Volume Grid Generation Applications

SolidMesh provides a user friendly front-end to the Advancing-Front/Local-Reconnection 3D (AFLR3) unstructured grid generator. AFLR3 is a stand-alone unstructured tetrahedral element grid generation system. AFLR3 generates a tetrahedral isotropic volume grid or a mixed element (tetrahedra, pentahedra and prisms) anisotropic boundary layer volume grid from a surface triangulation.

Global Grid Options:

Grid Type

The grid type selector allows the user to specify the type of volume grid:

• An Isotropic volume grid is used for Euler calculations. The volume grid point placement distances are interpolated from the surface grid.
• An Anisotropic-Laminar grid is used for Navier-Stokes viscous calculations where the boundary-layer must be resolved. An anisotropic grid needs a boundary layer spacing and growth rate in addition to the surface grid. A normal point placement technique is used to create right-angled tetrahedron, pentahedron or prism elements.  The growth of the boundary-layer assumes a laminar boundary-layer velocity profile.
• An Anisotropic-Turbulent grid is similar to the Anisotropic-Laminar except the growth of the boundary-layer growth assumes a turbulent boundary-layer velocity profile.

Volume Growth

The volume element size in the field is determined by using interpolation to smoothly propagate the point spacing within the field from the boundary surfaces. With optional growth, the element size is determined from interpolation and geometric growth normal to the boundary surfaces. There are four options available:

• Interpolated no growth specified, growth entirely determined by interpolation.
• Moderate growth normal to the surface at a geometric growth rate of 1.2.
• High growth normal to the surface at a geometric growth rate of 1.5.
• Very high growth normal to the surface at a geometric growth rate of 2.0.

Mixed Element toggle

Determines if a boundary-layer grid will output tetrahedra only or a mix element transition.

• Tet Tetrahedral elements only.
• Tet/Prism/Pyramid Tetrahedral, Prism, and Pyramid elements allowed.
• Tet/Prism/Pyramid/Hex Tetrahedral, Prism, Pyramid, and Hexahedral elements allowed.

Boundary Conditions:

Apply

Volume grid options may only be applied to Groups.  To apply a set of boundary conditions to a group, select the group, set all of the appropriate boundary condition toggles and select the Apply button.

Normal Growth toggle

The normal growth toggle specifies if a group of surfaces is a viscous boundary layer. If it is, and a boundary layer type grid is specified, the volume grid generator will build an anisotropic boundary layer grid off of these surfaces. NOTE: If you have specified an anisotropic type grid, there should be at least one group specified as a viscous boundary layer.

Rebuild toggle

The rebuild toggle allows surfaces in the applied group to be rebuilt by the volume grid generator to allow the viscous boundary-layer elements to cleanly connect and terminate.  This boundary condition should be used for planar surfaces that intersect surfaces with Normal Growth.

NOTE:  The precedence surface grid boundary condition should always be applied to symmetry plane surfaces.

Reconnect toggle

The reconnection toggle specifies if a Group's surface grid can be reconnected (points stay the same but edges are swapped on the triangles) during the volume grid generation process.  Reconnecting the surface is highly recommended as it allows the volume grid generator to produce higher-quality grids.  The Reconnect boundary condition will not be applied if the Group contains periodic surfaces.

Source toggle

The source toggle specifies if a Group's surface grid is used as a source control surface in the interior of the volume grid.  Each point on the surface grid is inserted in the volume grid as a point source.  The boundary faces of the source surfaces are not retained in the volume grid.  Individual line and point sources can also be created by placing a physical curve or vertex in a group with this boundary condition applied.  The physical curve edge grid and the vertex will be used as point sources in the volume grid.  Note that source points that interfere with the boundary layer will not be inserted.  See the FAQ on the differences among the Source, Transparent, and Transparent-Delete boundary conditions.

Transparent toggle

The transparency toggle specifies if a group's surface grid forms a solid wall or is a transparent control surface in the interior of the volume grid.  Transparent surfaces are retained in the volume grid and may impede the growth of the boundary-layer if they are in close proximity to Normal Growth surfaces.  See the FAQ on the differences among the Source, Transparent, and Transparent-Delete boundary conditions.  Normal growth is allowed on Transparent surfaces and they may intersect a normal growth boundary layer surface.  This may be particularly useful for refinement in wake regions.

