Checking grid quality is an area that is being investigated and is very difficult to quantify. The ultimate goal of grid quality checking is a quick, accurate flowfield solution; however, running the solution is a very expensive way to check a grid. The traditional way of checking a grid is to look at it. This can be very time consuming for large grids containing hundreds of thousands to millions of points, and it is very possible that a severe grid flaw could be missed. The grid quality checks of the QBERT Grid Evaluation Code have been implemented in the field grid generator. These checks are performed using the grid quality menu option. The user can perform these checks on the algebraic grid or at any stage of elliptic grid generation and can operate on part or all of a zone.
These checks will indicate the location of any negative volume cells, zero volume cells, collapsed faces, or crossed sides. The user may then look at the grid in that region to identify corrective action. Of all the problems, negative volume (negative Jacobian) cells are the most severe and should be corrected. Zero volume cells are generally an indication of a severe problem unless the user has intentionally chosen a topology with zero volume cells and the flow solver can handle that condition. Collapsed faces are not necessarily a problem as they occur naturally along a singular axis; however, the user should make sure that all of the collapsed faces are accounted for by the singular axes. The check for crossed sides indicates sides that are crossed or just highly skewed. This is a more rigorous test than most flow solvers employ. The user must visually inspect these areas to see what is causing the problem and how it may be corrected. Some codes have run grids with a small number of "crossed" sides with no noticeable degradation of the solution.
CHECK GRID - Perform grid checking
CHECK GRID [SCREEN|FILE filename [NEW|APPEND|OVERWRITE]]|
[PAUSE|NOPAUSE] [LINES lines]
Currently any combination of eight checks are available and the results can be displayed in any one of six output modes. Note that the output mode is no longer included in the CHECK GRID command. Also four of the check modes now have user defined tolerances.
Several of the checks are directional, i.e., they are calculated in each of the three coordinate directions of the grid, and reported as three separate values.
The grid quality checks in GMAN have been revised to work on 2-D (planer) grids. All check modes are available with areas replacing volumes (etc.). A new graphics display mode is available which places a star on each currently displayed node point which failed the check.
A new tolerance for the new SPACING check was added. The syntax for setting the tolerances was modified to be more user friendly.
To select the grid checking and output format modes the CKMODE command with the GRID qualifier is used. All previous grid quality modes are available as well as four new ones. The syntax for specifying the status of a check or output mode is shown below followed by a list and description of valid modes.
See the List and Check Input/Output Control section for the definitions of the SCREEN|FILE, NEW|APPEND|OVERWRITE, PAUSE|NOPAUSE, and LINES parameters.
CKMODE GRID - Select grid checking and output format modes
|CKMODE GRID [[ON|OFF] mode1 [mode2 [...]]]|
Each mode parameter is one of the following:
|ALL||Enables or disables all check modes.|
|VOLUMES_SIDES||Check for collapsed and crossed sides, and zero and negative volumes. Same as specifying NEGATIVE_VOLUMES, ZERO_VOLUMS, COLLAPSED_SIDES, and CROSSED_SIDES.|
|NEGATIVE_VOLUMES||Check for points with negative Jacobians.|
|ZERO_VOLUMS||Check for cells with zero volumes.|
|COLLAPSED_SIDES||Check for cells with collapsed sides. A collapsed side is a face of a cell with zero area, and most often occurs at a grid singularity. (Directional)|
|CROSSED_SIDES||Check for cells with crossed sides. Crossed sides are a by-product of the negative volume computation and may indicate a highly skewed or "nearly negative" volume. (Directional)|
|DISCONTINUITIES||Check for angles between cells which exceed the user tolerance. See the TOLERANCE command. (Directional)|
|ORTHOGONALITY||Check for cells whose angle between faces exceeds the user tolerance from 90°. See the TOLERANCE command. (Directional)|
|STRETCHING||Check for consecutive cells whose ratio of side lengths exceeds the user tolerance. See the TOLERANCE command. (Directional)|
|SPACING||Check for cells whose spacing normal to the boundary exceeds the user tolerance. See the TOLERANCE command. (Directional)|
|YPLUS||Check for cells whose y+ value normal to a wall boundary exceeds the user tolerance. The YPLUS check returns the actual y+ value based on the reference conditions. This check is only applied to boundary faces which have a wall boundary condition defined. To set the reference conditions see the REFERENCE command. See the TOLERANCE command. (Directional)|
|NODE_BY_NODE||Displays each node that fails the check criteria.|
|PLANE_BY_PLANE||Displays each plane that fails the check criteria.|
|MAXIMUM_PLANES||Displays the i, j, and k planes with the most failures.|
|REGIONS||Displays corner points of the volume that bounds each group of nodes that fail the check criteria.|
|SUMMARY||Displays number of failures, average and maximum values, and node location of maximum value. This is always displayed except when in interactive mode.|
|SHOW_WORK||Displays the calulated value for each selected mode at each node in the selected subregion. This data is not saved, and thus it is not available in interactive mode.|
|INTERACTIVE||The user is prompted for the desired output mode after the check computations have been completed. The results are displayed and the user is again prompted for the desired output mode. This loop continues until "quit" is selected. This allows multiple and varied viewings of check results without having to repeat the check calculations.|
TOLERANCE GRID - Specify tolerances
TOLERANCE GRID [DISCONTINUITES [tol] | ORTHOGONALITY [tol] ||
STRETCHING [tol] | YPLUS [tol] | SPACING [tol]]
Five new tolerances have been defined for orthogonality, discontinuities, stretching, y+, and spacing checks.
|DISCONTINUITIES||Sets the threshhold angle for determining discontinuity.|
|ORTHOGONALITY||Sets the threshhold angle, calculated between a normal to the surface and the actual grid line, for determining undesirable orthogonality.|
|SPACING||Sets the threshhold for determining undesirable spacing.|
|STRETCHING||Sets the threshhold for determining undesirable stretching (ratio of lengths of adjacent cells in a particular direction).|
|YPLUS||Sets the threshhold for determining undesirable wall spacing in terms of the reference value y+.|
LIST GRID - List grid
LIST GRID [SCREEN|FILE filename [NEW|APPEND|OVERWRITE]]|
[PAUSE|NOPAUSE] [LINES lines]
LIST NUMBER - List grid info
LIST NUMBER [OF POINTS] [PAUSE|NOPAUSE] [LINES lines]|
[SCREEN|FILE filename [NEW|APPEND|OVERWRITE]] [ZONE n1 [THRU n2]]
This command writes the zone name and size for each zone in the selected range, plus the total number of points in that range. Output may be directed to a file.
The CHECK and LIST commands are also used to check and list boundary conditions. These commands have common input and output modes. Once a list or check input mode (GRID, BOUNDARY, or FRINGE) has been selected, it will remain selected in all the checking and listing commands until changed. The number of lines per page, pause mode, output mode are also common between commands.
|SCREEN|FILE||If FILE is specified then all data will be dumped into the file filename until a new file name is entered or the mode is set to SCREEN. Changing the mode from FILE to SCREEN causes the file filename to be closed. When FILE is specified, the NEW|APPEND|OVERWRITE option may be used when to create a new file, append to an existing file, or overwrite an existing file. If NEW is specified, filename must not be an existing file.|
|PAUSE|NOPAUSE||Toggle determining whether the output pauses and prompts the user when a page of output has been written.|
|LINES||Number of lines defining a page of output.|
Last updated 16 Mar 1999