Prism NoPyramids

ICEM CFD Tetra/Prism For CFD++ or Fluent

Simon Pereira ANSYS ICEM CFD Product Manager

© 2007 ANSYS, Inc. All rights reserved.

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ANSYS, Inc. Proprietary

Large model Strategy • If your models are large and generating the mesh takes time. • Break out a chunk (most difficult chunk) chunk ) for optimizing setup and settings • With only a few million cells, your turn around time will be quicker • Apply lessons learned to the larger models • , apply that to other models within that class. class .


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Geometry Setup • Prism starts with geometry – For geometry patch independence, delete unnecessary curves and points. – Over constrained patches (too many curves) will make it difficult for the smoother to give you a smooth mesh. – an a so set s zes on curves or ens ty reg ons • Finer mesh in difficult areas gives more freedom to the smoother • Some users set “width” parameter on surfaces to keep the mesh size from coarsening for a number of layers beyond the surface. This puts more nodes in the volume and makes it easier for prism to connect up. – Don’t set width to more than 2 or 3 on surfaces your plan to prism. – Yes, it will increase your Octree tetra count – It only helps with octree tetra mesh (use density regions for a more general solution) © 2007 ANSYS, Inc. All rights reserved.

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Prism Preparation – It is difficult to smooth prisms, so we want to start with the best possible quality. Preparation is the key. – Start with good tetra or tri-surface mesh – Check aspect ratios / quality – Check and fix all diagnostics • Single edges, Non-manifold vertices, or Duplicate elements will crash the prism mesher

• Laplace smooth the surface tri’s • It may be a good idea to use one of the bottom up tetra methods (Advancing front, Tgrid, or Delaunay) to replace your octree tetras. • You could reduce the tetra element count by 30 to 50% and you will get better mesh transitions. • Be careful • Laplace smooth the surface mesh before filling • Make sure you have no single edges, non-manifold verts, etc. which could cause trouble for the bottom up tetra methods (and prism for similar reasons). • If you set “surface width” for volume refinement, it is not directly respected by the bottom up methods

– Visually scan the surface mesh – Look for kinks or sharp tent-like structures in the mesh • Diagnostics may not reveal all surface discrepancies

– Make sure part associations are correct – Look for a few elements in one part scattered among another part • This can sometimes happen and then when prism is generated it is tough to organize the transitions to the surrounding tetras between scattered prism columns. © 2007 ANSYS, Inc. All rights reserved.

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Mesh Smoothing • Use the Laplacian based surface smoother

•Laplacian smoother

p

i

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rp= Avg (ri) 1 – Some users like to float the tets and smooth the tris, then turn off Laplace and freeze the tri’s and smooth the tets. – Other users just delete the Octree tets, smooth the tri’s and then fill with a bottom up tetra method. • Laplace moves the point towards an “ideal” location defined as the average of the surrounding nodes – This procedure significantly improves the mesh – Has the effect of averaging the surface mesh transitions and triangles become more equilateral © 2007 ANSYS, Inc. All rights reserved.

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Tetra Fill • Octree tetra has sharp transitions and uses about 50% more nodes on these models than Delaunay Tetra • Advancing Front, TGRID Delaunay and TGRID AFT are also available – Advancing front methods give smoother mesh, but that means more elements. – AFT is an advancing front option that can be set for the TGRID mesher • My tests have shown it is roughly 6 times faster than the other advancing front algorithm

– 12.1 has a Delaunay with TGLib-AFT option (best) • We recommend doing the fill before Prism – Tetras enable collision detection – Bottom up methods may be more difficult after prism © 2007 ANSYS, Inc. All rights reserved.

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ICEM Tetra Meshing Delaunay

Octree

Expansion Ratio=1.2

Mix and Match with Hexa Core or Prism

Advancing Front


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Tetra Fill • Under global settins, you can change the growth ratio and adjust other properties for the fill. OCTREE Mesh

Delaunay Mesh


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Prism Requirements

• The settings are determined by your requirements. • In this case Fluent or CFD++ – You require All Prisms (Few or no Pyramids) • Can handle sharp high angles and relatively poor quality to achieve this

– You require smooth transitions • Cell to cell and between prisms and tetras

• You need to set the initial height. – Y+ © 2007 ANSYS, Inc. All rights reserved.

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Prism Settings • Leave initial height as “0” – This causes the initial height to float in order to reduce the volume change between the last prism and adjacent tetra.


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Pyramids due to intersection • If the height is floating, you must refine the mesh to control the prism height and prevent intersection from being resolved with pyramids

•Before refinement, prism layers intersected and pyramids were used •after refinement there is lots

Auto reduction (discussed later) can also prevent collision without pyramids © 2007 ANSYS, Inc. All rights reserved.