Transparent-Delete toggle

Similar to the Transparent boundary condition, except that the transparent surface is not retained in the volume grid.  Only the specification of the surface in the boundary surface face connectivity is removed.  The nodes remain part of the final interior volume grid.  See the FAQ on the differences among the Source, Transparent, and Transparent-Delete boundary conditions.  Normal growth is allowed on Transparent-Delete surfaces and they may intersect a normal growth boundary layer surface. This may be particularly useful for refinement in wake regions.

Isotropic Variables

Maximum Element Size

Specify the maximum element size in the field.  The element size in the field is determined using either interpolation or growth normal to the boundary surfaces.

• None The maximum element size is not limited by the maximum allowable distribution function (default).
• From Bndry The maximum element size is limited to be less than the maximum point spacing for the boundary surface grid.
• Specify Specify the maximum allowable distribution function and use it to limit the maximum element size.

Anisotropic Variables: Boundary-Layer Spacing

Boundary-Layer Fraction

Specify the fraction of laminar boundary-layer thickness to use in determining the initial spacing normal to boundary-layer surfaces. When the BL fraction value is changed, the Boundary-Layer Fraction text field becomes active and the Initial Normal Spacing field becomes inactive (grayed background).  The corresponding Initial Normal Spacing is computed using the specified Reynolds Number and Reference Length

y+ Value

Specify the y+ for a turbulent boundary-layer to use in determining the initial spacing normal to boundary-layer surfaces. When the y+ value is changed, the y+ text field becomes active and the Initial Normal Spacing field becomes inactive (grayed background). The corresponding Initial Normal Spacing is computed using the specified Reynolds Number and Reference Length.

Initial Normal Spacing

Specify the initial normal spacing normal to the boundary-layer surfaces. When the value of the initial normal spacing is changed, the Initial Normal Spacing text field becomes active and y+ (or BL Fraction) field becomes inactive. The corresponding y+ is computed using the specified Reynolds Number and Reference Length.

Boundary-Layer Thickness

Specify the boundary-layer thickness for boundary-layer surfaces.  If both the Specify and Ignore toggles are not set, a value based on either a laminar or turbulent velocity profile will be calucated using the specified Reynolds Number and Reference Length.

Ignore Boundary-Layer Thickness toggle

If the Ignore toggle is selected, the boundary-layer thickness (either specified or calculated from a laminar or turbulent profile) is ignored.  In this case, the boundary-layer normal generation will attempt to continue until the normal spacing is close to the nearby isotropic value.  This is particularly useful for constructing grids used in internal flow applications.  If both the Specify and Ignore toggles are not set, a value based on either a laminar or turblent velocity profile will be calculated automatically.

Specify Boundary-Layer Thickness toggle

Specify the boundary-layer thickness.  If the BL thickness is specified, the boundary-layer normal generation will attempt to continue beyond that thickness.  If both the Specify and Ignore toggles are not set, a value based on either a laminar or turbulent velocity profile will be calculated automatically.

Reynolds Number

Specify the reference Reynolds number to use in determining the parameters that control spacing normal to boundary-layer surfaces. The value of the Reynolds number specified is the Reynolds number at the Reference Length. When the value of the Reynolds number is changed, either the value of y+ or Initial Spacing is updated, depending on which is active.

Reference Length

Specifies the reference length in grid units where the Reynolds number is specified. When the value of the reference length is changed, either the value of y+ or Initial Spacing is updated, depending on which is active.

