Why Use T-Rex Hybrid Meshing?

As a CFD practitioner, have you experienced difficulty in generating meshes in regions where the flow changes rapidly, especially along the boundary layer or wall boundary? At your rescue is the Fidelity Pointwise T-Rex meshing for near-body or boundary layer meshing with special handling of symmetry boundaries.

T-Rex is an advanced and automated hybrid mesh generation method. T-Rex generates hybrid meshes that resolve boundary layers, wakes, and other phenomena in viscous flows by extruding layers of high-quality, high-aspect ratio tetrahedra that can be post-processed into stacks of prisms. The algorithm includes tools for optimizing cell quality and avoiding collisions of adjacent layers of cells. T-Rex has been used for many applications, including the sedan, in Figure 1.

Figure 1. A cut through of a T-Rex mesh around a generic automotive sedan geometry.

Overview of the T-Rex Algorithm

Let's look at how T-Rex works before delving deep into what you can do with it.

1. The algorithm starts with a distribution of points around the perimeter of the surface mesh. This is the initial extrusion front.
2. The boundary points are extruded (or advanced) one at a time into the surface mesh. The extrusion is directed normal to the boundary for a user-specified step size. This creates a candidate location for the extruded point.
3. The candidate point is checked to ensure it will not collide with any other extrusion front.
4. If the candidate point passes the collision test, it is connected to the previous front, forming a triangular cell. On the other hand, if the test fails, the candidate point is rejected, and extrusion stops locally for that point.
5. Extrusion continues, point by point, with the step size increasing at a user-specified rate until the extruded triangle is isotropic, the collision test fails, or Max Layers is reached. This is the final front.
6. A Delaunay-based, isotropic mesher fills the region enclosed by the final front.

How to Use T-Rex for Surface Meshing?

Unstructured surface meshes are initialized automatically with a Delaunay technique that generates isotropic cells throughout the entire surface. Use the T-Rex command in the Grid menu to set the T-Rex attributes and then re-initialize.

Figure 2. The T-Rex technique is applied to unstructured meshes via the Grid menu.

The two most important attributes in the T-Rex menu are the maximum number of layers to extrude (Max. Layers) and the desired number of full layers (Full Layers). For Max. Layers, keep in mind that T-Rex will keep extruding triangles until they become isotropic. After that, the extrusion stops, and a Delaunay-based mesher takes over. Therefore, Max. Layers is a way to stop the extrusion before isotropy is reached.

Boundary conditions (BCs) are the only other data you need to set for T-Rex. You define the edges from which the mesh should be extruded by setting them to the BC Type "Wall." Wall conditions also are where you set the size of the first extrusion step. There are two other T-Rex-specific BCs - Match and Adjacent Grid. "Match" indicates that the extrusion should match the distribution of points along that edge. "Adjacent Grid" indicates points will be extruded off this edge, and the initial step size will automatically be derived from an adjacent mesh.

With BCs and Max Layers set, you initialize the mesh and let T-Rex do the rest. A typical result is shown in Figure 3, a mesh on a slice through a blood vessel with an aneurysm.

Figure 3. T-Rex mesh on a slice through a blood vessel with an aneurysm.

T-Rex on Curved Surfaces

T-Rex meshing can be applied to any surface mesh, including those constrained to a CAD surface, such as the mixer blade in Figure 4. In this case, T-Rex gives high aspect ratio cells to resolve the leading-edge curvature before transitioning to an isotropic mesh.

Figure 4. T-Rex can be applied to 3D curved surfaces to resolve features like the leading edge of this mixer blade.

T-Rex for Complex Geometry

The T-Rex technique is very good at resolving complex geometry without much user intervention. Earlier in this article, the basic algorithm included a test for colliding fronts. The goal of this test is to stop the extrusion such that a large gap between fronts is left for smooth filling by the Delaunay mesher. A classic 2D test of this capability is a multi-element airfoil, as shown in Figure 5. You can see how the mesh smoothly blends from the extruded regions to the isotropic mesh between the slat and main element and the main element and the flap while the extrusion away from the collision region continues further out.

Figure 5. T-Rex automatically ensures a smooth transition from the anisotropic to isotropic mesh around complex geometry by automatically detecting collisions.

If you are working on viscous CFD solutions, prepare your meshes using the T-Rex tool in Fidelity Pointwise. The T-Rex tool offers highly clustered mesh with high-quality cells (triangles with included right angles) and the ability to handle complex geometry smoothly.

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Watch this video to learn more about applying 2D T-Rex to resolve areas of high curvature and provide smooth cell size transitions around convex corners: