Icemcfd Tips&Tricks2010

ICEM CFD Tips and Tricks 2010…

Simon Pereira © 2010 ANSYS, Inc. All rights reserved

11

ANSYS, Inc. Proprietary

Guiding principles of Workbench

CAD Model

Workbench Model

• Parametric; update in the CAD, DM or Spaceclaim Thisimagecann otcurrently bedisplayed.

• Persistent; meshing keeps up with model changes © 2010 ANSYS, Inc. All rights reserved

2



CAD Model

Workbench Model

• Parametric; update in the CAD, DM or Spaceclaim Thisimagecann otcurrently bedisplayed.

• Persistent; meshing keeps up with model changes © 2010 ANSYS, Inc. All rights reserved

2



• Highly-automated – Physics aware intelligent defaults

Automatic Contact (FEA),

 

Non-conformal interfaces (CFD), etc.

– Context sensitive

meshing (prism, contact, etc.) Automated Meshing

Apply loads on geometry Go straight to solution!

© 2010 ANSYS, Inc. All rights reserved.

3


Guiding principles of Workbench Hex-Dominant

• Flexibility:

Tet

– Breadth of mesh

methods/controls – Depth of basic/advanced tools

Swept

Free Mesh Type = Tetra

Free Mesh Type = Hexa Dominant

Free Mesh Type = Hexa Core © 2010 ANSYS, Inc. All rights reserved.

4


!"#" Meshing Workbench Integrated Meshing

Supports instances

Sold as part of integrated solver solution ►Can Add additional dedicated seats ►

ANSYS MAPDL

CFX-Mesh

Combine mesh methods CFD with “program controlled ” inflation

The Future of Meshing Combination of best in class technologies and concepts ►Integrated in a user friendly simulation environment ►For all Physics (CFD, FEA, etc.) ►

GAMBIT

ICEM CFD

Workbench Guiding Principles Parametric and Persistent ►Highly Automated, Physics Aware ►Flexible Controls ►Adaptive Architecture ► TGrid


5

FEA


!"#" TGrid E$%ends !"#" Meshing Capabili%ies Wi%h &o'er To( Mesh from Advanced Cartesian Hexa Core and Inflation

Facets and/or Mesh ►Poor or complex data ►

y t i l Efficiently Mesh large/ acomplex models n o ►Greater than 20 million cells i t c ►Extended mesh controls n uAdvanced Mesh options F ►Intelligent automated Prism d e ►HexCore to far-field planes d n ►Wrapper technology e t ►GoCart technology x EExtended mesh diagnostics

Interactive Mesh Editing Remeshing, etc.

►


Complex Geometry

Wrap any Complex Geometry

Cavity Re-meshing 6

Test drive GoCart now! ANSYS, Inc. Proprietary

!"#" ICEM CFD E$%ends !"#" Meshing Capabili%ies Wi%h &o'er To(

>250 Million Cells

Mesh from ►Dirty CAD, Third Party Geometry, etc. ►Faceted Data, Scan Data, STL ►Combinations of CAD, Facets & Mesh Efficiently Mesh large/complex models ►Greater than 20 million cells ►Extended mesh controls

Structured Hexa mesh

Hexa mesh ►Structured or Unstructured ►Advanced blocking control Extended mesh diagnostics Advanced Interactive Mesh Editing

Interactive Mesh Editing © 2010 ANSYS, Inc. All rights reserved.

Output to a wide variety of solver formats ►CFD, FEA, neutral formats 7

E x t e n d e d F u n c t i o n a l i t y


ICEM CFD Tips and Tricks 12)1 Simon Pereira

© 2010 ANSYS, Inc. All rights reserved

88


genda

• Tips and Tricks – General, Tetra/Prism – Hexa


9


Workbench *eaders • Workbench readers are available for use in ICEM CFD – Use the same license keys as the ICEM

CFD CAD interfaces – Available for CATIA, SolidWorks, UG, Pro/E, Inventor, OneSpace, SolidEdge, Bladegen, SpaceClaim, JTOpen, etc. • *.sat, *.x_t, *.dsdb, *.stp, *.step, *.igs, *.CATPart, *.ps, *.prt, *.dwg, *.sldprt, *.asm, *.agdb, *.model, *.par, *.scdoc, etc.)

