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March 31, 2006 The Focus Issue 46

By Doug Oatis

One of the betterfeatures ofWorkbench’s De-sign Modeler(besides face de-lete, face imprint-ing, slice, etc.) is

the Midsurface Extraction tool. This tool isfairly simple to use, and after some practice,you will be creating shell models in no time.

The Midsurface Extraction utility is locatedunderneath the ‘Tools’ Menu (or you canput it on your toolbar using the optionsmenu). The easiest way to create midsur-faces is to manually go through and selectthe face pairs. You will notice that afterselecting the “top” and “bottom” face, theywill appear as two different colors. It isimportant to note that the order in whichyou click the two faces determines the ‘top’and ‘bottom’ of the shell element(important for applying pressures/definingcontact). The shell normal is positive goingfrom the pink (2nd pick) to the purple (1stpick) surface.

You can select multiple face pairs, then goback and hit the apply button for “FacePairs” in the detail window. If there areseveral face pairs that have the same thick-ness, you can select one pair, then go backand change the selection type to be automat-ic. This will fill in some blanks in thedetails window (i.e. maximum and mini-mum threshold thicknesses). Finally,change “Find Face Pairs Now” to be “Yes”and DM will go out and find all face pairsthat meet this criteria. Hit generate and allof your midplanes will be generated, and allthicknesses will be stored for later use inSimulation.

One thing you need to keep in mind whilecreating midplanes is whether the selectedfaces belong to the same body or are sepa-rate bodies. One example is the creation ofa midsurface representation of an L-bracketwith ribs. If the rib is included in the L-bracket (unfreeze parts together), then therib geometry will be automatically extended

to the midsurface of the bracket. If the ribis a separate body, then there will be a gapbetween the rib and bracket midsurface(bracket thickness/2).

The surface extension tool is an extremelyrobust way to join midsurfaces together.Simply select the edge of the surface youwant to extend and select the method(specific offset, to plane, to face, etc), andthe surface will be extended. I’ve foundmyself using ‘To Surface’ most often, sinceit extends the edge to the projected surface,allowing you to join two midplanes usingtwo surface extensions. Another nice fea-ture of the surface extension tool is that itfollows the profile of the existing face.What this means is that if you have a ribwith a rounded edge, if you extend the edge,the surface will be extended and follow thepre-existing curvature.

After you’ve gone through and extracted allyour midplanes and done all your surfaceextensions/modifications, you probablywant to join all of them together (i.e. areashave continuous mesh). The first step indoing this is to form a new part of all theareas. Make sure you have your selectionfilter set to bodies, not faces, and select allthe areas you want to join. Right click in thegraphics window and select ‘Form NewPart’. Next, go Tools > Joint, which willinsert a joint object in the model tree. All ofyour areas should be selected from the newpart formation, but if not, select the multi-body part, and hit apply in the joint detailswindow. Hit generate and Workbench willhighlight all the continuous edges. Makesure you don’t create a multi-body part thatconsists of solids and shells, since this is notallowed in Simulation (you can have anassembly of shells/solids, just not a singlepart consisting of both).

When you bring this geometry into Simula-tion and mesh it, all shell/beam elementswill be shown with their real constant setturned on (/eshape,1). There is no togglefor this behavior, so you must manually gointo the geometry branch of Simulation and

set the thicknesses to be 0 to display theplanar shell elements.

If you have an assembly of shells and solids,Simulation does not automatically detect

March 31, 2006 A Publication for ANSYS Users Issue 46

ContentsMidsurface Extraction......................1Sub Modeling..................................2MOR for ANSYS..............................4ANSYS Conference Travel.............5

Midsurface Extraction with DesignModeler

(Continued on Page 2)

Figure 1: Manual Midplane Details View

Figure 2: Automatic Midplane Details View

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March 31, 2006 The Focus Issue 46

By Ted Harris

A submodel is a refined, separate finiteelement model that is typically used tomodel a portion of a coarser finite elementmodel with more detail and accuracy. Inthe old days when wavefront limits werefairly small, the frontal solver was the onlysolver, and computer resources were mea-ger by today’s standards submodeling wasroutinely used to get accurate results incomplex fillet areas or in other features thatjust couldn’t be meshed with sufficient re-finement to accurately predict stresseswithout ending up with a model that wastoo big to run.

