Application of “Objective Space Filtration” on Nacelle Cowl Design Variable Generation

Conventional approaches of design variable generation in nacelle design cases are usually proposed from either of two perspectives: (1) utilizing geometric resemblance to airfoil shape; or (2) selecting flexible geometric parametrization so that any potential geometry can be produced. However, the relationship between geometry and aerodynamic performance objectives is not emphasized in these approaches: design variables are likely not carrying the information that is most needed for aerodynamic optimization, as is the case in nacelle cowl design of drag reduction. In fact, an ideal aerodynamic information should be related with the move of samples toward better aerodynamic performance in design space. As an attempt to fix this issue, this paper proposes a new design variable generation method by extracting objective space (i.e., the aerodynamic performance of samples mapped from nacelle design space) information via linear discriminant analysis (LDA). By accomplishing this "objective space filtration, " the aerodynamic performance improvement potential is injected into design variable generation. Proof-of-concept studies of nacelle cowl design are provided in the paper demonstrating a more direct relationship that links: (1) LDA-aided design variables versus cowl drag, and (2) LDA-aided design variables versus shock strength and laminar length, respectively. LDA is used in the studies as a classic technique of classification, so that the analysis of objective space information finds a way to have impact on design space. The new approach outperforms a controlled test realized by Proper Orthogonal Decomposition in terms of reflecting aerodynamic information in design space, in that a localized improvement of aerodynamic objectives can be more clearly seen in the newly generated design variables. The result shows that the new design variable generation helps improve aerodynamic design and that it can be further extended to other cases beyond the scope of nacelle cowl design. (c) 2022American Society of Civil Engineers.