– Also read Workbench formats such as

Design Modeler, Simulation, etc. • *.dsdb, *.agdb

• Read in Mesh and/or Geometry! © 2010 ANSYS, Inc. All rights reserved

10


I+por% geo+e%r, using Workbench *eaders • You can import Geometry from Workbench Design Modeler – Only active bodies are imported (Bone Marrow was not

active) – Named selections are imported

1

• Create Subset from Named Selection – Named Selections can be setup in DM, Simulation or

2

CAD packages such as UG, Pro/E or SolidWorks – SpaceClaim too! – Option to filter by “Named Selection Prefixes”

• Accept the Defaults and apply

3 © 2010 ANSYS, Inc. All rights reserved.

11

Often easier to do CAD creation, simplification or repair in Workbench Design modeler and transfer the data to ICEM CFD ANSYS, Inc. Proprietary

!a+ed "elec%ions as "ubse%s

• Each Body comes in as a Part • The named Selections can appear as Geometry Subsets – These can be used to control display or selection

1

– These can be turned into Parts

• Make sure all the Subsets are active • Right Click on “Subsets” and choose “Create Part • The new parts can be displayed

2


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*epair Geo+e%r, • Build Topology establishes connectivity – Hole detection… • helpful for flood fill • But not required for patch independent methods

– MultiZone – Patch Dependent Meshing – Etc.

• Can cause trouble, use carefully • Single curve cleanup allows you to use two different tolerances, – Small size for the majority so you don’t collapse good features – Larger size for stubborn gaps © 2010 ANSYS, Inc. All rights reserved.

13


*epair Geo+e%r, • Once Diagnostic Topology is built options such as Flood fill and feature detection become available


14


*epair Geo+e%r, • Detection tools have obvious use for extracting, selecting and even removing features – Hole tool has advanced options to

remove holes or set sizes for patch based surface meshing – Fillet tool is usefull for putting fillets in a different subset or part so you can apply a different meshing method

• But what about nasty models where Build topology can’t help or causes more trouble then it is worth? © 2010 ANSYS, Inc. All rights reserved.

15


*epair Geo+e%r, -'.o /uild Topo • The Check Geometry option does *surface based* feature detection (not topology based) • If you have a scary model, such as a large complex cast engine block, – don’t build topology, it may take too long or cause trouble. – delete all the curves and points – extract all the flat items to a new part • Low curvature

– –

Extract points and curves from the flat parts Complete parameter setup and proceed to Octree tetra…

Area and Normal can help filter precisely


16


o%ke,s

• Hotkeys can make you much more efficient • Search the Help to get hotkey maps •There is consistency to the hotkeys •For instance “s” is for split. •On the geometry panel, it is split curve.

Hotkeys are Tab specific

•On the Mesh Edit Panel, it is for Split Edge •Control-hotkey is a special ability, such as control-s for split w/propagate •Shift-hokey is an alternate •“?” to list hotkeys © 2010 ANSYS, Inc. All rights reserved

17


*e+inder abou% c%ree "i3es • With Octree, it is important to understand mesh size… – Many long time users waste time setting wrong sizes • Sizes carefully set to 0.75, 0.5 and some 0.3 • Max deviation set to 0.05 and Min size set to 0.45…

• The 0.75 and 0.5 both round down to 0.5. • The smaller 0.3 would round down to 0.25, but since it is sill much larger than the small detail it is placed on, why no just set the entire part to 0.5 • The Min Size rounds down to 0.25 – –


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Since this is the smallest size set, all other sizes are related to this size Local Size = (Smallest Size)*2 n for n=0 to ∞


Densi%, regions… • A density region sets the Max local size within the volume – Use with Geometry Translation tool to copy/translate/rotate – Use line and point densities when advantageous – Use Width on surfaces to refine near the wall


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Working 'i%h Densi%, regions

• More here about various density types, show how to create a cone and a torus.


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Edge Cri%erion 4 Thin Cu%s –

Edge Criterion • Determines if a node projects to surface or the edge is split • Range from 0 to 1 –

0.2 ok for most cases.

• To increase nodes moving to fit to geometry, increase this criterion – Ideal for planned tet to hex conversion • To increase refinement near entities, reduce this –

Reduces non manifold verts in trailing edges

Edge Criterion = 0.2

Handy when thin cuts fail or are difficult to setup © 2010 ANSYS, Inc. All rights reserved

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Edge Criterion = 0.01


Edge Cri%erion 4 Thin Cu%s – Define Thin cuts • Tool for resolving gaps based on pairs of parts. • It is made necessary by the octree process – One way to avoid the problem is to use

patch based meshing

• Thin cuts have limits – If two surfaces approach asymptotically,

this may eventually break down – If the separation rule of thin cuts is violated at any point, all thin cuts are deactivated If the face of a tetra element has a surface/line node on part “A” then it may not have a surface/line node in part “B ” © 2010 ANSYS, Inc. All rights reserved.

