The *T-Rex* tab (figure below) allows you to generate anisotropic layers from the boundaries of unstructured blocks. Options are available to choose
the type of cells contained in those layers: tetrahedra and pyramids or a combination of tetrahedra, pyramids, prisms, and hexahedra. If you chose to generate
anisotropic layers containing only tetrahedra and pyramids, note that it is possible to combine these cells into prisms and hexahedra using
the *Combine T-Rex Cells* command. Combination into hexahedral cells can only occur where T-Rex
layers are grown from structured domains.

At the top of the *T-Rex* tab, a table provides information regarding structured domains which have been assigned to a *Match T-Rex* boundary
condition (refer to the Boundary Conditions section for more information). Since the *Push Attributes* feature
(described below) will not affect these domains in order to avoid unwanted modifications to the adjacent blocks using them, the information displayed on the
table is intended to help you determine the appropriate settings for the T-Rex algorithm.

The top of the table shows the total number of match structured domains. The table also shows the calculated *Layer Count*, the calculated *Growth
Rate*, and the *Initial Spacing* for such domains. Since multiple structured domains assigned to *T-Rex Match* boundary conditions may
exist, minimum, average, and maximum values are provided for each measure.

In the *Layers* frame you will find the basic settings for generating anisotropic tetrahedra layers in an unstructured block. These settings apply
specifically to those boundaries of a block which have been set to type *Wall* in the *Boundary Conditions* tab (refer to
the Boundary Conditions section for more information).

Use *Max. Layers* to set the total number of anisotropic tetrahedra layers you wish to generate. This is a target number only. Various physical
constraints and quality controls may prevent this target from being achieved. A value of 0 input for *Max. Layers* turns off the T-Rex anisotropic
meshing.

Use *Full Layers* to set a target for the number of layers you wish to generate
without any changes in the deforming front (i.e. the number of target layers to generate
prior to any refinement or decimation of the front). This parameter also controls whether
the T-Rex solver applies multiple normals (*Full Layers* = 0) along sharp edges or
not (*Full Layers* > 0).

**Tip:** Always set *Full Layers* to 0 (zero) when applying T-Rex layers to baffles. This allows the solver to apply multiple
normals along the sharp edges of a baffle which otherwise would not be applied, resulting in fewer layers on the baffle and significant scalloping of the
layers that are completed. See the image below for an example.

**Tip:** Always set *Full Layers* to 0 (zero) in situations where a growth domain (i.e. set to
a *Wall* T-Rex boundary condition) is nearly coplanar to a symmetry domain (i.e. set to
a *Match* T-Rex boundary condition). This allows the solver to apply multiple normals in this region greatly
improving the quality of the final volume cells.

The image below presents a sample case where the vertical stabilizer of an aircraft model has a sharp trailing edge. In cases where only half the model needs to be meshed, the result is a vertical stabilizer (growth) domain almost coplanar to the symmetry (match) domain. Compare the final volume grid with and without multiple T-Rex normals.

*Growth Rate* specifies the rate you wish to grow each anisotropic tetrahedral layer as layers deform from the boundary. Uncheck the *Use Default*
toggle to override this setting's default value of 1.2 with your own rate value.

*Push Attributes* automatically propagates the block T-Rex attributes to domains set to a *T-Rex Match* boundary condition (refer to
the Boundary Conditions section for more information). Specifically, the initial spacing, the number of layers, and
distribution of points on the interior of the T-Rex block will automatically be matched on these domains. In addition, the corresponding connectors on these
domains will also have their distributions updated to match the T-Rex volume. This option provides great time savings by avoiding the need for manual
matching of T-Rex blocks and their perimeter domains and connectors.

**Note:** If *Push Attributes* is on, and a domain with the *T-Rex Match* boundary condition
is shared with an extruded block, a warning dialog will be presented that the extruded block will be emptied and classified as *Undefined* in the *List*. You will have the
options to *Continue* or *Cancel*. This check and warning will occur when *Initialize* is used in the *Solve* command panel or on the toolbar.

