Contribution to GMGW‐1 - gmgworkshop.com · John P. Steinbrenner. ... meshed with quad‐dominant unstructured grid and T‐Rex method to keep a smooth cell size ratio

Contribution to GMGW‐1

• RENAUD Thomas• ONERA – The French Aerospace Lab• PID 05

1st Geometry and Mesh Generation WorkshopDenver, CO June 3‐4, 2017

Summary of grids generated:Case Code(s) Starting

Geometry Model

Grid Type Number Grid Levels

HL‐CRM full gap Pointwise IGES Overset Structured 1

HL‐CRM full gap Pointwise IGES Unstructured Hexa Tetra Prisms 2

GMGW‐1, Denver CO, June 2017 2

• Pointwise (http://www.pointwise.com):• Structured, unstructured, overset, and hybrid meshing software• Highly automated T‐Rex (anisotropic tetrahedral extrusion) technique for boundary layer resolved hybrid meshes

John P. Steinbrenner. "Construction of Prism and Hex Layers from Anisotropic Tetrahedra", 22nd AIAA Computational Fluid Dynamics Conference, AIAA AVIATION Forum, (AIAA 2015‐2296)

• Unstructured grid:• Mixed elements: hexa, tetra, prisms with hexa‐dom cells

• Structured grid:• Near‐body/off‐body grids with overlapping conditions• Background grid automatically generated and adapted with Cassiopée(ONERA in‐house suite of pre‐ and post‐processing tools)

Benoit, C., Péron, S., and Landier, S., “Cassiopee: A CFD pre‐ and post‐processing tool”, Aerospace Science and Technology, Vol. 45, 2015, pp. 272‐283.Péron, S. and Benoit, C., “Automatic off‐body overset adaptive Cartesian mesh method based on an octree approach”, Journal of Computational Physics, Vol. 232, No. 1, 2013, pp. 153‐173.

Geometry Import and Preparation


• Geometry preparation process• IGES import: no difficulty• Layering: hiding component to work on local grid• For overset structured grid, choice has been done to mesh the full wing and to

blank it with fluid blocks to generate the slat/flap cavities necessity to unhide some geometry patches

• No modification of the geometry

• Lessons learned• The layering could be automatically done either by CAD families or by adjacent

geometry entities

Mesh Generation (unstructured)


• Mesh generation process• Process flow:

• Automatic edges creation, merged within tolerance. Only characteristic edges are kept.• Surface meshes: structured grids are mainly done on intrados/extrados. Either surfaces are

meshed with quad‐dominant unstructured grid and T‐Rex method to keep a smooth cell size ratio.

• Volume mesh: with BL imposed parameters (no first uniform layers possible), T‐Rex method for the BL (hexa/prisms cells)

• Control of resolution: manual input for edges boundaries, imposed maximum cell size on surfaces and a smooth volume cell size decay (0.95)

• Mesh export file formats: CGNS

• Difficulties encountered• First uniform BLs not possible with T‐Rex method (uniform extrusion+T‐Rex

creates large cell size ratios)• Some difficulties to respect a smooth cell size ratio with multiple adjacent T‐Rex

Mesh Generation (unstructured)


• Difficulties encountered• Volume mesh generation crashed in the wing flaps cavity some iterations to

solve this issue but finally I needed to coarsen the wing cavity trailing edges to perform the volume grid

• Lessons learned• Volume mesh generation can be quite long… and eventually not work

optimization possible?• More information of reasons of crash (representation of the issuing cells and its

adjacent cells)• Local smoothing or improving of cells shapes could be useful

Mesh Generation (structured)


• Mesh generation process• Process flow:

• Choice has been made to create the wing cavities by blanking the full wing grid. A collar grid is used between fuselage and wing.

• Automatic edges creation, merged within tolerance. Only characteristic edges are kept.• Surface meshes: mainly by TFI. Some are done by extrusion on wall surfaces.• Near‐body volume mesh: by extrusion with BL imposed parameters (4 first uniform layers).

Manual TFI when extrusion is limited.• Off‐body background grid: automatically generated and adapted to the near‐body cell sizes by

Cassiopée in‐house tool.• Blanking for elsA CFD solver is done by Cassiopée.

• Control of resolution: manual input for edges boundaries, ratio of extrusion = 1/1.1/1.16 in BL, 1.2 outside BL

• Mesh export file formats: CGNS

Mesh Generation (structured)


• Difficulties encountered• Overset assembly can be tedious if too overlapping grids• So the main difficulty was the meshing of the flaps cavity: the chosen topology

for the surface mesh required to refine the wing trailing edge and the BL extrusion was tricky with 3 directions in the corners.

• Few process iterations to ensure a good overset assembly

• Lessons learned• The mesh generation process could be improved by accepting more

overlapping patches on wall surface.

Mesh Statistics


Geometry Model Grid Type Grid Level Nodes BFaces Volume Cells Jacobian Max. included angle<=0 Positive

skewMin max

HLCRM‐Full Gap Overset Structured Medium 52.0 M 370K 49.0M 0 2191

HLCRM‐Full Gap Unstructured HexaTetra Prism

Coarse 5.0 M 181K 7.9M 71.523 179.993

Medium 11.3 M 288K 18.6M 71.282 179.989

Max. included angle histogram for medium unstructured mesh

Surface MeshWing Upper Surface

9GMGW‐1, Denver CO, June 2017

Surface MeshWing Lower Surface


Surface Mesh ‐ Wing Slat LE at Root


Surface Mesh ‐ Wing Flap TE at Root


Surface Mesh ‐ Wing Tip LE


Surface Mesh ‐ Wing Tip TE


Flap Gap Upper Surface


Volume Mesh Cut at y=277.5


Volume Mesh Cut at y=638




Surface MeshWing Upper Surface

19GMGW‐1, Denver CO, June 2017

Surface MeshWing Lower Surface


Surface Mesh ‐ Wing Slat LE at Root


Surface Mesh ‐ Wing Flap TE at Root


Surface Mesh ‐ Wing Tip LE


Surface Mesh ‐ Wing Tip TE


Flap Gap Upper Surface


Volume Mesh Cut at y=277.5






Mesh Evaluation


• Evaluation• Overset structured mesh: Jacobian >0 (positive skew ok), no orphan points

during overset assembly• Unstructured mesh: decrease the maximum included angle• Solution has been computed on the medium unstructured grid (not presented

at HLPW)• Low turb. visc. on slat intrados

• Adherence to meshing guidelines• Overset structured: not adherence for the 4x multigridable grids (not full

validated for overset grids with elsA) and for the cells alignment with wakes (lack of time)

• Unstructured: not adherence for the 4x multigridable grids (not possible with elsA), for the cells alignment with wakes (lack of time) and for the first constant BL layers (not possible with T‐Rex functionality)

Additional Topics


• Future Technology • Improvement of unstructured mesh by local smoothing or cell shapes

deformation• ONERA works also on intersection of unstructured grids and on robustness of

extrusion for polyhedral meshes. It will be tested for HLCRM.

Summary

• What I learned from this exercise (first time I meshed a complex aircraft geometry)• What was the easiest? The geometry was clean (generally not the case)

• What was the hardest? For the structured grid, the extrusion in the flap cavity is tricky. For the unstructured grid, decreasing the max angle is difficult without crash. The cells alignment with component wakes can be tricky even with refinement source regions.


Documents

Contribution to GMGW‐1 - gmgworkshop.com · John P. Steinbrenner. ... meshed with quad‐dominant unstructured grid and T‐Rex method to keep a smooth cell size ratio