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Prism Settings

• Min Prism quality = 0.000001 – Default is 0.01; If you set this closer to zero, prism will permit lower quality prisms without resorting to r mids. – We set this low enough to get no pyramids and CFD++ can still handle the worst prisms.


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Prism Settings • On complex models, prism orthogonality and the quality of the inner tri surface are at odds. – Set this below 0.5 to favor tetra quality over orthogonality.

Ortho weight = 0.5

Ortho wei ht = 0.1

Ortho weight = 0.9

Poor quality tets Directional smoothing may also reduce orthogonality as was necessary in this example © 2007 ANSYS, Inc. All rights reserved.

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Prism Settings • Fillet ratio works together with Ortho weight to improve tetra quality in convex regions. – Larger fillet (number is a ratio to total prism height) means more inner surface area for the tops of the prism columns and their adjacent tetras

Fillet Ratio = 0.1 Fillet Ratio = 0.5

Fillet Ratio = 1.0


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Prism Settings • Max Prism Angle keeps the prism layers connected around tight bends. – Set this at 180 to prevent pyramids

Max Prism Angle = 140


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Max Prism Angle = 180


Prism Settings • Smoothing Options – Set surface smoothing steps to 0; you should smooth before running prism – Volume/directional Smoothing is the most time consuming part of Prism. If you turn these off, Prism generates very quickly – weight to be less than 1. For complex models with lots of tight spots, smoothing steps are important Without Directional smoothing steps, prisms are very orthogonal, but they will still bend for fillet ratio. Directional collisions result in pyramids © 2007 ANSYS, Inc. All rights reserved.

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Prism Settings • Advanced prism params, Auto Reduction – Proximity can cause prism to stop and give pyramids instead. – This option causes the prisms to squeeze down

– This is fixed with redistribute prisms (automatic at 12.1) yram s

Redistributed Reduced (Squished) and prisms No Pyramids


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Redistribute Prism • Edit Mesh, Redistribute Prism – Reset the initial prism height. – Solves the squished mesh from “auto reduction” – Allows the user to adjust for the desired y+ – Roughly maintains the smooth volume transition

– Auto reduced Prisms (Squished) – . – Prisms fixed – Initial height set

Before

• Issues • If the initial height is large, the min ratio will be 1 (uniform) distribution. • 11.0 could not redistribute layers containing pyramids. This is fixed in 12.0 • 12.0 changed the way Prisms were calculated to give the most precise column by column initial height • 12.0 could not redistribute hexas (inflated quads), or imported mesh, these have both been fixed for 12.1 © 2007 ANSYS, Inc. All rights reserved.

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After


Smooth Prism • Prism is difficult to smooth which is why we try to start with as good a surface mesh as possible. • Once the mesh is done, we recommend smoothing with the prisms (Penta_6) frozen. This will improve the tetra mesh. Do this until you don’t see any improvements.

– also, but use judiciously or you may add many new elements.


• If you want to smooth some more turn on the smooth for the Penta_6 elements, set the “up to Value” very low (< 0.05) so that most of the prism mesh is left alone. – I also recommend saving before smoothing prisms in case you don’t like the result. • From all accounts CFD++ is very robust and seems to be able to handle19even the worst prisms. ANSYS, Inc. Proprietary

One Layer/Multi Layer • The one layer, multi-layer argument has gone back and forth for a long time… – The one layer advantage is speed since it only needs to smooth the volume mesh out of the way one time and doesn’t do any directional smoothing. Smoothing is the most time consuming part. The downside is that your prisms are very simplistic and not good at capturing complex geometry… If that is all you wanted, then you may as well run prism with all the layers, but turn the smoothing steps way down (maybe to 0). – e a vantage o mu t p e ayers s t at t e smoot ng can e p en t e prisms around difficult areas, plus you can take advantage of a number of other intelligent features that the developers worked hard on (floating height, fillet ratio, etc.). The down side is that it is slow. – I usually try a middle solution. If you want 21 layers in the end, then maybe do three layers or seven layers instead. Use the max height over base to control the growth and then split and redistribute. We put a fair amount of development resources on split and redistribute prisms over the 12.0 and 12.1 releases.


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Closing • These settings should minimize the number of pyramids you produce, while allowing for a better Prism/tetra transition and control of the first cell height. • With more discussion, we can adjust these recommendations as needed • Contact us if you have any trouble • Tetra/Prism is a quick way to get a decent estimate. For more accuracy , methods.


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Prism NoPyramids

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