3.2. Automated Meshing

3.2.1. Overview

Flow360 offers automated meshing, from CAD geometry to a surface mesh and finally to a volume mesh. The supported CAD format is *.csm file which is Engineering Sketch Pad (EPS) format. The generated volume format is CGNS.

3.2.2. Geometry

The Engineering Sketch Pad is a solid-modeling, feature-based, web-enabled system for building parametric geometry. It can be downloaded from ESP’s website. Please check out pre-built ESP, you don’t need to compile software from source. The geometry in ESP is described in a text *.csm file containing all CAD instructions. See csm example:

# Branches:
sphere    0   0   0   1

attribute groupName $mysphere

end

which will create a sphere of radius = 1 at (0, 0, 0). The face of the sphere will be labelled as mysphere.

../_images/sphere_csm.png — Fig. 3.2.1 Engineering Sketch Pad view of the geometry.

3.2.3. Surface Meshing

The surface mesher takes the geometry file and configuration JSON file as input parameters and generates a surface mesh. The meshing configuration file (or dict in python), also called surfaceMesh.json, contains information such as maximum element edge length, curvature resolution angle or growth rate of 2D layers. See JSON example:

{
    "maxEdgeLength": 0.1, 
    "curvatureResolutionAngle": 15,
    "growthRate": 1.2, 
    "faces": {
        "mysphere": {
            "maxEdgeLength": 0.05,
            "adapt": false
        }
    }
}

The surface mesh is created by submitting a geometry file and JSON file using NewSurfaceMeshFromGeometry() function. See the example below:

import flow360client
surfaceMeshId = flow360client.NewSurfaceMeshFromGeometry("path/to/geometry.csm", "surfaceMesh.json", surfaceMeshName="my_surface_mesh", solverVersion='release-21.4.1.0')

The above code will create a surface mesh of geometry using "maxEdgeLength": 0.05 on a surface labelled as mysphere, see line 4 of the *.csm file.

../_images/sphere_surfMesh.png — Fig. 3.2.2 Auto-generated surface mesh.

Inputs:

geometry.csm file
surface mesh config.json or python dict

Outputs:

surface mesh
surfaceMeshId

For a full description of config.json for surface mesher see here.

Note

When using web interface make sure to select version during New Surface Mesh upload

3.2.4. Volume Meshing

The volume mesher takes a surfaceMeshId and config JSON as arguments and generates a CGNS mesh suitable for the Flow360 solver. The JSON configuration file (or dict in python) specifies the first layer thickness, growth rate, sizes and location of refinement zones (sources) and actuator disks. All geometry from ESP is treated as a no-slip wall therefore prism layers will be grown off geometry surfaces.

The farfield will be created automatically:

semi-spherical if the geometry is bounded by y=0 plane. The y=0 plane will be set as a symmetry plane boundary condition.
spherical otherwise.

Below is an example of a JSON configuration file:

{   
    "sources": [                                                                 
        {                                                                        
            "size": [4, 3, 2],                                                                                                              
            "axisOfRotation": [ 0, 0, 1 ],               
            "angleOfRotation": 45,                                               
            "center": [2, 0, 0],                 
            "spacing": 0.05                                               
        }                                                                                                                                                 
    ],                                                                                                                                                      
    "volume": {                                                                  
        "firstLayerThickness": 1e-3,                                             
        "TRexGrowthRate": 1.2                                                    
    }                                                                            
}                   

The volume mesh is created by NewMeshFromSurface() function using surfaceMeshId and config.json. See the example below:

volumeMeshId = flow360client.NewMeshFromSurface(surfaceMeshId, "volumeMesh.json", meshName="my_volume_mesh")

The above code will create a volume mesh out of the surface mesh. The refinement zone (source) of size=4x3x2 will be placed with its centre at (2,0,0). Additionally, it will be rotated by 45 degrees around the [0,0,1] axis. Spacing of 0.05 will be applied in this zone.

../_images/sphere_volMesh.png — Fig. 3.2.3 Auto-generated volume mesh.

Inputs:

surfaceMeshId
volume mesh config.json or python dict

Outputs:

volume CGNS mesh
Flow360Mesh.json
volumeMeshId

For a full description of config.json for volume mesher see here.

3.2.5. JSON surface mesher

Option	Type	Default	Example	Description
maxEdgeLength	float	REQUIRED	0.1	Maximum length of element edge.
curvatureResolutionAngle	float	REQUIRED	15	Maximum angular deviation in degrees. If the angle between the normal line segment and the normal to the underlying geometry curve at a grid point is larger than the specified angle, a new grid point will be added in the middle of the segment.
growthRate	float	REQUIRED	1.2	Growth rate of layers being grown from edges
edges	dict	{}		<edgeName>: {…} sets parameters for the edge named <edgeName>. The edge must be labelled in ESP (*.csm file). For details see here.
faces	dict	{}		<faceName>: {…} sets parameters for the face named <faceName>. The face must be labelled in ESP (*.csm file). For details see here.

