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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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Tutorial II - A Generic Wing/Body configuration using FELISA Sources

  This tutorial involves a simple wing/body configuration featuring a spherically blunted cylinder with conical tail and swept wing. The main body measures approximately 3.375 units, possesses a maximum diameter of 0.75 units, and has a wing span of approximately 2.25 units.

Model Import

The "Generic Wing/Body" sample model for the desired modeling kernel is loaded into the GridEx environment either by opening as a "CAPrI" type from the file dialog box which is accessed via the File->Open menu or by simply opening at startup via the -capri command line argument.  For example, assuming the Parasolid modeler is to be used:

    GridEx Parasolid -capri generic
(Note the model file extension is not required here)

The configuration is rendered within the Working Display as shown below.  Note that the solid model includes configuration surfaces only, and these have been triangulated by the software to support rendering requirements. To complete the solid model required for our flow analysis, we must extend this definition to encompass the desired computational domain.

 

 For our modeling efforts we will also assume a plane of symmetry to exist at the Z=0 plane, and our first effort will be to "trim" our model such that we retain only the required portion. We will perform this by accessing the CAD & Geometry panel whose facilities were demonstrated in Tutorial I.

Extending the Solid Model

 

 Our particular problem requires a far-field boundary represented by a rectangular channel of dimensions 12 x 10 x 5 units positioned 5 units upstream of the vehicle origin and centered vertically.  We select the Box facility from the toolkit and enter the initial coordinates as (-5,-5,0), and final coordinates as (7,5,5).  The resulting geometric entity is generated and rendered as shown above.  It's representation is also introduced into the Structure Tree as Volume 2.

  Though not demonstrated in this tutorial, the surfaces of the vehicle were, prior to the generation of the box, selected and rendered as shaded surfaces as shown in the view above. To duplicate this view, the user can either select the vehicle surfaces individually using the mouse, or as a collection using either the Structure Tree or the Edit facilities. The shaded mode selected from the Style pull-down menu then applies to all currently selected surfaces.


 To complete the definition of the solid model, we now use the subtract facility. The channel box is selected as the target volume, and the volume corresponding to the vehicle is selected as the tool volume.  Note in the figure above that the rendering color then changes to indicate that a new solid model exists. Furthermore, that portion of the vehicle beyond the confines of the channel box has been eliminated in this new representation.  Also note that the Structure Tree now displays a third volume entity (Volume 3) which corresponds to the newly defined solid model.  The original volumes remain within the tree though the rendering of such has been suppressed by the software.  We're now ready to begin the meshing effort.

Surface Meshing

 The meshing operation involves the introduction of discrete sources placed by the user at key positions within the domain to control local mesh resolution and its variation. In this tutorial, we have elected to employ FELISA-type sources, and we will use the corresponding meshing scheme for both surface and volume meshing operations.

 

  We introduce our first nodal source at the nose of the vehicle (as shown above) with the particular parameters as specified in the table below.   Once specified, we have the option of dynamically generating surface meshes anywhere within the domain, though it is usually a good practice to select only those surfaces local to the newly placed source in order to more quickly assess the efficacy of the source parameters.  As shown above, we have selected one surface (highlighted in red) which corresponds to the spherical cap on which to perform the mesh generation.  We see from the resulting mesh that the desired behavior has been obtained with the given parameters which are summarized in the table below.
 

Source Type
Family
      Coordinates 
dS
Constant Dist
Distance
Cell Size
Nodal
nose
(-0.375,0.0,0.0)
0.05
0.10
1.5
0.75

 

 We wish to further gauge the effects of the prescribed source parameters, and we now activate the symmetry plane to visually assess the extent of the influence of this source upstream of the vehicle. It should be apparent that it is an interactive procedure to determine an optimum or reasonable collection of sources and their defining parameters which produce the desired resolution and its variation in space.

 

 Our attention now shifts downstream to the wing/body juncture.  Since in this region we desire enhanced resolution at the leading and trailing edges as well as along the full chord, we introduce a linear source to achieve this.  Note that the termination points of this source are positioned slightly ahead and behind of the leading and training edges, respectively, so that its influence is manifested within the mesh in response to the expected flow field gradients.  The coordinates of the source termination points are given in the table below.

 

 Specification of the remaining source parameters and selection of the lower and upper surfaces of the vehicle as well as the spherical cap shows the resulting mesh on these surfaces. In this particular instance the source parameters at each termination point are identical though this need not necessarily be the case; these settings are defined to satisfy the requirements of the problem at hand.

