A clear resurgence in ducted fan research in recent years can be noticed due to the growing interest in UAV/MAV (Unmanned/Micro Aerial Vehicle), PAV (Personal Aerial Vehicle) and electric propulsion. Noise emission of fans enclosed by a duct of finite length has attracted great research interest during the past decades. Computational Fluid Dynamics (CFD) is becoming a powerful tool for choosing the aerodynamic configurations of new UAV/MAV concepts and their fans, as well as determining the optimal modes of their operation.
The simulation of ducted fans with blades and stators resolved is within the capability of modern CFD methods and computers. Also, with the rapid development of commercial codes, many CFD simulations on UAV configurations are carried out in combination with practical tests. However, simulations of full-scale ducted fans for propulsion purposes at high Reynolds numbers are less common, and the same is true for ducted fans with stators or guide vanes. Non-axial inflow results in not only strong aerodynamic forces and moments on the duct, which behaves like an annular wing, but also in imbalanced disk loading that further induces more severe problems such as vibration. Flow separation at the inner or outer duct surfaces may also be encountered. The clearance between the duct inner surface and the blade tip leads to tip leakage flow. The presence of the duct significantly surpasses the extent of the blade tip vortices and regulates the flow to align with the duct surface, given a small tip-duct clearance. The ducted fan noise mostly comes from the rotating fan and interactions of its wake with the stator/vane. The presence of the duct substantially modifies the acoustic characteristics of a rotating fan/propeller.
The aerodynamic characteristics of the ducted-propeller are determined in various flight modes of quadcopter including hovering and oblique airflow at a variable angle of deviation from the vertical. The influence of this modelling is analysed in detail with variation of angular velocity of propeller. Besides the distance between propeller tip and ducted barrier, the size of ducted is also an important part in thrust optimization and total weight minimization of quadcopter. To determine the aeroacoustics characteristics of a quadcopter, large-eddy simulation of a turbulent flow is performed. A spectral analysis of the acoustic noise generated by the ducted fan is carried out, and the directional patterns of acoustic radiation are plotted for various rotor rotation speeds that simulate vertical take-off and landing modes.
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