Numerical investigation on aerodynamic performance of a ducted fan under interferences from the ground, static water and dynamic waves

Abstract

Ducted fans have been widely applied in various VTOL (vertical takeoff and landing) vehicles due to their high efficiency and low noise. By using the sliding mesh technique to simulate propeller rotation, a ducted fan system is chosen and the effects of the ground, static water and dynamic waves on its aerodynamic performance are evaluated, thereby exploring the adaptability of the system to complicated environments. The ground is set to slip wall moving with the same speed as the free stream and the static water surface is identified by using the VOF model. To generate dynamic waves, the open channel wave boundary is investigated to simulate a Stokes wave combined with the VOF model. A series of calculation cases consisting of free space, the ground effect, the static water effect and the wave effect are observed and analyzed in detail. The results indicate that ground, water surface and wave can strongly affect the aerodynamic performance of the ducted fan, as they interrupt the jet flow from the outlet of the duct and further pull the rebounded flow into the system. A decrease in the height between the system and the surface leads to a greater influence on the ducted fan. The strong effect obtained by the rigid ground benefits the lift of the propeller the most but also greatly weakens the lift of the duct; ultimately, the total lift of the system is lost. The torque changes by a law similar to that of the lift. The soft water surface is impacted to form a “drainage area”, which can be considered to increase the distance between the system and the water. Therefore, the water effect is not as strong as the ground effect. However, the change rules of the lift and the torque are nearly the same as those for the ground effect. The dynamic wave has the most complicated effect on the system. The periodic wave causes unpredictable periodic changes on the lift and the torque of the duct and the propeller, which may be related to numerical wave attenuation and the real-time changes in the soft wave surface. A larger wave height and a shorter distance between the wave and the system lead to a greater influence on the ducted fan. Moreover, the dynamic wave can induce “dynamic uplift” and hysteresis phenomena on the aerodynamic performance of the system.