High-fidelity CFD-based Shape Optimization of a Blended-Wing-Body Aircraft for Improved Aerodynamic Performance, Considering Engine Integration Effects.

Abstract

Aerodynamic shape optimization is performed on the half model of the Blended Wing Body (BWB) with high-fidelity CFD computation including engine integration effects. It has been observed that the optimization has ran as expected showing that the DLR MDO chain which was tested many times for the tube-wing configurations also works for unconventional aircraft configurations. The history plot of the shape optimization is observed to have a sudden jump in the objective function, thus two critical design points are compared. As the decomposition of the drag during optimization was not performed, the critical design points are verified again with suitable drag computation thus calculating aerodynamic efficiency. The convergence history shows that the optimization was successful. During the optimization the changes in the design shape were observed, across the wing section in both the airfoil shape and twist angles and the thickness of the engine integration section. Along with these changes deformation in the wingtip was also observed. This deformation is generated from the reduced order model of the parameterized geometry (CAD-ROM). To validate this change in geometry, the updated design parameters of the critical design points are extracted and remodeled in CAD, where, after performing the CFD computations, it is observed that the wingtip deformation did not originate from CAD models. The CFD computations also showed that the coefficient of drag ($C_D$) has significantly increased when compared to its counterparts which has wingtip deformation. The corresponding coefficient of pressure ($C_p$) distributions which were extracted at three locations along the span of the wing, showed similar distributions in the first two sections, whereas a completely different $C_p$ distribution near the wingtip. The wing cross-sections are also plotted at these three locations, where a comparable difference exists between the two models of with- and without- wingtip deformation. The optimized models that were generated by the strange behavior of the CAD-ROM, seem to have produced better results The study was also extended to perform MDA on the BWB full model where a high-fidelity CFD-CSM coupling is performed along with trimmed AoA. The results of coupling show displacement along the vertical direction, as a result of which an increase in the coefficient of drag ($C_D$) is observed. Whereas the coefficient of lift ($C_L$) remains constant for all the cases. Similar to the increasing trend of the aerodynamic efficiency for the critical design points of shape optimization, the aerodynamic efficiency of the MDA is also increasing. This shows that the MDA provides better physical representation of the problem as it includes the interaction between the effects of one discipline with the other. The results of the shape optimization at Iteration 176 showed the best result among them all, where the optimized result is 36.6\% less in coefficient of drag ($C_D$) when compared to the baseline model. As the scope of the study is for aerodynamic shape optimization, the results can be further investigated by performing an MDO for better comprehensive results. As the baseline model was from the DLR-SIAM project, whose major goal was reduction of the noise impact, the flow separation was initially not reduced. As the optimization is sensitive to the baseline model and the focus was on the methodology, the flow separation could not be reduced during the optimization as well. The future work can be realised to be more focused on reducing flow separation by better parameterization of the pylon and engine integration with its specific constraints, along with constraints on the angle of attack. It is also important to validate the shape displacement field reconstruction of the CAD-ROM for higher accuracy.