Mesh Generation of Large Size Industrial CFD Applications using a Cartesian Grid based Shrink Wrap approach

Transcription

1 Mesh Generation of Large Size Industrial CFD Applications using a Cartesian Grid based Shrink Wrap approach October 17, 2007 Tetrahedron II Erling Eklund, ANSYS Fluent France Y. K. Lee, ANSYS Inc., Evanston, USA H. Vardhan, ANSYS Inc., Pune, India H. Ghazialam, ANSYS Inc., AnnArbor, USA Please enter the conference name, etc. 1 ANSYS, Inc.

2 Outline Background and Objectives Mesh Generation Procedure Pre-wrapping: octree technique and leakage treatment Wrapping: interface creation and shrink wrapping Post-wrapping improvements: feature recovery, zone recovery, face quality improvement, inflating, etc. Mesh generation experiments Examples and performances Summary 2 ANSYS, Inc.

3 Background: We assume a dirty geometry Overlap Penetration Gaps Disconnected edges between adjacent surfaces Excessive details and/or irrelevant geometries, ETC 3 ANSYS, Inc.

4 Objectives A tailored mesh generator for Auto UTM (Underhood Thermal Management) analysis where geometric dirtiness is abundant. To deliver a dirtiness tolerant surface mesh generator that significantly shortens engineering time from raw CAD models to CFD-ready meshes by eliminating extensive manual cleanups. 4 ANSYS, Inc.

5 Schematic Meshing Procedure Input Dirty Geometry Pre-Wrapping Wrapping Region 2 Region 1 Region 3 Final Wrapped Mesh Post-Wrapping 5 ANSYS, Inc.

6 Scopes Pre-Wrapping Wrapping Post-Wrapping octree grid generation resolution controls leakage detection and hole patching cell overlaying &partitioning shell generation and projection face quality and geometric approximation improvement preparing volume meshing 6 ANSYS, Inc.

7 Cartesian Grid Generation : combined octree data structure overview OctDomain Overlaying with Intersection checks CartOctree CartOctant OctDomain CartOctree 1 CartOctree 2 Master Octant (0) CartOctree I Oct 0 CartOctree N (1) Oct 0 (1) Oct 1 Oct i (1) (1) Oct 7 Refinement level ( R 1) Oct j Oct Oct Oct i Oct ( R) 0 ( R) 1 (R) ( R) 7 7 ANSYS, Inc.

8 Cartesian Grid : integer coordinate in the octree Octant Ref. Level R Coord. (I,J,K) Ref. levels Spans(δ) /2 /2 2 /2 6 Pre-computed spans Pre-computed translations y oct x oct z oct δ For =1m, the max. resolution is mm. Octree Octant (I,J,K) (x tree,y tree,z tree ) 0 I, J, K 16 2 (x+δ,y+δ,z+δ) (x,y,z) =(x tree +x oct,y tree +y oct,zt ree +z oct ) 8 ANSYS, Inc.

9 Cartesian Grid : query to neighbors x+ neighbor of Q 1? Q 4 Q Q 3 Q 6 Q Q 7 y Q x 9 ANSYS, Inc.

10 Cartesian Grid Generation : refinement strategies Curvature and proximity size functions Zhu, Blacker and Smith (2002) Fixed sizes for geometric surfaces or using predefined boxes Using buffer layer for mesh smoothness Refinements (local) can be used for automatic hole detection and resolution G D curvature sf. proximity sf. D G ANSYS, Inc.

11 Cell Region Partitioning 1. Take any nonintersecting cell which has not been visited. 2. Flood-fill while not crossing intersected cells 3. Go to (1) until there is no such a cell. Regions of contiguous cells bounded by intersected cells 11 ANSYS, Inc.

12 Manual Leakage detection : Tracing between inside and outside location 12 ANSYS, Inc.

13 Manual Leakage detection : Tracing is locating the hole in the geometry 13 ANSYS, Inc.

14 Manual Leakage detection : After hole fixing and updating the regions 14 ANSYS, Inc.

15 Auto Hole Detection : Detection during Region refinements Region refinement do not create any new regions. 15 ANSYS, Inc.

