FINAL WORKSHOP OF GRID PROJECTS, ”PON RICERCA 2000-2006, AVVISO 1575” 1

OpenFOAM and Fluent Features inCFD Simulations on CRESCO High

Power Computing SystemF. Ambrosino1, A. Funel1

1ENEA, Via Vecchio Macello - 80055 Portici (Naples, Italy)E-mail: fiorenzo.ambrosino@enea.it, agostino.funel@enea.it

Abstract—We present the results of a CFDsimulation obtained with Fluent and OpenFOAM,an open source code. Our aim is to compareOpenFOAM capabilities to that of a well knowncommercial CFD code like Fluent. Both the twocodes have been run on CRESCO, the new ENEAhigh power computing cluster.

I. I NTRODUCTION

I N the last years high quality CFD tech-niques have become necessary and essential

tools for tasks such as aircraft industry aerody-namic development, combustion research, fluid-structure interaction problem solving etc. Thisrequires an ongoing struggle for computationalefficiency improvements where parallel comput-ing is one of the major issues. As the CRESCOsupercomputer has been integrated in ENEAGRID a considerable high power computing canbe achieved. Unfortunately, commercial CFDsolvers like Fluent have an expensive licensecost and generally this cost is directly relatedto the number of parallel cores needed to runthe program. OpenFOAM is an open sourceCFD solver, it can simulate anything from com-plex fluid flows involving chemical reactions,turbulence and heat transfer, to solid dynam-ics, electromagnetics and molecular dynamics.Being an open source toolbox it does not needany proprietary license and therefore it is a validFluent alternative during all the phases of a CFDsolution development.

Fig. 1. Fluent mesh snapshot.

II. PROBLEM SPECIFICATION

T HE case study examined here concernsthe exterior flow field around a simplified

passenger sedan geometry that proceeds witha steady and straight velocity in forward frontdirection.

The case was produced by ANSYS-Fluentand used as a benckmark of the standardbenchmark collection provided by Fluent to testparallel performances on various high powercomputing systems [1]. It belongs to the “large”class of benchmarks: a viscous-hybrid grid withabout 3.6 million prismatic cells is used to ad-equately model the region of interest. In Fig. 1is shown the computational domain.

The geometry is outlined in Fig. 2. The carhas a velocityv of 27.8 m/s, the pressurep isthe atmospheric ambient pressure that is 101325

F. AMBROSINO AND A. FUNEL: OPENFOAM AND FLUENT FEATURES IN CFD SIMULATIONS ON CRESCO HPC SYSTEM

Pa. The flow is assumed to be steady turbulent.Moreover, due to its low velocity, it can also beconsidered incompressible.

The equations of motion are:

∇ · U = 0 (1)

U · ∇U +1

ρ∇ p = ν∇2U + g (2)

where U is the velocity,ρ the density,p thepressure,ν the cinematic viscosity andg is thegravitational field.

Both for Fluent and OpenFOAM the adoptedturbulence model is the standard K-ǫ.

III. O PENFOAM CASE SETTING

I N order to run the case with Open-FOAM, the first step is to convert the

computational domain in OpenFOAM for-mat. This can be accomplished by using thefluentMeshToFoam utility [2]. In Fig. 3 isshown the result for the body car.The case requires initial and boundary condi-tions settings for all the involved fields. Forapplying boundary conditions a boundary isbroken up into a set of patches. In OpenFOAMphysical boundary conditions are specified bysetting appropriate keywords in a file whichdescribes the type of the patches. Moreover, foreach involved field a file exists where initialnumerical conditions have to be set [2]. In thiscase the air enters perpendicularly the inlet (y

direction) with an initial velocity of27.8 m/s.The initial pressure field is 1 atm everywhereand at the boundaries we require the condi-tion ∇ p = 0. The coefficients used for theturbulence model areCµ = 0.09, C1 = 1.44,C2 = 1.92, αk = 1, αǫ = 7.69. The valueof the turbulent kinetic energy isk = 0.02898

m2s−2 at the inlet, the same value has been setat the outlet for modeling the turbulence in caseof backflow. The value of the energy dissipationrate isǫ = 1.8239 m2s−3.The OpenFOAM solver used issimpleFoam,implemented for steady incompressible flow.We adopted as numerical scheme the standardfinite volume discretization of Gaussian integra-tion.

Fig. 2. The geometry.

