Ducted fans have been extensively used in Unmanned Aerial Vehicles (UAVs) for a variety of missions because of high efficiency, high safety and low noise. Wind, as a kind of typical meteorological condition, brings significant aerodynamic interference to the ducted fan, which seriously threatens flight stability and safety. In this work, the numerical simulation with the Unsteady Reynolds Averaged Navier-Stokes (URANS) method and the sliding mesh technique is performed to evaluate the steady wind effect. The results show that the wind will lead to serious unsteady effects in the flow field, and the thrust fluctuates at the blade passing frequency of 200 Hz. As the wind speed increases, the rotor thrust increases, the duct thrust decreases, and the total thrust changes little. Flow instability may occur when the wind speed exceeds 8 m/s. As the angle of low-speed wind increases, the rotor thrust changes little, the duct thrust increases, and the total thrust increases. In addition, we figure out that cases with the same crosswind ratio are similar in results, and increasing the rotating speed or fan radius is beneficial to performance improvement in wind. The findings are essential to the ducted fan design and UAV flight control design for stable and safe operations in wind conditions.

Ducted fans are widely used in various applications of Unmanned Aerial Vehicles (UAVs) due to the high efficiency, low noise and high safety. The unsteady characteristics of ducted fans flying near the ground are significant, which may bring stability problems. In this paper, the sliding mesh technology is applied and the Unsteady Reynolds Averaged Navier-Stokes (URANS) method is adopted to evaluate the influence of ground on the aerodynamic performance of ducted fans. The time-averaged results show that the ground leads to the decrease of duct thrust, the increase of rotor thrust and the decrease of total thrust. The transient results show that there exist small-scale stall cells with circumferential movements in ground effect. The stall cells start to appear at the blade root when the height is 0.8 rotor radius distance, and arise at both the blade root and tip when the height drops to 0.2. It is found that the unsteady cells rotate between blade passages with an approximate relative speed of 30%-80% of the fan speed, and lead to thrust fluctuations up to 37% of the total thrust. The results are essential to the flight control design of the ducted fan flying vehicle, to ensure its stability in ground effect.