T11~ P:ir; i i i~etric G r i d G e n e r a t i o n S y s t e n i M e g a C a d s

M. Hepper1e1. 0 . Brodersen', A. Ronzheimerl, B. Schöningl, C.-C. Rossow'

L DLR, Institute of Design Aerodynamics

Lilienthalplatz 7, D-38108 Brauiischweig, Germany

During the last decade, advasices iri the field of numerical aerodynamics lead the way to very

powerful codes capable of handling problems in two and three dimensioiis [I, 21. The solution

of the Navier-Stokes equatioiis for a complex configuration, including the wall boundary layers,

requires structiired grids of high resolution. Even today the creation of such grids of high quality

is very costly isi time and tlius iri manpower. On the one hand, the long development times can be

the result of using batch geiierators which usually can handle variations in parameters (like a flap

deflection angle) easily, hut are tailored to specific topologies which cannot be changed quickly.

On the other iiaiid, i ~ i t e r a ~ t i v e metliods, ofteri integrated into existing CAD systems, are time

constimiiig if modificatioiis in geometry or parameters force the user to do a redesign of large parts

of a grid.

Over the last years, tlie DLR Institute of Design Aerodyiiamics has used commercial procluct,~

and lias developed batch generators for grids around complex transport configurations [3, 41. Spe-

cial interactive tools were developed for the smoothing and refining of algebraic grids [5]. The

experiences gained while usiiig these systems were not fully satisfactory and formed the base for

the requirements for a new grid generatiosi tool which should be oriented towards tlie rieeds of a

research institute as well as iiidustrial applications [6] :

Opeii grid generatioii software developme~lt platform

Parametric desigii aiid replay mode concept

C:apability to handle rriultiblock grids

Elliptic a ~ i d biharmoriic smoothing of grids

Basic CAD tecliriiques

Machine iridepeiidesit source code

UNIX, X-Wisidows, OperiGL standards

All these requirenients are combined into the Multiblock-Elliptic-Grid-Generatiori-Arid-C' A D

System (MegaCads), whicli is ciirrently under development.

MegaCJads is an iiiteractive grid gerieratiori system, which enables tlie User to build blocli houii-

daries, ciistribute points o i ~ tlieni, create surface grids and fill volume bloclrs with grids, hased oll

simple represeritations of shapes through the use of basic objects like polygons arid splines iri 2D

aiid 3D. Tlie resultiiig algebraic grids can be smoothed using elliptic or biharsnoiiic tecliniques.

The wliole desigii is dosie in a parametric fasliion and each design step is stored in a script file. Tlie

hierarchical structure of tlie ciesign process is reflected in tlie script file, which can be visualised

aiid edited usiiig tile iiiteractive MegaEdit applicatiosi. Using the built in restart capability, it is

quiclr and easy to cliasige paraineters and replay the script file.

In the past, a combination of elliptic sinoothing in all three dimensions has proveri to be

successful for tlie creation of smooth grids. MegaCads distributes points by solvirig tlie poissori

equatiori aloiig spline curves, which form the edges of blocks. These block boundaries are filled

with elliptic/biharmonic smoothed grids, which form tlie boundary faces for the volume grids

[7, 8, 91.

Tlie Dc~vc.lopilleiit of Mc,gaCads

Tlie first versiori of MegaCads liad been developed in 1992 usirig FORTRAN 77 and simple, proprie-

tary graphics software. Tlie concept proved to be very successful for 2D and simple 3D applications,

but riecessary exterisions to more complex 3D configurations forced a redesign [7, 81. After eva-

luatirig various programn-iing languages arid graphics software systems, the decision was made to

develop a new versiori using a completely new data structure, user iriterface and staridard graphics

software. Startirig in mid 1994, the development uses the C lariguage, the Motif tool kit for X11

arid OpenGL/GL compatible graphics software. Tlie graphical user interface (GUI) is created by

rising a commercial interface huilder. The standard behavio~ir of the User interface results in a

sliort learriing period, and easy user customisation through resource files. Tlie graphics system is

based 011 a subset of the OperiGL/GL language, which cari easily be replaced by vendor specific

implementatioris arid optimised for specific hardware. The portable software implementation is

based ori tlie basic 2D graphics commands contained in tlie X11 protocol arid is thus capable of

driving eveii tlie simplest X-Wiridow terminals.

