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-   -   how to deremine boundary layer height? (http://www.cfd-online.com/Forums/cfx/22399-how-deremine-boundary-layer-height.html)

Mike April 5, 2006 01:08

how to deremine boundary layer height?
 
I have faecd a fact that from manual thet when u are going to capture near wall region you have to refine mesh at near wall region ,and your y plus should be in limit of 11.In your boundary layer there should be 10 nodes.but how to determine this hight.i.e. there should be these much nodes in this hight so that we can mesh according to that. Thaks in advance.

Chebeba April 5, 2006 02:45

Re: how to deremine boundary layer height?
 
I don't know if this is the proper mathematical way, but what I do is run a solution, and if the turbulence doesn't change much from the outer-most boundary layer to the next node of the mesh, then the BL is high enough. /C

Glenn Horrocks April 5, 2006 18:19

Re: how to deremine boundary layer height?
 
Hi,

The issue of how to determine near wall mesh spacing is discussed in the CFX Modelling manual, page 126 (Guidelines for Mesh Generation in the Modelling Flow Near the Wall section).

You want y+>11 (or preferably 20) if you are using standard wall functions. Using the CFX automatic wall function treatment you can use y+<11 and it still functions well. If you use the k-omega based models (SST being the main one) then it can integrate to the wall with y+<1 approx. Thus the wall resolution you need depends on the turbulence model and flow type.

I have no idea where Chebeba's idea about zero turbulence gradient came from and I doubt it has any physical basis. It does not sound like a good approach to me. The approach recommended in the manual is my suggestion.

Regards, Glenn Horrocks

vijesh joshi April 6, 2006 03:06

Re: how to deremine boundary layer height?
 
hi, what i feel is you want to know the number of nodes to be put in the interested region (boundary layer). Get the two farmulas of Geometric progression -- 1. series summing and 2. last term obtaining. Ratio of last term to first term = L2 / L1 (CFX). and sum = total length of the boundary layer (guessing). With these two equations and set the value of first term-a, and assume ratio-r to get the number of nodes-n. I think it will work.

Mike April 6, 2006 04:02

Re: how to deremine boundary layer height?
 
Thaks sir,but what i want to know that how can i know that boudary layer height will b ethis much?should i calculte manually?and according to that i have to put no of nodes required by y plus criteria?is this the procedure?pls let me know.

longbow April 6, 2006 08:15

Re: how to deremine boundary layer height?
 
y plus value only tells you how far the first node away from wall surface. It does not control how many nodes are allocated in boundary layer. A simple and dirty way to figure it out is to generate a mesh and run it. The vector plot near surface will give you an idea how many nodes are in BL.


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