A Product Model based Approach to Interactive CAE Design Optimization

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1 A Product Model based Approach to Interactve CAE Desgn Optmzaton T. Fahrg, B. Nachtwey, S. Geller, J. Tölke, M. Krafczyk Insttut für Computeranwendungen m Baungeneurwesen (CAB) Technsche Unverstät Braunschweg, Pockelsstraße 3, Braunschweg, Germany {fahrg/nachtwey/geller/toelke/kraft}@cab.bau.tu-bs.de Summary We present a software prototype for flud flow problems n cvl engneerng, whch combnes essental features of Computatonal Steerng approaches wth effcent methods for model transfer and hgh performance computng. The man components of the system are descrbed: - The modeler wth a focus on the data management of the product model - The pre-processng and the post-processng toolkt - The smulaton kernel based on the Lattce Boltzmann method - The requred hardware for real-tme computng 1 Introducton Functonal Desgn of complex CAE objects requres a close co-operaton of specalsts n varous dscplnes. Especally n the early desgn phase, the effort to correct errors or nadequaces s relatvely small. As the desgn goals of the dfferent engneerng dscplnes often tend to be mutually exclusve, effcent communcaton between the engneerng specalsts s essental for an effectve desgn process and mandatory for an economc optmzaton durng the desgn and the economc lfetme of the constructon. An automated computer-asssted system servng as a framework for ths process would presuppose a consstent theoretcal descrpton of all partal models as part of an object and process orented generalzed theory about the nteractons of all buldng components and processes. Such a theory s not yet avalable. 2 Applcaton The presented prototype descrbes a heurstc strategy: A group of engneers from dfferent dscplnes come together n an nteractve and vrtual desgn space representng an open-plan offce and evaluate n close cooperaton the mutual mplcatons of ther specfc desgn varants onlne. Dependng on the postonng of the offce furnture and an HVAC (heat-condtonng, ventlaton, ar-condtonng) nstallaton a human wellness ndex s nteractvely computed. The wellness factor based on the PMV- Index (Predcted Mean Vote; Rtschel, 1994), dependng on the temperature, the densty and the velocty of the arflow nsde the offce, as well as addtonal values defned by the user. Page 1 of 9

The engneers are provded the opportunty to locally or globally optmze the PMV-Index by movng objects and modfyng parameters of the HVAC nstallaton wthn the modeler.

2 The engneers are provded the opportunty to locally or globally optmze the PMV-Index by movng objects and modfyng parameters of the HVAC nstallaton wthn the modeler. If the geometry or the boundary condtons change, the ndex wll be recomputed automatcally and the results are vsualsed n real tme. The optmzaton process s controlled by dedcated functons mplemented n the modelng tool. If a modfcaton exceeds a defned crtcal value, the system wll nform the user or, n smpler cases, correct the modfcaton. Fg. 1: Ar-flow n an open-plant offce Page 2 of 9

3 System layout Fg. 2: System layout 3.1 Hardware For modelng and vsualsaton we use a 64-Bt dual-cpu PC and a professonal hgh performance graphcs card.

3 3 System layout Fg. 2: System layout 3.1 Hardware For modelng and vsualsaton we use a 64-Bt dual-cpu PC and a professonal hgh performance graphcs card. Ths PC s connected drectly to a vrtual realty envronment where the numercal results are dsplayed. The vrtual realty envronment s based on a stereoscopc vsualsaton system. Two beamers equpped wth drected polarzaton flters project two mages (one separate mage from each of the two outputs of the graphcs card) on a transparent screen. Polarzaton flters allow to separate the two mages for the left and the rght eye. For operatng the Vrtual Realty envronment, a 3D moton tracker based on a magnetc system s used. To obtan an acceptable response tme between the modfcaton of the geometry and the vsualsaton of the results (.e. wthn a mnute), hgh performance computng s requred. Therefore, the smulaton kernel runs on a PC Cluster equpped wth Myrnet2000 network wth a peak performance of 250 GFLOPS delvered by ts 96 CPU s and approxmately 150 GB RAM. An addtonal hgh-speed optcal Ggabt network drectly and exclusvely connects the cluster and the Front-End PC. The vrtual desgn space and the vsualsaton can be modfed by addtonal engneers usng PCs connected to the Front-End-PC over the network or nternet. Ths s possble by utlzng a mult-user- envronment based on the freeware VNC. Addtonal software and hardware s used for audo- and vdeo-communcaton. 3.2 Software Components The prototype s based on the modules as llustrated n Fgure 2. The vrtual desgn space s generated n the modeler for whch we use the Autodesk Archtectural Desktop. The smulaton kernel s a research CFD prototype mplementaton based on the Lattce-Boltzmann method and uses smoothed unstructured Cartesan octree meshes. For vsualsaton of the flow and other computed quanttes the AVS/Express toolkt s used. All the modules are descrbed n more detal n the followng text. Page 3 of 9