That scenario doesn’t happen as much thesedays. However, for large models solutiontime, memory or disk space limitations canstill become the limiting factor in getting

accurate results. Submodeling becomesuseful especially for simulations that don’tlend themselves to substructuring but havevery tiny yet important features containedwithin a much larger geometric envelope.In other words, the overall model cannot besimplified due to boundary condition re-quirements, but the small features of inter-est cannot be meshed with sufficientrefinement for accurate results without theoverall model becoming to big to solve.

In such cases a relatively coarse overallmodel can be constructed which accuratelycaptures the stiffness and deflections of thestructure. A submodel of a small portion ofthe overall geometry that contains the de-tailed region of interest is then created.This submodel might have more geometricdetail, such as fillets, than in the coarsemodel as long as the overall stiffness of thestructure is not significantly affected. Thesubmodel can have a much finer mesh thanthe coarse model to help minimize meshdiscretization error.

Submodeling takes advantage of St.Venant’s principle. Recall that this princi-ple states that once you get away from theimmediate vicinity of load application, thestress distribution is the same regardless of

Definitionsub \s under or below [1]

model \m Aschematic description of a system, theo-ry, or phenomenon that accounts for itsknown or inferred properties and may beused for further study of its characteris-tics [2]

submodeling \s m A finite ele-ment technique that you can use to ob-tain more accurate results in a particularregion of a model [3]. It involves buildinga more detailed local model with bound-ary conditions mapped from a larger,typically coarser model. No, it has noth-ing to do with submarines or The Hunt forRed October,en.wikipedia.org/wiki/The_Hunt_For_Red_October although I must say I enjoyedthe book far more than the movie.

substructuring \sAn essentially unrelated but similar-sounding finite element concept involv-ing breaking up an FE model into smallerlinear ‘chunks’, which are generated in-dependently but linked through masterdegrees of freedom. Please see RodScholl’s Focus articlewww.padtinc.com/epubs/focus/2006/0045_0228/TheFocus_45.pdf#Page=2 for more information on the applica-bility and implementation of substructur-ing.

Sub Modeling in ANSYS

(“Midplane” cont.)

(Continued on Page 3)

edge/face contact (unless you’ve changedthis setting in ‘Options’ beforehand). Youcan change this option in the contact branchdetails window, then regenerate all contactpairs. If you want to define edge/face con-tact, the edge must be the contact and theface must be the target. If you have shellface/edge or shell face/solid contact, payattention to the contact normal directions(displayed red and blue). The contact nor-mals are going to be aligned with the shell

normals by default(esurf,top), which aredetermined by the faceselection order when themidplane was created. Ifyou need to reverse thenormals, you can insert acommand snippet underthe offending contactpair to reverse the con-tact and/or target nor-mals.

Here are a few of the tips/tricks I’ve pickedup:

You can easily fill in “voids” by creatingsketches and then forming surfaces fromsketches (Concept > Sketches from sur-faces).

Don’t be afraid to use the slice tool to cutup surfaces, then extend individual sur-face edges, and the unfreeze the surfacesback into one

Don’t be afraid to create excess areas. I willsometimes overextend an area, slice it insome way, then suppress the area I no longerneed. The joint tool created edges and pointsthat can be used for future CAD operations

After enough practice, you can impress yourfriends and make fairly complex models likea tractor bucket. This mixed shell/solidmodel was created from a solid CAD model.In case you’re wondering, this picture looksgreat hanging on my fridge…

Workbench (and Design Modeler) ofcourse comes with your ANSYS instal-lation CD’s. However there is a license(and associated fee) to use DesignModeler. Talk to your ASD about get-ting a demo key. With the cost ofMid-Plane extraction (and waiting onthe designer to get around to doing it)this feature alone can justify the rela-tively low cost of Design Modeler.