The *Cell Types* frame provides five options (organized in two groups) to specify the
type of cells to be generated by the T-Rex algorithm (i.e. the types of cells in the anisotropic
region of the volume grid):

**Standard:**Creates anisotropic layers containing all of the volume cell types currently supported: tetrahedra, pyramids, prisms, and hexahedra. Stacks of anisotropic tetrahedra are grown off of surface triangles and quads (represented with 2 triangles) until a satisfactory transition cell is achieved. Subject to quality criteria, T-Rex anisotropic tetrahedral cell stacks which originate from triangles and quads are combined into prisms and hexes, respectively. At the top of each hex stack a cap is created from a pyramid surrounded by four side pyramids and topped by two inverted tetrahedra which interface with the isotropic volume of the block outside the T-Rex layers.**Reduce Pyramids:**Creates anisotropic layers using approximately the same workflow as the*Standard*option with one difference — the cap at the top of each hex stack is created from a single height-optimized pyramid whose four exposed triangles interface with the isotropic volume of the block outside the T-Rex layers.**Convert Wall Domains:**Creates anisotropic layers using approximately the same workflow as the*Standard*option with one difference — all fully triangular unstructured domains assigned to T-Rex boundary conditions of type*Wall*(refer to the Boundary Conditions section for more information) are converted to quad-dominant (refer to the Algorithm section for more information).**Legacy:**Creates anisotropic layers using approximately the same workflow as the*Standard*option with one difference — each surface quad is represented with 4 triangles instead of 2. This older variation on the algorithm is provided for backwards compatibility for legacy meshes.**Tets and Surface Pyramids:**Creates anisotropic layers containing only tetrahedra and pyramids. Note that the T-Rex algorithm includes pyramid cells only if any of the boundaries of an unstructured block contains structured or quad-dominant domains. Wherever quad cells are included in an unstructured face, pyramid cells are used to transition from the quad cells in the face to the unstructured (tetrahedral) volume grid.

**Caution:** Note that in isolated cases, the T-Rex algorithm could diagonalize quadrilateral cells in unstructured wall domains in order
to improve the quality of the generated volume isotropic cells.

In general, when any of the *Cell Types* options other than *Convert Wall Domains* is selected, the T-Rex algorithm does not change the cell
topology of the unstructured bounding domains assigned to a T-Rex boundary condition of type *Wall* (refer to
the Boundary Conditions section for more information). However, in isolated cases, a reduced number of quadrilateral
cells in these domains may be split in order to improve the quality of the created anisotropic volume cells.

The image below illustrates a typical case where the T-Rex algorithm splits a reduced number of quadrilateral cells in the bounding unstructured domains (shown in bright green) in order to improve the quality of the final (prism) anisotropic cells. In the image, we compare two cases with the exact same geometry. In one case, some of the bounding domains are structured (shown in red); their cells cannot be split. In the other case, the same bounding domains are quad-dominant unstructured (shown in bright green) and their quadrilateral cells can be split by the T-Rex algorithm as needed. The two enlarged images on the right side show that when the quadrilateral cells cannot be split (red structured domain), the T-Rex algorithm places pyramid volume cells which reach a maximum included angle of 172 degrees. On the other hand, when the quadrilateral cells can be split as needed (green unstructured domain), the T-Rex algorithm, splits them in order to place anisotropic prisms with a much lower maximum included angle of 92 degrees.

From the comments above, it can be inferred that one way to prevent the T-Rex algorithm from splitting quadrilateral cells in wall domains to improve
quality it by making those domains structured. An alternative way to reduce the number of quadrilateral cells split by the algorithm is to relax the quality
constraints; this could be accomplished by increasing the specified *Max. Angle* parameter in the *Skew Criteria* frame described below.

Use the options in the *Advanced* frame to change the *Isotropic Seed Layers*, *Collision Buffer*, *Aniso-Iso Blend*, and
*Isotropic Height* attributes.

*Isotropic Seed Layers* sets the maximum number of layers of seed points, or vertices, to be created in the isotropic regions adjacent to anisotropic
cells that have stopped before reaching isotropy due to one of the following reasons: achievement of the specified maximum number of anisotropic layers
(*Max. Layers*), cell collision, or failure of cell skew criteria. These additional vertices are marched out from the anisotropic tetrahedra based on
their defined attributes such as growth rate and layer height. This input defaults to off, or 0. If the *Use Remaining* check box is checked on, this
parameter will be set locally to the difference between the value specified in *Max. Layers* and the maximum number of anisotropic
layers actually achieved. Note that the T-Rex algorithm will locally stop adding seed points, independently of the value of this parameter, once isotropy is
locally reached.

*Collision Buffer* specifies the minimum buffer to be maintained between encroaching advancing tetrahedra, in terms of multiplicative factors of the
current cell height. For example, with a factor of 0.5, a grid point will be advanced by a distance of 0.01 only if it may also be advanced by a distance of
0.015 without intersecting any other portion of the front. Values must be non-negative. The default value of this parameter is 2.0.