4 As shown n Fgure 2 addtonal communcaton modules and nterfaces are requred to allow drect network based data exchange between the modules. Ther mplementaton s based on the TCP/IP socket communcaton protocol. 3.3 Modeler The bass for the modeler s the CAD-system Autodesk Archtectural Desktop (ADT), an extenson of the AutoCAD software whch can be regarded as the de-facto standard n cvl engneerng. Ths choce s motvated by the systems powerful modeler whch comes wth a powerful object-orented programmng nterface allowng an effcent data exchange wth the controllng process, the communcaton modules and allows drect access to a facet model of the object geometry. In addton, ADT can be utlzed for the whole data management ncludng mport, export and converson between the product model and other model representatons. Fg. 3: Snapshot of a model developed usng Autodesk Archtectural Desktop (ADT) Fgure 3 shows the user nterface of the vrtual desgn space of the ADT. The optons of modfyng the geometry and the manpulaton of the vew are reduced to the essental functons for optmal usablty. The user has the possblty to move, delete or add objects or change attrbutes for the smulaton of specal objects or object-types. For a fast workflow the user can drag a prepared object from an object lbrary nto the desgn space. New objects can be created externally and mported nto the lbrary durng the work process. Addtonal attrbutes are requred for each object whch provde nformaton for the dedcated controllng functons. For example, a power socket may only be placed on a wall n a dstance of 30 cm above the floor. Such bult-n rules for the behavour of objects are stored as addtonal object propertes. Page 4 of 9

5 3.4 Data management The challenge of the data management concept of our prototype s the keepng of all requred data durng the hole lfe cycle of the product model. Thus smulatons can be run on every status of development or also for faclty management purposes durng the lfe tme of the offce buldng. Generally, the followng addtonal object data are requred for the prototype: 1. Attrbutes for the flud smulaton, e.g. boundary condton attrbutes such as temperature, velocty, pressure or correspondng materal propertes such as thermal conductvty 2. Attrbutes for vsualsaton purposes, e.g. colour or transparency 3. Attrbutes for the control functons,.e. the objects behavour wth respect to ts poston n the system. 4. The geometry of each object defned as a consstent volume model n order to allow an automatc mappng of a surface representaton to an octree mesh for the numercal smulaton kernel. For an effcent data management the modeler has to be fully compatble wth the product model standard ncludng the mport and export of data wth addtonal object attrbutes requred for the smulaton process. The latest verson of ADT does not support product models out-of-the-box. Wth a specal plugn (IFC-Utlty 2x for ADT 2004) the ADT s able to work wth product model complant data based on the Industry Foundaton Classes (IFC) of the IAI. The IFC are the most open product modelng technology for the buldng constructon today, but object attrbutes requred for numercal smulatons such as temperature, velocty or surface propertes are not yet ncluded n the IFC product model. To extend ths product model wth addtonal data, lsts of varables and attrbutes can be attached to every component. The IfcPropertySets defned n the IfcXML format offer such a functonalty. IfcPropertyDefnton Identfcaton IfcGloballyUnqueId IfcPropertySet Set of S[1:n] Propertes Descrpton IfcProperty IfcText IfcPropertySetDefnton Name IfcLabel Fg. 4: Dagram of the IfcProperySet The property set s a contaner class that holds propertes wthn a property tree. An dentfer, a name and addtonal text descrbe each property set assgned to an object. Each set can have an unlmted lst of propertes. The Autodesk AEC Object Modelng Framework (OMF), the C++ Runtme Extenson program of Autodesk ADT, can access the data of the IFC product model. All addtonal attrbutes needed for the prototype and attached to an OMF-object are mported or exported as IfcPropertySets. Ths also ncludes the attrbutes for the behavour of an object wth respect to ts locaton n the system. Ths s based on the use of OMF anchor elements that allow movng Page 5 of 9

