Wireframe

Contents

A Wireframe component is a structured grid of points: rows of points that form a surface of quadrilaterals, the way legacy geometry and many CFD tools describe a shape. It comes from an XSec (.hrm) or Plot3D (.p3d) file, and the controls on its WireFrame tab rearrange and tidy the grid -- turn it, trim it, thin it and close its open edges -- so that it can take part in OpenVSP's analyses. The points themselves cannot be moved one by one.

The Gen and XForm tabs are the ones every component has; see Gen and XForm. A wireframe has no mass, symmetry or sub-surfaces.

Quick Start

1. Choose File > Import and XSec as Wireframe (.hrm) or Plot3D as Wireframe (.p3d). Each component of the file becomes a wireframe, under a new Blank.
2. Place the Blank with the XForm tab; the wireframes move with it.
3. Check each wireframe's Type and normals, and close its open edges with the Patch choices.

Importing a Grid

XSec files

The XSec (.hrm) format is OpenVSP's own cross section format, also written by File > Export > XSec. Each component is a set of cross sections with the same number of points in each:

 HERMITE INPUT FILE

 NUMBER OF COMPONENTS = 2

WingGeom
 GROUP NUMBER      = 0
 TYPE              = 0
 CROSS SECTIONS    = 10
 PTS/CROSS SECTION = 33
  4.00000000000000000e+00    0.00000000000000000e+00    0.00000000000000000e+00
  3.92363234218945145e+00    0.00000000000000000e+00   -3.09158494470971589e-16
  ...

The name line names the wireframe, with its spaces turned to underscores, and TYPE sets its Type: 0 for lifting, 1 for non-lifting. The cross sections are the grid's rows in I and the points of each section its columns in J. The same file imported as XSec as Tri Mesh becomes one Mesh of all its components instead.

Plot3D files

A Plot3D (.p3d) grid file holds one or more blocks, each a grid of $n_i \times n_j \times n_k$ points. The file may be formatted (text) or unformatted (Fortran records), in single or double precision, with or without the iblank array, and with $x$ and $y$ only (a planar grid, at $z = 0$) or $x$, $y$ and $z$; all of these are told apart by the file's content. Each block becomes a wireframe with I along the block's first index and J along its second. Only the first $k$ plane of each block is kept, so a block should be a surface grid, $n_k = 1$.

A block with only one row or column of points is a curve, not a surface: it is drawn as a line and takes no part in the analyses.

Orientation on import

On import each wireframe checks which way its grid faces, from the sign of the volume it encloses, and sets Invert so that its normals point outward. For a closed body this is right; for an open grid such as a thin wing it is a guess.

The Grid: I and J

The grid is a set of rows: I counts the rows and J the points along each row. Each cell of four neighboring points is a quadrilateral of the surface, and analyses split each into two triangles. With the feature lines on (Feature in the Geom Browser), a red line runs from the grid's first point in the direction of I and a green one in the direction of J, each half way across the grid. Figure 1 marks I and J in the same colors.

Type and Normals

Type says what the grid represents: Lifting a wing and Non-Lifting a body. It decides how the degenerate geometry is built from the grid, and so how the analyses built on it see the wireframe: in VSPAERO's thin set, a lifting wireframe becomes a camber surface and a non-lifting one a set of plates. The type is also written to an XSec export.

Invert reverses the normals, the side of the surface that counts as outside. The analyses that intersect the model, such as CompGeom and Mass Properties, depend on it. CompGeom's report is a quick check: a closed wireframe whose normals point in has a negative volume.

Rearranging the Grid

The controls act in this order, each on the result of the one before.

Swapping and reversing turn the grid over, so each also reverses the normals internally to keep the outside where it was; Invert does not change. Skips trim rows of points that are not wanted, such as a duplicated section or the wake rows of a CFD grid. A skip that leaves no rows leaves no grid. Strides thin a dense grid to keep the analyses fast; a stride of 1 keeps every row.

Closing Open Edges

A grid made for another tool often has open edges: a wing whose tip is not closed, a blunt trailing edge, a body of revolution whose sections stop short of repeating their first point. Each Patch choice adds one row of points at its edge of the grid, built from the edge row itself or from the opposite edge:

The patches act in the order I Start, I End, J Start, J End, each on the grid the one before made, so a patch can build on another. Halfway Opposite at J Start and Copy Opposite at J End close a blunt trailing edge to a sharp one: the first adds a row on the trailing edge's midline, the second a copy of that row at the other end. Halfway Opposite at both edges does not close the gap, since the second midpoint is taken to the first (Figure 2). Figure 1 shows the tip of a wing closed with Point and with Line.

Figure 1. A wing grid imported from an XSec file. I runs root to tip and J round each section from the lower trailing edge, both from the grid's first point. I End Point closes the tip with a flat cap to its centroid; I End Line closes it onto the camber line.

Figure 2. The root section of the wing grid of Figure 1, with its blunt trailing edge open, closed on its midline, and left open by Halfway Opposite at both edges, whose J End point lies three quarters of the way up the trailing edge.

Using a Wireframe

A wireframe is placed like any component, with the XForm tab, including Scale and Flip. Its grid takes part in the analyses as a surface of quadrilaterals:

• CompGeom, Mass Properties, the slicing analyses and the mesh exports take its triangles, two per cell. For these the grid should be closed and its normals should point out.
• Degenerate geometry and the analyses built on it take the grid as a wing or a body, by its Type.
• An XSec export writes the grid as it has been rearranged and placed, so a wireframe cleaned up here can be written out for another tool.

It is not a smooth surface: it has no sub-surfaces and takes no part in CFD Mesh, FEA Mesh, Fit Model or projections. The model file keeps the points as they were imported, and the controls are applied again each time it is opened, so any of them can be changed later.

Scripting

A wireframe has no functions of its own in the API: import it with ImportFile and IMPORT_XSEC_WIRE or IMPORT_P3D_WIRE, which returns the ID of the Blank holding the wireframes, and set the Parms in the table above. They are in the group WireFrame, except FlipIJFlag, RevIFlag, RevJFlag and InvertFlag, which are in the group Wireframe. Type is WireType, 0 for lifting and 1 for non-lifting, and the patch types are PATCH_NONE, PATCH_POINT, PATCH_LINE, PATCH_COPY and PATCH_HALFWAY.

import openvsp as vsp

blank = vsp.ImportFile( "wing.hrm", vsp.IMPORT_XSEC_WIRE, "" )
wire = vsp.GetGeomChildren( blank )[ 0 ]
vsp.SetParmVal( wire, "IEndPatchType", "WireFrame", vsp.PATCH_LINE )
vsp.Update()