Description

The Global panel provides tools for defining the farfield shape and extents, specifying the isotropic cell type on the interior of the volume, and defining resolution goals for the anisotropic boundary layer.

The Global panel provides slightly different tools depending on which mode is being used. Create Boundaries mode (shown left) provides tools for defining the shape and extents of the farfield. These tools are not available when in Use Existing Boundaries mode (shown right).

Press the Create Volume Mesh button to create a volume mesh according to your current settings. Once finished, the volume mesh is shown in the Display window. Press the OK button to accept the volume mesh and exit the command. Otherwise, you can change the parameters and run Create Volume Mesh again or press Cancel to discard all work and exit the command.

Once a volume mesh has been created, the Populate Volume Mesh on Command Completion checkbox becomes enabled. When checked, this option specifies that the newly created volume block will be initialized immediately upon pressing the OK button. Initialization can be a lengthy process, so the Populate Volume Mesh on Command Completion option is turned off by default.

 

Tip: Initialization can be a computationally expensive and time consuming process. The Populate Volume Mesh on Command Completion option is turned off by default so that you can go into Grid, Solve and double check the initialization settings or save your file and move it to a machine with more resources before initializing the block.

Remember, though, blocks must be initialized before you can export your mesh to your desired CAE format! This can be done either by going into Grid, Solve or by pressing the toolbar icon (Initialize Unstructured Grid Icon).

Farfield Mesh / Interior Mesh

The first frame on the Global tab is labeled based on which mode is being used in Automatic Volume Mesh. The frame is labeled Farfield Mesh when using the Create Boundaries mode and contains tools for both specifying the cell type and for defining the farfield boundaries. On the other hand, it is labeled Interior Mesh when using the Use Existing Boundaries mode and contains a reduced set of parameters.

The Farfield Mesh frame provides tools for defining the length, width, and height for box shaped farfields (left); length, radius, and alignment for cylindrical farfields (center); and the radius for spherical farfields (right).

Use the Type parameter to specify the cell type for the interior of the isotropic region of the volume mesh from the choices described below. For more information, please see the Algorithm section in this User Manual.

  • Unstructured (Unstructured Grid Type Icon): Fills the isotropic region with unstructured tetrahedra using a modified Delaunay algorithm. Unstructured is the default option.
  • Voxel (Voxel Grid Type Icon): Fills the isotropic region with Cartesian aligned isotropic hexahedra with transitions composed of pyramids and tetrahedra.
 

Tip: Use the Voxel type to fill the interior of the volume with isotropic flow-aligned hexahedral cells. If using the Voxel cell type, it is also recommended to use the Box option for Shape as this allows the voxel cells to match up against the boundaries of the volume.

The Shape parameter allows you to specify the desired shape of the farfield boundaries:

Icon Type Description
Icon Farfield Shape Box Box Creates a box-shaped farfield. This is the default option.
Icon Farfield Shape Cylinder Cylinder Creates a cylindrically shaped farfield.
Icon Farfield Shape Sphere Sphere Creates a spherical shaped farfield.
 

Note: The Display window shows a preview of the farfield shape to be created. This preview also displays arrows for each direction emanating either from the Margin or Centroid, depending on the selection for Center. Hover over one of the direction text fields to highlight the corresponding arrow in the Display window.

Use the Center parameter to choose how the farfield extents are drawn relative to the shape:

Icon Type Description
Icon Center: Margin Margins Draws the farfield extents relative to the faces of the extents box for the primary selection. This option is available for Box and Cylinder shapes only.
Icon Center: Centroid Centroid Draws the farfield extents relative to the centroid of the extents box for the primary selection. This is the default option.
Icon Center: User Specified User Specified Draws the farfield extents relative to a user specified center point. By default, the center is placed at the centroid of the extents box for the primary selection.
User Specified Center Options
When using the User Specified option for Center, the center point can be reset either by using the XYZ parameter in the panel or interactively by selecting points in the Display window.

Use the XYZ parameter to change the center point location when using the User Specified option for Center. The center point can also be updated interactively by selecting points in the Display window.

Use the Align parameter (for cylindrical farfields only) to specify the alignment of the cylindrical axis. Automatic aligns the cylindrical axis with the direction of the longest extent and is the default setting. Use the X, Y, and Z radio buttons to instead align the cylindrical axis with the X, Y, or Z axes, respectively.

 

Note: After creating the volume mesh, newly created symmetry and farfield domains are named symmetry-* and farfield-*, respectively. If the domain is planar, the * is set according to the domain's relative alignment with the principal axes (i.e. xmin, xmax, ymin, ymax, zmin or zmax), otherwise it is set to a unique index. Any planes created by Automatic Volume Mesh are also named in a similar fashion (e.g. plane-symmetry-xmin). This naming convention makes it easy to find these entities in the List panel once you have exited the Automatic Volume Mesh command.