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A c B

Note; If the surfaces of the two parts, A and B , meet, then the contact curve must be in a third part, c , or the thin cut will fail. ANSYS, Inc. Proprietary

Ignore Wall Thickness

• You don’t always want thin gaps refined • FEA – higher Aspect Ratio • CFD – ignore wall thickness FEA

CFD

Off

On


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ccurac,

• The mesh projection module in ICEM CFD facets b-spline surfaces and curves • Tri tolerance determines the degree of facetization, accuracy

Facetized behind the scenes

Tri t olerance = 0.1 © 2010 ANSYS, Inc. All rights reserved.

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ccurac, 



Lower tri tolerance = more precise curvature 

Greater accuracy



Slower Rendering

Increase tri tolerance to work on large models, but put it back at the default when computing the mesh Tri tolerance =0.1

Tri tolerance =0.01

Tri tolerance =0.001

(default) © 2010 ANSYS, Inc. All rights reserved.

25


ccurac,

Important for High resolution NavierStokes grids with very fine near wall spacing (1e-6)

• For greater accuracy without increasing tri-tolerance, there is a hexa option to project directly to B-Splines (no facetization) © 2010 ANSYS, Inc. All rights reserved

26


"urface Mesh T,pe

Surface based meshing controls: • Mesh Type • Mesh Method – Transitions between mesh methods and

types are handled automatically

Set different mesh methods or types for each surface? Mapped/paved bosses, patch independent tetra everywhere else? © 2010 ANSYS, Inc. All rights reserved

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!"#" ICEM CFD De5elop+en%

Surface based meshing controls: • Use Patch Independent on surfaces with poor connectivity. – Common edges are meshed with Octree method.

This method can be used to pave fillets, thin regions, etc. Note; For High aspect ratio elements along wings, etc. We recommend the Hexa merge method

• Paved with quads or tris on more important regions, – fillets, bosses, etc. © 2010 ANSYS, Inc. All rights reserved.

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&a%ch Independen% "urface Meshing

• Need a surface mesh for rough geometry? – Patch Independent uses Octree approach for all the Octree advantages – Huge time saver


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e$a for "urface +eshing • It is a little more work, but for stamped parts, Hexa may be well worth while. • Controlled wrapping Just use Remesh to fix any difficult areas


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Co+ple$ Geo+e%r,

• Often we have complex geometry, but the details are not critical to our analysis. • Try shrinkwrap. – Essentially it is a Cartesian based surface mesher ICEM CFD Shrinkwrap is good for capturing rough blockage quickly TGrid Shrinkwrap is good for capturing details more precisely (more like ICEM Octree Tetra) © 2010 ANSYS, Inc. All rights reserved

31


/o%%o+ 6p Te%ra +e%hods • Use these methods in conjunction with the other ICEM CFD methods – “top down” or “patch independent” tetra – Patch based surface meshing • Each method has its own specific requirements/Benefits – All require a closed outer volume (no single edges). • Run all the checks on the surface mesh • Internal single edges are ok (baffles)

– No sudden changes either along a surface or across a

gap • use Laplace smooth to prep the surface

– Advancing Front mesher only works with all Tris


32


/o%%o+ up Te%ra +e%hods •Advancing Front Mesh ignores Volume density control, but is affected by surface density •Delaunay respects Volume control, but not Surface Width control

Advancing front Delaunay is grows more slowly generally coarser, from the walls, but but it does does not respect density density regions regions


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ICEM Te%ra Meshing Delaunay

Octree

Expansion Ratio=1.2

Mix and Match with Hexa Core or Prism

Advancing Front


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TGrid Te%ra

• TGrid Tetra was also added to ICEM CFD at V11. • At R12, we also include the new TGrid AF algorithm which is advancing front based (instead of skewness based) – It actually generates more quickly that the previous scheme – Greater memory requirements

than Standard Delaunay © 2010 ANSYS, Inc. All rights reserved

35


Te%2e$ • Tet2Hex (12 to 1) works with Octree tetra mesh and converts it back to cartesian • Transitions and smoothed mesh are not easily converted – Turn off smoothing – Increasing Edge Criterion can help somewhat


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*12 I+pro5e+en%

• Orient Octree by LCS • Tet to Hex by LCS

Oriented Octree Tetra more easily converts to oriented ANSYS ICEM CFD Hexa hybrid mesh


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Mesh Edi%ing 'i%h "ubse%s • Subsets are non exclusive groups that you can use to work with geometry and Mesh • Experts use Subsets for mesh editing – Display mesh Quality • Use Histogram to create subsets of the worst elements • Add layers to subsets to get a better view • Repair a few of the worst elements and stuck elements • Run smoother – Check Mesh • For many of the checks a Subset is the best option • Add layers to the subset • Repair issues

• Right click on subsets to add a layer or remove elements or choose modify for more options, such as adding volume elements. © 2010 ANSYS, Inc. All rights reserved.

