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ICEM CFD Tetra/Prism For CFD++ or Fluent
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Simon PereiraANSYS ICEM CFDProduct Manager
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Large model Strategy
If your models are large and generating the mesh takestime.
Break out a chunk (most difficult chunk) for optimizing setup andsettings
With only a few million cells, your turn around time will be quicker Apply lessons learned to the larger models
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,apply that to other models within that 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.
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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 sizefrom 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. Dont 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)
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Prism Preparation
It is difficult to smooth prisms, so we want to start with the best possible quality. Preparation isthe 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 tris 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.
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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.
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Mesh Smoothing
Use the Laplacian based surface smoother
pLaplacian smootherrp= Avg (r i)
1
2i
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Some users like to float the tets and smooth the tris, then turn offLaplace and freeze the tris and smooth the tets. Other users just delete the Octree tets, smooth the tris and then fill
with a bottom up tetra method.
Laplace moves the point towards an ideal location defined as theaverage of the surrounding nodes This procedure significantly improves the mesh Has the effect of averaging the surface mesh transitions and
triangles become more equilateral
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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 alsoavailable Advancing front methods give smoother mesh, but that
means more elements.
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AFT is an advancing front option that can be set for theTGRID 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
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ICEM Tetra Meshing
Octree Delaunay
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Expansion Ratio=1.2
Advancing Front Mix and Match
with Hexa Core or
Prism
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Tetra Fill
Under global settins, you can change the growth ratioand adjust other properties for the fill.
OCTREE Mesh
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Delaunay Mesh
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Prism Requirements
The settings are determined by yourrequirements.
In this case Fluent or CFD++ You require All Prisms (Few or no
Pyramids)
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Can handle sharp high angles andrelatively poor quality to achieve this You require smooth transitions
Cell to cell and between prisms andtetras
You need to set the initial height. Y+
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Pyramids due to intersection
If the height is floating, you must refinethe mesh to control the prism height andprevent intersection from being resolvedwith pyramids Before refinement, prismlayers intersected and
pyramids were used
after refinement there is lots
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Auto reduction (discussedlater) can also preventcollision without pyramids
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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 lowerquality prisms without resorting to
r mids.
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We set this low enough to get nopyramids and CFD++ can still handlethe worst prisms.
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Prism Settings
On complex models, prism orthogonality and thequality of the inner tri surface are at odds. Set this below 0.5 to favor tetra quality over
orthogonality.
Ortho wei ht = 0.1 Ortho weight = 0.5 Ortho weight = 0.9
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Poor quality tets
Directional smoothing may also
reduce orthogonality as wasnecessary in this example
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Prism Settings
Fillet ratio works together with Ortho weight toimprove tetra quality in convex regions. Larger fillet (number is a ratio to total prism height)
means more inner surface area for the tops of theprism columns and their adjacent tetras
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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 connectedaround tight bends. Set this at 180 to prevent pyramids
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Max Prism Angle = 140 Max Prism Angle = 180
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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, Prismgenerates very quickly
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weight to be less than 1. For complex models withlots of tight spots, smoothing steps are important
Without Directional
smoothing steps, prisms arevery orthogonal, but theywill still bend for fillet ratio.
Directional collisions resultin pyramids
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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)
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yram s
Reduced (Squished) prismsRedistributed andNo 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)
Before
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Issues If the initial height is large, the min ratio will be 1
(uniform) distribution.
11.0 could not redistribute layers containingpyramids. This is fixed in 12.0
12.0 changed the way Prisms were calculated to givethe most precise column by column initial height
12.0 could not redistribute hexas (inflated quads), orimported mesh, these have both been fixed for 12.1
.
Prisms fixed Initial height set
After
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Smooth Prism
Prism is difficult to smooth which is why we try to startwith as good a surface mesh as possible.
Once the mesh is done, we recommend smoothingwith the prisms (Penta_6) frozen. This will improve thetetra mesh. Do this until you dont see anyimprovements.
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also, but use judiciously or you may add many newelements.
If you want to smooth some more turn on the smoothfor the Penta_6 elements, set the up to Value very
low (< 0.05) so that most of the prism mesh is leftalone. I also recommend saving before smoothing prisms
in case you dont like the result.
From all accounts CFD++ is very robust and seems tobe able to handle even the worst prisms.
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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 doesnt do any directionalsmoothing. Smoothing is the most time consuming part. The downside is thatyour prisms are very simplistic and not good at capturing complexgeometry If that is all you wanted, then you may as well run prism with allthe layers, but turn the smoothing steps way down (maybe to 0).
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e a vantage o mu t p e ayers s t at t e smoot ng can e p en t eprisms around difficult areas, plus you can take advantage of a number of otherintelligent features that the developers worked hard on (floating height, filletratio, 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 controlthe growth and then split and redistribute. We put a fair amount ofdevelopment resources on split and redistribute prisms over the 12.0 and 12.1releases.
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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 cellheight.
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
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,methods.
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