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refinement of mesh around a half-cylinder-using blockMesh

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Old   July 15, 2013, 08:25
Default refinement of mesh around a half-cylinder-using blockMesh
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Amin
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i would like to refine the mesh around a half cylinder, that is why i used simpleGrading to refine the area near to the surface of the half-cylinder, in order to capture the boundary layer. however it is not working ...

here is my blockMeshDict :

Code:
// The OpenFOAM Project // File: blockMeshDict
  /*
  -------------------------------------------------------------------------------
   =========         | dictionary
   \\      /         |
    \\    /          | Name:   blockMeshDict
     \\  /           | Family: FoamX configuration file
      \\/            |
      F ield         | OpenFOAM version: 2.3
      O peration     | Product of Nabla Ltd.
      A and          |
      M anipulation  | author: Amine Abd.
  -------------------------------------------------------------------------------
  */
  // FoamX Case Dictionary.
  
  FoamFile
  {
      version         2.0;
      format          ascii;
  
      root            "";
      case            "";
      instance        "";
      local           "";
  
      class           dictionary;
      object          blockMeshDict;
  }
 
  // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
  
 convertToMeters 1;
  
  vertices
  (

    (-0.5 0 0)//0
    (-0.353553 0.353553 0)//1
    (0 0.5 0)//2
    (24 0.5 0)//3
    (24 -0.5 0)//4
    (0 -0.5 0)//5
    (-0.353553 -0.353553 0)//6
    (-1 0 0)//7
    (-0.707107 0.707107 0)//8
    (0 1 0)//9
    (24 1 0)//10
    (24 -1 0)//11
    (0 -1 0)//12
    (-0.707107 -0.707107 0)//13
    (-8 0 0)//14
    (-8 0.707107 0)//15
    (-8 8 0)//16
    (-0.707107 8 0)//17
    (0 8 0)//18
    (24 8 0)//19
    (24 -8 0)//20
    (0 -8 0)//21
    (-0.707107 -8 0)//22
    (-8 -8 0)//23
    (-8 -0.707107 0)//24-------------
    (-0.5 0 0.1)//25
    (-0.353553 0.353553 0.1)//26
    (0 0.5 0.1)//27
    (24 0.5 0.1)//28
    (24 -0.5 0.1)//29
    (0 -0.5 0.1)//30
    (-0.353553 -0.353553 0.1)//31
    (-1 0 0.1)//32
    (-0.707107 0.707107 0.1)//33
    (0 1 0.1)//34
    (24 1 0.1)//35
    (24 -1 0.1)//36
    (0 -1 0.1)//37
    (-0.707107 -0.707107 0.1)//38
    (-8 0 0.1)//39
    (-8 0.707107 0.1)//40
    (-8 8 0.1)//41
    (-0.707107 8 0.1)//42
    (0 8 0.1)//43
    (24 8 0.1)//44
    (24 -8 0.1)//45
    (0 -8 0.1)//46
    (-0.707107 -8 0.1)//47
    (-8 -8 0.1)//48
    (-8 -0.707107 0.1)//49
    




  );
  
  blocks
  (


    hex (7 0 1 8 32 25 26 33) (50 50 1) simpleGrading (1 1 1)//1
    hex (8 1 2 9 33 26 27 34) (50 50 1) simpleGrading (1 1 1)//2
    hex (2 3 10 9 27 28 35 34) (200 50 1) simpleGrading (16469 1 1)//3
    hex (12 11 4 5 37 36 29 30) (200 50 1) simpleGrading (16469 1 1)//4
    hex (13 12 5 6 38 37 30 31) (50 50 1) simpleGrading (1 1 1)//5
    hex (13 6 0 7 38 31 25 32) (50 50 1) simpleGrading (1 1 1)//6
    hex (14 7 8 15 39 32 33 40) (50 50 1) simpleGrading (0.03 1 1)//7
    hex (15 8 17 16 40 33 42 41) (50 80 1) simpleGrading (0.03 20 1)//8
    hex (8 9  18 17 33 34 43 42) (50 80 1) simpleGrading (1 20 1)//9
    hex (9 10 19 18 34 35 44 43) (200 80 1) simpleGrading (16469 20 1)//10
    hex (21 20 11 12 46 45 36 37) (200 50 1) simpleGrading (16469 0.05 1)//11
    hex (22 21 12 13 47 46 37 38) (50 50 1) simpleGrading (1 0.05 1)//12
    hex (23 22 13 24 48 47 38 49) (50 50 1) simpleGrading (0.03 0.05 1)//13
    hex (24 13 7 14 49 38 32 39) (50 50 1) simpleGrading (0.03 1 1)//14
    hex (5 4 3 2 30 29 28 27) (200 200 1) simpleGrading (16469 1 1)//15