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e$a FarField • Hybridize your models – tunnel is usually simple and doesn’t change from run to run • Use hexa for max efficiency – Detailed region may be complex in shape • Use Tetra/Prism for max ease of mesh generation – Merge together

Convert Tetra to Hexa Run Prism thru Both © 2010 ANSYS, Inc. All rights reserved.

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e$a Far Field

Some Models have regions of complexity and regions where Hexa would be more appropriate and relatively easy to apply


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&u%%ing i% all %og%her


41


e$a Wings.irfoils

• Here it is in reverse – Hexa mesh inside the airfoils, Tetra mesh outside. Then merge mesh together – Delete Hexas, Pyramids and Tetras, grow prisms

from quads and tris, then generate tetras again.


42


6nderhood

• In this case, a hexa region around the cooling system has been merged into the rest of this under hood model (done with octree tetra)


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7e, *ules for a successful Merge

• Perimeter must match – Merged surface parts must share a perimeter 100% – Both meshes must be projected to this perimeter • Curves in tetra, curves plus edge associations in hexa • Can not simply merge the bottom of the cylinder with the top of the box. They do not share a perimeter. • Must first intersect them to subdivide the top of the box. Then mesh. Then merge the circle on the top of the box with the bottom of the cylinder • Check for green nodes/edges around perimeter

– Size diff between Hexa/tetras will reduce quality. • Try to stay within 3 to 1 either way; 0.7 (hexa) to 1 (tetra) is best. © 2010 ANSYS, Inc. All rights reserved

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1 to 1 or ANSYS, Inc. Proprietary

"lide abou% Merging

• More to come


45


e$a Tunnels • For External Aero, – tunnel is usually simple geometrically and doesn’t change from run to

run • Use hexa for max efficeincy

– Vehicle changes frequently and can be complex in shape • Use Tetra for max ease of mesh generation – Merge together

Tetra Vehicle Hexa Tunnel


46


e$a Tunnels

• For External Aero, – Merge together – Also hybridize the Tetra

portion with Tet to hex…

Merge Hexa and Tetra


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Flexible Hybrid Mesh Mul%i8one Meshing • Automatic hybrid blocking consisting of Unstructured, Swept and Structured Blocks to handle anisotropic meshing of wings etc. • Unstructured blocks can be filled with tetra, hexa core or hex dominant mesh • O-grid boundary layers • control mesh at the block level • Flexible blocking structure makes it the ultimate flexible hybrid mesh © 2010 ANSYS, Inc. All rights reserved

48


Flexible Hybrid Mesh Mul%i8one Meshing


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Mul%i8one &rocess Geometry Import & Repair

Mesh Params

Bottom Up (automatic patch based)

Top Down (Manual patch Independent)

Interactively modify surface blocking and adjust edge params

More interactive adjustment

2D to 3D Fill w Boundary layers

Generate Mesh

Output to Solver

Check Mesh © 2010 ANSYS, Inc. All rights reserved

50

Adjust Geometry ANSYS, Inc. Proprietary

• Sometimes a Simple shape is complicated by unnecessary patches. • Use Top Down blocking instead • It will save time in the long run Patchwork wing

Manual component approach may be much cleaner, then “sew” into rest of blocking

Automatic Bottom up approach may give a very complex blocking


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Mul%i8one "'eep De+o

• Sweep with multiple sources and targets…


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Mul%i8one De+o

• Tidal Turbine


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&ris+ &repara%ion – Start with good tetra or tri-surface mesh • It may be difficult to smooth prism after it is generated, it is relatively easy to improve the surface mesh before hand – Check aspect ratios / quality – Check and fix all diagnostics • Single/multiple edges, Non-manifold vertices, Duplicate elements

• Laplace smooth • often better without tetra, then fill w/ Delaunay after smooth • Delaunay mesh will have fewer nodes and smoother transitions; easier for prism – 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 – Extruding from a few isolated elements and none of their neighbors will likely crash – Modify part assignments of offending elements


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Mesh "+oo%hing • Use the Laplacian Smoother •Laplacian smoother

p

i

rp= Avg (ri) 2

1 • Moves the point towards an “ideal” location defined as the average of the surrounding nodes – Has the effect of averaging the surface mesh transitions and

triangles become more equilateral • Better for bottom up fill and prism © 2010 ANSYS, Inc. All rights reserved

55


&ris+ "e%%ings • 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.