16 Auto Hole Detection : automatically patched holes Local intersection check and region update with the new facets. 16 ANSYS, Inc.

17 Cartesian Grid Generation : Boundary Extraction and wrapping Extract boundary of this cell region regions of contiguous cells bounded by intersected cells with different color codes rugged but watertight skin surface of wrapping region initial wrapped surface after projection and improvement 17 ANSYS, Inc.

18 Projection : nearest projection and exceptional case Projection to the nearest location on the geometry Range search with a tree Pre/post smoothing of the wrapper surface Exceptional handling using face normal and projection direction N r r u 1 > u 2 u1 u 2 N An exceptional case in the nearest projection 18 ANSYS, Inc.

19 Post-Wrapping Improve face quality Smoothing Weighted Laplacian smoothing with reprojection to the geometry Edge swapping/collapsing Improve face skewness Coarsening: Modified Garland and Heckbert (1997) Local Skewness based improve Improve geometric accuracy Feature imprinting Zone separation and boundary straightening Volume meshing preparation inflating to remove extremely thin gaps and self-intersections 19 ANSYS, Inc.

20 Feature Imprinting 1. For a given feature line extracted from the geometry 2. Trace a node path close in the guidance of the geometric feature 3. Project the path nodes onto the feature The compulsive projection may deteriorate validity of mesh. Controlling node projection with regard to the projection vector and local mesh size plays a key role for success Initial wrapped surface feature to recover Starting from the starting node, trace the best path using the distance and inner product. After feature recovery wind shield v g v m (1) d v m (4) v m (2) v m (3) stretched faces 20 ANSYS, Inc.

21 Zone Treatment : zone separation and boundary straightening Zone separation Group faces in terms of the closest geometric surfaces from their centroids. Boundary straightening For each node on the boundary of zones Iteratively project it onto the geometric surfaces which are associated with its incident zones 21 ANSYS, Inc.

22 Inflating : remove extremely thin gaps and self-intersections Problematic regions in subsequent volume-meshing A single (dangling) surface in the geometry is resulted in a region having meshes on two sides Large faces in thin curved region may intersect other wrapper faces on the other side. Sacrifice geometric accuracy to avoid volume meshing failures Inflating 22 ANSYS, Inc.

23 Industrial Wrapper applications A simplified Truck Model External Aero + UTM + Cabin HVAC Demonstrate capability on real world applications Performance 23 ANSYS, Inc.

24 Wrapping Truck : original STL model 1250 Assembly parts 473,156 triangles and 237,761 vertices 24 ANSYS, Inc.

25 Wrapping Truck : initialization and performance 60M cells in the octree grid for wrapping Took 1 hour and consume 15GB memory on an AMD 64 bit Linux machine with a facebased Cartesian grid data structure with integer coordinate 5GB in the improved octree data structure CPU Time (sec.) Cell Counts (Thousands) Memory (MB) CPU Time , , ,461 1, refined corner 0 10,000 20,000 30,000 40,000 Cell Counts (Thousands) Intel P4, Linux CPU time complexity of octree refinement and intersection checks 25 ANSYS, Inc.

26 Wrapping Truck : final wrapper surface 26 ANSYS, Inc.

27 27 ANSYS, Inc.

28 Wrapping Truck : volume mesh generation Prism boundary layer if needed Tetrahedral meshes using the constrained Delaunay triangulation 28 ANSYS, Inc.

29 Wrapping Truck : External Aero + UTM + Cabin HVAC analysis 29 ANSYS, Inc.

30 Summary Developed a surface mesh generator by (1) Overlaying Cartesian cells with intersection checks, (2) region partitioning with intersected cells, (3) shrink wrapping and (4) post-wrap improvements Important characteristics Dirty geometry tolerant and ignoring irrelevant small features Ignoring irrelevant external and internal geometries Using the simple facetted representation as input facilitates surface mesh generation Significant reduction of engineering time for meshing while sacrificing little geometric accuracy 30 ANSYS, Inc.

31 Thank you! 31 ANSYS, Inc.