Mesh

Contents

A Mesh component holds a fixed set of triangles. It is how OpenVSP brings in geometry from other tools -- STL, Cart3D and NASCART files -- and it is what several analyses leave behind: the intersected and trimmed outer mold line from CompGeom, the slices from Mass Properties and Planar Slice, a projected area. A mesh can be moved, turned, scaled, saved and used in further analyses, but its shape cannot be edited: it has no parameters of its own beyond how it is drawn.

Topic	Tab
Quick Start
Where a Mesh Comes From
Importing a Mesh
Tags
Converting a Mesh	Other
Mesh and Slices	Other
Using a Mesh
Scripting

The Gen, XForm and Mass tabs are the ones every component has; see Gen, XForm and Mass. A Mesh has no Sub tab.

Quick Start

An imported mesh

Choose File > Import and click the file's format: Stereolith (.stl), Cart3D (.tri), NASCART (.dat) or XSec as Tri Mesh (.hrm).
Place it with the XForm tab, and scale it to the model's units with Scale.
Set its density on the Mass tab if it is to count in the mass properties.

The result of an analysis

Run the analysis -- CompGeom, Mass Properties, Planar Slice, Projected Area. The mesh it makes is added to the model. Run from their screens, CompGeom, Mass Properties, Planar Slice and Wave Drag hide the components the mesh was made from.
Inspect it in the 3D view: turn on Sub in the Geom Browser to color it by component and sub-surface.
Delete it when done, or keep it as a record: it is saved with the model.

Where a Mesh Comes From

A mesh is made by an import, by an analysis, or from another component.

Source	The mesh
File > Import	The file's triangles.
CompGeom	The outer mold line: every component intersected with every other, inside parts removed, negative volumes cut out.
Mass Properties	The components' triangles, and the slices the mass was integrated over.
Planar Slice	The components' triangles, and the cutting planes' cross sections.
Wave Drag	The components' triangles, and the slices at the Mach angle.
Projected Area	The projected area, flat, and its outline.
Geometry Analysis, Make MeshGeom	The triangles the analysis case worked on, and any slices.
DegenGeom, Make Mesh	The degenerate geometry as triangles: plates for bodies, camber surfaces for wings.
Point Cloud, Create Convex Hull Mesh	The convex hull of the points.
File > Export, the mesh formats	The triangles of the exported set.

Mass Properties removes the mesh it made the previous time it ran, so only the latest is kept. The mesh exports (STL, Cart3D, OBJ and others) also build a mesh, and leave it in the model with every other component hidden. An export whose set already holds a mesh writes that mesh instead of building a new one, so a second export after the model has changed writes the old shape; delete the mesh before exporting again.

Importing a Mesh

The importers read triangles and nothing else: no units, no names of parts and no surface curvature. OpenVSP's model has no units, so a mesh comes in at the numbers in the file; scale it to match the model (Scale on the XForm tab).

Format	Notes
Stereolith (.stl)	ASCII or binary, told apart by the file's content. Several solids in one file make one mesh, in ASCII or as binary blocks, one per solid, as DATCOM writes them. A binary file whose size does not match its facet count, such as one cut short, is refused. The facet normals are read as given.
Cart3D (.tri)	Formatted or unformatted, single or double precision, told apart by the file's content. The component number of each triangle is kept as its tag.
NASCART (.dat)	Rotated as it is read, from NASCART's axes ( $Y$ up) to OpenVSP's ( $Z$ up): the file's $(x, y, z)$ becomes $(x, -z, y)$ .
XSec (.hrm)	OpenVSP's cross section format. Each component's grid of points is split into triangles, two per quadrilateral, and all components go into one mesh. To keep the components apart as grids, import the same file as XSec as Wireframe instead (see Wireframe).

A file that holds no triangles is not imported.

Converting a Mesh

The Other tab converts a mesh into two other components. Each is a new component; the mesh is left as it was.

Convert under Convert to Point Cloud makes a Point Cloud of the mesh's vertices, each once, where the mesh is placed. The vertices of the slices are included when Slices is on.
Convert under Convert to NGon Mesh makes an NGon Mesh: the triangles merged into polygons, with the wing wakes found, ready to write as a VSPAERO geometry file. The NGon mesh is made where the mesh is placed. The cleanup passes that collapse short segments along intersections and merge the fans of thin triangles at poles and tip caps are not run (see How the Polygons Are Made); they are options of VSPAERO's own conversion. A mesh opened from a model file has lost the tables that say which component each triangle came from, so its conversion finds no wakes; convert a mesh in the session that made it.

Mesh and Slices

A mesh from Mass Properties, Planar Slice or Wave Drag holds two sets of triangles: the components' mesh and the slices. Mesh and Slices on the Other tab choose which are drawn. They also choose what an STL export of the mesh writes and what a point cloud made from it holds, so turn Slices off to export the shape alone.

Using a Mesh

A mesh takes part wherever triangles are enough.

It is placed like any component: location, rotation, attachment, Scale and Flip on the XForm tab. It makes no symmetric copies, so the XForm tab's symmetry controls are greyed, and so is Rot Origin(X).
CompGeom, Mass Properties, the slicing analyses, Projected Area, interference checks and the mesh exports take it as they take any component. A mesh run through CompGeom again is intersected with the other components like any of them; it should be closed for the result to be right.
Mass Properties gives it the density on its Mass tab, like any component.
VSPAERO takes it as one or more components.
It is saved with the model, triangles and all.

It is not a surface. It has no sub-surfaces, its degenerate geometry is empty, and it takes no part where a surface is needed: CFD Mesh, FEA Mesh, Fit Model, or as the boundary or direction of a projection. Its triangles cannot be edited in OpenVSP; change the geometry it came from and run the analysis again, or edit the file and import it again.

Scripting

A mesh is made by importing a file or by running an analysis; each returns the new mesh's ID.

Function	Use
`ImportFile( file, IMPORT_STL, "" )`	Import a file as a mesh (`IMPORT_CART3D_TRI`, `IMPORT_NASCART`, `IMPORT_XSEC_MESH` for the other formats).
`ComputeCompGeom( set, half, types )`	Run CompGeom and return the mesh's ID.
`CreatePtCloudGeom( mesh_id )`	Convert to a point cloud, as the Convert to Point Cloud button.
`CreateNGonMeshGeom( mesh_id )`	Convert to an NGon mesh, as the Convert to NGon Mesh button.

The Other tab's toggles are the Parms MeshFlag, SliceFlag and StartColorDegree in the group Draw. The scale the mesh has been given in total is Scale_From_Original in the group XForm.

import openvsp as vsp

mesh_id = vsp.ImportFile( "part.stl", vsp.IMPORT_STL, "" )
vsp.SetParmVal( mesh_id, "Scale", "XForm", 0.0254 )     # inches to meters
vsp.Update()