56


&,ra+ids due %o in%ersec%ion • If the height is floating, you can 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 of clearance (no pyramids)

Auto reduction (discussed later) can also prevent collision without pyramids © 2010 ANSYS, Inc. All rights reserved

57


&ris+ "e%%ings

• 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 pyramids. • Solver preference

• If Min Prism Quality is too high, it may adjust Max Prism angle down to keep quality up, this will result in pyramids near sharp trailing edges. – Common mistake © 2010 ANSYS, Inc. All rights reserved

58


&ris+ "e%%ings • Max Prism Angle keeps the prism layers connected around tight bends. – Set this at 180 to prevent pyramids

Max Prism Angle = 140


59

Max Prism Angle = 180


&ris+ "e%%ings • 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 Pyramids

Redistributed Reduced (Squished) and prisms No Pyramids


60


*edis%ribu%e &ris+ • Redistribute prism improvements at R12 – Mesh more quickly with fewer layers, then split and redistribute prisms for a fixed initial height or ratio. – The new distribution is calculated column by column • Works on partial layers, etc.


61


"+oo%h &ris+ • 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 without the prisms (Penta_6). Set them to frozen. This will only improve the tetra mesh. Do this until you don’t see any further improvements. – Turning on the allow refinement option may help 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.

• In ICEM CFD prism quality is judged more harshly than TGrid and other meshers. – 0.01 or even 0.0001 is ok depending on your solver © 2010 ANSYS, Inc. All rights reserved

62


Infla%ion fro+ 9uads

• You can Inflate from quads. – Prism will not move pyramids (yet) so you

must delete the volume mesh first, or just start with a surface mesh. – Grow Prism without a volume mesh – Tetra mesh using

the Delaunay mesher • It will fill the void without damaging the inflation layers © 2010 ANSYS, Inc. All rights reserved

63


6se E$is%ing 9uad :a,ers

• You can grow Prism along quads. – Need to turn on Prism advanced option for “Use Existing Quad Layers” – Tricky to set up…


64


6se E$is%ing 9uad :a,ers • Generate Hexa portion and tetra portion, Merge – This is to connect the surface tris with the quads

• Delete all the hexas, keep only the envelope around the tetra region, also delete the tetras – It will not align if there are Hexas behind the quads – It will not inflate along Pyramids – Be careful not to smooth QUAD faces so we can merge with Hexas

again later – We hope to remove these tricky requirements at R13

• • • •

Setup prism params to ~match quad layers. Run Prism without volume mesh Fill remaining volume with Delaunay tetra Load previous Hexa portion and Merge nodes with a small tolerance.


65


"e%%ing &ris+ &ara+e%ers on Cur5es • Mesh > Curve Mesh Setup • You can get Prism to transition linearly across a surface by not setting a height on the surface, but instead set a different height on each curve on the opposite sides of the prism surface • Height ratio and Num. of layers have no affect on prism for curve settings

Height = 0.01

Height = 0 on surface


66

Height = 0.003


&ris+ Transi%ion

• You can get Prism to transition linearly by not setting a height on the wall, but instead set a different height on each curve on the opposite edges of the prism surface…

Height = 0.01 © 2010 ANSYS, Inc. All rights reserved

Height = 0 on surface

Height = 0.003 67


igh spec% *a%io +esh

• Several ways – Stretch and squish works if the high aspect ratio is in one direction only • Squish geometry, mesh with uniform mesh and then stretch (scale in one direction) the mesh to get high aspect ratio.

– Surface mesh with high aspect ratio quads, grow

prisms and bottom up tetra… Good for some external aero applications – MultiZone, use hexa edge params to adjust mesh aspect ratio. Works on certain cases if the geometry is good quality. © 2010 ANSYS, Inc. All rights reserved

68


";uish and "%re%ch – Stretch and squish

Octree generates Uniform tetras

works if the high aspect ratio is in one direction only • Squish geometry (in one or two directions) • mesh with uniform mesh • then stretch (scale back) the mesh to get high aspect ratio.