Local Spacing Toggle

Set the specified value of either the Initial Spacing or the Boundary-Layer thickness to the selected surface(s).  If the Specify Boundary-Layer Thickness toggle is set, then the Boundary-Layer Thickness value will be applied to the surface(s); otherwise the Initial Spacing will be applied.  This toggle must be selected when the surface grid is written in order to use the Initial Spacing or B-L Thickness values that have been set.  If the toggle is not selected when the surface grid is written, then a uniform Initial Spacing and Boundary-Layer Thickness (assuming the Ignore is turned off) will be used on all surfaces.  The initial spacing and Boundary-Layer Thickness values between adjacent surfaces are smoothed over several layers to avoid large changes in spacing values which can lead to poor volume grid quality.  Initial spacing and Boundary-Layer Thickness may be applied only to Groups in the geometry or surface grid.

NOTE:  If set, variable initial spacing should be applied to ALL Groups having the viscous boundary condition. 

Apply Variable Locally to Group

• Applies the Initial Spacing/Boundary-Layer Thickness to the selected Group if the Set Local Spacing toggle is turned on.
• Applies maximum number of boundary layers to the selected group.

Anisotropic Variables: Boundary-Layer Growth Rates

Maximum Growth Rate

The maximum boundary-layer growth rate allowed.  Both the normal spacing and the maximum growth rate increase geometrically.

Manual Growth Parameters

Number of Layers

This field controls the maximum number of boundary layers that are going to be generated on a given Group. This is not a required value for mesh generation. For most cases the default value is useful.

• Each Group can be assigned and individual number of maximum boundary layers by inputting a value in this field, selecting a Group, and pushing the Apply Locally button.
• To assign a maximum number of layers for the entire grid (for every Group): make sure that no group has an assigned value for Number of Layers; input a value into the field; Toggle Only output script and surface grid; Output surface grid and script file. The script will now have an extra option "nbl"--more information can be found in related AFLR3 documentation.

Boundary Layer Termination Chooser

This chooser sets the mode for boundary layer mesh termination. For more information about this option and how it affects the boundary layer mesh termination see the related AFLR3 documentation.

Number of Constant Layers

Specifies the number of constant boundary-layer normal spacing layers adjacent to the boundary-layer surface.  Boundary-layer normal spacing growth is applied after the specified number of constant spacing layers.

Initial Growth Rate

Specifies the initial boundary-layer normal spacing growth multiplier. The multiplier determines the geometrical growth rate at which the boundary layer grid grows.

Growth Multiplier

The growth multiplier adds an additional geometrical growth to the boundary-layer normal spacing growth rate.

Graphical Explanation of Boundary-Layer Growth

ds0 = Absolute Initial Spacing ds1 = r1*ds0 ds2 = r2*ds1 ds3 = r3*ds2            .            .            .

r1 = r0 = Initial Growth r2 = Growth Mult * r1 r3 = Growth Mult * r2          .          .          .

Volume Grid:

Host field

The host field allows you to specify which machine you want to run the volume grid generator.  If "Local" is specified, the machine that SolidMesh is currently running on will be used. If a valid machine name is specified (one that is accessible to you and that has an executable version of AFLR3) a script file will be generated and a "rsh xterm" command will be generated.  A window will be started with the output of the AFLR3 program.  When AFLR3 has completed generating the grid, an output file will be generated that can then be read into SolidMesh for visualization of grid quality.  NOTE: You probably want to quit SolidMesh and use the -grid option to read in the volume grid due to the amount of memory needed to compute volume grid quality.

Partition Mode toggle

Use multi-pass partion mode grid generation.  In partition mode, the grid generation is performed one stage at a time and intermediate data is stored in temporary files instead of memory (out of core).  This option should be used for large cases with BL generation as it will substantially reduce memory requirements.

Tags File toggle

Specify that AFLR3 use the boundary conditions, initial spacing, and boundary-layer thickness specified in the tags file instead of those specified in the surface grid file.

Output option toggle

Writes only the surface grid, tags file, and the AFLR3 run script file.  This allows manual modification of the run script and/or launching of the AFLR3 volume grid generator on a specified machine.

Volume

Launches the AFLR3 volume grid generator using the specified options described above as well as the options specified in the Volume Options application.  Launching the volume grid generator will write out a surface grid, tags file, and a script with all of the specified options and (if specified) "rsh xterm" the script on the specified machine.  Once the volume grid generator is launched, you can still work within SolidMesh.