Mesh parameters

The Mesh parameters in Flow360 provide fundamental control over mesh generation, affecting both surface and volume mesh characteristics. These settings serve as global defaults that determine the quality and characteristics of the computational mesh.

Available Options

Surface mesh

Option	Description	Applicable
Surface max edge length	Maximum allowable edge length for surface elements	always
Geometry accuracy	Smallest length scale to be resolved by surface meshing	Geometry AI
Curvature resolution angle	Maximum angle a single surface mesh element can span	always
Target surface node count	Target number of nodes for the surface mesh	beta mesher, Geometry AI
Surface edge growth rate	Controls the size progression of elements grown from edges	advanced
Surface max aspect ratio	Maximum aspect ratio for surface cells	advanced, Geometry AI
Resolve face boundaries	Whether to resolve boundaries between adjacent faces	advanced, Geometry AI
Surface max adaptation iterations	Maximum iterations for surface mesh adaptation	advanced, Geometry AI

Volume mesh

Option	Description	Applicable
Boundary layer first layer thickness	First layer thickness for volumetric anisotropic layers	always
Boundary layer growth rate	Growth rate for prismatic boundary layer elements	advanced
Refinement factor	Global scaling factor for mesh refinement	advanced
Gap treatment strength	Controls mesh behavior in narrow gaps	advanced
Number of boundary layers	Fixed number of boundary layer elements	advanced, beta mesher
Sliding interface tolerance	Tolerance for sliding interface detection	advanced
Edge split layers at corners	Number of boundary layer edge-splitting layers at geometric corners	advanced, beta mesher
Octree base spacing	Base spacing for the octree volume mesher	advanced, beta mesher

Geometry

Option	Description	Applicable
Sealing size	Threshold size below which geometry gaps are closed	advanced, Geometry AI
Preserve thin geometry	Whether thin geometry features should be resolved	advanced, Geometry AI
Remove hidden geometry	Removes internal geometry not visible to flow	advanced, Geometry AI
Min passage size	Minimum passage size that hidden geometry removal can resolve	advanced, Geometry AI, Remove hidden geometry is On
Planar face tolerance	Tolerance for detecting planar faces	advanced

Detailed Descriptions

Surface mesh

Surface max edge length

Defines the maximum allowable length for any mesh edge on surfaces.

• Required

• Units: Length

Notes:

• Can be overridden using Surface Refinement

• Should be chosen based on geometry scale and required resolution

• Affects overall mesh density and computational cost

Geometry accuracy

The smallest length scale that will be resolved accurately by the surface meshing process.

• Required (when using Geometry AI)

• Units: Length

Notes:

• Only available when using Geometry AI

• Can be overridden with Geometry Refinement

• A smaller value results in a finer mesh and higher cell count

• The Geometry AI mesher may stop early due to resource limits before fully achieving the requested accuracy; check the process log for an indication of early termination

⚠️ Geometry accuracy warnings

The meshing form evaluates your Geometry accuracy value against the geometry bounding box (BBox) diagonal and displays a warning indicator on the field when the value falls outside the recommended range:

Condition	Warning	Consequence
Value > 0.01 × BBox diagonal	Geometry accuracy too coarse	The mesher may fail to resolve small geometric features, causing it to “eat into” the underlying geometry.
Value < 1e-5 × BBox diagonal	Geometry accuracy too fine	The resulting surface mesh will be unnecessarily large, significantly increasing cell count and meshing time.

These are warnings only — the mesh may still complete successfully. However, values outside this range often lead to suboptimal results.

How to resolve:

• If the warning indicates the value is too coarse, reduce Geometry accuracy or, preferably, use a Geometry Refinement to apply a finer accuracy locally on the specific faces or regions that require it, rather than lowering the global value.

• If the warning indicates the value is too fine, increase Geometry accuracy or use local Geometry Refinement only on the faces that can have a coarser resolution.

Using local geometry refinements on specific faces is strongly recommended over adjusting the global Geometry accuracy value, as it gives you precise control without unnecessarily inflating the overall mesh size.

