The *Boundary Conditions* tab allows you to specify how each bounding connector of a domain will be treated when using the T-Rex anisotropic
meshing tool. The list operates nearly identically to the *Set Boundary Conditions*
command for CAE use. It displays information on all the boundary conditions that have been set (including for blocks) regardless of your current domain
selection.

This is the basic workflow:

- Use the
*New*command to create a new T-Rex boundary condition. - Select the connectors to be assigned to the new T-Rex boundary condition.
- Check the appropriate
*Set*checkbox to effectively assign the selected connectors to the T-Rex boundary condition.

**Tip:** The boundary conditions table has quite a few columns, but all of
the columns to the right of the *Value* column are optional and can be hidden to
declutter your view. Simply right-click in
the header row of the table to bring up a menu to hide/show the optional columns.

When setting T-Rex boundary conditions, the currently selected connectors are highlighted in white (see figure below). Note that when there is either a connection between two domains or a baffle connector, arrows normal to the boundary are rendered to indicate which side of the connection or the baffle you are currently selecting.

There are seven types of boundary conditions available for the T-Rex algorithm:

**Wall:**Sets bounding connectors to have anisotropic cells (quads and/or triangles) grown off of them. Use the*Value*column to define a fixed initial height for the anisotropic cells.**Angle:**Sets bounding connectors to have anisotropic cells (quads and/or triangles) grown off of them. Use the*Value*column to define the 2D T-Rex turning angle (expressed in degrees) used to compute a variable initial height for the anisotropic cells along the bounding connectors.This computation consists of walking along the bounding connectors assigned to this T-Rex boundary condition and sampling the underlying database surface in the transverse direction. For each grid point on the connectors (Point A), the algorithm will find the closest point on the underlying database surface (Point B) for which the angle between the normals to the underlying database surface at Point A and Point B is smaller than or equal to the specified turning angle. The distance between Point A and Point B is the calculated initial height at that particular grid point.

The final result is a smoothed distribution of T-Rex initial height values at each grid point along the connectors assigned to this T-Rex boundary condition. In the case when this T-Rex boundary condition is applied to both sides of a shared connector, the minimum spacing will be used in both domains.

**Tip:** The *Angle* T-Rex boundary condition calculates a distribution of T-Rex initial heights along the connectors bounding a
given domain based on the curvature of the underlying database surface. This functionality therefore requires the domain in question to be constrained to
such database surface.

**Aspect Ratio:**Sets bounding connectors to have anisotropic cells (quads and/or triangles) grown off of them. Use the*Value*column to define the aspect ratio used to compute a variable initial height of the anisotropic cells along the bounding connectors.This computation consists of walking along the bounding connectors assigned to this T-Rex boundary condition and calculating an initial height for each line segment in the connector (i.e. the segment between two consecutive grid points) based on the length of the line segment and the specified aspect ratio. The final result is a smoothed distribution of T-Rex initial height values at each grid point along the connectors assigned to this T-Rex boundary condition.

**Max Aspect Ratio:**Sets bounding connectors to have anisotropic cells (quads and/or triangles) grown off of them. Use the*Value*column to define the aspect ratio used to compute the fixed initial height of the anisotropic cells based on the maximum edge length along the connectors (i.e. the maximum distance between two consecutive grid points). The resulting initial height is then smoothed to better match the surrounding mesh while not exceeding the user-specified aspect ratio.**Match:**Sets bounding connectors to have a grid point distribution that matches the growth of the interior anisotropic cells (quads and/or triangles). Note that for the connectors to be modified, you need to have the*Push Attributes*option checked on.**Adjacent Grid:**Sets bounding connectors to have anisotropic cells (quads and/or triangles) grown off of them while matching the initial cell height to an adjacent domain.**Off:**Sets bounding connectors to not have anisotropic cells (quads and/or triangles) grown off of them. Note that these connectors will have isotropic triangles applied along the entire boundary.

**Tip:** When applying T-Rex boundary conditions, you can always leave connectors as *Unspecified* when you want to use
type *Off* since *Unspecified* defaults to the *Off* boundary condition type.

The *Adapt* column specifies that connectors assigned to the boundary
condition have their point clustering adapted (points will be added or removed) in order to meet
the target cell sizes specified by any selected size field influencers on the
*Size Field* tab. The *Adapt* column is set to *Off*
by default and can be toggled *On* by double clicking in the field. Adaptation is not
available for the *Unspecified* boundary condition.

**Note:** Even though T-Rex layers are not directly influenced by the
size field, the *Adapt* option can still be applied to any T-Rex condition
that grows anisotropic cells (*Wall*, *Angle*, *Aspect Ratio*, *Max.
Aspect Ratio*, and *Adjacent Grid*). In this case, the connectors will be adapted prior to marching T-Rex
layers, thus picking up the influence from the size field.

The remaining columns (*Growth Rate*, *Constant Layers*, *Min. Edge
Length*, and *Min. Radius of Curvature*) are only available for T-Rex boundary
conditions that grow anisotropic cells (i.e. *Wall*, *Angle*, *Aspect
Ratio*, *Max. Aspect Ratio*, and *Adjacent Grid*) and function similarly to
the *Automatic Surface
Mesh* Stretching Filter parameters of the same names.

The *Growth Rate* and *Constant Layers* columns provide more fine-tuned control
of the growth schedule for individual T-Rex boundary conditions. By default, T-Rex uses the
growth rate specified on the *T-Rex* tab. The *Constant
Layers* column allows you to specify an integer number of constant growth layers before
the *Growth Rate* is applied and has a default value of 0. Double click in the
corresponding field to specify a different value for the assigned connectors.

**Note:** The *Constant Layers* attribute tells T-Rex how many layers
of anisotropic cells to grow using the prescribed initial spacing before applying the
*Growth Rate*. For a value of 1, T-Rex will grow 1 layer using the initial spacing
and apply the *Growth Rate* to layers >= 2. For a value of 2, T-Rex will grow 2
layers using the initial spacing and apply the *Growth Rate* to layers >= 3, and so
on. The default value of 0 is a special value that indicates that T-Rex should use the
legacy behavior. This produces the same result as specifying a value of 1, i.e. the first
layer uses the prescribed initial spacing and subsequent layers apply the *Growth Rate*.

The *Min. Edge Length* and *Min. Radius of Curvature* columns allow you to
specify a minimum edge length and radius of curvature, respectively, to
be used with adaptation for the assigned connectors. These are intended to limit excessive
over-refinement of the connectors due to inconsistencies in the geometry definition.