3. Summary CFD Workshop : 3.1.2 Part. 1

3. Summary of CFD Workshop

3.1.2. Result Comparison

 1) Summary of 1st Workshop

Detailed explanation for the DLR F4 model for the 1st workshop is described by Redeker [1] as shown in Fig. 3 and 4; Fig. 4 specify position of the sting which minimize the interference while transition strips are shown for both upper and lower side of the wing and body. The transition strip on the body is at about half of the nose to describe the real air-flow condition. Strip of the upper side of the wing is located at much less than the quarter of the chord length while portion become much smaller for root and wing-tip. It would be interesting that amount of the portion position of the strip is different because complex behavior of flow around the tip and near body makes easier transition than the cleaned-section. However, there is no kind of complexity at the lower side, transition strip is located very strict manner of rule. 

Part of the WT test result is shown at Fig. 5 and 6; noticeable difference occurs at high AoA or Cm value region. Cm value is different for each WT facility less than -0.12. Although the same model was used for the testing, there is no description about the discrepancy between the facilities. Detailed information about the wind-tunnels are shown by Wahls’ report [2]. For their size, NLR and ONERA has similar size while all three WT used their own sting. Correction of the WT facilities are done respectively; details of them are describe in the Fig. 8. Variation of the drag is similar for more than 10M Re# with free and fixed transition while aero-elastic effect of wing-deformation is accounted for reference geometry generation. Effect of the interference and blockage is about 5.9 counts while correction for the sting support is 19.2 counts (Fig. 8). 

Fig. 3. WT model geometry of DLR-F4

Fig. 4. WT model specification of DLR-F4

Fig. 5. WT test result of DLR-F4

Fig. 6. WT test result of DLR-F4 : Wing-deformation during the test

Fig. 7. WT facility comparison for Bedford, NLR-HST, and ONERA-52MA

Fig. 8. WT facility correction for the experiment

Size of the far-field boundary of the CFD is for more than 50 times of the chord length, and whole grid point is about 3M [3]. Guidelines for the airfoil section are 0.1% chord for leading section, 0.5%, 0.125%, and 0.0625% chord section respectively. Spanwise spacing is about 0.5% of semi-span about 3mm while fuselage is about 10mm. Summary of the result is reported by Levy [4]; comparison between the experiment and several CFD cases are shown in Fig. 9. Result shows that CL value is different for turbulence model and grid style however CL_a is similar for all cases except Euler model. k-omega and k-omega-SST model are the best option for the AoA while S-A model, and k-omega is the best fit for lift-drag curve which represents induced drag. Error for some cases about 20 counts of drag for given CL.

For pitching moments, discrepancy between experiment and CFD become larger which is about 0.02 Cm for given CL. That report[4] evaluates the CFD result as good-agreement with the experiment, however, generally, minimum drag is higher, and induced drag is lower than experiment. Grid type shows no-clear tendency for the CFD while turbulence model affects. Type comparison is shown at Fig. 10. The report recommends more study for complex configuration, transition prediction, separation, extrapolation for flight condition, and visualization. 

Fig. 9. Comparison CFD and WT results

Fig. 10. Comparison CFD and WT results II

2) Summary of 2nd Workshop

As described in the previous sections, addition of the nacelle is noticeable for the 2nd workshop [5]. Also, grid convergence, transition strip, and drag rise in given CL are given for deep understanding of the CFD. Fig. 11 shows WT model of the 2nd Workshop, typical representation of narrow-body airliners. 

Laflin’s report [6] summarized the numerous CFD result from participants, and effect of grid size for case 1 is shown in Fig. 12. Convergence tendency via increase of grid size can be definitely shown; about 60 counts divergence is reduced to about 10 counts for fine grid. Also, average value of the drag is almost same as the experiment value. Fig. 13 represents drag value of each case; nacelle/pylon makes more divergence on the whole participant’s data. Richardson extrapolation of the CFD data made not-good result because convergence of the data is not enough to predict better value. 

