Automated Meshing

3.1. Automated Meshing#

Overview#

Flow360 offers automated meshing, from a CAD geometry to a surface mesh and finally to a volume mesh. The supported CAD formats are CSM and EGADS. The output volume meshes are in CGNS format.

Meshing settings#

Mesh in flow360 can be defined using settings, which fall into one of the three categories visible below:

Setting	Default	Description
`defaults`	REQUIRED	Default/global settings for meshing.
`volume_zones`	REQUIRED	Creation of new volume_zones. Can be of type `RotationCylinder` or `AutomatedFarfield`.
`refinements`	OPTIONAL	Additional fine-tunning for the mesh on top of default settings.

More details about each of those categories can be found here:

defaults
volume_zones
refinements

Surface Meshing#

The surface mesher takes the geometry file and user defined settings as inputs to generate a surface mesh. Here, settings can be defined either in WebUI or through Python API, and are used to control the surface mesh resolution. Example of defining surface meshing settings in Python API:

import flow360 as fl
with fl.SI_unit_system:
    params = fl.SimulationParams(
        meshing=fl.MeshingParams(
            defaults=fl.MeshingDefaults(
                surface_max_edge_length=1,
                surface_edge_growth_rate=1.2,
                curvature_resolution_angle=12 * fl.u.deg
            )
        ),
        ...
    )

The surface mesh can be created by submitting the project with a geometry file and providing previously set up SimulationParams containing MeshingParams via the Python API:

import flow360 as fl
project = fl.Project.from_file("path/to/geometry.csm", "MyProject")
project.generate_surface_mesh(params, "MySurfaceMesh")
print(project.surface_mesh.id)

Inputs

WebUI project that contains an uploaded geometry
params: SimulationParams

Outputs

surface mesh on cluster
return the surface_mesh.id

../../_images/surfaceMesh.png — Fig. 3.1.1 Auto-generated surface mesh. An extra refinement is applied to the top face of the box.#

../../_images/surfaceMesh_firstLayerThickness.png — Fig. 3.1.2 Surface mesh on a wing. Anisotropic layers are growing from leading and trailing edges.#

../../_images/projectAnisoSpacing.png — Fig. 3.1.3 Surface mesh on a hub. Anisotropic layers are growing from `hubCircle` (green). The anisotropic spacing on the neighboring patches is “projected” to `hubSplitEdge` (red) and updates the nodes distribution along `hubSplitEdge`.#

Volume Meshing#

The volume mesher takes a surface mesh and user defined settings to generate a volume mesh. These settings are defined either through WebUI or Python API and determine the volume mesh resolution. They contain boundary_layer_first_layer_thickness and boundary_layer_growth_rate of 3D anisotropic layers, the size/location/resolution of refinements and etc. Below is an example of defining volume meshing settings in Python API:

import flow360 as fl
with fl.SI_unit_system:
    cylinder1 = fl.Cylinder(name="Cylinder1", axis=(0, 1, 0), center=(1, 0.2, 0), outer_radius=3, height=2)
    farfield = fl.AutomatedFarfield(name="farfield", method="auto")
    params = fl.SimulationParams(
        meshing=fl.MeshingParams(
            defaults=fl.MeshingDefaults(
                boundary_layer_growth_rate=1.15,
                boundary_layer_first_layer_thickness=3e-6,
            ),
            refinement_factor=1.25,
            gap_treatment_strength=0.7,
            volume_zones=[farfield],
            refinements=[
                fl.UniformRefinement(name="refinement1", spacing=0.2, entities=[cylinder1])
            ]
        ),
        ...
    )

In the above Python script, there is a cylindrical volume mesh refinement zone centered at (1, 0.2, 0). This refinement will have a uniform spacing of 0.2 m in this volume zone. The volume mesh can be created from an existing surface mesh and parameters for the volume meshing, for example, using the Python API:

import flow360 as fl
project = fl.Project.from_file("path/to/geometry.csm", "MyProject")
project.generate_volume_mesh(params, "MyVolumeMesh")
print(project.volume_mesh.id)

Inputs

WebUI project that contains an uploaded geometry
params: SimulationParams

Outputs

volume mesh on cluster (in CGNS format)
Flow360Mesh.json on cluster
return the volume_mesh.id

../../_images/volumeMesh3DView.svg — Fig. 3.1.4 Left: WebUI showing the refinement zone. Right: auto-generated volume mesh.#

../../_images/volumeMeshSlices.svg — Fig. 3.1.5 Auto-generated volume mesh. Left: sliced at y=1. Right: sliced at y=-1.#

`defaults`#

This section describes the default settings for both surface and volume meshing.