 

Source Type
Family
      Coordinates 
dS
Constant Dist
Distance
Cell Size
Nodal
nose
(-0.375,0.0,0.0)
0.05
0.10
1.5
0.75
Linear
wing root
(0.65,0.0,0.375)
0.04
0.10
0.5
0.5
-
-
(1.6,0.0,0.375)
0.04
0.10
0.5
0.5


  The process described above is repeated with sources systematically introduced at strategic locations within the domain. It's hopefully apparent that these locations usually correspond to geometric features of the configuration though there is no implied limitation in this regard; sources can be placed anywhere to exert the appropriate influence on the mesh.

 We continue this process and introduce sources at the wing leading and trailing edges, the wing tip, the vehicle tail, and along the fuselage. At the conclusion, we have introduced 8 additional sources as shown in the figure below with the corresponding source parameters for each listed in the accompanying table. Note that at the conical tail a triangle source has been introduced as it offers an efficient means to control the resolution about the full circumference of the vehicle.  A similar effect could, of course, have been introduced with the use of two linear sources. The resulting FELISA background file is available here.



Source Type
Name
      Coordinates 
dS
Constant Dist
Distance
Cell Size
Nodal
nose
(-0.375,0.0,0.0)
0.05
0.10
1.5
0.75
Linear
wing root
(0.65,0.0,0.375)
0.04
0.10
0.5
0.5
-
-
(1.6,0.0,0.375)
0.04
0.10
0.5
0.5
Triangle
base shoulder
(2.0,0.375,0.0)
0.05
0.10
1.5
1.0
-
-
(2.0,0.0,0.375)
0.05
0.10
1.5
1.0
-
-
(2.0,-0.375,0.0)
0.05
0.10
1.5
1.0
Nodal
base tip
(3.0,0.0,0.0)
0.05
0.10
2.0
0.5
Linear
inboard LE
(0.75,0.0,0.375)
0.02
0.10
0.5
0.5
-
-
(0.94,0.0,1.125)
0.02
0.10
0.5
0.5
Linear
outboard LE
(0.94,0.0,1.125)
0.02
0.07
0.4
0.5
-
-
(0.95,0.0,2.25)
0.02
0.07
0.4
0.5
Linear
trailing edge
(1.5,0.0,0.375)
0.02
0.07
0.5
0.5
-
-
(1.5,0.0,2.25)
0.02
0.07
0.5
0.5
Linear
wing tip
(0.94,0.0,2.25)
0.02
0.05
0.5
0.5
-
-
(1.5,0.0,2.25)
0.02
0.05
0.5
0.5
Linear
lee side
(0.0,0.375,0.0)
0.07
0.10
2.0
1.0
-
-
(2.0,0.375,0.0)
0.05
0.10
1.5
1.0
Linear
wind side
(0.0,-0.375,0.0)
0.07
0.10
2.0
1.0
-
-
(2.0,-0.375,0.0)
0.05
0.10
1.5
1.0

  The combined effect of this collection of sources is shown on all vehicle surfaces in the figure below.

 Note that with this particular arrangement of sources, we have achieved a relatively uniform distribution along the fuselage with higher resolution at the leading and trailing edges of the wing.  Note also that the resolution at the rearward vehicle shoulder is also enhanced due to the presence of the triangle source. 

 The behavior of the far-field surface meshes are shown above.  Note that in this particular problem, all sources were placed at the surfaces of the configuration; none were placed within the far-field as the formulation of the FELISA sources contains a term by which the far-field resolution may be prescribed.  Additional sources could certainly have been placed at domain corners, for instance, if the requirements of the problem had warranted enhanced resolution in any of these areas.

  At this point, with the boundaries of the domain satisfactorily discretized, we are ready for volume mesh generation.

Volume Meshing

 The volume mesh to be generated is a function of the surface discretizations as well as the sources which have been placed to control those meshes. In the current GridEx release, the user need do nothing more than initiate the volume mesher using the appropriate button. It is noteworthy to mention at this point that there are currently no provisions by which the user may assess a priori the nature of the volume mesh which will result given the collection of sources. Thus the volume meshing operation may also become something of an iterative process requiring the refinement of the sources (in terms of their number, placement, and relative strength) in order to obtain a volume mesh which exhibits the desired qualities. Facilities offering a means to dynamically evaluate the source field without having to repeatedly generate the volume mesh are expected to be available within GridEx in the near future, and these will largely obviate the need for this potentially time consuming iteration.

Visualization

 After the volume mesh has been generated, facilities available within the Visualization panel may be used to conduct a detailed inspection and assessment of the mesh. A sample crinkle cut generated along the span of the wing for our generic vehicle is shown in the figure below. Note that the crinkle cut clearly shows the presence of "crows feet" a short distance off the wing surface which suggests that an adjustment of the local source strengths is in order to smooth the rather abrupt transition in cell size.

 The process continues in this fashion until the arrangement of sources yields the desired volume mesh. The final mesh may be written using the facilities within the File pull menu located in the main menu bar.


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