Flow Field Analysis in Vortex Ring State Using Small Diameter Rotor by Descent Simulation

While the unstable turbulence condition known as Vortex Ring State in rotorcraft has been studied mainly in helicopters, there have not been many studies of drones, which have become more active in recent years. In particular, there are few studies using numerical simulations focusing on small diameter rotors such as quadcopters. In this paper, descent simulations are performed using a rotor model with a diameter of 8 inches and a propeller pitch of 4.5 inches, which is used for quadcopters. In this study, Moving Computational Domain method is used to reproduce the descent motion of the rotor over the entire moving computational domain. In addition, Sliding mesh method is applied to reproduce the rotor rotation within the computational grid. This method allows flow field analysis under free rigid body motion of the analytical model. The displacement of the computational domain itself is applied at each step to reproduce the descent conditions. By combining these methods, fluid flow simulations under vertical descent and conditions are performed to visualize the flow field. Flow field evaluation using the Q criterion showed that even in a small rotor, circulating vortices are generated at a velocity close to the induced velocity \({v}_{h}\) . VRS was also observed around the rotor under the conditions of horizontal speed \(V_H \; = \;2.0v_h ,\; descent\;speed\; V_Y \, = \,1.0v_h\) , and descent angle of 26.6°, but at the same time vortex divergence was also observed. It was inferred that the forward velocity component helps to avoid Vortex Ring State.