Option	Default	Type	Description
`surface_edge_growth_rate`	1.2	SURFACE	[float] Growth rate of the anisotropic layers grown from the edges.
`surface_max_edge_length`	REQUIRED	SURFACE	[float] Default maximum edge length for surface cells. This can be overridden with `SurfaceRefinement`.
`curvature_resolution_angle`	12	SURFACE	[float] Default maximum angular deviation in degrees. This value will restrict: 1. The angle between a cell’s normal and its underlying surface normal. 2. The angle between a line segment’s normal and its underlying curve normal. This can not be overridden per face.
`boundary_layer_growth_rate`	1.2	VOLUME	[float] Default growth rate for volume prism layers. This can not be overridden per face.
`boundary_layer_first_layer_thickness`	REQUIRED	VOLUME	[float] Default first layer thickness for volumetric anisotropic layers. This can be overridden with `BoundaryLayer`.
`refinement_factor`	1	VOLUME (ADVANCED)	Adjusts all spacings in refinement regions and first layer thickness to generate r-times finer mesh, where r i s the `refinement_factor` value.
`gap_treatment_strength`	0	VOLUME (ADVANCED)	Narrow gap treatment strength used when two surfaces are in close proximity. Takes a value between 0 and 1, where 0 is no treatment and 1 is the most conservative treatment. This parameter has a global impact where the anisotropic transition into the isotropic mesh. Its impact on regions without close proximity is negligible.

Example of defining MeshingDefaults in Python API:

import flow360 as fl
with fl.SI_unit_system:
    params = fl.SimulationParams(
        meshing=fl.MeshingParams(
            defaults=fl.MeshingDefaults(
                surface_max_edge_length=1,
                surface_edge_growth_rate=1.2,
                curvature_resolution_angle=12 * fl.u.deg,
                boundary_layer_growth_rate=1.1,
                boundary_layer_first_layer_thickness=1e-5
            ),
            refinement_factor=1.1,
            gap_treatment_strength=0.2
        )
        ...
    )

Gap treatment examples:

../../_images/gap.svg — Fig. 3.1.6 Different `gap_treatment_strength` strength. The larger `gap_treatment_strength` is, the more space is dedicated to isotropic mesh than anisotropic mesh in narrow gaps.#

`volume_zones`#

Settings used for defining AutomatedFarfield and RotationCylinder volume zones.

Volume zone	Default	Description
`AutomatedFarfield`	REQUIRED	Automatically created farfield volume zone.
`RotationCylinder`	OPTIONAL	Rotation cylinder volume zone.

Example of defining volume_zones in Python API:

import flow360 as fl
with fl.SI_unit_system:
    farfield = fl.AutomatedFarfield(name="Farfield", method="auto")
    cylinder = fl.Cylinder(name="Cylinder", axis=[0, 1, 0], center=[1, 1, 1], outer_radius=10, height=3)
    rotation_cylinder = fl.RotationCylinder(
        name="RotationCylinder",
        spacing_axial=0.5,
        spacing_circumferential=0.3,
        spacing_radial=1.5,
        entities=cylinder
    )
    params = fl.SimulationParams(
        meshing=fl.MeshingParams(
            ...
            volume_zones=[farfield, rotation_cylinder]
        ),
        ...
    )

`AutomatedFarfield`#

Settings for automatic farfield volume zone generation.

Option

Default

Description

name

Farfield

Name of the AutomatedFarfield.

method

auto

By default, the farfield shape will be automatically determined by the volume mesher. The user can prescribe the farfield shape if needed.

auto: The mesher will generate a Sphere or a semi-sphere based on the bounding box of the geometry
- Full sphere if min{Y} < 0 and max{Y} > 0
- +Y semi sphere if min{Y} = 0 and max{Y} > 0
- -Y semi sphere if min{Y} <> 0 and max{Y} = 0
quasi-3d: Thin disk will be generated for quasi 3D cases. Both sides of the farfield disk will be treated as “symmetric plane”.

`RotationCylinder`#

The mesh on RotationCylinder is guaranteed to be concentric.
The RotationCylinder is designed to enclose other objects, but it can’t intersect with other objects.
Users could create a donut-shape RotationCylinder and put their stationary centerbody in the middle.
This type of volume zone can be used to generate volume zone compatible with Rotation model.

Option	Default	Description
`name`	Rotation cylinder	Name of the `RotationCylinder`.
`spacing_axial`	REQUIRED	[float] Spacing along the axial direction.
`spacing_radial`	REQUIRED	[float] Spacing along the radial direction.
`spacing_circumferential`	REQUIRED	[float] Spacing along the circumferential direction.
`entities` (`Cylinder`)	REQUIRED	List of volumes where `RotationCylinder` will be applied.
`enclosed_entities` (`Cylinder`, `Surface`)	OPTIONAL	List of entities enclosed by `RotationCylinder`, can be a `Cylinder` and/or `Surface`.

`refinements`#

Describes the settings used for defining additional settings for refining the mesh on top of MeshingDefaults.