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March 31, 2006 The Focus Issue 46

the manner in which the load is applied.This holds as long as the loading in thesubmodel is statically equivalent to theloading in the coarse model. The boundar-ies of the submodel should be sufficientlyremoved from local stress gradients.

Submodeling in ANSYS is implemented bymapping displacement boundary conditionsfrom the coarse model onto the exterior‘cut’ faces of the submodel. This is accom-plished by selecting the cut face nodes ofthe submodel and writing them to a nodefile with the NWRITE command in/PREP7. Submodeling in ANSYS is ac-complished in the General Postprocessor.The displacement boundary condition map-ping is accomplished with the CBDSP com-mand in /POST1.

If the coarse model has an applied tempera-ture distribution this can be mapped to thesubmodel as well. The process here is towrite all the submodel nodes to a node filewith NWRITE. The temperature mappingfrom the coarse model is then accomplishedwith the BFINT command in /POST1. Notethat any other loads that were applied in thecoarse model should be applied to the sub-model as well, such as pressure loading orrotation velocity.

An assumption of the submodeling methodis that the analysis is linear. However, if thenonlinear effects are contained within thesubmodel and do not affect the stiffness ofthe structure, it is possible to consider themin the submodel. Please see SheldonImaoka’s excellent article on submodelingwith nonlinearities:

ansys.net/ansys/tips_sheldon/STI02_Nonlinear_Submodeling.zip for more informa-tion.Once CBDSP or BFINT are complete, theresulting input files can be read into thesubmodel database in ANSYS using the/INPUT command in /PREP7. Once anyother required loads are applied the solutioncan be solved and postprocessed in normalfashion.

Let’s say our task is to perform a stressanalysis on a submarine, with particularinterest in the axial stresses where the for-ward part of the conning tower intersectsthe hull. Even though submodeling is cur-rently supported only in the ANSYS envi-ronment and not in Workbench, we canleverage Workbench to make the coarsemodel and submodel setup easier. Ourinitial geometry was constructed in Design-Modeler and an enclosure was used tomodel the water surrounding our subma-rine, as shown in Figure 1. CFX Mesh andCFX were utilized within the WorkbenchEnvironment to perform a CFD analysis onthe submarine, results of which can be seenin Figure 2.

Workbench Simulation was then used forthe initial stress analysis of the sub, withpressures from the CFX simulation easilytransferred to the structural model to pro-vide the structural loading environment.Results of the coarse model stress solutionare shown in Figures 3 and 4.

DesignModeler was used again to create thesubmodel geometry from the full model, asshown in Figure 5.

The submodel was meshed in WorkbenchSimulation (see Figure 6) and the pressureload was once again easily applied from theCFX Simulation. A Named Selection of thecut boundary faces was created to facilitateselection of the appropriate cut face nodesin ANSYS. The FE model was then ex-ported to ANSYS by highlighting the Solu-tion branch and clicking on Tools > WriteANSYS Input File. This file was then readinto ANSYS and the boundary conditionmapping was accomplished using thecoarse model ANSYS results file. That fileis automatically saved during the Work-bench Simulation solution as long as theSimulation Solution option to save ANSYSfiles is checked.

The mapped displacements were then readinto the ANSYS submodel database and thesubmodel was solved and postprocessed inANSYS, as shown in Figure 7. In all sevensubmodels with increasing mesh density

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Figure 1: DM Geometry - Submarine and Enclosure

(“Sub Modeling,” Cont.)

(Continued on Page 4)

Figure 3: Coarse Model Axial Stress Results

Figure 4: Coarse Model, SZ in Region of Interest (ANSYS)

Figure 5: Submodel Geometry for Region of Interest

Figure 2: CFX Simulation Pressure Calculations. RunningSilent, Running Deep.