MegaCadc is separated into tl-iree large blocks: the GUI part , the graphics subsystem and the CAD

arid grid geiieration routiiies. Tlie parts are coririected by a few well-defined interfaces to guararitee

tliat tlie grid generatioii can also be used without tlie GUI and graphics part as a suhf~inctiori iri

design and optirnisatioii software.

Tlie C:AD and grid generatioii routiries are distributed in manager, workirig, arid service fiiiic-

tioris. The mariager friiictions are responsible for the evaluatiori of tlie commarids coming frorn

tlie GUI block or from the script file processor and for the mariagement of the geometric da ta

base. Tlie workirig fiinctioris perform mathematical operations on basic geometric objects sucli as

calculatiori of poiiit distributioiis along a curve or locating the intersection of two surfaces. Tlie

cervice frinctioiis handle tlie iriteraction witli tlie basic objects. Tliey can provide, for example the

arclerigtli of poirits on a splirie curve or tlie local slopes and curvature of a point on a surface.

GUI / Gr;tl)llic.s Striic.tiirc,

Tlie user interface corisists of a mairi window for displaying the geometry and for tlie selectiori of

functions fror11 tlie menu bar (Figiire 1). Tlie implemented functions are attached to the followiiig

six main riierius :

s File rnenii : incliide5 fiirictioiis like choosing a work directory, input/outprit of gronietrv data .

user-specific coiifiguratioii da ta and exit.

s Edit : provides the possibilit,~ to edit tlie script file and to make a restart frorn a specific

process. An LTrido feature is also implemented.

CAD arid Grid : see below.

e Optioris : allows to switch on/off visualisatiori resources (e.g coordinate axis, marker sizes).

Help : offers assistarice with a context sensitive on-line help system MegaHelp (Figure 2) on

all relevant topics. The systern uses a network wide database which can be viewed using a

hypertext browser. Also available is a library of examples which can be copied and rnodified

to fit a specific user need.

All functions have its owri dialog window with the riecessary text fields and option buttons to

read the user iriput (see distribute points dialog window in Figure 1). The dialog windows can be

placed ori tlie screen by the user so that no main window area has to be permanently reserved.

The graphic fiiriction buttoris cari he found in tlie graphic iitility window. Together with tlie mouse

these buttoris provide the hasic tecliniques such as translation, rotation, Zoom, grid browsing etc.

To find freqiiently used functions more easily the short-key feature is implemerited and the user

has tlie possihility to put f~inctioris in a favorite list located on the right of the rnairi wiridow.

The result of every C:AD or grid process will be displayed in the main wiridow arid in a n

entity list window. Tlie list helps the user to mernorize the generated geometries and it offers

the possihility to switcli tliem ori arid off using different selection criterias (see entity list wiridow

in Figure 1). In additiori to tlie Edit-function of tlie main menu the interactive tool MegaEdit

displays tlie hierarchical structure of the grid generation process arid supports an edit mode for the

parameters. Figure 3 sliows a geometric object (ciirve with point distribution) which is currently

heirig edited.

CAD Trrllili(liic~s

Basic CAD techiiiqiies are riecessary to design block topologies in 2D/3D arid on siirfaces. Chrreritly

the followirig furictioris are available :

Parametrisatiori of iriput da ta for the represeritation as spline curves or Coons surfaces.

Split/Trim tecliniques for curves and grids.

Concaterlate furictioris for curves arid grids.

Definition of vectors for direction specifications of new curves.

Definition of ciirves

C~lone/Translation for copying arid t,ranslating of geometries.

Ciurve/curve arid surface/surface intersection.

Curreritly a sirnplified version of the surface/surface intersectiori algorithm of [10] is tested.

A working plane coricept allows the user to define whether the resulting geometry of a selected

CAD functiori cari he found ori a parametrised surface or somewhere else in 3D physical space.