Which can then be stretched out

69


igh spec% ra%io Tri
On convex features, Aspect ratio is less severe as Prism radiates tris outward

Fill volume with bottom up tetra method


70


=olu+e Change • With bottom up tetra methods, high aspect ratio tri’s, quads or prisms result in adjacent high aspect ratio tetras or pyramids. • However the Elements between and adjacent to those high aspect ratio elements may have a lower aspect ratio and therefore much more volume. • TGrid Pyramids may be better… – (hope to improve for R12)


71


&ris+ • Prism can run from Quads or Tris – But not from pyramids (yet), so run prism before tetra if starting with a quad mesh – You can see that as the prism radiates outward, the element length stays the

same, but the width increases, which reduces the aspect ratio on convex surfaces


72


Mul%i8one

• Semi-automatic blocking (discussed later) – Easy control of distribution, shape, etc. of the zones – Zones can be structured, swept or unstructured


73


Mul%i8one

• Change the shape of mesh by simply contouring the “ZONE” edges.


74


"+oo%hing

• Smoothing can adjust the angle or volume ratio of elements


75


e$a /locking in si+ple s%eps… • Once you decide on the blocking topology you want (the hardest part), generating the Hexa blocking is just a series of the same steps – Split blocks (could be an Ogrid split) to get the topology you want. – Delete blocks that are not part of your topology – Associate the blocks with the geometry – Move the vertices into place so they fit to the geometry and blocks are good quality – Adjust edge params for quality and transition • There is a lot of iteration, but generally, it pays to go as far as you can with each split before things go to complicated… – Delete unneded blocks as soon as you can to reduce complexity – Associated the blocking as soon as an edge can be matched with a curve (or chain of curves) so you will have less work to do after you make further splits. – Move the verts into place while you only have a few splits so future splits will already be aligned. – Get as much done before you add a boundary OGrid so you won’t have to move OGrid verts later. © 2010 ANSYS, Inc. All rights reserved

76


Grids • 2 main uses for OGRID… – Capture Topology: • OGrid can be used to get the basic blocking shape laid out. Is your basic topology an OGrid or a Cgrid? Do you have features within your model (such as drilled holes) that would best be captured with an Ogrid? • Use these as early in the blocking process as possible

– Boundary layer: • OGrid used to aling mesh with the walls. Solves problem of Hexas mapped to a curve and helps align the besh for boundary layer analysis. • Best if used late in the blocking process after all other steps are complete.


77


>Grid Defini%ion • An O-grid is a series of blocks created in one step which arranges grid lines into an “O” shape or a wrapping nature •

3 basic types created through the same operation all referred to as “O-grids” • O-grid • C-grid (half O-grid)

O-grid

C-grid

L-grid

• L-grid (quarter O-grid) •

Reduce skew where a block corner must lie on a continuous curve/surface • Cylinders • Complex geometries

•

Improves efficiency of node clustering near walls for CFD applications

No O-grid (H-grid)


O-grid

78


Crea%ing an >Grid • Select blocks for O-grid – Can select by visible, part, around face, around edge, around vertex, 2 corner method, or individual selection

5 blocks in 2D

7 blocks in 3D

Select specific blocks or around face, edge, or vertex © 2010 ANSYS, Inc. All rights reserved.

Note: Internal block has all internal (blue) edges and vertices 79


>Grid>dding Faces • Adding faces during O-grid creation – O-grid “passes through” the selected block faces – In general, add faces on the “flat parts” – Adding a face actually adds blocks on both sides of the face O-grid passes through this face Half O-grid (C-grid)

• Examples of uses – Pipe ends – Symmetry planes – Complex geometries © 2010 ANSYS, Inc. All rights reserved.

O-grid passes through this face 80


dding Mul%iple Faces • Any number of faces can be added around a selected block – If all the faces are added around a block, the result is no change since the O-grid passes through all the faces

Quarter O-grid (L-grid) Quarter O-grids can be used to block triangular shapes Seen as a Cgrid in one direction and an L-grid in another direction


81


>Grid>round /locks • Select Around block(s) to create the O-grid around the selected blocks – Useful for creating wrap-around grid around a solid object – Examples • Opposite order of creation from block around/O-grid inside • Flow over a cylinder • Boundary layer resolution around an airplane or car body


82


"caling an >Grid • O-grids can be re-sized after or during creation – By default the O-grid size is set to minimize block distortion – You are actually scaling all parallel O-grid (radial) edges to the selected edge – The selected edge is given a factor of 1 – Numbers < 1 will shrink the edge and thus create a larger inner block

Selected edge factor = 1


83

Factor = 0.3


Wh, >Grid?