3.2.5.1. edges

Option	Type	Default	Example	Description
<edgeName>-> firstLayerThickness	float	REQUIRED	1e-3	First layer thickness of the anisotropic surface cells growing along the orthogonal direction to the edge named <edgeName>. See Fig. 3.2.4.

Example:

"edges": {
    "leadingEdge":  {
        "firstLayerThickness": 1e-3
    },
    "trailingEdge":  {
        "firstLayerThickness": 1e-3
    }
}

../_images/surfaceMesh_firstLayerThickness.png — Fig. 3.2.4 Surface mesh of a wing. Leading edge and trailing edge are labelled and first layer thickness is applied to the layers grown from the edges.

3.2.5.2. faces

Option	Type	Default	Example	Description
<faceName>-> maxEdgeLength	float	REQUIRED	0.05	Maximum length of element edge for the face named <faceName>.
<faceName>-> adapt	boolean	REQUIRED	FALSE	Turn on/off the extra refinement based on surface proximity. [This argument will be removed in the next release and the proximity refinement will be automatically decided.]

Example:

"faces": {
    "rightWing": {
        "maxEdgeLength": 0.05,
        "adapt": false
    },
    "fuselage": {
        "maxEdgeLength": 0.05,
        "adapt": false
    }
}

3.2.6. JSON volume mesher

Option	Type	Default	Example	Description
refinementFactor	float	1	2	If refinementFactor=r is provided all spacings will be adjusted to generate r-times finer mesh everywhere. For example, if refinementFactor=2, all spacings will be divided by \(2^{1/3}\approx 1.26\) , so the resulting mesh will have approximately 2 times more nodes.
volume	dict	REQUIRED		First layer thickness and growth rate for prism layers being grown out of the surface. It is applied to all surface mesh faces.
volume-> firstLayerThickness	float	REQUIRED	1e-5	First layer thickness for volume prism layers.
volume-> TRexGrowthRate	float	REQUIRED	1.2	Growth rate for volume prism layers.
sources	list(dict)	[]		Description of refinement zones. For details see here.
actuatorDisks	list(dict)	[]		Description of actuator disks. This generates a cylindrical shape (hollowed or not) with structured mesh in it. Used for actuator disks. For details see here.

3.2.6.1. sources (list)

Option	Type	Default	Example	Description
type	[“box”, “cylinder”]	“box”	“box”	Type of source. Supported source shapes: box and cylinder. Cylinder’s axis is along the z-direction.
size	3-array	REQUIRED	[4, 3, 2]	For type: box. Refinement zone size in x, y and z directions.
radius	float	REQUIRED	4	For type: cylinder. Radius of the cylinder.
lengthZ	float	REQUIRED	5	For type: cylinder. Length of the cylinder. Cylinder’s axis is along the z-direction.
center	3-array	REQUIRED	[2, 0, 0]	Position of geometrical center of the primitive, i.e., box or cylinder.
spacing	float	REQUIRED	0.05	Cell spacing applied in the refinement zone.
axisOfRotation	3-array		[0, 0, 1]	Axis of rotation for rotation transform of the primitive (box or cylinder). Requires angleOfRotation.
angleOfRotation	float		45	The angle of rotation (in degrees) for rotation transform of the primitive (box or cylinder). Requires axisOfRotation.

Example:

"sources": [
 {
     "size": [4, 3, 2],
     "center": [2, 0, 0],
     "spacing": 0.05,
     "axisOfRotation": [ 0, 0, 1 ],
     "angleOfRotation": 45
 },
 {
     "type": "cylinder",
     "radius": 4,
     "lengthZ": 5,
     "center": [5, 0, 0],
     "spacing": 0.05,
     "axisOfRotation": [0, 1, 0],
     "angleOfRotation": 90
 }]

3.2.6.2. actuatorDisks (list)

Option	Type	Default	Example	Description
innerRadius	float	REQUIRED	0	Inner radius of actuator disk, if greater than 0 then “donut” shape is created.
outerRadius	float	REQUIRED	10	Outer radius of actuator disk.
thickness	float	REQUIRED	0.5	Thickness of actuator disk.
axisThrust	3-array	REQUIRED	[1, 0, 0]	Axis of thrust of actuator disk. The cylindrical shape is rotated such its axis is aligned with axisThrust.
center	3-array	REQUIRED	[0, 0, 0]	Position of center of the actuator disk.
spacingAxial	float	REQUIRED	0.01	Spacing along the axial direction.
spacingRadial	float	REQUIRED	0.03	Spacing along the radial direction.
spacingCircumferential	float	REQUIRED	0.03	Spacing along the circumferential direction.

Example:

"actuatorDisks": [
 {
     "innerRadius": 0,
     "outerRadius": 10,
     "thickness": 0.5,
     "axisThrust": [1, 0, 0],
     "center": [0, 0, 0],
     "spacingAxial": 0.01,
     "spacingRadial": 0.03,
     "spacingCircumferential": 0.03
 }]