6 an object to specfc postons or ts behavour towards other elements. For example, an anchor can be defned n ts geometry as a pont, a lne, a surface or a volume and acts lke a spatal lmter for the correspondng object t s attached to. The user has drect access to these attrbutes wthn ADT by OMF menus. product model IfcObject / fcxml IfcPropertySet mport / export OMF Object wthn ADT Attrbutes needed for flud flow calculaton.e. temperature, velocty, pressure Smulaton IfcPropertySet Attrbutes needed for vsualsaton.e. object color, type, transparency Vsualsaton IfcPropertySet Attrbutes needed for modellng control.e. ancor elements, type Fg. 5: product model data mangement The ADT programmng nterface allows drect access to the object geometry and can transform t nto a Brep-model (3D volume model, see Autodesk ADN). Ths model s based on facets and s requred for the automatc mesh generaton of the flud solver. The robustness and effcency of the automatc mesh generaton process s ensured by addtonal functons ntegrated wthn ADT to control the consstency of ths model converson wthout gaps and overlaps. 4 Pre-Processng and CFD-Kernel The smulaton process tself does not receve any product model specfc data. Only geometrc objects and ther addtonal physcal attrbutes for flud smulaton are sent from the modeler to the pre-processor. The pre-processng s an octree based grd generator whch converts the facet model data to a grd requred by the CFD-kernel. Ths automatc 3D-grd generaton substantally optmzes the nteractve workflow of the prototype and allows short desgn cycles. The attrbutes of the facet model objects are adapted to the correspondng nodes of the CFDgrd and wll trgger a consstent behavour n the CFD-kernel. 4.1 Grd Generaton and Octree Herarchcal octree grds are wdely used and powerful approaches to model flud flows n complex 3D domans. Here we use a generalzed octree component approach wth base components for modelng physcal and geometrc problems based on (Brüggemann, 2004). Page 6 of 9

7 By a straght-forward extenson wth desred addtonal components ths general octree can be extended to store all necessary nodal quanttes for the CFD-kernel. 4.2 CFD-Kernel and communcaton nterfaces The CFD-kernel conssts of three major components whch obtan nformaton from the modeler, dstrbute and collect the data and do the computaton. They are mplemented as a set of threads communcatng wth each other usng the message passng nterface (MPI). The frst component s wrtten n ANSI-C, because the operatng system of the Modeler s a 32 bt Wndows whereas the CFD-kernel s runnng on 64 bt Lnux. To the best of our knowledge, no standard communcaton-toolkt allows the automatc mappng between these envronments, so we use basc socket communcaton whch s only avalable n ANSI-C or C++. One part of the automatc grd generator s also ncluded n ths thread because t transforms the real coordnates of each data pont nto grd coordnates. For optmum performance the other components are wrtten n Fortran90. The second component contans the second/remanng part(s) of the grd generator transformng object propertes nto computatonal propertes of the grd nodes. It also dstrbutes these data to the correspondng computng nodes. Thrdly ths thread collects the data and sends t as a sngle dataset to the vsualsaton module usng the Vsualsaton Interface Toolkt (Vst) developed at the Research Centre Juelch. The man effort s spent on the thrd component that s responsble for the computaton of the flud dynamcs ncludng the energy equaton. The flud dynamcs s computed by a Lattce-Boltzmann approach usng the so-called D3Q19 model whch ncludes nteracton wth 18 neghbourng nodes, the energy computaton s based on a Fnte-Dfference method usng the same 19-pont star. For both parts the requred parallel communcaton s accelerated by asynchronous communcaton whch means that non-blockng communcaton routnes are used. Frst the boundares of each subdoman are treated and whle watng for the ncomng data the nner nodes of each subdoman are computed Lattce-Boltzmann method It s well known for many decades (Chapman 1990), that the Naver-Stokes equatons r u r r 1 r (1) + ( u ) u = p + ν u t ρ and the contnuty equaton u r = 0 (here both wrtten for an ncompressble flud) can be derved from the Boltzmann equaton f r (2) + v f = Ω t where f s the probablty to fnd a partcle wth a mcroscopc velocty v r at ( x r, t) gven a sutable collson operator Ω descrbng the nfluence of partcle collsons. The connecton between the mcroscopc quantty f and the macroscopc felds s defned va moments of the form r r Φ v = fdv where Φ 0 = ρ and Φ = ρu r 1 and for weakly compressble flows by an equaton of state of state p( ρ, T ) = 0. Thus n prncple t s possble to obtan solutons for the Naver- Stokes equatons by solvng the Boltzmann equaton. As ths latter equaton s much harder to solve n general, there s nothng to gan from such an approach. Fortunately, t turns out that the Boltzmann equaton can be drastcally smplfed and stll can act as a hyperbolc superset of the Naver-Stokes equatons. For ths purpose one can dscretze the mcroscopc velocty space of the partcles (Grad, 1949),.e. the contnuous varable v r s replaced by a set of dscrete, constant veloctes { e r j }, j = 0, L, b whch typcally span a space fllng unt cell n two or three dmensons. Page 7 of 9

