Steady-Adjoint Optimization of Tonal and Trailing-Edge Propeller Noise

Abstract

This paper presents an efficient optimization framework for reducing tonal and turbulent boundary-layer trailing-edge noise (TE) of a subscale propeller operating in forward flight. The steady-state Reynolds-averaged Navier–Stokes solver is coupled with the Ffowcs Williams–Hawkings acoustic analogy to predict thickness and loading noise. Broadband TE noise is estimated using Amiet’s analytical model, with wall pressure spectra provided by Goody’s model. The boundary-layer thickness near the trailing edge is computed using a novel extension of a general method for nonequilibrium flows, adapted to a rotating reference frame. A discrete adjoint solver, generated using automatic differentiation, provides sensitivity information for two objective functions targeting tonal noise and TE self-noise. These sensitivities guide a gradient-based algorithm to search for local minima. The numerical approach is validated against NASA data for the C24ND and COPR-3 rotor geometries. The framework is then applied to the COPR-3 blade, achieving significant reductions in tonal and broadband noise. The results demonstrate how different optimization objectives, such as noise minimization and torque reduction, lead to distinct geometries, boundary-layer characteristics, and acoustic signatures. This highlights the proposed method’s ability to improve performance efficiently while revealing physical design tradeoffs relevant to urban air mobility applications.