Multi-objective shape optimization of doubly offset serpentine diffuser using Adjoint method

Abstract

Doubly offset serpentine diffusers are needed in the compact designs of modern stealth fighters with highly integrated propulsion systems. In addition, these doubly offset serpentine diffusers are also essential to provide reduced radar cross-section (RCS) due to the blockage of a complete line of sight. In this research, shape optimization of the doubly offset serpentine diffuser is achieved using a multi-objective approach to maximize the pressure recovery (PR) and minimize the flow swirl angle (α) and distortion coefficient (DC) at the aerodynamic interface plane (AIP). A computationally efficient Adjoint-based method was implemented for the optimization. For baseline geometry, an additional offset is added to NASA’s M2129 single serpentine duct to make it doubly offset serpentine diffusers. The freeform deformation technique was used to parametrize the baseline shape using 180 control points. A steady-state flow solution through the diffuser was achieved by solving Reynolds-Averaged Navier-Stokes (RANS) equations using a general-purpose computational analysis tool, i.e., ANSYS Fluent. The sensitivity of objective functions from the Adjoint solver was used to compute the required design change in the direction of minima or maxima of the objective functions using the steepest descent optimization technique. Subsequently, the calculated design changes were implemented through mesh Morpher. The Adjoint-based approach and freeform deformation resulted in a powerful and efficient optimization framework. The optimized shape showed a significant reduction in distortion coefficient and flow swirl angle, along with a slight increase in pressure recovery.