Distributed electric propulsion is widely recognized as a disruptive technology in aviation, and the aerodynamic characteristics of a multi-propeller are critical to the design of such configurations. A reformulated vortex particle method is adopted to provide an in-depth analysis of the aerodynamic characteristics between the multiple propellers operating in close proximity. According to the symmetrical distribution features of the particle field, the entire flowfield is divided into a noninterference region and an interference region. The results show that the aerodynamic performance of the middle propeller fluctuates more than the other two adjacent propellers in the hover state, and the flowfield in the noninterference zone exhibits time-independent characteristics. As the advance ratio increases, the performance fluctuations decrease, the radial contraction of the tip vortex is gradually attenuated, and the initial vortex strength decreases. For the interference zone in hover, the aerodynamic load of each blade drops as the propellers approach the interference region and the wake geometry shows asymmetry. The slipstream deformation of the multi-propeller is more pronounced in hover. The downwash from the previous propeller is responsible for the sequential decrease in thrust coefficient for multi-propeller systems operating at different sideslip angles, and the middle propeller experiences the greatest thrust fluctuation.

A vortex ring is a flow phenomenon involving a complex toroidal vortex system. When a rotor is in descending flight, it may enter the vortex ring state (VRS), which will affect the rotor’s aerodynamic characteristics and even endanger it. In this paper, to clarify the aerodynamic mechanism by which the vortex ring exerts its effects on an axial descending rotor, an unsteady numerical simulation method for the rotor integrated with structured moving overset grids is proposed and validated using experimental data. This numerical simulation method is then applied to analyze the aerodynamic characteristics of the rotor in descending flight. The variations of the aerodynamic forces and the flow characteristics in the slipstream are analyzed to elucidate the physical mechanisms responsible for the relationships between the aerodynamic loads, flow field, and vortices when the rotor is in the VRS. The effects of the VRS cause a sharp drop in the average aerodynamic forces, which directly affects the safety and reliability of a rotor aircraft. In the slipstream of the descending rotor, a distinct vortex ring forms and moves upward as the velocity of descent increases. The most severe VRS occurs at a nondimensional velocity of descent of 1.13 when the center of the highly unsteady vortex ring is right at the blade tip, and this can be used as an indicator of the VRS. The physical mechanism by which the VRS exerts its effects can be attributed to the low pressure induced by strong vortices. The VRS decreases the pressure on the lower surface of the blade and increases the pressure on the upper surface, resulting in a reduction in the aerodynamic loads. In comparison with the hovering state, the VRS results in a much larger vortex strength and possesses a different vortex structure.

Aircraft secondary flow systems are small-flow circulation devices that are used for thermal and cold management, flow control, and energy generation on aircraft. The aerodynamic characteristics of main-flow-based inlets have been widely studied, but the secondary-flow-based small inlets, jets, and blowing and suction devices have seldom been studied. Two types of secondary flow systems embedded in a supersonic aircraft wing, a ram-air intake and a submerged intake, are researched here. Firstly, wind tunnel tests under subsonic, transonic, and supersonic conditions are carried out to test the total pressure recovery and total pressure distortion. Secondly, numerical simulations are used to analyze the flow characteristics in the secondary flow systems. The numerical results are validated with experimental data. The calculating errors of the total pressure recovery on the ram-air and submerged secondary flow systems are 8% and 10%, respectively. The simulation results demonstrate that the total pressure distortion tends to grow while the total pressure recovery drops with the increasing Mach number. As the Mach number increases from 0.4 to 2, the total pressure recovery of the ram-air secondary flow system decreases by 68% and 71% for the submerged system. Moreover, the total pressure distortion of the ram-air and submerged secondary flow systems is increased by 19.7 times and 8.3 times, respectively. Thirdly, a detailed flow mechanism is studied based on the simulation method. It is found that the flow separation at the front part of the tube is induced by adverse pressure gradients, which primarily determine the total pressure recovery at the outlet. The three-dimensional vortex in the tube is mainly caused by the change in cross-sectional shape, which influences the total pressure distortion.

The wingtip-mounted tractor propeller configuration has been extensively studied due to its great potential for improving wing aerodynamic performance. In the present work, the aerodynamic interaction mechanisms in typical wingtip-mounted tractor propeller configurations are analyzed in depth using the reformulated vortex particle method. The results show that the elliptical lift distribution over the entire wing is altered by the installation of the tip-mounted propeller. The lift coefficient increases with inboard-up rotation and decreases with outboard-up rotation. Three key aerodynamic interaction mechanisms, swirl recovery, slipstream distortion, and slipstream impingement, are investigated. The swirl recovery mechanism generated by the inboard-up rotating propeller positively contributes to the performance of downstream wing. For the outboard-up rotation, the propeller-induced downwash results in reduced lift and increased induced drag. The interaction between the propeller-induced spanwise velocity and the wingtip-induced crossflow results in slipstream distortion. At α = 0°, for either inboard-up or outboard-up rotation, the slipstream on the retreating side is moved toward the propeller axis and away from the propeller axis on the advancing side. With the change of the rotation direction and angle of attack, the slipstream geometry exhibits different features. The slipstream impingement induced by the propeller generates time-varying loads on the downstream wing at the blade passing frequency. The propeller-induced tip vortices start to bend close to the leading edge and their vorticity increases, and then the vortex tube begins to deform. A higher angle of attack results in larger fluctuations in the downstream wing's drag coefficient.