Before O-grid

This mesh can be improved by using an O-grid – An example of bad mesh in the block corners


84

Right mouse click on the histogram to access options like show show,, replot replot,, or done


Mul%iple Wa,s of /locking %he "a+e Geo+e%r, • Creati Creating ng a fork fork by Merg Merge e verti vertices ces Splits

#1

#2

Delete block

#3


#4

85

Merge vertices #5 Or Collapse Block


Mul%iple Wa,s of /locking %he "a+e Geo+e%r, • Creati Creating ng a fork fork by Extr Extrude ude face faces s

#3

#2

#1

Extrude 1

Extrude 2

#5

#4


86

Associate


Mul%iple Wa,s of /locking %he "a+e Geo+e%r, • Creating a fork with Top-down methods

split

Two quarter O-grids

Delete blocks

Move vertices

One quarter O-grid

One quarter O-grid © 2010 ANSYS, Inc. All rights reserved

87


Topolog,

• What is common in these? – Their block topology


88


Topolog, • All these parts have the same basic topology

This helical blocking is quickly created with extrude along curve

– Blocking strategy for all pipes are

similar. – Single block with o-grid – The only difference is the number of splits added to help control the blocking – Create the one block, then split, and add O-grid last

splits Single block with o-grid © 2010 ANSYS, Inc. All rights reserved.

Single block with 5 splits and o-grid 89

Single block with multiple splits and ogrid ANSYS, Inc. Proprietary

/asic /lock "%ruc%ures

Bad quality Bad quality

Bad quality

Mesh inside fillet


O-grid necessary O-grid necessary

O-grid necessary

O-grid is required unless approximating a corner using large mesh size

Acceptable quality

90

O-grid


/asic /lock "%ruc%ures However, an O-grid makes normal grid lines for boundary layer resolution

Quality is good without an O-grid

Mesh outside fillet

Mesh inside and outside fillet. Combine previous examples This method needs large mesh size

This is not preferable because quality is lower

Vertex at end of fillet

Delete center block


Mesh only in ring

There should be one vertex at the center of every approximate 90 degree bend

Mesh in both

Split any radial edge 91



Split

Split

Mesh inside tube

Tubes are one-block topologies

Splits can be added for more control

O-grids can make this work although it is not the preferred topology

Mesh inside and outside tube

Many students mistakenly just split at geometry features


Delete blocks

Bad quality

Bad angle

92


/asic /lock "%ruc%ures We must use the straight-through split strategy as in the previous example for grid lines to flow with the inner tube

Bad angle

Add O-grid for meshing inside curvature

Delete block

Blocks selected for creating O-grid

2 splits

Method 1 Quarter O-grid

Split for better blocking control

These may also be called wedge blocks. They can be used to effectively terminate mesh from continuing through the geometry Move vertices

There are multiple methods of producing the Y-block structure


93


/asic /lock "%ruc%ures Move vertices

split Method 2

Method 3 Inside H-grid ( bottomup)

Collapse block

Method 5 Create block function (quarter Ogrid)


2 splits

Move vertices

Delete blocks “permanently”

Method 4 Inside H-grid (topdown)

Create block

Create block

1st quarter O-grid

2nd quarter O-grid

Move vertices

5 2

4 6

1

3

94


/asic /lock "%ruc%ures Add quarter O-grid

Move vertices

Add O-grid for round

Since the round is closer to 180 degrees than 90 degrees, having two block corners may be better

Add O-grid for round

Add O-grid A Y-block structure can also be used for a circle


Move vertices

Useful for filling the center of geometry that is periodic in multiples of 3

95


/asic /lock "%ruc%ures split

Delete blocks “permanently”

Merge vertices

Move vertices

Create blocks

Add more splits for control

Method 1

Method 2


Splits for more control

Move vertices

O-grid

Delete block

96


/asic /lock "%ruc%ures First block the outside

Thin thicknesses are easily done with an O-grid

Then put an O-grid inside all blocks (add appropriate faces)

Delete the inside blocks

Resulting blocks

Scan plane

Vertex moved slightly off intersection for better angles

Method 1

Method 2


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/asic /lock "%ruc%ures Method 3 Using Yblocks

Half O-grid

O-grid

Delete blocks

In 3D model, interpolate face association Method 4 Leaving small ledge Delete edge association Merge vertices Method 5 Degenerate blocks

Method 6 Convert block


Start from method 4 above

Face interpolation not necessary when merging vertices

Convert block to free for 2D or swept for 3D

Start from method 5 above

98

Quad dominant

All tri


/asic /lock "%ruc%ures O-grid

Filleted wings are easily done with a simple block. The block can be made larger than the wing and an O-grid made inside, or the block can be on the wing and the O-grid made around.

