Natural-Laminar-Flow Airfoil Design Optimization Considering Crossflow Instability When Configured on a Swept Wing

This paper proposes a novel design optimization method for transonic natural-laminar-flow (NLF) airfoils allowing for suppressing crossflow instability (CFI) when configured on a three-dimensional (3-D) swept wing. The distribution of the nondimensionalized crossflow pressure gradient (CFPG), a newly proposed flow parameter by the authors, is used as the objective flow characteristics related to crossflow in the corresponding 3-D case and combined with the total drag as the weighted objective function. The flow field and transition location are obtained by an extensively verified Reynolds averaged Navier-Stokes equations flow solver coupled an e^N method based on the linear stability theory. A surrogate-based optimization framework with Kriging model and parallel infill-sampling method is used to solve the optimization problem. The proposed method is demonstrated by the design optimization of a transonic NLF airfoil named NPU-LSC-72613, which is assumed to be configured on an infinite wing with a 30 deg sweep angle. Damped CFI and extensive laminar flow are obtained on the wing's upper surface, which demonstrates the feasibility of using the nondimensionalized CFPG to enable a 2-D NLF airfoil design optimization considering 3-D effects of crossflow.