Regime-Dependent Thrust-Driven Aerodynamic Optimization of a Two-Element Wingsail with a Bayesian–URANS Approach

Journal article, 2026

Despite increasing research on rigid wingsail aerodynamics for wind-assisted ship propulsion, optimization of two-element configurations remains insufficiently explored, particularly under a wide range of apparent wind angles (AWA) and considering dynamic interactions between the elements. This study establishes a geometrically interpretable parameterization and develops an optimization framework using a regime-dependent thrust formulation that enables consistent optimization across AWA regimes. The parameterization comprises inter-element gap, main element thickness, flap thickness, flap-to-main chord ratio, flap hinge position, and flap deflection. Three-dimensional unsteady Reynolds-averaged Navier–Stokes equations, coupled with Bayesian optimization, are used. The thrust objective is defined through weighted averaging over AWA intervals, under fixed main-element angle of attack of 12° and Reynolds number of 5.2E6. A full AWA range results in lift-dominated optimization. However, when emphasizing regimes to upwind or downwind (AWA below or above 90°), optimal configurations are different due to increased drag contribution to thrust. This infers the critical role of the flap deflection in drag production. Sensitivity analysis identifies the hinge position and gap as dominant parameters across regimes, while flap-related parameters are secondary and regime-dependent. This study demonstrates the importance of regime-dependence in wingsail optimization and provide physical insight into the transition between lift-dominated and drag-assisted propulsion mechanisms.