For example, a polygori hetween two poirits will he tlie euclidean connection if tlie workirig plane

is tlie 3D space. Biit it cari also be a liiie ori a surface when this is the current worliirig plane. All

processes use tlie current workirig plane until a different one is selected.

Tlie main objectives of the grid fiinctioris ar r to distribute points on curves, 011 block houridaries

arid in 3D blocks following the philosopliy of MegaCads. The following functions are curreritly

implemerit,ed :

e Point distrihutions 011 curves. The first and the last spacings can be specified.

Algebraic grids (2D13D).

Definition of first layers to define spacings and angles as boundary conditions for ellip-

tic/biharmonic smoothing (2D/3D).

Elliptic/biharmonic smoothiiig using techniques described in [4, 8 , 9, 111.

Figure 1 and figure 4 show the definition of a C-0-topology for the upper part of a wing grid.

Based on wing sectioris, the wing surface grid and six block faces can be generated.

Figure 5 and figure 6 show tlie grid of the rear part of an airfoil with slotted flap. Setting a iiew

flap angle arid running the script file agairi produced the grid shown in figure G. Other parameter

also could have heeri chariged (e.g. riurnher of grid cells, spacings, etc.).

MegaCads is aii iriteractive paranietric grid generation system for cornplex geometries. The irnple-

mented CAD arid grid generatiori techniques give the possibility to create arbitrary grid topologies.

All processes are stored in a script file. Tlie replay mode together with the interactive variation

of the script file supports parameter variations iri an easy way. The script files for different grid

topologies are stored in a grid generation lihrary. The files are available for users to simplify fritiire

applications to similar problerns.

Tlie curreiit work focuses 011 tlie iritegration of addit io~ial 3 0 functioris arid tlie application of

tlie riew versioii of MegaC'ads for complex 3D configuratioiis. Furthermore the system is undergoing

tlie iritegration of macro tecliniques arid variable substitution into the script file parser. This lias

already beeri completed for tlie MegaEdit applicatiori.

A final paper would iiiclude a detailed discriptiori of MegaCads and examples of generated grids for

complex geometries. Tlie iriteractive grid generation process and the variatioii of grid pararneters

could be sliown in a demoristratiori session.

Literatur

[I] R. Radespiel, C:.-C'. Rossow, R.C. Swanson : Elfici<,rii ( ' ( . / I \,isrtex .\lultigritl ,S(.Ii(~iii(~ i i ~ r ilit.

1'1~rrc~--l~iiiic~ii,~ioi1i1l .\-;ii.ic,r .SioX-0,s kqualiori.5. AIAA Journal Vol. 28. No. 8, 1464-1472, 1990

[2] B.L. Atliiiis : . \ , \ i i i l i i i i l o ~ k .\liilii,<i.iil _\lctliorl f ~ r f I i ( ' Soliifioir o f ' t l ie Crr1i.r ; i i i c / AYiii it'r S f ~ k i , , ,

l:~(li i;~rioiis ioi ~ l ~ l i i c ~ i ~ l I ir i ic~~i ,~iorin/ l"loir-.5. 29th AIAA Aerospace Sciences Meeting, AIAA 91-

0101, 199i

[3] R. Radespiel : ( f r i r l ( r i i ~ i ~ i ~ i i i o i i ,irolirlti I \ irig no t l y ( ori i l~i i i~i t ioii i uyiri<: a \ l i i l i i Illoc h \ i i i i (

tiirctl ( ciiiiiiilt~iiioii,il 1)orri;iiii DLR-IB 129-87116, DLR Institute of Design Aeroclynamics,

1987

[4] C.-C. Rossow, A. Ronzheimer : Allul t i -BlotX C i i i d (;c~iic~rctliori A r o ~ i n t l \ \ r n g - U o t / \ l i r iqi i i i

P ~ r i o r i ( ' o r i i i g ~ i r a t i o i ~ i . Numerical Grid Gerieratiori iri Computational Fluid Dynamics arid

Related Fields, ed. A.S. Arcliilla, J . Häuser, P.R. Eiseman, J .F . Thompson, Elsevier, 357-368,

