add virtual topology demo

Author: mkraska

CAD/OnshapeTutorial/README.md

@@ -1,5 +1,5 @@
 # Meshing a CAD Geometry with Gmsh
-Tested with CGX/CCX 2.10, Gmsh 2.12
+Tested with CGX/CCX 2.10, Gmsh 2.13.1
 
 This demonstrates a possible workflow for a CalculiX analysis of a CAD
 generated part.
@@ -9,6 +9,9 @@ generated part.
 * Import and meshing in gmsh, export as .inp file with volume and surface meshes.
   In gmsh, define physical surfaces for boundary conditions and export the mesh
   with appropriate settings (to ensure that node sets are written)
+* There are two versions for gmsh pre-processing:
+  * `part.geo`: mesh the geometry as it is, leading to small elements at short edges and narrow surfaces
+  * `partVT.geo`: Compounding lines and geometry before meshing.  
 * Open the inp file in cgx and remove all surface elements. Eventually extend node sets to
   face sets for surface definition or pressure application.
 * Write the mesh and required set definitions
@@ -21,6 +24,9 @@ generated part.
 | [part.step](part.step) | STEP geometry exported from Onshape |
 | [part.geo](part.geo) | Gmsh control file for meshing and model display |
 | [run.fbd](run.fbd) | CGX control file for preprocessing, solving and postprocessing |
+| [partVT.geo](partVT.geo) | Gmsh control file with geometry cleaning (virtual topology)|
+| [runVT.fbd](runVT.fbd) | CGX control file for the VT version|
+| [VTdemo.fbd](VTdemo.fbd) | CGX file for the mesh plots (original and VT version) |
 | [solve.inp](solve.inp) | CCX input file |
 
 # Run the analysis
@@ -53,6 +59,37 @@ The gmsh geo file can be executed separately if you want to play around with the
 > gmsh part.geo
 ```
 The result of the meshing is the file `gmsh.inp`
+
+## Virtual Topology
+
+The CAD model contains short edges and narrow faces, which locally enforce small elements.
+
+Spurious edges and points can be removed by joining and re-parametrization of adjacent lines or surfaces using the commands
+```
+Compound Line(new#) = {old#1,old#2};
+Compound Surface(new#) = {old#1,old#2};
+
+```
+The Gmsh command file `partVT.geo` uses these commands to produce a mesh without spurious refinement spots.
+
+There is one limitation: The midside nodes of second order elements don't follow the geometry, the edges of such elements are always straight except at boundaries. In Gmsh 3.0 this is announced to be fixed.
+
+Execute
+```
+> cgx -b VTdemo.fbd
+```
+to produce images for comparison of the meshes produced with the original geometry or with the cleaned geometry (virtual topology).
+
+Original geometry (1856 nodes, 998 elements)
+
+<img src="mesh.png" width="400" title="Mesh based on the original geometry">
+<img src="mesh1.png" width="400" title="Mesh based on the original geometry">
+
+Virtual topology (1425 nodes, 665 elements):
+
+<img src="VTmesh.png" width="400" title="Mesh based on the modified geometry, no local refinement spot any more">
+<img src="VTmesh1.png" width="400" title="Mesh based on the modified geometry, note the straight element edges on the arched surface.">
+
 # Application of Boundary Conditions
 After closing the gmsh window, CGX takes over control again and reads `gmsh.inp`.
 
@@ -67,12 +104,12 @@ Some cleanup is required, because gmsh writes 2D elements for the physical surfa
 
 Remove the 2D elements (we address them by type here):
 ```
-zap +CPS6
+(cgx window) zap +CPS6
 ```
 Extend node sets to face sets if required (here we need the set `load` for
 pressure application)
 ```
-comp load do
+(cgx window) comp load do
 ```
 The following image shows the nodes of the support surface and the faces of the pressure
 application surface.
@@ -81,6 +118,7 @@ application surface.
 
 Once the sets are defined, there is no particular challenge any more with setting up the simulation.
 
+
 # Results
 
 von Mises stress, displaced geometry

CAD/OnshapeTutorial/VTdemo.fbd

@@ -0,0 +1,48 @@
+# sys gmsh part.step -clcurv -3 -o gmsh.inp -order 2
+sys gmsh part.geo
+read gmsh.inp
+# remove surface elements
+zap +CPS6
+plot f all n
+view elem
+rot y
+rot d 20
+zoom 2
+plus n all
+hcpy png
+sys mv hcpy_1.png mesh.png
+frame
+zoom 5
+rot y
+rot u 20
+tra l 30
+tra d 30
+plot f all n
+plus n all
+hcpy png
+sys mv hcpy_2.png mesh1.png
+del mesh
+
+sys gmsh partVT.geo
+read gmshVT.inp new
+# remove surface elements
+zap +CPS6
+plot f all n
+view elem
+frame
+rot y
+rot d 20
+zoom 2
+plus n all
+hcpy png
+sys mv hcpy_3.png VTmesh.png
+frame
+zoom 5
+rot y
+rot u 20
+tra l 30
+tra d 30
+plot f all n
+plus n all
+hcpy png
+sys mv hcpy_4.png VTmesh1.png

CAD/OnshapeTutorial/VTmesh.png

@@ -0,0 +1,41 @@
+Merge "part.step";
+// Virtual topology to avoid enforcing small elements
+Compound Line(1000)={63,64};
+Compound Line(1001)={65,66};
+Compound Surface(30) = {23,25};
+Compound Surface(31) = {9,24,26};
+Compound Surface(32) = {21};
+Compound Surface(33) = {22};
+// Set definitions
+Physical Surface("support") = {5};
+Physical Surface("load") = {17};
+Physical Volume("part") = {1};
+// Mesh control and meshing
+Mesh.SaveGroupsOfNodes = 1;
+Mesh.ElementOrder=2;
+Mesh.Optimize=1;
+Mesh 3;
+Save "gmshVT.inp";
+// image
+// in script mode, there is bad image quality
+Mesh.SurfaceEdges = 1;
+Mesh.SurfaceFaces = 1;
+Mesh.VolumeEdges = 0;
+Mesh.VolumeFaces = 0;
+General.Trackball=0;
+General.RotationX=-50;
+Geometry.HideCompounds = 1;
+
+Save "gmshVT.png";
+Save "gmshVT.gif";
+Save "gmshVT.pdf";
+General.RotationX=-260;
+General.RotationY=23;
+General.ScaleX = 2.5;
+General.ScaleY = 2.5;
+General.ScaleZ = 2.5;
+General.TranslationX = -40; // X-axis translation (in model units)
+General.TranslationY = 10; // Y-axis translation (in model units)
+Save "gmshVT-zoom.png";
+Save "gmshVT-zoom.gif";
+Save "gmshVT-zoom.pdf";