Optimization and Adjoint-Based CFD for the Conceptual Design of Low Sonic Boom Aircraft

Abstract

Sonic boom tailoring techniques for aircraft roughly the size and weight of a business jet have been demonstrated in flight-test,1 suggesting that the design of low-boom or even boom-less aircraft may be possible; however, the computational cost and complexity associated with current state-of-the-art tools makes e↵ective conceptual-level design infeasible when using traditional optimization approaches. Ongoing e↵orts in low sonic boom design have focused on gradient-driven shaping of the signature in the near-field2,3 or on the ground4,5 to directly drive the high-fidelity shape optimization. Gradient-driven and gradient-free approaches incorporating an integrated objective in the form of a noise level are described by Alonso6 et al. Low-fidelity, linearized (area-rule) approaches are also described in Ref. 6; however, these methods by definition are unable to accurately capture the non-linear features — namely, shocks — present in supersonic flows. While invaluable during design exploration, details of the shape produced by such approaches do not readily carry over to equivalent high-fidelity representations of the geometry. This paper presents an approach that decouples the low-boom aircraft design problem into two discrete components. Sonic boom minimization is performed using an approach based on linear theory, where a low-dimensionality, parametrized form of the near-field pressure signal enables rapid discovery of candidate target signal shapes. These target signals are then used in an inverse-design CFD framework, where a gradient-driven optimizer coupled with a discrete adjoint approach e ciently solves for the vehicle shape whose near-field signal matches the supplied target. A parametric geometry modeler is used to generate the surface triangulations and surface sensitivities required by this approach. A multi-disciplinary conceptual design tool is used to provide bounds for the high-fidelity shape optimization, ensuring that the final design is able to meet performance goals such as cruise range and takeo↵ field length. The current work starts with a discussion of the components comprising the design approach. Particular attention is paid to the linear component, which integrates a variety of tools to provide a robust, automated system for sonic boom minimization. Studies with simple axisymmetric bodies are then performed to characterize the mesh required for accurate resolution of near-field pressure signals in the CFD domain. This is followed by application of the adjoint-driven inverse-design component, again using a simple axisymmetric