Use the field entries to define the extents for the farfield's length (L1 and L2), width (W1 and W2), height (H1 and H2), and radius (R1 and R2 for cylindrical farfields and R for spherical farfields). The Uniform entry field (for box-shaped farfields only) allows you to enter a single number to define all three coordinate directions. By default the values for each direction (e.g. W1 and W2 for box-shaped farfields) are locked together. The button to the right shows whether the fields are currently locked (Icon: Fields are Locked) or unlocked (Icon: Fields are Unlocked). Press the button to toggle between modes.

 

Caution: Moving the center point (when using the User Specified option for Center) may cause the extent box to no longer encapsulate the primary selection, which would produce an invalid topology. When this happens, the appropriate arrows in the Display window will be rendered in red and a message will be printed to the Messages window with the minimum required offset for each invalid extent. Simply edit the appropriate extents to regain a valid block definition.

Boundary Mesh

The Boundary Mesh frame provides settings for controlling the anisotropic boundary layer. These include settings for specifying the initial wall spacing, growth rate, and stop conditions for controlling cell quality. Global Tab: Boundary Mesh Frame
The Boundary Mesh frame provides settings for controlling the growth of the anisotropic boundary layer.

Use the Boundary Layer dropdown list to specify whether or not the volume mesh should have a boundary layer:

  • Off: Specifies that the block should not have a boundary layer. The interior of the volume is filled with isotropic cells (either tetrahedra or voxel cells, depending on the choice of Type in the Farfield Mesh/Interior Mesh frame described above. Off is the default setting for Boundary Layer.
  • Geometric Growth: Specifies that layers of anisotropic cells should be grown off of surface domains that are set to either a Wall or Adjacent Grid boundary condition on the Boundary Conditions tab to form the boundary layer using the T-Rex functionality. Changing Boundary Layer to Geometric Growth brings up additional settings (described below) for the boundary layer growth. For more information on T-Rex, please see the T-Rex section of this User Manual.

The Wall Normal Spacing field is used to specify the height normal to the surface for the first layer of cells in the boundary layer. Updating this field also updates the Δ column of the Boundary Layer filter on the Boundary Conditions tab. Use the Growth Rate field to specify the rate at which the cell height grows as the anisotropic layers deform away from the surface. Wall Normal Spacing has a default value of 0.1 and Growth Rate has a default value of 1.2

The Extrusion Stop Conditions sub-frame contains additional settings for controlling the quality of cells within the boundary layer region. Use the Automatic checkbox next to each parameter to have the value automatically computed by the Automatic Volume Mesh command. Otherwise, uncheck the checkbox to edit the value.

 

Tip: The more quality measures you enforce on T-Rex, the fewer layers you will likely get as more stringent criteria usually results in more layers being forced to stop locally. Therefore, try to be as relaxed as possible on quality criteria settings. Remember, you can always edit these settings and more after creating the volume mesh by selecting the block and going to Grid, T-Rex.

The Collision Buffer parameter specifies a minimum buffer size between encroaching fronts of anisotropic cells. Collision Buffer is specified as a multiplicative factor of the current layer's cell height and has a default value of 2.0 when using Create Boundaries mode and 4.0 when using Use Existing Boundaries mode. For more information on Collision Buffer (including example images), please see the description of Collision Buffer in the T-Rex section of this User Manual.

The Final Cell Aspect Ratio parameter specifies the desired aspect ratio for the final layer of anisotropic cells in the boundary layer region. Values smaller than 1.0 stop the growth of anisotropic cells before they reach the isotropic state (i.e. cells are wider than they are tall) and values larger than 1.0 allow anisotropic layers to continue growing past the isotropic state (i.e. cells are taller than they are wide). The default values are 1.0 for Create Boundaries mode and 0.5 for Use Existing Boundaries mode. For more information pertaining to Final Cell Aspect Ratio (including example images), please see the description for Isotropic Height in the T-Rex section of this User Manual.

The Max. Included Angle and Centroid Skewness parameters set the maximum included face/dihedral angle and maximum centroid skew quality thresholds for anisotropic cells, respectively. Anisotropic cells with measures above the threshold are modified locally in an attempt to satisfy the condition. Otherwise, if the criterion cannot be met, the anisotropic element is discarded and the front is stopped locally. The default values are 170.0 for Max. Included Angle and 0.8 for Centroid Skewness. For more information about how these quality metrics are computed, please review the descriptions of the Maximum Included Angle and Centroid Skewness examine functions. Analogously, for more information about how these settings are used to control the anisotropic layer growth, please review the Max Angle and Centroid descriptions in the T-Rex section of this User Manual.

Demonstration

Automatic Volume Mesh: Global Tab

Automatic Volume Mesh: Global Tab - Part 2