Fig. 3. Body car mesh obtained by converting the originalGAMBIT file in OpenFOAM format.

IV. CROSSCHECKING RESULTS

BOTH Fluent and OpenFOAM got con-vergence. Fig. 4 shows the residuals of

the Fluent simulation while Fig. 5 shows theOpenFoam ones.

The viscous-hybrid mesh used had not a goodaccuracy in modelling the behavior of the flowat the boundary layer. In fact, they+ scale valueon the sedan surface is of order100, much moreof the viscous sublayer scale value. Fig. 6 showsa map ofy+ on the sedan surface. However, wedo not want to study boundary layer effects. Ourattention focused on comparing computationalperformances, thus we considered only global

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FINAL WORKSHOP OF GRID PROJECTS, ”PON RICERCA 2000-2006, AVVISO 1575”

Fig. 4. Fluent residuals.

0.0001

0.001

0.01

0.1

1

0 2000 4000 6000 8000 10000

RE

SID

UA

LS

ITERATIONS

3D CAR

UxUyUzpkepsilon

Fig. 5. OpenFOAM residuals.

properties of the flow motion.In order to compare Fluent with OpenFOAM

results we shall analyse the shape of the fieldson the central symmetry plane. Fig. 7 andFig. 8 show the contour map of the velocitymagnitude field around the sedan for Fluent andOpenFOAM solvers, respectively.

We observe that the magnitude of the velocityfields is of the same order. The velocity fieldaround the car has a similar shape in bothsimulations.

Fig. 9 anf Fig. 10 show the contour map ofthe pressure field for Fluent and OpenFOAMsolvers, respectively. The Fluent picture reportsthe pressure values referred to 1 atm, thusnegative values indicate a pressure lower than 1atm. On the other hand, the OpenFOAM picturereports the absolute pressure values as displayedin the scale. In this case an overall agreement

Fig. 6. Map ofy+.

Fig. 7. Contour map of Fluent velocity field.

Fig. 8. Contour map of OpenFOAM velocity field.

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F. AMBROSINO AND A. FUNEL: OPENFOAM AND FLUENT FEATURES IN CFD SIMULATIONS ON CRESCO HPC SYSTEM

is observed, too.

Fig. 9. Contour map of Fluent pressure field.

Fig. 10. Contour map of OpenFOAM pressure field.

All simulations have been run on theCRESCO cluster architecture at the ENEA-Portici Research Center. Performances and scal-ability tests were made both for Fluent andOpenFOAM solvers. Parallel efficiency de-creases as the number of computing cores in-creases. Moreover, parallel efficiency enhanceswith the mesh size. Fig. 11 shows the speed-upof the simulations up to 512 cores. The speed-up is defined as a ratio of serial to parallelexecution time.

The shape of the speed-up depends on themethod used for decomponing the computa-tional domain. In the case of Fluent we useda standard decomposition, as comes with thebenchmark, where the mesh is subdivided ac-cording to Cartesian directions, while in the

Fig. 11. Speed-up for Fluent and OpenFOAM solvers.The simulations have been run on the ENEA CRESCO highpower computing system.

OpenFOAM case we used an optimized decom-position method which attempts to minimize thenumber of common faces between processors.As Fig. 11 shows, the speed-up increases up to256 cores with the OpenFOAM solver and upabout 64 cores with Fluent.

V. CONCLUSION

In summary we have shown that OpenFOAMmay be suitable for running CFD simulationsin place of a commercial code like Fluent. Themain advantage is that of not having to pay anylicense cost. This aspect is particularly impor-tant when one wishes to run a CFD simulationwith many cores. Moreover, being an opensource toolbox OpenFOAM is fully extensibleand provides the possibility of writing newlibraries. We have shown that similar results areobtained by running OpenFOAM with a Fluentbenchmark. Tests have been made on the ENEACRESCO high power computing system whichallowed us to establish that OpenFOAM codehas a good scalability.

ACKNOWLEDGMENTS

We thank Silvio Migliori, Salvatore Podda,Pietro D’Angelo and Paolo Roccetti for helpingus with their support. We also wish to thank allthe staff of the ENEA-Portici Research Center.

REFERENCES

[1] http://www.fluent.com/software/fluent/fl5bench/flbench 6.3/problems/fl5l2.htm[2] http://www.opencfd.co.uk/openfoam/doc/user.html

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