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How to decide the number of grid lines in boundary layer

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Old   January 20, 2013, 16:21
Question How to decide the number of grid lines in boundary layer
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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.
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Old   January 21, 2013, 02:52
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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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Old   January 24, 2013, 16:08
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Quote:
Originally Posted by cfdnewbie View Post
That depends on the order of your scheme and on the type of flow you are considering. Impossible to answer in general....
Could you please recommend any paper about this?
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Old   January 24, 2013, 21:25
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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.
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Old   January 25, 2013, 04:04
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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.
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Old   January 25, 2013, 16:20
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Quote:
Originally Posted by venkateshaero View Post
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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Old   January 26, 2013, 07:48
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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. .
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Old   January 26, 2013, 09:42
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Quote:
Originally Posted by venkateshaero View Post
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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Old   January 27, 2013, 04:38
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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 .
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Old   June 7, 2013, 18:04
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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.
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Old   June 8, 2013, 09:04
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Quote:
Originally Posted by Anna Tian View Post
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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Old   June 13, 2013, 05:10
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Quote:
Originally Posted by immortality View Post
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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