Towards Incorporating Aerodynamic Flow Separation Constraints into Aero-Structural Wing Optimization

Kurzfassung

Multidisciplinary optimization subject to local aerodynamic flow feature-related constraints for ensuring flow separation-free designs, is still challenging even for state-of-the-art approaches. This is attributed to both the number of design variables and the number of constraints required to capture local flow features being very large. The problem presented in Establishing a Joint Research-Industry MDO Benchmark based on the DLR-F25 Aircraft Configuration identified local flow separation at elevated lift as one issue of high aspect ratio wing designs. Gradient-based optimization approaches that do not explicitly constrain the flow separation and instead aim to eliminate unwanted flow artifacts implicitly by reducing the aircraft’s drag were shown to be successful in reducing the area of flow separation, but proved to be incapable of removing the flow separation completely. The design task presented here is the aero-structural design of high aspect ratio wings using predominantly high-fidelity solvers, involving the disciplines of aerodynamic performance prediction, structural sizing, geometry parametrization, and local aerodynamic flow feature constraints. Gradient-based optimizations are performed, exploiting efficient numerical methods to handle large numbers of design variables and constraints such as the adjoint method and forward differentiation. The structural sizing model employed in this paper considers industrial manufacturing constraints in combination with a comprehensive set of strength and static stability constraints defined for a large set of load cases. A new methodology to constrain optimizations by local flow features is developed and the sensitivities are validated against finite difference approximations. After describing the involved models, methods and the design task, the model is optimized with a large number of design variables reflecting aerodynamic shape and structural properties using an MDO chain co-developed by DLR and Airbus Defence and Space. The developed constraints for eliminating local flow features such as flow separation at high-lift are included into the shape optimization of a high aspect ratio wing. It is shown that the optimized wing with the applied flow-feature constraints at off-design conditions creates a wing that is completely free of flow separation and fulfills all flow feature requirements with only a negligible increase in drag in cruise.