The mesh entities may be referenced by the user during the mesh creation. These references may then be used to mark out some mesh entities according to the need (specification of boundary conditions, pressure drop zones, ...). The references are generally of one of the two following types:
Using the GUI or high-levels mesh element selections in user-defined functions, selection criteria allow defining a selection of mesh entities (usually cells, boundary faces, or interior faces) in a simple and consistent manner.
Typically, a selection criteria is simply a string containing the required group names (or color numbers for older formats), possibly combined using boolean expressions. Simple geometric criteria are also available.
A few examples are given below:
ENTRY
1 or 7
all[]
3.1 >= z >= -2 or not (15 or entry)
range[04, 13, attribute]
contains[X]
sphere[0, 0, 0, 2] and (not no_group[])
Strings such as group names containing white-space or having names similar to reserved operators may be protected using "escape characters".
In C++ (or C), to define a string such as \plane, the string \\plane must be used, as the first \ character is used by the compiler itself. Using the GUI, either notation is easy. More complex examples of strings with protected strings are given here:
"First entry" or Wall\ or\ sym entry or \plane or "noone's output"
The following operators and syntaxes are allowed (fully capitalized versions of keywords are also allowed, but mixed upper-case/lower-case versions are not):
| Escape characters | |
|---|---|
| protect next character only | \ |
| protect string | ‘'string’ <br/> "string" <tr><th> Basic operators <th> <tr><td> priority <td> ( ) <tr><td> not <td> not ! != <tr><td> and <td> and & && <tr><td> or <td> or | || , ; <tr><td> xor <td> xor ^ <tr><th> General functions <th> <tr><td> select all <td> all[] <tr><td> entities having no group or color <td> no_group[] <tr><td> select a all groups or containing a substring <td> contains[ *substring* ] <br/> <tr><td> select a range of groups or colors <td> range[ *first*, *last*] <br/> range[ *first*, *last*, group] <br/> range[ *first*, *last*, attribute]` |
For the range operator, first and last values are inclusive. For attribute (color) numbers, natural integer value ordering is used, while for group names, alphabetical ordering is used. Note also that in the bizarre (not recommended) case in which a mesh would contain for example both a color number 15 and a group named "15", using range[15, 15, group] or range[15, 15, attribute] could be used to distinguish the two.
Geometric functions are also available. The coordinates considered are those of the cell or face centres. Normals are of course usable only for face selections, not cell selections.
| Geometric functions | |
|---|---|
| face normals | normal[ x, y, z, epsilon] normal[ x, y, z, epsilon = epsilon] |
| plane, ax + by + cz + d = 0 form | plane[ a, b, c, d, epsilon] plane[ a, b, c, d, epsilon = epsilon] plane[ a, b, c, d, inside] plane[ a, b, c, d, outside] |
| plane, normal + point in plane form | plane[ nx, ny, nz, x, y, z, epsilon] plane[ nx, ny, nz, x, y, z, epsilon = epsilon] plane[ nx, ny, nz, x, y, z, inside] plane[ nx, ny, nz, x, y, z, outside] |
| box, extents (axis-aligned) form | box[ xmin, ymin, zmin, xmax, ymax, zmax] |
| box, origin + axes form | box[ x0, y0, z0, dx1, dy1, dy1, dx2, dy2, dz2, dx3, dy3, dz3] |
| cylinder | cylinder[ x0, y0, z0, x1, y1, z1, radius] |
| sphere | sphere[ x, y, z, radius] |
| inequalities | >, <, >=, <= associated with x, y, z or X, Y, Z keywords and coordinate value xmin <= x xmax type syntax is allowed |
All selection criteria used are maintained in a list, so that re-interpreting a criterion already encountered (such as at the previous time step) is avoided. Lists of entities corresponding to a criteria containing no geometric functions are also saved in a compact manner, so re-using a previously used selection should be very fast. For criteria containing geometric functions, the full list of corresponding entities is not maintained, so each entity must be compared to the criterion at each time step. Heavy use of many selection criteria containing geometric functions may thus lead to reduced performance.
In order to use selection criteria in user-defined functions, a collection of utility subroutines is provided.
for example:
This section explains how to define surface or volume sections, in the form of lists lstelt of nlelt elements (internal faces, boundary faces or cells). For each type of element, the user calls the appropriate function:
Several examples of possible selections are given here:
The user may then use a loop on the selected elements. For instance, in the subroutine cs_user_boundary_y_conditions used to impose boundary conditions, let us consider the boundary faces of color number 2 and which have the coordinate X <= 0.01 (so that ‘call getfbr('2 and x <= 0.01’, nlelt,lstelt)); we can do a loop (do ilelt = 1, nlelt) and obtain ifac = lstelt(ilelt)`.
More examples are available in the User examples section.
Though selection criteria can and should be used directly in mesh preprocessing operations and in some postprocessing subset extractions, for regions with a specific meaning, it is preferable to build zones.
In most cases, zones can be mapped directly to mesh regions and groups, but can be defined in a more complex manner using selection criteria.
Once built, boundary and volume zones can be used to quickly access all matching elements, as they maintain lists of corresponding elements.
The GUI naturally builds and associates zones for boundary and volume conditions. The cs_user_zones user-defined functions (from cs_user_zones.c) can be used to build such zones.
1.14.0 