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Model Requirements for a Flow Analysis

Midplane . Fusion . 3D . Model Requirements for a Flow Analysis. Introduction. Aim Understand the requirements of a good mesh for midplane, Fusion and 3D Why do it A good mesh is critical for accurate results Get better models from CAD when requirements are understood

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Model Requirements for a Flow Analysis

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  1. Midplane  Fusion  3D  Model Requirements for a Flow Analysis

  2. Introduction • Aim • Understand the requirements of a good mesh for midplane, Fusion and 3D • Why do it • A good mesh is critical for accurate results • Get better models from CAD when requirements are understood • Easier translation • Overview • General mesh requirements • 3D specific requirements

  3. Mesh Types & Model Requirements • Often same requirements for different mesh types • Fusion • 3D • Most of the time 3D models are built from Fusion meshes • Must have good Fusion model to create 3D model • Midplane • Some requirements specific to the mesh type

  4. Free Edges • Edge of an element that does not touch another element • Fusion & 3D • Must not have any free edges • Midplane • Will occur at partedges

  5. Non-Manifold Edges • Edge where 3 or more elements share the edge • A “T” intersection • Fusion & 3D • Must NOT have any non-manifold edges • Midplane • Will have non-manifold edgesat all rib intersections

  6. Unmatched Matched Match Ratio – Fusion Only • An element on one side of wall thickness matched to another element on the other side of the wall • Fusion • Poor match relates to poor thickness determination • Should be least 85% for Flow, 90% for Warp

  7. 1 3 Matched but not reciprocally matched Reciprocally Matched 2 4 1 3 2 4 Reciprocal Match Ratio – Fusion Only • When two elements on opposite walls are matched with each other • 100% reciprocal match is when all matched elements are reciprocally matched • Should be over 90% reciprocal match • For good warpage results Nodes do not have to be aligned to be reciprocally matched

  8. h L Aspect Ratio • Ratio of Length to Height of an element • Fusion & Midplane • Average < 3:1 • Maximum < 6:1 • Higher can be tolerated for flow analysis, • High ratios cause problems with cool and warp • 3D • Maximum 30:1 on Fusion mesh to be converted • Tetrahedral mesh ratio < 50:1 is best

  9. Connectivity Regions • A group of elements that are connected together • All mesh types must have one connectivity region for the part • Mesh statistics will report more than one region if cooling geometry is in the study

  10. Element Orientation • Define the “Top” positive normal side and “Bottom” side of elements • Fusion • Must have the Top (Blue) side pointing outward • Midplane • Orientation should be consistent • 3D • Not applicable Two elements not correct

  11. Intersecting Elements • Intersections • Elements intersect the plane of other elements • Shown in Red • Fusion & 3D & Midplane • Must not have any

  12. Overlapping Elements • Overlaps • Elements in the sameplane intersect (takethe same space) • Shown in Blue • Fusion & 3D & Midplane • Must not have any

  13. Zero Area Elements • Zero Area Element • Very low volume or edge length • Normally caused by very high aspect ratio elements • Fusion & 3D & Midplane • Must not have any

  14. Thickness Representation • Ensure the thickness of the model is correct • “Chunky” geometry • Fusion will not calculate the thickness correctly • Must be represented by a 3D model

  15. TH 10x TH 4x TH 2x TH Chunky Geometry • Less than 4:1 width : thickness ratio • Prefer a ratio of greater than 10:1 • Heat Transfer only done on faces (blue) not in thickness direction (Green) • 2:1 ratio - thickness is 33% of the perimeter • 4:1 ratio - thickness is 20% of the perimeter • 10:1 ratio - thickness is 9% of the perimeter

  16. 1 2 Correct Inverted Node 1 and 2 on the same side of the face shared by the two elements 3D only – Inverted Elements • Must not have inverted elements

  17. 3D only - Number of Element Layers • Default number of rows 6 • Used for most applications • 8 rows better for 3D fiber orientation • As number of rows increases so does the aspect ratio • This can lead to problems with Navier-Stokes solver

  18. 3D Only • Collapsed faces • A node is defined on two opposing faces • Local zero thickness • Must not have • Internal long edges • Edge length of element inside part to the edge length on the part surface • Should be <2.5:1

  19. 3D Only • Extremely large volumes • Ratio of volume of element to the average • Should be under 20:1 • High aspect ratios • Maximum aspect ratio should be < 50:1 • Small angle between faces • Node close to the plane formed by the plane of the other elements • Should be > 2 degrees Normal Small angle

  20. Summary of Model Requirements

  21. Summary of Model Requirements

  22. Summary of Model Requirements

  23. Summary of Model Requirements

  24. 2.0 mm 2.0 mm 1.0 mm 1.0 mm 3.0 mm 3.0 mm Mesh Density Effects - Hesitation • To pick up hesitation, three rows of elements across a major change in thickness are required

  25. Mesh Density Effects - Weld Lines • The mesh at the weld line location must be dense enough to pick up the weld line

  26. Nominal wall 2.5 mm Thin Region 1.25 mm Mesh Density Effects - Air Traps • Air traps may not be predicted if the mesh is not fine enough in thin regions

  27. Mesh Detail • Model must represent FLOW characteristics • Thickness • Flow length • Volume • Small features of a part should be eliminated from a flow model • Blends • Radii • Fillets

  28. Effect of Geometry on Fill Pressure • Thickness • Greatest effect on pressure • Flow length • Second greatest effect • Volume • Virtually no effect

  29. 1.0 mm 1.5 mm R 0.25 mm Effect of Corner Radii • Fusion & Midplane • No effect on pressure • Purple dots are nodes • To represent radii, small node spacing is required • Creates high aspect ratio for very minor thickness changes • 3D • Can see local effect of corner radii but must have very high mesh density

  30. A Model With and Without Radii • No Radii 14 elements > 6:1 aspect ratio • With Radii 561 elements >6:1 aspect ratio No Radii With Radii

  31. Shear Rate (3D) [1/sec] A Model With and Without Radii • 3D can pick up local high shear at sharp corners • Requires very fine mesh • Only has local affect

  32. Compute Time, Mesh Density and Accuracy • As mesh density increases • The compute time increases exponentially • Limited accuracy improvement Material, ABS 1.9 mm nominal wall Processing Cond. 60-235-1 Computer 2.8 GHz, 1 Gig Ram

  33. QUESTIONS?

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