The Geometry accuracy field displays a yellow warning indicator when the value falls outside the recommended range relative to the geometry bounding box. The tooltip shown here reads “Geometry accuracy too fine”.

Curvature resolution angle

Specifies the maximum angular deviation allowed in the surface mesh.

• Default: 12 degrees

• Units: Angle

Notes:

• Controls both:

  1. Angle between cell normal and underlying surface normal

  2. Angle between line segment normal and underlying curve normal

• Can be overridden per face only when using Geometry AI

• Lower values capture curvature more accurately but increase mesh density

Target surface node count

Target number of nodes for the surface mesh. When specified, the surface mesher scales the initial metric to achieve approximately this number of surface mesh nodes.

• Default: None

• Units: Count

Notes:

• Available with Geometry AI and beta surface meshers

• Cannot be overridden per face

• The specified parameters are not modified directly, the initial metric is scaled to achieve the target count, so resulting Curvature resolution angle may be different than specified. Surface max edge length is enforced regardless, which may lead to discrepancies between the target node count and the resulting node count.

• This setting does not affect the geometry-resolving behaviour of the mesher (controlled by parameters like Geometry accuracy).

• Useful for quickly controlling overall mesh density without manually tuning individual length-scale parameters

Surface edge growth rate

Controls the size progression of mesh elements grown from edges, affecting the transition between regions of different refinement (anisotropic to isotropic).

• Default: 1.2

• Units: Dimensionless

• Advanced option — found under the Advanced section in the UI

Notes:

• Values must be greater than or equal to 1.0, typically 1.1–1.25

• Cannot be overridden per edge

• Higher values lead to faster mesh size growth but may reduce quality

Surface max aspect ratio

Maximum aspect ratio allowed for surface mesh cells.

• Default: 10

• Units: Dimensionless

• Advanced option — found under the Advanced section in the UI

Notes:

• Only available when using Geometry AI

• Cannot be overridden per face

• Lower values enforce more isotropic surface cells

Resolve face boundaries

Toggle to specify whether boundaries between adjacent faces should be resolved accurately using anisotropic mesh refinement.

• Default: Off

• Advanced option — found under the Advanced section in the UI

Notes:

• Only applicable to Geometry AI

• Can be overridden per face with Surface Refinement

• Improves mesh quality at face intersections

Surface max adaptation iterations

Maximum number of adaptation iterations for the surface mesher.

• Default: 50

• Units: Count

• Advanced option — found under the Advanced section in the UI

Notes:

• Only available when using Geometry AI

• Higher values may improve mesh quality but increase meshing time

Volume mesh

Boundary layer first layer thickness

Specifies the height of the first prismatic layer adjacent to wall surfaces.

• Required

• Units: Length

Notes:

• Can be overridden using Boundary Layer Refinement

• Critical for achieving desired y+ values

• Should be calculated based on Reynolds number and desired y+ value

Boundary layer growth rate

Defines the growth rate of prismatic layers in the boundary layer region.

• Default: 1.2

• Units: Dimensionless

• Advanced option — found under the Advanced section in the UI

Notes:

• Controls thickness progression of successive boundary layer elements

• Values must be greater than or equal to 1.0, typically 1.1–1.25

• Lower values create more gradual growth but increase total element count

• Critical for accurate boundary layer resolution

Refinement factor

Global scaling factor that affects the overall mesh refinement level.

• Default: 1

• Units: Dimensionless

• Advanced option — found under the Advanced section in the UI

Notes:

• Not supported with beta mesher

• Adjusts all spacings in refinement regions and first layer thickness to generate r-times finer mesh

• For example, if refinement factor = 2, all spacings will be divided by ( 2^{1/3} ), resulting in approximately 2× more nodes

• Useful for quick mesh resolution studies

Gap treatment strength

Controls how the mesh transitions in narrow gaps between surfaces.

