CFD Update: Whats New in Computational Fluid Dynamics

AIAA (American Institute of Aeronautics andAstronautics) has held two workshops in the last threeyears to evaluate the ability of modern CFD methodsto accurately predict lift and drag of complete aircraftconfigurations. The first workshop was held in 2001and had an active participation of 18 CFD groups. ADLR-F4 wing-body configuration test case was usedwith structured and unstructured grids provided by theorganization and by ANSYS ICEM CFD. The results ofthis first comparison showed a wide spread betweenthe results produced with different codes and turbu-lence models. Even different codes with the same tur-bulence model produced significantly different results.As a consequence of this outcome, it was decided tohold a second workshop on the same topic, but withdifferent test cases.

The second workshop was held in Orlando on June21 and 22, 2003. Due to experience gained by participa-tion in the EU aeronautics project FLOMANIA, CFX fromANSYS was able to compute the matrix of specified testcases. The geometries selected were the DLR-F6 wing-body (WB) and DLR-F6 wing-body-nacelle-pylon(WBNP) configuration. The goal of the comparison wasto compute lift, drag and pitching moments of both con-figurations for a series of seven angles of attack.

To ensure the quality of the numerical results, par-ticipants also were required to perform a grid refine-ment study for each configuration. The structured gridsprovided by ANSYS ICEM CFD were:

All of the grids were of the low-Re type with y+

values of the order of one. Transition was specified inthe numerical tests according to the transitions stripson the wind-tunnel models. The grid refinement studyshowed that the main solution parameters did notchange between the medium and the fine grid. All sim-ulations shown are computed on the medium grid.

CFX Simulation for Predicting Aircraft Drag

Grids Coarse Medium Fine WB 3,450,000 5,820,000 10,130,000WBNP 4,890,000 8,430,000 13,690,000

Accurate results from a fully coupled formulation andalgebraic multi-grid solverBy Florian Menter and Robin Lantry, ANSYS CFX

Table 1: Number of nodes for different grids for WB and WBNPconfiguration

Lift vs. angle of attack for WB test case. Angles of attack were -3, -2, -1.5, -1.0, -0.0, 1.0, and 1.5. Modification to the SSTmodel, which mimics the adverse pressure gradient performance of the S-A model, produced an appropriate shift as shown by red triangle.

DLR-F6 configuration used as test case at the 2003 AIAA DragPrediction Workshop. Left: wing body (WB); Right: wing body-nacelle-pylon (WBNP)

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Numerical MethodAll simulations were computed with CFX-5.6. CFX-5 isa finite, volume-based CFD method for unstructuredhybrid meshes. It features some of the most advancedCFD techniques available today, namely a fully coupledformulation in combination with an algebraic multi-gridsolver. For a pressure-based method, this combinationhas proven essential to obtain high-quality convergedresults for the current transonic test cases. CFX-5 ishighly optimized in its physical modeling. The currentsimulations have been carried out using the SST turbu-lence model in combination with the CFX-automaticwall treatment.

ResultsThe computed lift, Cl, vs. the angle of attack, a, for thewing-body test case (WB) is in very good agreementwith the experimental data over the entire a-range.Surprisingly, it was found that the simulation of the liftcurve proved difficult for most other codes in the com-parison. In many cases, the lift curves were shifted by~0.25-0.5, which is a significant deviation in aero-nautical simulations. As the majority of these simula-tions have been computed with the Spalart-Allmaras(S-A) one-equation turbulence model, the sensitivity ofthe simulations to the turbulence model formulationhas been investigated after the workshop using CFX.It was found that a modification to the SST model,which mimics the adverse pressure gradient perform-ance of the S-A model, did indeed produce a shift ofthe expected magnitude.

The results for the lift-curve for the WBNP caseare again in good agreement with the experiments,except for negative angles of attack. Compared to most of the other codes, CFX was still quite closeto the data. All simulations showed a tendency ofover-predicting the lift at negative angles of attack.The main reason seems to be an over-prediction of a separation at the wing-pylon intersection, whichaffects the circulation around the wing. This issue iscurrently being investigated in more detail by severalof the participating groups.

For an airplane, the drag is one order of magni-tude smaller than the lift. It is therefore a notoriouslydifficult quantity to compute. The agreement of theCFX results with the experimental data for the drag-polar for both configurations is therefore surprisinglygood for all angles of attack. It was possible to clearlyidentify the additional installation drag due to pylonand nacelle. The CFX results could therefore satisfythe main demand to accurately predict lift and drag ofcomplete aircraft configurations using CFD posed bythe workshop.

The shock on the upper surface, which is cap-tured quite well by the simulations, can clearly beidentified in the chart of surface pressure distributions at different span-wise locations on the wing of theWBNP case.

Lift vs. angle of attack for WBNP test case

Drag polar for WB and WBNP test case

Wall pressure distributions at two span-wise locations for WBNP test case (yb/0.377 and yb/0.847) show the shock at the upper surface.

CFD Update: Whats New in Computational Fluid Dynamics

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Separation details on top-side of wing. Left: experiment with oil flow. Right: CFX-5 simulation. Region of reverse flow can be identified.

Convergence of forces for WB test case

In the CFX analysis of flow details on the upperside of the wing for the WBNP configuration, smallregions of flow reversal, which are in good agreementwith experiment, can be identified.

Numerical PerformanceOne of the main limiting factors in CFD simulations forcomplete aircraft configurations is the relatively highcomputing requirements. It is therefore important thatthe nonlinear iterative CFD procedure converges withhigh efficiency. For the lift and the drag force for theWB case, CFX typically converged for all cases inabout 100-150 iterative steps. This was one of thelowest iteration counts in the workshop comparison,

although the comparison is not straightforward dueto the many different numerical schemes involved.Due to the scalable parallelization of CFX-5, simula-tions for a grid of about 6 million nodes can be con-verged overnight on a modern 16-process PC cluster.

ConclusionsCFX from ANSYS has participated successfully inthe validation study of AIAA. The studies showedthat pressure-based methods are very well suitedfor the simulation of aeronautical flows at transon-ic conditions. Of the 25 participants, CFX softwareproduced one of the most consistent sets ofresults in good general agreement with the experi-mental data.

Web sites for More Information

Links

Results of First AIAA CFD Drag Prediction Workshop,Anaheim, CA June 910, 2001aaac.larc.nasa.gov/tsab/cfdlarc/aiaadpw/Workshop1/Final_Schedule_and_Results.html

FLOMANIA cfd.me.umist.ac.uk/flomania/

Results of Second AIAA CFD Drag Prediction Workshop,Orlando, CA June 2112, 2003aaac.larc.nasa.gov/tsab/cfdlarc/aiaadpw/Workshop2/Final_Schedule_and_Results.htm

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