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Table of Contents
Software Execution
GUI Design & Operation
CAD & Geometry
Element Sizing
VGRID Sources
FELISA Sources
Quick-Link Reference

GridEx Tutorials
Geometric Primitives & Solid Operations
Generic Wing/Body - FELISA Sourcing
Generic Wing/Body - VGRID Sourcing
Feature Suppression & Regeneration
Parameter Modification & Regeneration
Transformations & Displacement

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Element Sizing

  Element sizing for both surface and volume meshing operations is controlled using the facilities provided within the Background Grid panel. The constraints which are imposed to achieve the desired variation relate to mesh length scales and stretching (or aspect ratio) are prescribed using an entity referred to as a source.  These sources are placed by the user anywhere within the domain to satisfy resolution requirements of the problem at hand.  Local mesh characteristics are thus determined from an integrated effect of all sources placed.  A variety of source types are at the user's disposal to provide the extensive control over the mesh.  The use of these sources is described below.  The terminology employed is intentionally analogous to the classical heat conduction problem containing discrete heat sources - the solution to this problem is a strong function of the location and energy capacity of the sources - the same is true with the sources employed in the meshing operation.

  User specified sources are maintained within a background grid which is transparently generated by the software.  The background grid encloses the entire physical domain as represented by the solid model, but does not conform to the geometric configuration for which the unstructured mesh is to be developed.  Though sources are placed discretely, a Poisson equation for the constraints is solved to provide a smooth variation throughout the domain.  During the mesh generation process, the meshing algorithms sample the background mesh for information on desired local mesh characteristics and build the mesh to satisfy these constraints.  Sources influence an infinite region of 3D space, and thus sources placed on domain boundaries will not only influence the local surface mesh but also the local volume mesh.  The extent and direction of this influence are, of course, under user control.

  GridEx currently incorporates VGRID, FELISA, and AFLR3 as the mesh generation options. The first two algorithms make use of information available from the background grid during mesh construction while AFLR3 does not.  Though the manner in which sources are placed within the domain is independent of the meshing approach, the specific formulation of VGRID sources and FELISA sources differ.  The use of both types of sources is described below.

  The Background Grid form, seen here, provides for source creation and management.  The Source frame enables the creation/management of sources, while the Node frame provides the common controls necessary to determine the applicable node and place the node in 3D space.  The tabs on the right-hand side manage forms that enable the definition of additional parameters specific to the respective sourcing algorithms.

Source Creation

  The controls in the Source frame support source management and include the following capabilities:

  The New button introduces a source whose type is to be specified within either the VGRID or FELISA tabbed forms.  Note that as a source is introduced, it is transparently tagged as either a VGRID or FELISA source depending upon which of the latter forms is active.  Sources are not interchangeable between the meshing algorithms, so ensure that the proper tab is active before introducing the source.  Only sources of the active tab are displayed.  Note also that the identifier displayed in the Number field is also incremented and is relative to the active tab.

  The Copy button introduces a new source which is an exact copy in terms of placement, type, and properties of the current or active source.  The active source is not necessarily the last source entered; the Number field indicates which source is active and the adjacent arrow buttons permit the user to step through all sources currently defined.

  The Select button offers a short-cut to identifying the active source as such may then be selected interactively from the Working Display.  Simply position the bulls eye cursor over the desired source and press the right mouse button.  The active source becomes highlighted in yellow with the active node in red and the Number field along with the other form fields are updated to reflect the selection.

  The Delete button simply deletes the active source.  Note that source identifiers are not renumbered though they remain ordered in the same sequence.

  The Show toggle simply toggles the display of the active source.  The source need not be rendered for it to be active, and all sources defined are used in the mesh generation process regardless of whether they are displayed.

  The Family text field permits the user to group any number of sources into a common family.  For VGRID sources, the "Box" family has the usual special meaning.

  The Node frame permits the assignment of the physical coordinates for all points required to define the active source.  The left- and right-arrow buttons permit the user to cycle through the defining nodes of the active source.  The Pick button enables positioning via the direct selection of a point from the Working Display at which the source node is to be placed.  Alternatively, the coordinates for the active node may be entered directly into the text fields or may be defined by pressing the adjacent up- or down-arrow keys to move the point incrementally in the respective direction.  Lastly, for sources which are also functionally dependent upon a vector, the components may also be specified within this form as the Node identified will appear as a P or U in the text field.  Placement of these points is identical to the placement of regular source nodes.

Responsible NASA Official: William T. Jones
Site Curator: William T. Jones
Comments and Questions
Last Updated: Dec 18, 2001