Unstructured Mesh Generation

We now open the NURBS from the STEP file in Gmsh and generate an unstructured surface mesh with refinement along the leading edge of the wing.

NOTE: the CurveLists described below are valid for the file zeroebwb.STEP and Gmsh v4.8.4

1. Import STEP file

   • File > Merge

2. Create physical group with all NURBS surfaces

   • Make sure that geometric surfaces are visible: Tools > Options > Geometry > Visibility > Surfaces
   • Click the Y button in the bottom left corner to get a top view
   • Create physical group: Modules > Geometry > Physical Groups > Add > Surface
   • Press CTRL + Right Click to box-select only half of the airframe (we will export half the mesh and will later mirror it to obtain a mesh that is exactly symmetric)

   

3. Create criterion for refinement at LE

   • Modules > Mesh > Define > Size fields
   • Define leading edge lines: Distance with CurvesList set to 33, 25, 53, 45 and 1000 for NumPointsPercurve
   • Define transition lines: Distance with CurvesList set to 18, 19, 56, 54 and 1000 for NumPointsPercurve
   • Define upper-bound lines of leading edge: Distance with CurvesList set to 30, 22, 55, 47 and 1000 for NumPointsPercurve
   • Define lower-bound lines of leading edge: Distance with CurvesList set to 35, 28, 51, 43 and 1000 for NumPointsPercurve
   • Define cell size based on distance to leading edge: MathEval with the formula 1.5*(F1^1.2 + 500/F2^0.5)/2 + 30
   • Define cell size based on thickness of leading edge: MathEval with the formula 3*((5*F1+F3+F4)/7)^1.5 + 5
   • Mix both cell sizes: Min with FieldsList set to 5, 6 select "Set as background field" and click Apply. Defined this way, it refines the mesh in the proximity of the LE lines based on LE thickness, while transitioning the refinement close to the transition lines.

4. Set mesher settings

   • Set size factor: Tools > Options > Mesh > General > Element size factor and set to 0.1
   • Select MeshAdapt for the 2D algorithm

5. Mesh the NURBS surface

   • Discretize 1D curves: Modules > Mesh > 1D
   • Discretize 2D surfaces: Modules > Mesh > 2D
   • Smooth out the discretization by clicking Modules > Mesh > Smooth 2D a few times

   

6. Export mesh as .msh in ASCII format v4: File > Export and select Mesh - Gmsh MSH (*.msh) in the dropdown menu