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gmsh boundary layer mesh for channel flow

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Old   November 4, 2022, 07:58
Question gmsh boundary layer mesh for channel flow
  #1
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Hi,

I would like to mesh a periodic channel flow with structured grid using gmsh.


I firstly mesh it with transfinite lines and it works but I do not succeed in keeping the first cell size to obtain y+=1 when refining the mesh:
Code:
n = 50;
nb_points = n;
r=1.02;
 nb_points_centre = 26;

SetFactory("OpenCASCADE");
//+
Point(1) = {0, 1/3, 0, 1.0};
//+
Point(2) = {0, 1, 0, 1.0};
//+
Point(3) = {1, 1, 0, 1.0};
//+
Point(4) = {1, 1/3, 0, 1.0};
//+
Point(11) = {1, 0.7, 0, 1.0};
//+
Point(13) = {0, 0.7, 0, 1.0};
//+
Line(1) = {4, 11};
//+
Line(2) = {11, 3};
//+
Line(4) = {1, 13};
//+
Line(5) = {13, 2};
//+
//+
Line(8) = {1, 4};
//+
Line(9) = {13, 11};
//+
Line(10) = {2, 3};
Line Loop(13) = {8, 1, -9, -4};
//+
Plane Surface(1) = {13};
//+
Line Loop(14) = {9, 2, -10, -5};
//+
Plane Surface(2) = {14};
Symmetry {0, 1, 0, -1} {
  Duplicata { Surface{1}; Surface{2}; }
}
Coherence;
Transfinite Line {1, 4} = nb_points Using Progression r;
Transfinite Line {10, 11} = nb_points_centre;
Transfinite Line {8,9} = 1;
Transfinite Line {9, 12} = 1;
Transfinite Line {17, 18} = nb_points_centre;
//+
Transfinite Line {14, 15} = nb_points Using Progression r;
//+
Transfinite Line {13, 16} = 1;
//+
Transfinite Line {16, 12} = 1;

Extrude {0, 0, 1} {
//  Surface{1, 2, 3, 4}; Recombine;
  Surface{1, 2, 3, 4}; Layers{1}; Recombine;
}
Recombine Surface "*";
 Transfinite Surface "*";
Could you please help me to mesh my domain with fixed first cell size using transfinite lines ?


I also tried to use the Boundarylayer function but I do not manage to set the thickness of the boundary layer equal to the half of my channel and/or to specify the size of the cells that are beyond the boundary layer. Below is a code example for a simple rectangle:

Code:
SetFactory("OpenCASCADE");
Rectangle(1) = {0, 0, 0, 1, 1, 0}; 

Transfinite Surface "*";
Recombine Surface "*";


Field[1] = BoundaryLayer;
Field[1].EdgesList = {1}; 
Field[1].NodesList = {1, 2}; 
Field[1].hwall_n = 0.01;
Field[1].thickness = 1;
Field[1].ratio = 1.1;
Field[1].Quads = 1;
BoundaryLayer Field = 1;
This gives me the attached mesh (see picture: Capture d’écran du 2022-11-04 13-51-47.png). In the picture we see that the boundary layer thickness is about 1/3 of the rectangle height despite the
Code:
Field[1].thickness = 1;
Could you please help me to solve that issue ?


How can I set the size of the cells that are above the boundary layer ?


Thanks a lot,

Boone
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Old   November 4, 2022, 10:19
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use this for the overall mesh size:

Code:
Mesh.CharacteristicLengthMin = 0.02;
Mesh.CharacteristicLengthMax = 0.02;

I think there is a limit to the size of the boundary layer thickness with respect to the total size of the domain, I could not make the boundary layer thickness larger than 0.4. For larger values, the thickness of the boundary layer was the same.
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Old   November 4, 2022, 11:39
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Thanks for your answer. In the meantime I found a way to solve this issue with the background function. It works well for a single boundary layer but when multiple boundaries are used it does not work. I read that we should use the Field[] = Min; to solve this but I do not manage to make it work.


I give my script below. If someone knows how to make it works with several backgronud functions it would be great =).

Code:
SetFactory("OpenCASCADE");

Dext=0.023;
dy0=0.00219133;
r=1.05;
Ly_wall=0.7-1/3;
Ly_center=0.3;
Rectangle(1) = {0, 1/3, 0, 1, Ly_wall, 0}; 
Rectangle(2) = {0, 0.7, 0, 1, Ly_center, 0}; 
Rectangle(3) = {0, 1, 0, 1, Ly_center, 0}; 
Rectangle(4) = {0, 1+Ly_center, 0, 1, Ly_wall, 0}; 


Coherence;

Transfinite Surface "*";
Recombine Surface "*";

Transfinite Line {5, 6} = Ly_center/Dext;
Transfinite Line {8, 9} = Ly_center/Dext;

Field[4] = BoundaryLayer;
Field[4].EdgesList = {13}; 
Field[4].NodesList = {9, 10}; 
Field[4].hwall_n = dy0;
Field[4].hfar = Dext;
Field[4].thickness = Ly_wall;
Field[4].ratio = r;
Field[4].Quads = 1;
BoundaryLayer Field = 4;
// Background Field = 4;

Field[1] = BoundaryLayer;
Field[1].EdgesList = {1}; 
Field[1].NodesList = {1, 2}; 
Field[1].hwall_n = dy0;
Field[1].hfar = Dext;
Field[1].thickness = Ly_wall;
Field[1].ratio = r;
Field[1].Quads = 1;
BoundaryLayer Field = 1;
//Background Field = 1;

//+
Field[5] = Min;
//+
Field[5].FieldsList = {1};
//+
Background Field = 5;
Thanks again
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boundary layer grid, channel flow, gmsh, mesh, structure mesh

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