Aerodynamic Shape Optimization of Transonic Swept Wings for Natural Laminar Flow

Optimizing transonic swept wings for natural laminar flow (NLF) presents a significant aerodynamic challenge, particularly at high Reynolds numbers and sweep angles where crossflow instabilities are important. In this study, lift-constrained drag minimization of infinite swept and finite wings is performed at operating conditions representative of a small jet, a business jet, and a regional jet using a Reynolds-averaged Navier-Stokes based aerodynamic shape optimization framework coupled with the SA-sLM2015cc local correlation-based transition model. To ensure robustness of the NLF optimum to varying operating conditions a multipoint optimization is performed. In addition, a gradient-based multistart optimization is carried out to address the multimodality observed for free-transition optimizations. For the infinite swept wing, a single-point optimization is performed at cruise conditions for a small jet and an aircraft-level drag reduction of roughly 15% is achieved compared to an infinite swept wing optimized and analysed under fully turbulent conditions. For the infinite swept wing at business jet conditions, a multipoint optimization gives a NLF optimum that maintains an aircraft-level drag reduction of roughly 10% across all 5 design points compared to a fully turbulent optimized and analysed infinite swept wing. For the finite wing, at regional jet conditions, an aircraft-level drag reduction of roughly 8% is achieved compared to a finite wing optimized and analysed under fully turbulent conditions. Lastly, a multimodality study performed for an airfoil under typical cruise conditions for a Dash8-Q400 aircraft identified three distinct local minima, with substantial differences in performance.