• Default: 0 (beta mesher uses 1.0)

• Units: Dimensionless (range: 0–1)

• Advanced option — found under the Advanced section in the UI

Notes:

• 0 = no treatment, 1 = most conservative treatment

• Higher values dedicate more space to isotropic mesh in narrow gaps

• Critical for mesh quality in tight geometric spaces

Number of boundary layers

Specifies a fixed number of volumetric anisotropic (boundary layer) elements.

• Default: Automatic (mesher calculates required layers)

• Units: Count

• Advanced option — found under the Advanced section in the UI

Notes:

• Only supported by the beta mesher

• Cannot be overridden per face

• When not specified, the volume mesher automatically calculates the required number of layers to grow boundary layer elements to isotropic size

Sliding interface tolerance

Tolerance used for detecting or creating curves lying on sliding interfaces.

• Default: 0.01

• Units: Dimensionless (relative to smallest sliding interface radius defined within a case)

• Advanced option — found under the Advanced section in the UI

Notes:

• Cannot be overridden per sliding interface

• Used when setting up rotating zones with sliding mesh interfaces

Edge split layers at corners

Number of layers in the boundary layer mesh that are considered for edge splitting at geometric corners.

• Default: 1

• Units: Count

• Advanced option — found under the Advanced section in the UI

Notes:

• Only supported by the beta mesher

• Set to 0 to disable edge splitting at corners

Octree base spacing

Sets the base cell size for the octree volume mesher, from which all element sizes in the volume mesh are derived.

• Default: 1 × project length unit

• Units: Length

• Advanced option — found under the Advanced section in the UI

Notes:

• Available with the beta mesher

• The octree mesher generates a hierarchy of cell sizes that are successive halves or doubles of the base spacing (e.g., base/2, base/4, … or base*2, base*4, …)

• To achieve the desired mesh resolution, set the base spacing so that the target refinement spacings align with values in this series

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Geometry

Sealing size

Threshold size below which all geometry gaps are automatically closed.

• Default: 0 m

• Units: Length

• Advanced option — found under the Advanced section in the UI

Notes:

• Only available when using Geometry AI

• Can be overridden with Geometry Refinement

• Useful for closing small holes or gaps in imperfect CAD geometry

Preserve thin geometry

Toggle to specify whether thin geometry features should be resolved accurately during the surface meshing process.

• Default: Off

• Advanced option — found under the Advanced section in the UI

Notes:

• Only available when using Geometry AI

• Resolves features with thickness roughly equal to Geometry accuracy

• Can be overridden with Geometry Refinement

• May improve mesh quality but increase meshing time

Remove hidden geometry

Toggle to specify whether internal geometry not visible to the flow (e.g., a steering wheel inside a car body) should be removed before meshing.

• Default: Off

• Advanced option — found under the Advanced section in the UI

Notes:

• Only available when using Geometry AI

• Removes enclosed geometry that has no influence on the external flow, reducing mesh complexity

• Use Min passage size to control the minimum passage size that the removal algorithm can resolve

Min passage size

Minimum passage size that the hidden geometry removal algorithm can resolve.

• Default: Derived automatically from Geometry accuracy and Sealing size

• Units: Length

• Advanced option — found under the Advanced section in the UI

Notes:

• Only available when using Geometry AI

• Can only be specified when Remove hidden geometry is enabled

• Internal regions connected by passages smaller than this value may not be detected and removed

• If not specified, the value is derived from Geometry accuracy and Sealing size

• Can be overridden per face via Geometry Refinement

Planar face tolerance

Tolerance used for detecting planar faces in the input geometry that need to be remeshed, such as symmetry planes.

