The sector parameters give the possibility to specify the revolution axis, the origin of the axis, the minimum and maximum radius and the angle of revolution.
The revolution axis must be X, Y or Z axis.
The minimum radius must be bigger than 0 and smaller than the maximum radius.6-5.3.3 Number of Divisions
It is possible to change the number of cells for the radial, tangential and axial directions. The Total
Number of Cells indicates the total number of cells for the initial mesh.
FIGURE 6.5.3-1 Initial cylindrical mesh
The refinement parameters menu is then modified according to the choice of an initial cylindrical mesh: the target cell sizes X, Y and Z are replaced by R, Theta and Z in the adaptation criteria.
FIGURE 6.5.3-2 Refinement parameters menu for an initial cylindrical mesh
6-5.4
Import Mesh File
The user can import ( ) an external mesh as an initial mesh for the mesh generation process. Any hexahedral mesh (i.e. suffix ".hex") converted to the HEXPRESS format is valid. A file name can be directly specified or a file selection box is activated.
The mesh refinement algorithm used in HEXPRESS (see section 6-6 on page 6-10) assumes that all cells of the initial mesh are oriented in the same way with respect to the X, Y and Z axis. This is true for the initial mesh obtained by subdivision of the computa- tional domain bounding box. However, this is not necessarily the case for a HEX- PRESS mesh obtained after snapping to the geometry. Indeed, the additional cells introduced during buffer insertion (see section 6-7 on page 6-33) may be randomly ori- ented. In this situation, anisotropic refinement may lead to unexpected refinement pat- terns and isotropic refinement should be enforced.
A converted structured project from IGG™ or AutoGrid5™ can be used as initial mesh. Before converting, the IGG™ or AutoGrid5™ mesh should not contain undefined patches (UND) otherwise the concerned patches will be considered as internal surface and may lead to problem during adaptation step. All "UND" have to be changed in another type (for example CON if at the connection between blocks) before loading into HEXPRESS™.6-6
Adapt To Geometry
6-6.1
Overview
This mesh generation action is probably the most important in terms of user interaction in HEX- PRESS. Indeed, the parameters selection in this action directly dictates the computed mesh obtained within HEXPRESS.
In this action, cells are successively subdivided such that specific geometrical criteria are satisfied. Two classes of criteria are available:
• Curve criteria
Cells which intersect curves are considered as candidates for refinement. Three criteria are implemented and are respectively based on:
—a proximity testing between curves
—a specification of cell target sizes on the curves
• Surface criteria
Cells which intersect surfaces and geometrical surfaces created with the Geometry menu (more details in section 2-3.2 and section 2-5.2) are considered as candidates for refinement (Figure6.6.1-1). Three criteria are implemented and are respectively based on:
—a proximity testing between surfaces
—a surface curvature measurement
—a specification of cell target sizes on the surfaces
• Volume criterion (Box)
For this criterion, cells which are intersecting or located inside a three-dimensional volume are flagged for refinement if their cell size is larger than some user prescribed target size.
FIGURE 6.6.1-1 Cells flagged for refinement by surface intersection
Proximity measurement saved in ".dist" file (distance adaptation criterion) is also applied between solid surfaces and non solid surfaces, e.g. between a solid surface (SOL) and any other surface type (MIR, EXT,...).In order to limit the number of cells created during refinement, HEXPRESS subdivides cells ani- sotropically as often as possible. A cell can thus be subdivided in 2, 4 or 8 cells (see Figure 6.6.1-2)
FIGURE 6.6.1-2 Refinement modes of a hexahedron
For the criteria involving the user specification of target sizes (along the X, Y and Z axis) such as the volume criterion and surface target size criterion, the mechanism used for anisotropic refine- ment depends on the orientation of the cell with respect to the Cartesian axes. In case of the default initial mesh, the cells are simply oriented along the axes (X, Y and Z axis). Thus, cell sizes are inde- pendently compared to the target sizes and the cell refinement is triggered accordingly along each axis.
For surface refinement criteria, such as the distance and the curvature criteria, HEXPRESS com- putes target sizes in a reference framework attached to the surface with base vectors formed by the two surface tangents and normal. These target sizes are mapped onto the cell reference axis and compared to the actual cell size in order to determine the type of refinement to apply to the cell. In practice, cells which are oriented sufficiently parallel to surfaces may be subdivided anisotropically. A cell, oriented at 45 degrees between the surface tangent and the cell principal axis, will never be refined anisotropically.
The anisotropic refinement can be controlled by a parameter available in the list of expert parame- ters of the surface adaptation criteria (see Figure6.6.4-8 on page 6-26).
The Refinement and trimming parameters dialog box contains refinement and trimming parameters as shown in Figure
6.6.1-3.
FIGURE 6.6.1-3 Mesh refinement and trimming parameters dialog box
6-6.1.1 Refinement
This action consists of successive refinements of the initial mesh in order to satisfy the adaptation criteria (Figure
6.6.1-4).
The refinement parameters (available when clicking on Parameters... button - Figure 6.6.1-3) are organized into a notebook, divided into 4 panels qualified as global, curve adaptation, surface
adaptation and box adaptation parameters (see Figure 6.6.2-1).
6-6.1.2 Trimming
The trimming step removes all the cells intersecting or located outside of the geometry. HEX- PRESS automatically finds the cell located inside the computational domain. At the end of the process, a staircase mesh is obtained including all the interior cells (see Figure
6.6.1-5).
FIGURE 6.6.1-5 Trimming of surface intersecting cells