Component-Based Aerodynamic Shape Optimization using Overset Meshes

Abstract

Advances in computational power allow the increase in the fidelity level of analysis tools used in conceptual aircraft design and optimization. These tools not only give more accurate assessments of aircraft efficiency, but also provide insights to improve the performance of next-generation aircraft. Aerodynamic shape optimization involves the inclusion of aerodynamic analysis tools in optimization frameworks to maximize the aerodynamic efficiency of an aircraft configuration via modifications of its outer mold line.

When using CFD-based aerodynamic shape optimization, generating high-quality structured meshes for complex aircraft configurations becomes challenging, especially near junctions. Furthermore, mesh deformation procedures frequently generate negative volume cells when applied to these structured meshes during optimization. Complex geometries can be accurately modeled using overset meshes, whereby multiple high-quality structured meshes corresponding to different aircraft components overlap to model the complete aircraft configuration. However, from the standpoint of geometry manipulation, most methods operate on the entire geometry rather than on separate components, which diminishes the advantages of overset meshes.

Tracking intersections among multiple components is a key challenge in the implementation of component-based geometry manipulation methods. The mesh nodes should also be updated in accordance to the intersection curves.

This thesis addresses this issue by introducing of a geometry module that operates on individual components and uses triangulated surfaces to automatically compute intersections during optimization. A modified hyperbolic mesh marching algorithm is used to regenerate the overset meshes near intersections. The reverse-mode automatic differentiation is used to compute partial derivatives across this geometry module, so that it fits into an optimization framework that uses a hybrid adjoint method (ADjoint) to efficiently compute gradients for a large number of design variables. Particularities of the automatic differentiation of the geometry module are detailed in this thesis.

By using these automatically updated meshes and the corresponding derivatives, the aerodynamic shape of the DLR-F6 geometry is optimized while allowing changes in the wing-fuselage intersection. Sixteen design variables control the fuselage shape and 128 design variables determine the wing surface. Under transonic flight conditions, the optimization reduces drag by 16 counts (5%) compared with the baseline design.

This approach is also used to minimize drag of the PADRI 2017 strut-braced wing benchmark for a fixed lift constraint at transonic flight conditions. The drag of the optimized configuration is 15% lower than the baseline due to reduction of shocks and separation in the wing-strut junction region. This result is an example where high-fidelity modeling is required to quantify the benefits of a new aircraft configuration and address potential issues during the conceptual design.

The methodologies developed in this work give additional flexibility for geometry and mesh manipulation tools used in aerodynamic shape optimization frameworks. This extends the applicability of design optimization tools to provide insights to more complex cases involving multiple components, including unconventional aircraft configurations.