  );
  
  edges

  (
    arc 5 6 (-0.17101 -0.469846 0)
    arc 6 0 (-0.469846 -0.17101 0)
    arc 0 1 (-0.469846 0.17101 0)
    arc 1 2 (-0.17101 0.469846 0)
    arc 9 8 (-0.34202 0.939693 0)
    arc 8 7 (-0.939693 0.34202 0)
    arc 7 13 (-0.939693 -0.34202 0)
    arc 13 12 (-0.34202 -0.939693 0)//----
    arc 30 31 (-0.17101 -0.469846 0.1)
    arc 31 25 (-0.469846 -0.17101 0.1)
        arc 25 26 (-0.469846 0.17101 0.1)
    arc 26 27 (-0.17101 0.469846 0.1)
    arc 34 33 (-0.34202 0.939693 0.1)
    arc 33 32 (-0.939693 0.34202 0.1)
    arc 32 38 (-0.939693 -0.34202 0.1)
    arc 38 37 (-0.34202 -0.939693 0.1)
    
    

  );
  

boundary
(

 inlet
    {
        type patch;
        faces
        (
          
    (16 15 40 41)
    (15 14 39 40)
    (14 24 49 39)
    (24 23 48 49)      
  );
    }
    
 outlet
    {
        type patch;
        faces
        ( 
            (10 19 44 35)
            (3 10 35 28)
            (4 3 28 29)
        (11 4 29 36)
        (20 11 36 45)    
            
       
               
        );
    }
 
 fixedWalls
    {
        type wall;
        faces
        (
    (17 16 41 42)
    (18 17 42 43)
    (19 18 43 44)
    (21 20 45 46)
    (22 21 46 47)
    (23 22 47 48)
    (2 5 30 27)
    (0 1 26 25)
    (1 2 27 26)
    (6 0 25 31)
    (5 6 31 30)

         
           );
    }      

    frontAndBack
    {
        type empty;
        faces
        ( 

    (20 21 12 11)//b11
    (11 12 5 4)//b4
    (4 5 2 3)//b15
    (3 2 9 10)//b3
    (10 9 18 19)//b10
    (9 8 17 18)//b9
    (8 15 16 17)//b8
    (7 14 15 8)//b7
    (13 24 14 7)//b14
    (22 23 24 13)//b13
    (21 22 13 12)//b12
    (12 13 6 5)//b5
    (13 7 0 6)//b6
    (0 7 8 1)//b1
    (1 8 9 2)//b2-------spiegel
    (46 45 36 37)//b11
    (37 36 29 30)//b4
    (30 29 28 27)//b15
    (27 28 35 34)//b3
    (34 35 44 43)//b10
    (33 34 43 42)//b9
    (40 33 42 41)//b8
    (39 32 33 40)//b7
    (49 38 32 39)//b14
    (48 47 38 49)//b13
    (47 46 37 38)//b12
    (38 37 30 31)//b5
    (38 31 25 32)//b6
    (32 25 26 33)//b1
    (33 26 27 34)//b2


           
       
        );
    }
);


mergePatchPairs
(
);

// ************************************************************************* //

any idea, how to refine the mesh ?

Thx
Attached Images
File Type: jpg mesh.jpg (96.1 KB, 78 views)
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Old   July 15, 2013, 10:26
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well you do some refinement, but in the wrong block.

Be aware that the blocks are counting from 0
so you have 15 blocks but the lowest number is zero
and the highest number is 14

It is the same with the points and the patches as well.

Hint for the future: have look at the whole mesh and see what
it looks like so you can easily figure out what is wrong and do not
only focus in the part that doesn't work, like the picture you displayed.

success
regards
Colin
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Old   July 16, 2013, 00:58
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Amin
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Concerning the numbering of the blocks..you are right i was too lazy to edit it
However capturing the boundary layer is very important for me, that is why i want to refine the mesh close to surface of the geometry....

Any Idea ??
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Old   July 16, 2013, 02:53
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Hey,

for a good resolution of the boundary layer the y+ value
gives a good indication.

so when you re done with meshing you type

yPlusRAS

in your terminal and check the values which are displayed.
(I assume you are doing a RANS calculation if you
are performing a LES calculation use yPlusLES instead)

dependant on whether you are using wall functions or not
the max values should be between 1 (no wall functions) and
300 (wall functions)

If you know your desired yPlus you can easily calculate the required
cell height with the formula mentioned here:

http://www.cfd-online.com/Wiki/Dimen...e_%28y_plus%29

I hope I could contribute,
regards
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Old   July 16, 2013, 03:55
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Amin
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Hey colin


Thx for ur answer

I am using icoFoam, so i think using a turbulence model is not required.

Do you know how to use yPlus for laminar flow ?

Thx
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Old   July 17, 2013, 04:49
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Hi

icoFoam is indeed for laminar flow,

so for the boundary layer resolution you should
estimate the thickness of your boundary layer
as described e.g. here:

http://en.wikipedia.org/wiki/Boundary-layer_thickness

in a different forum post it is suggested to have 10 layers
of cells within the viscous layer (see below post #4):

Prism layer needed to resolve laminar boundary (lowRe)

this should give you a good starting point.

regrads
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Old   July 17, 2013, 06:11
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Hello Colin

It is for me clear that #the mesh, close to the surface, should be fine, otherwise it will be hard to converge#.

I estimated the thickness of the boundary layer, and my question is, is there any feature to specify that the mesh should be fine near to surface ?

Thx
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