8 Thus we obtan a set of coupled equatons of the form f r (3) + e f = Ω ({ f j }) t where, j = 0, L, b. Ths set of equatons s referred to as the dscrete Boltzmann equaton due to ts dscreteness of the mcroscopc velocty space. Another substantal smplfcaton s to replace the collson operator by the so-called BGK approxmaton, whch merely ncludes the tendency of the system to develop towards an equlbrum state. f r 1 eq (4) + e f = ( f f ) t τ where τ s a mcroscopc relaxaton tme. For an deal gas the famous Maxwell dstrbuton can be chosen as the equlbrum functon. Macroscopc quanttes can now be obtaned from summatons n analogy to the moments obtaned va ntegraton as above: r r (5) ρ = f and ρ u = f e. A straght forward approach to solve the dscrete Boltzmann equatons s to use an explct frst order upwnd Fnte Dfference scheme on a unform grd wth grd spacng x. One obtans the followng set of algebrac relatons: r r r t r eq r (6) f ( x + e t, t + t) f ( x, t) = ( f ( x, t) f ( x, t)) τ Ths equaton s frequently referred to as the Lattce-BGK-equaton or Lattce-Boltzmann equaton. Alternatve dscretzaton technques for eq. (4) have been developed optmzed for specfc problems. 5 Post-Processng and Vsualsaton AVS/EXPRESS (AVS, 2004) s used for the vsualsaton of the systems physcal behavour as computed by the CFD kernel. Ths program ncludes a powerful programmng nterface and many modules are avalable to expand ts bult-n lbrary functons for vsualsaton and communcaton wth other programs. Even for large datasets AVS allows an effcent vsualsaton of scalar and vector felds. Isosurfaces, sovolumes and streamlnes are utlzed to dsplay the arflow and the dstrbuton of the PMV-Index nsde the offce. A specal AVS module allows to mport the CAD geometry (va DXF fles) to be drectly mported nto AVS. The extracted dataset based on a vector format ncludng addtonal vsualsaton attrbutes such as colors and transparency. The data of the smulaton kernel and the modeler are combned wthn AVS and vsualsed. 6 Concluson A prototype of a computatonal steerng envronment as the one descrbed above can substantally reduce the tme for many complex optmzaton processes whch requre human expertse durng the desgn process. The use of thermal smulatons n general can reduce the lfe cycle costs of a buldng up to 5% (Perre Barles, 2000). Ths calculaton ncludes the cost Page 8 of 9

9 of the HVAC system and ts consumpton costs. Especally n the desgn phase of complex buldngs wth a hgh budget, functonal optmzaton by CFD analyss wll lkely gan more mportance. Ths development wll be accelerated by further progress wth respect to computer hardware and effcent numercal technques the combnaton of whch wll offer reasonable capabltes to predct the behavour of complex engneerng systems n the early desgn phase. 7 References Autodesk Archtectural Desktop / AutoCAD, Autodesk Archtectural Desktop - OMF Developer's Gude 3.3, Autodesk Developer Network, Autodesk ObjectARX Developer's Gude, Autodesk Developer Network, AVS/EXPRESS, Barles (2000): Etude de l usage de la Smulaton Dynamque dans la Concepton des Bâtments en France, PBC Brüggemann, Holz (2004): Quadtree based hydronformatcs smulaton systems: A component-orented fnte volume toolkt for shallow water equatons, 6 th Internatonal Conference on Hydronformatcs (submtted) Chapman, Cowlng (1990): The Mathematcal Theory of Non-Unform Gases. Cambrdge, Unversty Press Grad (1949): On the Knetc Theory of Rarfed Gases, Communcatons on Pure and Appled Mathematcs, pp IAC Internatonal AVS Centre, IFC-Utlty 2x for ADT G.E.M. Team Solutons, IFC / fcxml, IAI Internatonal Allance for Interoperablty, Kühner, Krafczyk, Tölke, (2002): Towards nteractve comfort optmzaton of Indoor Flows usng Vrtual Realty based analyss of Large-Eddy smulaton results, n proceedngs of ICCCBE-IX, The 9th Internatonal Conference on Computng n Cvl and Buldng Engneerng, Tape, Tawan, pp MPI-Forum, Neuberg, Hoffmann (2001): Smulaton von Energeströmen n Gebäuden auf der Grundlage enes IFC baserten Produktmodells, proceedngs Forum Baunformatk Rtschel (1994), Raumklmatechnk, Band 1, Sprnger Verlag, Berln, pp Tölke, Fahrg, Nachtwey, Krafczyk (2003): Computatonal Steerng n Cvl Engneerng, IKM - Internatonales Kolloquum über Anwendungen der Informatk und Mathematk n Archtektur und Bauwesen Wemar Vst at FZ Jülch, VNC Software by RealVNC, Page 9 of 9

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