Edge splits to shape edges Filleted wings are easily done with a block on the wing then add an O-grid around

Mesh region beyond the blade-tip is bad quality

Knife-edge wings pose a problem if you collapse. The collapse behind the wing must propagate through the blocking, or invalid elements result in a corner. This means mesh beyond the blade tip will have the same mesh pattern as on the blade tip, as if the blade were meshed inside. © 2010 ANSYS, Inc. All rights reserved

99

Very low angles and unnecessary mesh density result from a simple collapse ANSYS, Inc. Proprietary


O-grid with split

Collapse block Collapse blocks

3 splits

Delete blocks permanently

Move vertices

Create 4 blocks


100


/asic /lock "%ruc%ures A knife-edge blade can also be handled by collapsing all around the blade if the complicated strategy of the previous example is not wanted.


101


o' To s l i High or low curvature, etc. o ►May require different blocking strategy (Topology) f r i A ►Requires some robust scripting f o y t e i r a V


102


o' To s l i Trailing edge? o ►Blunt f r i ►Deal with size transitions A f ►Sharp o ►Collapse trailing edge block y t e i r a V


103


o' To sFar Fields d l e ►Square, round, half circle, Wedge etc. i F ►HGrid, OGrid, CGrid, YGrid, etc. r a F ►Periodic? f o y t e i r a V

Note: the FF scale has be dramatically reduced for illustration © 2010 ANSYS, Inc. All rights reserved

104


o' To sPeriodic? d l e ►Setup in Global Params i F ►apply under Edit Block => r a Make Verts Periodic F f o y t e i r a V


105


o' To sPeriodic? d l e ►Around Blades or Between Blades i F ►Between Blades if NO Tip Gap r a F f o y t e i r a V

Channel between blades? Multi-Block with OGrid ►HGrid converted to a single output block ►


106


"hif%ed &eriodic… V a Periodic r i ►High curvature or blade angle? e t y ►Try Shifted periodic o f F a r F i e l d s

No Tip Gap makes it much easier Edges don’t need to cross the airfoil ►More careful with a tip gap ►


107


r%hogonali%, "+oo%her • Improves volume transition • Equalizes angles between elements • Can affect just the mesh or adjust blocking

• Saves time otherwise spent on adjusting edge parameters • Can be used to set orthogonal initial height • Works on many mesh types – – – – –

Structured Multi-block Hexa Unstructured Hexa hexa dominant BF-Cart Hybrid mesh

Equalizes angles, smoothes volume transitions, makes the mesh orthogonal © 2010 ANSYS, Inc. All rights reserved

108


r%hogonali%, "+oo%her

• Users run into trouble because not all the mesh is supposed to be orthogonal – Release orthogonality • Also can use this to set initial height off the wall • Note: this may reduce min quality… Its goal is to improve transitions and the overall quality of the mesh


109


E$%ended Meshing( ICEM CFD In%erac%i5e

Using ICEM CFD Interactive w/replay control, user can block out a mesh and commit it back to Meshing

• Options under MultiZone Method Change model parameters, and run ICEM CFD in batch to update parameter change

Yes: Writes ICEM CFD Files for use in ICEM CFD Interactive: Runs interactive ICEM CFD Batch: Runs ICEM CFD in Batch © 2010 ANSYS, Inc. All rights reserved.

110


/F>Car%

• Works well for models without sharp edges


111


/F>Car% I+pro5e+en%s

• Stretched Cartesian – Aspect ratio or Hexa Blocking file based – Biasing

• Automatic Material Flood-fill

Focus the mesh refinement and smoothly transition


Mesh multiple regions at once with connected mesh

Material Flood fill

112


Car%esian Mesh

• Do you have complex aerospace geometry, early design phase • Consider Cart3D – Inviscid flow Analysis package – Cartesian Mesh based – Developed at NASA Ames

Research center – Integrated into ANSYS ICEM CFD (6dof) – Extensively validated in subsonic and low supersonic regimes After you narrow down your design, finalize with more labor intensive /accurate body fitting mesh and solve in ANSYS CFD. © 2010 ANSYS, Inc. All rights reserved.

113


"elf Training *esources


114 114


"elf Training *esources

•

•

•

ICEM CFD has many options. – Flexibility, Power – Harder to learn on your own Ideally, Ideally, you can get training from an instructor, but not everyone has this option. Self training resources include – Built in tutorials and Help – Programmers Guide


115


"elf Training *esources Web resources include – The Demo Room www.ansys.com/demoroom – The Customer Portal – Tech Support 1 800 937 3321 – Website – www.youtube/ansysinc/ – www.cfd-online.com


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Icemcfd Tips&Tricks2010

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