Modal analysis of the unsteady flow dynamics of a ducted fan in the ground effect

Ducted fans have been widely adopted in electric vertical takeoff and landing aircraft due to their high hover efficiency and low noise. In practice, the unsteady aerodynamic performance of ducted fans operating in proximity to the ground is of critical importance. However, the ground effect can induce aerodynamic instabilities during operation. In this study, the influence of the ground effect on ducted fan performance is evaluated using the sliding mesh technique coupled with the unsteady Reynolds-averaged Navier–Stokes method, along with experimental measurements. Dynamic Mode Decomposition (DMD) is further applied to investigate the three-dimensional flow structures. The results show that the ground effect increases blade thrust, reduces duct thrust, and leads to an increase in total thrust. Part-span stall occurs near the blade root, accompanied by low-frequency fluctuations in ducted fan performance. Circumferential mode decomposition and correlation analyses reveal the low-frequency disturbances propagate circumferentially. Furthermore, DMD in three dimension is employed to identify the low-frequency flow structures and to characterize the disturbance propagation patterns induced by the ground effect.