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March 31, 2006 The Focus Issue 46

By: Evgenii B. Rudnyi, Jan G. Korvink

Lab of Simulation, Department of Micro-system Engineering, University of Freiburg- IMTEK,

Model reduction is not something new forengineers. The mode superposition methodand Guyan reduction methods have beenimplemented in ANSYS almost since thebeginning. Yet, a topic of model reductionhas recently received much attention frommathematicians and nowadays this makes alarge difference. We suggest you a test:compare two recent books on model reduc-tion, one written by an engineer [1] and oneby a mathematician [2]. You will see thatnow mathematicians have much deeper un-derstanding on why model reduction is pos-sible and how to perform it in the mostefficient way.

MOR for ANSYS is open-source softwaredeveloped at IMTEK to research the possi-bilities of modern model reduction for engi-neering design and system level simulation.It reads binary ANSYS FULL and EMATfiles in order to extract the system matricesand then runs the block Arnoldi algorithmto build a reduced model.

We have started the research on modernmodel reduction in 2001 and in our subse-quent experience modern model reductionhas proven to hold great potential for engi-neering. The key observations are listed be-low:

a) The Arnoldi algorithm is the most effi-cient computational method to performmodel reduction. Time to perform modelreduction is comparable with that for astatic solution provided there is a plentyof memory (with 4 Gb of RAM thisobservation is valid for many finite ele-ment models up to 500 000 degrees offreedom).

b) Because of a), MOR for ANSYS allowsus to obtain harmonic or transient simu-lation for a linear model for the computa-tional cost comparable with a staticsolution!

c) MOR for ANSYS can be used for anylinear ANSYS model: thermal, mechan-ical, piezoelectric, acoustic includingstructure-fluid interaction, electromag-netic, even thermomechanical with not-uniform temperature.

Research papers describing results obtainedwith MOR for ANSYS are available fromthe MOR for ANSYS site.

Finally, we would like to direct your atten-tion to new results on parametric modelreduction. It happens that one can formallypreserve several parameters within systemmatrices in the symbolic form during amodified model reduction procedure. Thisfeature has not been implemented in MORfor ANSYS yet but the research phase hasalmost been completed and an update willfollow.

were completed to demonstrate the meshconvergence behavior seen in Figure 8.

So, was a submodel required for our subma-rine? The answer is probably not since wecould have created an accurate enoughmesh using local mesh controls in our filletof interest.

Note that it’s also possible to nest submod-els to capture features that are extremelysmall and detailed relative to the requiredcoarse model. An example implementationof this is shown in Figure 9. You canexperiment with submodeling and nestedsubmodeling yourself by downloading the

Figure 7: Submodel SZ

MOR for ANSYS: Model OrderReduction for ANSYS

More information onMOR for ANSYS

MOR for ANSYS site:www.imtek.uni-freiburg.de/simulation/mor4ansys/

MOR for ANSYS tutorial:www.imtek.uni-freiburg.de/simulation/mor4ansys/lab/thruster

Introduction to modern model reductionat engineering level:

E. B. Rudnyi, J. G. Korvink, Review:Automatic Model Reduction for Tran-sient Simulation of MEMS-based De-vices. Sensors Update v. 11, p.3-33, 2002.dx.doi.org/10.1002/seup.200211105or preprint atwww.imtek.uni-freiburg.de/simulation/mor4ansys/pdf/rudnyi02SU.pdf

Recent results on parametric model re-duction:

www.imtek.uni-freiburg.de/simulation/mor4ansys/publications.html#parametric

(“Sub Modeling,” Cont.)

Figure 2: Submodel Mesh Created in WB Simulation

Figure 8: SZ vs. Mesh Density for Coarse and Submodel

Figure 9: Nested Submodel Example

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March 31, 2006 The Focus Issue 46

The Focus is a periodic publication of Phoenix Analysis & Design Technologies (PADT).Its goal is to educate and entertain the worldwide ANSYS user community. More informa-tion on this publication can be found at: http://www.padtinc.com/epubs/focus/about

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Computer Benchmark Data for ANSYS?www.padtinc.com/support/benchmark

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Upcoming Training ClassesMonth Start End # Title Location

