Description

When we look at the volume cells inside a block, we notice that, in general, not all cells are the same size. In parts of the block where more refinement is needed, the cells are smaller while in other parts of the block where a coarser grid is sufficient, the cells are larger.

The cell size distribution inside a block is driven by a size field (i.e. target spacing field) which is computed using three components: the background spacing, the calculation method, and the influencers. Note that the combination of the size field influencers along with the background spacing allows the control of grid spacing away from grid topology where spacing is defined explicitly.

 

Tip: You can visualize the size field for a block even before it is initialized using the Size Field diagnostic function.

The Size Field panel provides tools for controlling both global size field properties and individual influencers.

Build Farfield: Size Field Tab
The Size Field tab provides tools for selecting size field source or grid entities, controlling their attributes, and setting global parameters.

At the top of the Size Field panel, there is a Decay entry field where you can specify the decay factor to be applied to the bounding topology of the block(s) being created. The Decay parameter varies from 0.0 to 1.0; a value of 0.0 indicates no influence beyond the boundary of the influence topology while a value of 1.0 indicates maximum influence. This parameter defaults to the value specified in the Defaults panel. Uncheck Use Defaults to enter a new value.

The Background Spacing entry field allows you to specify a constant target spacing throughout the interior of a block. Keep in mind that if you are creating a T-Rex unstructured farfield block, the size field will only affect its isotropic portion. This parameter defaults to the value specified in the Defaults panel. Uncheck Use Defaults to enter a new value. When left as its default value of 0.0, the Background Spacing is determined automatically in different ways depending on the type of farfield block being created:

  • Unstructured: The Background Spacing is automatically determined as a blend of the maximum and average edge lengths on the volume's boundary.
  • Voxel: The Background Spacing is set to the default value of the Max. Edge parameter if this has been explicitly defined. On the other hand, if the Max. Edge's default value is set to Automatic (i.e. not explicitly defined), the Background Spacing is determined automatically as a blend of the influencers (sources and gird entities if any) and the default number of Transition Layers. Refer to the Unstructured Block, Voxel section of the Defaults tab for further information on setting default values for voxel blocks.
 

Caution: The final size field can locally differ from the Background Spacing due to the influence of sources and grid entities marked as influencers (described below).

Once the source and/or grid influencers (described below) and the Background Spacing have been defined (or computed), the size field is calculated throughout the block(s) as an interpolation of radial basis functions (RBFs) represented at discrete points. This means that a source created as a cylinder shape, for example, will have a discrete representation used for size field calculations. Furthermore, this point cloud representation will increase its density as spacing values get smaller. This, in turn, increases the time necessary to initialize the interior of a block.

Note that the set of RBFs used by Pointwise take into account the background edge target (i.e. Background Spacing), the source edge target (i.e. the sources), distance to source, and source decay, among other attributes. The Calculation pull-down list provides a set of choices for how the spacing target will be determined at any given location in the grid:

  • Minimum Distance: Uses the closest source point to the grid point being influenced to determine the local target. This setting works well where there is strong variation in source influence. Could produce target flipping when equally close RBF points have significantly different targets. This is the default Calculation method.
  • Inverse Distance: Uses a weighted average of local source points where the weighting function is 1/(distance to source)2 This option will not work as well where there is strong variation in target element size due to the weighting. This weighting can smear the strong variation locally.
  • Blend Distance: Blends the Minimum Distance value for near cloud points with the Inverse Distance value for far cloud points where near and far are a function of the local source distance. Works well for wall bounded type cases.
  • Minimum Value: Simply takes the actual smallest target spacing from the nearby RBF points.

Sources

Block Farfield, Size Field Tab: Source
The Sources frame provides tools to enable/disable sources for use as influencers by the solver and changing their attributes.
 

Tip: Sources can be used to control the grid density in blocks while remaining independent of the database geometry and the grid topology.

At the top of the Sources frame, you will find the Ignore All Sources checkbox. You can use this option to quickly deactivate the use of all sources, regardless of their status in the table below. For a selected source, or sources, the Reverse option will swap the Begin and End spacings and decay factors. Start Interactive Edit provides graphical objects in the Display window which you can click with the mouse and drag to change the spacing values in a visual style. Below these three commands, there is a Legend frame showing the symbols representing the beginning (Begin) and ending (End) of sources as rendered in the Display window.

The table of sources at the bottom of the Sources frame provides a Use checkbox to either enable (checked) or disable (unchecked) the use of individual sources. By default, any source(s) located inside a region computed as twice the size of the bounding box of the selected block(s) will be enabled for influence. Therefore uncheck any individual source you wish not to affect the cell size of the selected blocks.

In the second column of the table, the source Name is shown. These default names can be edited by double clicking the field and typing in a new name. In the third column, you can see the current source Spacing Type. Simply, double click the field to change type if desired. See the Grid, Sources command for further information. The final four columns show the spacing and decay at the beginning and end of the sources; you can be edit their values in the same way already described for the other columns.

 

Note: When the source Spacing Type is set to Constant, the Reverse command button will be unavailable.

Grid

Block Farfield, Size Field Tab: Grid
The Grid frame provides tools to select grid entities for use as influencers by the solver and changing their attributes.

At the top of the Grid frame, you will find the Sync with Selected Entities checkbox. You can use this option to quickly synchronize the list of influence grid entities with the list of grid entities selected before entering the Create, Build Farfield functionality (i.e. enclosed by the farfield block being created). Keep in mind that this selection can be updated while inside the functionality using the commands in the Selection frame located inside the Build tab.

The Ignore Selected Grid Entities checkbox allows you to quickly deactivate the use of all grid entities currently listed in the table at the bottom of this panel. Right after this option, you will find a Selection frame where Begin starts a selection mode for selecting grid entities to be used as influencers. The command button becomes an End command to be clicked once the desired grid entities have been selected. Selected entities will then show up in the table below with Name, entity Type, and minimum and maximum cell edge lengths (Min. Edge and Max. Edge respectively). All of these columns are for information purposes only and cannot be edited.