....

January 24, 2013, 17:08
#3
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Meimei Wang
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 Originally Posted by cfdnewbie That depends on the order of your scheme and on the type of flow you are considering. Impossible to answer in general....
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 January 24, 2013, 22:25 #4 Senior Member   Join Date: Jul 2009 Posts: 238 Rep Power: 11 If you are performing a RANS or URANS simulation the standard approach is to settle on the distance of the first point off the wall based on an acceptable y+ value, and then keep any grid stretching ratio below 1.2. The resulting simulation should always be reviewed after the fact to ensure that the actual value of y+ is still acceptable and the boundary layer is well-resolved.

 January 25, 2013, 05:04 #5 Member   venki Join Date: May 2011 Posts: 85 Rep Power: 7 After running problem in solver. you will get actual value of Y+ . before running also we can calculate by Y+ estimatation calculater in CFD online(Rough). based on calculation you can find first cell distance.

January 25, 2013, 17:20
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Meimei Wang
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Quote:
 Originally Posted by venkateshaero After running problem in solver. you will get actual value of Y+ . before running also we can calculate by Y+ estimatation calculater in CFD online(Rough). based on calculation you can find first cell distance.
Thanks. May I ask another question?

If I use laminar model(no model), do I need to have Y+ smaller or equal to 1?

Thank you.
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 January 26, 2013, 08:48 #7 Member   venki Join Date: May 2011 Posts: 85 Rep Power: 7 please post your problem brief. whether its Heat transfer or only fluid flow problem. I will send some data based on that. that data will answer your question. .

January 26, 2013, 10:42
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Meimei Wang
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Quote:
 Originally Posted by venkateshaero please post your problem brief. whether its Heat transfer or only fluid flow problem. I will send some data based on that. that data will answer your question. .
It's a very simple case. Just laminar flow go through a rectangular pipe. But there's a 90 degree turning in the middle. There's nothing about heat transfer. Do I need to consider Y+ for laminar flow?

Just wondering: If it's for heat transfer, Y+ will need to be below 1, right?
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 January 27, 2013, 05:38 #9 Member   venki Join Date: May 2011 Posts: 85 Rep Power: 7 Based Turbulence models Y+ limitation is there for each models(like K-omega ,K-epsilon). if u want caputure flow parameter along vertical. u have make fine mesh near wall.For Y+ consideration i will refer and answer your question later .

 June 7, 2013, 18:04 #10 Senior Member     Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,209 Rep Power: 19 Hi I have a question like Anna. Anna did you find an answer to your question about importance of y+ in laminar cases? and is it important that flow be compressible or incompressible? thanks. __________________ Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked.

June 8, 2013, 09:04
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Lefteris
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Quote:
 Originally Posted by Anna Tian Hi, May I ask how to decide how many grid lines(at the direction proportional to the wall) do I need(to resolve boundary layer well)? Of course grid independence study is always good practice. But I always need a first guess of this number to have my first grids before the grid independence study. Thank you very much.
There are formulas for the estimation of the boundary layer thickness whether that is laminar or turbulent. Use them.
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June 13, 2013, 05:10
#12
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Meimei Wang
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Quote:
 Originally Posted by immortality Hi I have a question like Anna. Anna did you find an answer to your question about importance of y+ in laminar cases? and is it important that flow be compressible or incompressible? thanks.
I think usually the boundary layer thickness doesn't strongly depend on whether the flow is compressible or incompressible. Y+ can be defined for laminar flow but doesn't make too much sense. We can use Blasius theory to estimate the laminar boundary thickness then put 11 points to the boundary layer. This could be a good starting point. Checking and grid independence study need to be done afterwards.
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