• Default: 1e-6

• Units: Dimensionless (relative to largest bounding box dimension)

• Advanced option — found under the Advanced section in the UI

Notes:

• Cannot be overridden per face

• Used to identify and preserve flat surfaces during meshing

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💡 Tips

• Use Surface edge growth rate between 1.1–1.3 for most applications

• Set Surface max edge length based on the largest geometric features requiring resolution

• For Geometry AI workflows, start with a larger Geometry accuracy and decrease iteratively to find the optimal balance

• Calculate Boundary layer first layer thickness based on desired y+ value using Reynolds number and flow conditions

• Use Gap treatment strength of 1.0 for automotive geometries with tight clearances

• Enable Preserve thin geometry when your model has thin features (e.g., trailing edges, fins) that must be captured

• Enable Remove hidden geometry when your geometry contains internal cavities or parts (e.g., engine compartments, interior trim) that are hidden from the flow and should not affect the mesh

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❓ Frequently Asked Questions

• How do I determine the appropriate first layer thickness?

  Calculate based on desired y+ value using Reynolds number and flow conditions. For turbulent flow with wall functions, target y+ ≈ 30–100. For resolving the viscous sublayer, target y+ < 1.

• What is Geometry accuracy and when do I need it?

  Geometry accuracy defines the smallest feature size the surface mesher will resolve. It is required when using Geometry AI and should be set based on the smallest important geometric features in your model.

• What do the geometry accuracy warning indicators mean?

  The meshing form compares your Geometry accuracy value to the geometry bounding box (BBox) diagonal and shows a yellow warning on the field if the value is outside the recommended range:

  • Too coarse (value > 0.01 × BBox diagonal): the mesher may not resolve small features, potentially consuming part of the underlying geometry.

  • Too fine (value < 1e-5 × BBox diagonal): the mesh will be unnecessarily detailed, leading to a very high cell count and long meshing times.

  Seeing this warning does not mean the mesh will necessarily fail — it is advisory only. To resolve it, consider applying a local Geometry Refinement on specific faces that need tighter resolution instead of adjusting the global value.

• When should I use Preserve thin geometry?

  Enable this when your geometry contains thin features (like trailing edges or cooling fins) with thickness close to the Geometry accuracy value that must be captured accurately.

• What surface edge growth rate should I use?

  Start with the default of 1.2. Use lower values (1.1–1.15) for more gradual transitions in critical regions, and higher values (up to 1.3) where rapid growth is acceptable.

• How does gap treatment strength affect my mesh?

  Higher values create more isotropic cells in narrow gaps, improving mesh quality but potentially increasing cell count. For automotive simulations, a value of 1.0 is recommended.

• When should I use Remove hidden geometry?

  Enable this when your geometry contains internal parts that are enclosed and not exposed to the flow (e.g., a steering column inside a car cabin). Removing such geometry reduces mesh complexity and meshing time without affecting solution accuracy.

• What should I set Min passage size to?

  In most cases, leaving it unset is sufficient — the mesher derives an appropriate value from Geometry accuracy and Sealing size. Set it explicitly only when internal cavities are connected by very thin passages that you need to ensure are detected.

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🐍 Python Example Usage

import flow360 as fl

# Basic meshing defaults
meshing_defaults = fl.MeshingDefaults(
    surface_edge_growth_rate=1.2,
    surface_max_edge_length=0.1 * fl.u.m,
    curvature_resolution_angle=12 * fl.u.deg,
    boundary_layer_growth_rate=1.2,
    boundary_layer_first_layer_thickness=0.01 * fl.u.mm,
    octree_spacing=fl.OctreeSpacing(base_spacing=0.1 * fl.u.m),
)

# With Geometry AI features
meshing_defaults_gai = fl.MeshingDefaults(
    geometry_accuracy=0.001 * fl.u.m,
    surface_edge_growth_rate=1.2,
    surface_max_edge_length=0.1 * fl.u.m,
    surface_max_aspect_ratio=10,
    curvature_resolution_angle=12 * fl.u.deg,
    boundary_layer_growth_rate=1.2,
    boundary_layer_first_layer_thickness=0.01 * fl.u.mm,
    octree_spacing=fl.OctreeSpacing(base_spacing=0.1 * fl.u.m),
    preserve_thin_geometry=True,
    sealing_size=0.0001 * fl.u.m,
    remove_hidden_geometry=True,
    min_passage_size=0.005 * fl.u.m,
    target_surface_node_count=500000,
)