Apr '06 6-Apr 7-Apr 107 ANSYS WB DsgnMdlr Tempe, AZ10-Apr 12-Apr 401 Introduction to EMAG Tempe, AZ13-Apr 14-Apr 604 Introduction to CFX Tempe, AZ18-Apr 19-Apr 201 Basic Structural Nonlinearities Tempe, AZ20-Apr 21-Apr 204 Adv. Cont. & Bolt Pretension Tempe, AZ26-Apr 28-Apr 152 AI*Environment Tempe, AZ

May ‘06 1-May 3-May 104 ANSYS WB Sim Intro Tempe, AZ4-May 4-May 105 ANSYS WB Sim Struct Nonlin Tempe, AZ8-May 9-May 100 Engineering with FE Analysis Tempe, AZ

11-May 12-May 803 Tcl/Tk for ANSYS Tempe, AZ15-May 16-May 604 Introduction to CFX Irvine, CA18-may 19-May 203 Dynamics Irvine, CA25-May 25-May 106 ANSYS WB DesignXplorer Tempe, AZ31-May 31-May 108 Paramesh Tempe, AZ

- ANSYS Stock Price Breaks $50/Share

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News

An Engineer’s Travelogue:

Interacting with the Natives andUnderstanding What they Worship

“Sub Structuring” References

[1] My high school Latin classes.

[2] The American Heritage® Dictionaryof the English Language, Fourth Edi-tion copyright ©2000 by Houghton Mif-flin Company.

[3] The ANSYS 10.0 Help System,Chapter 9 of the Advanced AnalysisTechniques Guide

“MOR for ANSYS” References:

[1] Zu-Qing Qu, Model Order Reduc-tion Techniques : with Applications inFinite Element Analysis. Springer,2004, ISBN: 1852338075.

[2] Athanasios C. Antoulas, Approxi-mation of Large-Scale Dynamical Sys-tems.Society for Industrial and AppliedMathematic, 2005, ISBN:0898715296.

By: Egbertius Fog,MBE, PE

I have just re-turned from myoutfitter and all

is prepared for my pending journey to Pitts-burgh for my favorite biennial event: the2006 International ANSYS Conference. Inmy last dispatch to you, my faithful readers,I shared the anticipation of the sojourn tothis congress of calculators. And, as prom-ised, I would now like to prepare you fur-ther by sharing some insight into the nativesand their tribes.

The first thing to be prepared for is howthey worship. They will begin their meet-ing by taking part in ritual training sessions.Gathering in large rooms, the wisest andmost experienced amongst them share theirknowledge with others. In many ways thisis the most powerful event at these gather-ings. With the lights dimmed the attendeesingest large quantities of caffeineted bever-ages and stare at images projected on a largewhite rectangle for three to four hours. Thewords and paintings are indecipherable tomost, but to the initiate they are not justinteresting, but so fascinating that they of-ten illicit cheers and clapping. Interviewsof those who participate often result in com-ments similar to “that was the most usefulclass I have ever taken.”

After the first two day of ritual training, theentire congress moves towards the mainconvention. Gathering in a single roomthey listen to stories from people both insideand outside the tribe about how the toolsthey all worship can make life better. Somespeakers are leaders within the group, someare leaders from other groups. But all sharea passion for engineering and creating prod-ucts and the stories relate to how the worldof engineering is changing and how mem-bers can be better at it and can contributemore to the business tribes for whom theywork.

The other ritual that you should be aware ofis a special village in the midst of the facil-ity for another tribe referred to as vendors.As with many societies, trade between dif-ferent groups is key to the success of all.This agora is a place for the users to meetvendors who have products that they canuse to improve the quality, speed and effi-ciency of ANSYS usage. This year isespecially exciting because of the strongfocus on the growing need for high perfor-mance computing.

There is much more for you to know, but Imust be off to my doctor for my vaccina-tions. I hope that this little taste is enoughto convince you to join me at the confer-ence.

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March 31, 2006 The Focus Issue 46

Be Cool andKnow whatDay it Is!

Download the 2006 PADTANSYS Calendar