The whole lift/drag prediction and its comparison are shown in Fig. 14; discrepancy between cases are significantly reduced although about 40 counts of divergence is still shown for each CL value. More detailed comparison for type comparison is shown for Fig. 15. Although 1st DPW did not show noticeable difference between cases except turbulence models, result of the 2nd one shows discrimination among case set up. More statistical analysis was conducted by Hemsch [7], and he compared the two DPW as shown in Fig. 16. Result from 2nd DPW shows smaller variance and spread than the 1st one. Additionally, as shown in Fig. 17, there is a separation bubble which potentially degraded prediction performance of the CFD. 

Fig. 11. WT model of DLR-F6 with nacelles

Fig. 12. Effect of grid size for drag value

Fig. 13. Grid convergence (left), Extrapolation of delta drag (right)

Fig. 14. Drag polar of whole CFD result with experiment for both wing-body only and nacelle-pylon cases

Fig. 15. Drag polar of whole CFD result with experiment for both wing-body only and nacelle-pylon cases

Fig. 16. Statistical spread comparison between the 1st and 2nd DPW

Fig. 17. Overall evaluation of the result comparison (left), separation bubble on wing-root

3) Summary of 3rd Workshop

Tinocco [8] reports summarized report for the 3rd workshop which represents addition of body-fairing. Separation bubble on the wing-root can be considered in more-realistic geometry. He announced overall result convergence is disappointing as shown in Fig. 18 and 19. Contrary to the addition of nacelle in 2nd workshop, the addition of the fairing did not affect significantly for the convergence. Performance of the overset grid is the best among the grid types, however, there is an only slight convergence as increase of the grid size. Then, author reported that kind of weak-trend make hard to set the rule for the grid. Major problem of the prediction is separation bubble of the wing-root as shown in the Fig. 20; CFD also predict occurrence of the bubble however exact size and position of the detail is different for each cases. 

Fig. 18. Grid convergence

Fig. 19. Grid convergence of skin-friction drag

Fig. 20. Separation bubble on the upper surface of the wing-root

* Reference

[1] Redeker, G., 1994, A Selection of Experimental Test Case for the Validation of CFD Codes, AGARD-AR-303 Vol. II

[2] Wahls, R. A., 2001, AIAA Drag Prediction Workshop Wind-Tunnel Data, AIAA CFD Prediction Workshop

[3] Vassberg, J. C., 2001, Guidelines for Baseline Grids, AIAA CFD Drag Prediction Workshop, 19th Applied Aerodynamic Conference

[4] Levy, D. W., 2001, AIAA CFD Drag Prediction Workshop : Data Summary Comparison

[5] Broderson, O., et al., 2001, Drag Prediction of Engine-Airframe Interference Effects Using Unstructured Navier-Stokes Calculations, AIAA 2001-2414

[6] Laflin, K., 2003, 2nd AIAA CFD Drag Prediction Workshop Data Summary and Comparison

[7] Hemsch, M. J., 2003, Statistical Analysis of CFD Solutions

[8] Tinocco, E. N., 2006, DLR F6/FX2B Summary, 3rd CFD Drag Prediction Workshop

[9] Vassberg, J. C., et al., 2008, Development of a Common Research Model for Applied CFD Validation Studies, AIAA-2008-6919

[10] Sclafani, A. J., et al., 2010, Drag Prediction for the NASA CRM Wing-Body-Tail using CFL3D and Overflow on an Overset Mesh

[11] Rider B. J., 2010, Structured Grid Summary for the 4th Drag Prediction Workshop

[12] Pirzadeh, S., 2010, Baseline Unstructured Grids, AIAA DPW4

[13] Oswald, M., 2010, 4th AIAA CFD Drag Prediction Workshop

[14] Tinoco, E. N., et al., 2009, DPW-IV Summary of Participants Data

[15] Levy, D., et al., 2013, Summary of Data from the Fifth AIAA CFD Drag Prediction Workshop

[16] https://allaboutairplanes.wordpress.com/2012/03/03/the-coffin-corner/

[17] Roy, C., 2017, DPW 6 Summary of Participant Data – Case 1: Code Verification

[18] Tinoco, E. N., 2017, Summary of Data from the Sixth AIAA CFD Drag Prediction Workshop: CRM Cases 2to 5

[19] Derlaga, J., et al., 2017, Preliminary Statistical Analysis of CFD Solutions

[20] Laflin, K., and Orderson, O., 2017, Side-of-Body & Trailing Edge Separations