1991

[5] U. Herrmann : ~ ~ ~ l : ' S I ~ '111 I r l t e r a c t ~ t ~ ~ 12105/1 (:r.ri< r ~ ~ l l o r i ['dckagc fc)r , i ~ ~ h l t 5i i l i t I' \ \ o r X

i t , i t i o i i , Numerical Grid Generation in Computational Fluid Dynamics and Related Fields,

ed. A.S. Arcliilla, J . Häuser, P.R. Eiseman, J .F . Thompsori, Elsevier, 357-368, 1991

[6] K. Becker, L. Fornasier, H. Rieger : f r i r l ~ i i t r i a l A ' h l ~ r c t i ol ( ' /C ' / ) ~ / ) / ) / ~ ~ ~ L ~ I o I J ~ 1 0 ( ' o i r i l ~ / c 'i

C ' o i i l i q i i r ~ t r o r ~ i Proceedings of 1993 European Forum : Recent Developmerits arid Applicatioris

in Aeronautical CFD, 1.1-1.14, RAE U K , 1993

[7] A. Ronzheimer, 0. Brodersen, R. Rudnik, A. Findling, C.-C. Rossow : i 2c r r I i i l c ~ i , t c ~ l i r c

Io(11 ~ ( J I i l i t L\l:ir~ag( I I I ( 111 of L'r~cl C ~ ~ I J ~ ~ ~ ~ I ~ I O I I l l roc(>\w\ ~ I O L U I ( ~ A 4 r l ) i t ~ a r ~ ( o ~ i h q i i i ~ i / i o i i ~

Numerical Grid Generation in C:omputaional Fluid Dynamics and Related Fields, ed. N.P.

Weatherill, P.R. Eiseman, J . Häuser, Pineridge Press, 441-452, 1994

[8] 0. Brodersen, 8. Findling, A. Ronzheimer : I i i 1 ( r , i c l l r e ( T I J ( / ~ ~ I I C I < I / I O I I 101 ( 'OI I I / J /C \ ( 01111g11-

i < i ( i o r ~ i , Cloniplltatiorial Fluid Dynamits '94, Proceedings of Second European Computatiorial

Fluid Dyriamics Coiiference, ed. C. Wagner, E.H. Hirschel, J.Pkriaux, R. Piva, John Wiley &

Soris, 265-272, 1994

[9] A. Findling, U . Herrmann : I ) o \ r l o ~ ~ r i i c r i t ol dri I" i f ic icr~t a i l d Rol)rr\i 5oli (,I l o r I , / / r / ~ l I ( t r i i r l

(:i r r c r < i l l o r ~ . Niiinerical Grici C+erieration in Computatiorial Fluid Dynamics and Related Fields,

ed. A.S. Arcliilla, J . Häuser, P.R. Eiseman, J .F . Thompson, Elsevier, 781-792, I991

[I01 R.E. Barnliill, S.K. Icersey : 1 , \ l , i r i I i ~ i i q JI( II iot l T o r I ' , i r ~ i r i ~ ~ t r i ~ 5111f~1ce / S I I I ~ < I ~ i 111i( I \ ( ( i 1 o 1 1

C:omputer Aidecl Geometrie Design, 7, 257-280, 1990

[ I I ] Z.1J.A. wars; . \ iiiii( I I ( ? I I (:I[(/ (;( ~ ~ C ' I C I ~ I O I I 111 ~ l ~ ~ l / l ~ t l J S L i ~ f a ( ? b T / ~ I < ~ I I ~ / I < I SC ( 0 1 1 ~ / OI(/('I

l)i//c 'ri rii iiil ( , r o i i i i t r i c Ilorlc I Journal of Computational Physics, 64, 82-96, I986

File Edit CAD Grid Options Online help

Figure 1: MegaCads User interface, block topology definition for C - 0 wing grid (upper side).

Figure 2: MegaHelp browser Figure 3: MegaEdit tool

Figure 4: Algebraic grids on 3 block faces for a C - 0 wing grid (upper side)

7