*Aniso-Iso Blend* specifies the floating-point rate with which anisotropic 2D cells on a block's T-Rex front are blended into isotropic 2D cells with
each marching step. This blending of the 2D cells is achieved by means of a local decimation on the T-Rex front as the algorithm progresses. The image bellow
shows the effect of this parameter on a volume grid. In particular, the central image displays the decimation of the T-Rex front enclosed in the red circle.

Note that larger values of this parameter decrease the number of layers and distance over which the anisotropic to isotropic transition occurs (resulting in a faster transition). Values must lie between 0 (disabled blending) and 1 (maximum blending).

*Isotropic Height* specifies a scaling factor used to scale the normal isotropic cell height of a vertex. This will allow the T-Rex algorithm to grow
anisotropic cells past the isotropic state (factor larger than one) or to stop the growth of anisotropic cells before they reach the isotropic state (factor
smaller than one).

Use the *Skew Criteria* attributes to enforce additional quality control measures on the tetrahedra, pyramids, prisms and hexahedral cells formed by
the T-Rex algorithm.

The *Skew Criteria* frame (figure above) provides input fields for quality measures applied to all newly formed tetrahedra, pyramids, prisms, and
hexahedra as the T-Rex layers advance. The parameters on this frame are all disabled by default. *Delay Skew Criteria* delays the use of all the skew
quality constraints for the specified number of layers. The input value must be less than or equal to *Max. Layers*. Once the input number of layers
have been formed, cells failing the specified skew criteria will not be added to the mesh.

*Max. Angle* specifies the maximum included face and dihedral angle quality threshold for anisotropic cells (refer to
the Maximum Included Angle section for more information). Anisotropic cells with
angles above the threshold will be modified locally in an attempt to satisfy the criterion. If the criterion cannot be met, the anisotropic element is
discarded and the front is stopped locally. The specified threshold must be a floating-point number between 60 (i.e. isotropic tetrahedron) and 180
(i.e. collapsed). The default value is 170.0.

*Equi-Volume* specifies the volume skew quality threshold for anisotropic cells (refer to
the Equiarea, Equivolume Skewness section for more information). Anisotropic
cells with measures above the threshold will be modified locally in an attempt to satisfy the criterion. If the criterion cannot be met, the anisotropic
element is discarded and the front is stopped locally. The specified threshold must be a floating-point number between 0 (i.e.perfect quality) and 1
(i.e. collapsed). The default value is 1 (i.e. no quality check).

*Equi-Angle* specifies the angle skew quality threshold for anisotropic cells (refer to
the Equiangle Skewness section for more information). Anisotropic cells with measures
above the threshold will be modified locally in an attempt to satisfy the criterion. If the criterion cannot be met, the anisotropic element is discarded and
the front is stopped locally. The specified threshold must be a floating-point number between 0 (i.e. perfect quality) and 1 (i.e. collapsed). The default
value is 1 (i.e. no quality check).

*Centroid* specifies the centroid skew quality threshold for anisotropic cells (refer to
the Centroid Skewness section for more information). Anisotropic cells with measures
above the threshold will be modified locally in an attempt to satisfy the criterion. If the criterion cannot be met, the anisotropic element is discarded and
the front is stopped locally. The specified threshold must be a floating-point number between 0 (i.e. perfect quality) and 1 (i.e. collapsed). The default
value is 1 (i.e. no quality check).

Use the *Smoothing* frame to adjust T-Rex smoothing parameters.

The *Smoothing* frame (figure above) provides input fields for *Smooth* and *Relax* controlling local cell height Laplacian
smoothing. *Smooth* is the number of smoothing iterations to be applied. *Relax* is a factor controlling the influence of the smoothing, varying
between 0 and 1. For both parameters, uncheck *Use Default* to override the default values.

Use the *Volume Criteria* attributes for further volume quality settings.

In the *Volume Criteria* frame, the *Volume Computation Method* pull-down allows you to choose a method by which volumes are calculated for
quality verification. The default, *Min. Component Volume*, is determined in the same fashion as for the *Examine* diagnostic of the same name
(refer to the Component Volume section for more information).

*Green-Gauss Volume* is a much less demanding volume quality check which may result in more anisotropic layers but probably with a higher number
inferior cells. This volume is computed by decomposing any quad cell faces into triangles with a common vertex at the quad’s weighted centroid. All cell face
triangles then have their area multiplied by the triangle normal, dotted with the X directed unit vector and summed to form the cell’s volume.

**Tip:** The more quality measures you enforce on your T-Rex layers the less layers you will likely get as more stringent criteria force the
checking to stop advancing the front in more areas. Therefore, try to be as relaxed as possible on quality criteria settings.