Refinement	Default	Description
`SurfaceEdgeRefinement`	OPTIONAL	Setting for growing anisotropic layers orthogonal to the specified `Edge` (s).
`SurfaceRefinement`	OPTIONAL	Setting for refining surface elements for given `Surface` (s).
`BoundaryLayer`	OPTIONAL	Setting for growing anisotropic layers orthogonal to the specified `Surface` (s).
`PassiveSpacing`	OPTIONAL	Setting for controling the mesh spacing either through adjacent `Surface`’s meshing settings or retaining the current surface mesh.
`UniformRefinement`	OPTIONAL	Setting for uniform spacing refinement inside specified region of mesh.
`AxisymmetricRefinement`	OPTIONAL	Setting for axisymmetric refinement inside specified region of mesh.

Example of defining refinements in Python API:

import flow360 as fl
with fl.SI_unit_system:
    farfield = fl.AutomatedFarfield(name="Farfield", method="auto")
    cylinder = fl.Cylinder(name="Cylinder", axis=[0, 1, 0], center=[1, 1, 1], outer_radius=10, height=3)
    rotation_cylinder = fl.RotationCylinder(
        name="RotationCylinder",
        spacing_axial=0.5,
        spacing_circumferential=0.3,
        spacing_radial=1.5,
        entities=cylinder
    )
    params = fl.SimulationParams(
        meshing=fl.MeshingParams(
            ...
            volume_zones=[farfield, rotation_cylinder]
        ),
        ...
    )

`SurfaceEdgeRefinement`#

Setting for growing anisotropic layers orthogonal to the specified Edge (s).

Option

Default

Description

name

SurfaceEdgeRefinement

Name of the SurfaceEdgeRefinement.

method

REQUIRED

Method for determining the spacing. Can be one of:

AngleBasedRefinement: Surface edge refinement by specifying curvature resolution angle.
HeightBasedRefinement: Surface edge refinement by specifying first layer height of the anisotropic layers.
AspectRatioBasedRefinement: Surface edge refinement by specifying maximum aspect ratio of the anisotropic cells.
ProjectAnisoSpacing: Project the anisotropic spacing from neighboring faces to the edge.

entities (Edge)

REQUIRED

List of Edge (s), which will be used for SurfaceEdgeRefinement.

`SurfaceRefinement`#

Setting for refining surface elements for given Surface.

Option	Default	Description
`name`	SurfaceRefinement	Name of the `SurfaceRefinement`.
`max_edge_length`	REQUIRED	[float] Maximum edge length of surface cells.
`entities` (`Surface`)	REQUIRED	List of `Surface` (s), which will be used for `SurfaceRefinement`.

`BoundaryLayer`#

Setting for growing anisotropic layers orthogonal to the specified Surface (s).

Option	Default	Description
`name`	BoundaryLayerRefinement	Name of the `BoundaryLayer`.
`first_layer_thickness`	REQUIRED	[float] First layer thickness for volumetric anisotropic layers grown from given `Surface` (s).
`entities` (`Surface`)	REQUIRED	List of `Surface` (s), which will be used for `BoundaryLayer`.

`PassiveSpacing`#

Passively control the mesh spacing either through the adjacent Surface’s meshing settings or doing nothing to change existing surface mesh at all.

Option

Default

Description

name

PassiveSpacingRefinement

Name of the PassiveSpacing.

type

REQUIRED

[str] Type of the PassiveSpacing refinement. Can be one of:

projected: Turns off anisotropic layers growing for selected Surface (s) and projects anisotropic spacing from the neighboring volumes to this face.
unchanged: Turns off anisotropic layers growing for selected Surface (s), the surface mesh will remain unaltered when populating the volume mesh.

entities (Surface)

REQUIRED

List of Surface (s), which will be used for PassiveSpacing.

`UniformRefinement`#

Uniform spacing refinement inside specified region of mesh.

Option	Default	Description
`name`	UniformRefinement	Name of the `UniformRefinement`.
`spacing`	REQUIRED	The required refinement spacing.
`entities` (`Cylinder`, `Box`)	REQUIRED	List of `Cylinder` (s) and/or `Box` (s), which will be used for `UniformRefinement`.

`AxisymmetricRefinement`#

The mesh inside the AxisymmetricRefinement is semi-structured.
The AxisymmetricRefinement cannot enclose/intersect with other objects.
Users could create a donut-shaped AxisymmetricRefinement and place their hub/centerbody in the middle.
AxisymmetricRefinement can be used for resolving the strong flow gradient along the axial direction for actuator or BET disks.
The spacings along the axial, radial and circumferential directions can be adjusted independently.

Option	Default	Description
`name`	AxisymmetricRefinement	Name of the `AxisymmetricRefinement`.
`spacing_axial`	REQUIRED	[float] Spacing along the axial direction.
`spacing_radial`	REQUIRED	[float] Spacing along the radial direction.
`spacing_circumferential`	REQUIRED	[float] Spacing along the circumferential direction.
`entities` (`Cylinder`)	REQUIRED	List of `Cylinder` (s), which will be used for `AxisymmetricRefinement`.

Automated Meshing

Contents

3.1. Automated Meshing#

Overview#

Meshing settings#

Surface Meshing#

Volume Meshing#