Investigations of the vortex ring state on a helicopter main rotor based on computational methodology using URANS solver

Computational investigations of the Vortex Ring State (VRS) on a helicopter main rotor have been conducted. The VRS phenomenon is a condition of powered flight that occurs most frequently during the vertical or nearly vertical descent of a rotorcraft. The characteristic feature of the VRS is a torus-shaped vortex around a rotor. The occurrence of this extensive vortex structure is a dangerous phenomenon that usually causes sudden decrease of main-rotor thrust, finally leading to an increase of the rate of descent and vibration level, disturbances of a helicopter balance, deterioration of manoeuvrability and deficit of power. The investigations presented in the paper, have been conducted based on computational methodology developed and implemented by the Authors. The methodology is based on a coupling of several methods of Computational Fluid Dynamics and Flight Dynamic. The approach consists of calculation of unsteady aerodynamic forces acting on the flying rotorcraft by simultaneous solution of the URANS equations, the equations of motion of the helicopter as well as the equations describing fluid-structure-interaction phenomena. Flow effects caused by rotating rotor blades, are modelled using a simplified approach based on the Virtual Blade Model. Using the described methodology, a series of helicopter flight simulations, in a vicinity of the VRS boundaries, have been conducted. Selected results of these simulations have been discussed in the paper