Numerical investigation of ducted propellers for novel rotorcraft configurations

The ducted propeller is a promising propulsion or lift generator for novel rotorcraft configurations, considering the stringent restrictions on safety, efficiency, and noise/carbon emissions. However, extensive research work is still needed to further understand the aerodynamic and acoustic characteristics of ducted propellers at various conditions. This thesis aims to deliver highfidelity and systematic investigations of the aerodynamics, acoustics, and optimisation of ducted/open propellers at various conditions. A detail survey of past works on ducted propellers was first performed to analyse the research status and challenges. Critical assessments of available data sets for validation were also carried out. Numerical validation was then performed to verify the meshing, numerical methods, and simulation strategies for ducted propellers using a test case by NASA. High-fidelity CFD methods and lower-order tools were employed and compared at a range of conditions. Detailed analyses of the aerodynamic performance of ducted/open propellers were later performed at various advance ratios, pitch angles, and crosswind angles. The near- and far-field acoustic features of the ducted/open propellers in axial flight was also computed and inspected closely. A gradient-based design optimisation framework was also compiled to improve the ducted propeller performance at high advance ratios by varying the duct and blade shapes. The gradients of aerodynamic performance with respect to the design variables were computed using the discrete adjoint CFD methods. The ducted propeller thrust was successfully increased at high advance ratios after the optimisation. The far-field acoustics of the optimised designs was only mildly affected by the optimisation. A parametric study of the equivalent ducted/open propellers was also conducted to further evaluate the influence of different design and operating conditions. An automatic mesh generation tool chain was developed to ease the efforts required for the mesh generation. The ducted/open propellers were then installed under a main rotor to investigate performance changes due to the aerodynamic interactions. The main rotor downwash induced imbalanced disk loadings and loading variations with complex frequency compositions. The duct was found to provide aerodynamic shielding for the blades enclosed, but it also created considerable blockage to the downwash flow. A simplified modelling approach for the rotor/propeller interactions using actuator disk models was later put forward. By introducing an inflow distortion metric quantifying the aerodynamic interactions, an optimisation framework was compiled to minimise the rotor/propeller interference by changing the propeller position, i.e. the configuration optimisation. The inflow distortion factor was used as the objective, and its gradients with respect to the propeller position were computed using the adjoint method. Gradient-based and gradient-free optimisation approaches were proposed and assessed. With constraints on the pitching and rolling moments, the optimisation managed to effectively reduce the rotor/propeller interference. The optimisation results were further verified using blade-resolved simulations