This post explains briefly how can we get a smooth mesh when using the snappyHexMesh in OpenFOAM 2.0.

The main ingredient is the ‘surfaceFeatureExtract’ utility.

Here are the steps:

1- blockMesh: the BCs can be defined here, example: inlet and outlet patches.

2- extract the surface edge from the .stl file, example: tank.stl
command : “surfaceFeatureExtract -includedAngle 150 -writeObj constant/triSurface/tank.stl features”

this utiliy will create .eMesh file in /constant/triSurface/ : example tank.eMesh

3- in the snappyHexMeshDict, edge refinement is made by referring the eMesh file name, example: tank.eMesh.

4- run the snappyHexmesh utility.

Enjoy!

This post is about how to calculate yPlus in OpenFOAM.

For RANS
1) with wall function; simply type ‘yPlusRAS’
2) without wall function;
i- type wallShearStress
ii- calculate manually: y+ = sqrt(wallShearStress/rho) * y/nu
y = height of the cell near to the wall
nu = kinematic viscosity.

For LES
1_ simply type ‘yPlusLES’

enjoy!

This post is for OpenFOAM user.

Most of the times, we need to import CAD drawing to OpenFOAM, especially for the complex geomtery. The dimension in the drawing somehow may changes after we imported into OpenFOAM.

with ‘transformPoints’ utilitiy, we can manipulate the coordinate of the imported file to our requirement. Refer to this site for complete explanation:

https://openfoamwiki.net/index.php/TransformPoints

Example: if we want to scale the CAD drawing to a factor of 0.01 in x and y directions:

>> tranformPoints -scale ‘(0.01 0.01 1)’

if the surface is not smooth (due to the sudden change in geometry) use surfaceFeatureExtractDict, e.g.: tutorials/mesh/snappyHexMesh/flange

or snapEdgeDict http://openfoamwiki.net/index.php/Contrib_snapEdge

/*——————————–*- C++ -*———————————-*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  1.7                                   |
|   \\  /    A nd           | Web:      http://www.OpenFOAM.org               |
|    \\/     M anipulation  |                                                 |
\*—————————————————————————*/
FoamFile
{
version     2.0;
format      ascii;
class       dictionary;
location    “constant”;
object      snapEdgeDict;
}

snapEdgeDict
{

/*snapPatches
(
);

snapZones
(
);*/

stlFileNames
(
square.stl
);

// only move those points that are closer to the features than the tolerance*| edge length |
tolerance 1.9;

// 1.0 = move the points to the edge, 0 = dont move the point
// under-relaxation works better with many iterations to capture corners
relaxation 0.1;

// number of iterations to move points towards edge
nIterations 15;

// set to yes if edges inside a patch should be included
// set to no if only the edges of the patch should be moved
// in general: set this to no to get optimal mapping of inlet/outlet patches
// this ensures that wall-faces and inlet/outlet-faces are mapped to correct positions
includeInterior yes;

// resolve all features with greater angle than this
featureAngle 90.0;

// do not snap edges with an angle greater than this to the feature line
excludeEdgeAngle 60;

// used when selecting between multiple edges to map to the same position
// if the angle is above this value
//     the choice will be based on which edge has the smallest angle to feature line
// otherwise
//     the choice will be based on which edge is closest
parallelAngle 50.0;

// allow for small overlap when fitting the edges to feature lines
fitFactor 1.0e-2;

}

enjoy!