Helicopters in steep descending flight experience a flow known as vortex ring state, which is characterized by inherent unsteadiness and instability. Rotors operating under vortex ring state show abrupt thrust reduction and fluctuation of airloads due to the increased downwash induced by the vortex ring and the blade-vortex interaction. In the present thesis, vortex ring state of the helicopter rotors is comprehensively studied by using a three-dimensional, compressible, invisid flow solver on unstructured meshes. Calculations were made for the Azuma rotor where extensive experimental data is available for comparison. Mesh refinement and adaptation is also made to examine the effect of mesh density on the vortex ring strength and the aerodynamic loads of the rotor. The results are in good agreement with experimental data, demonstrating the capability of the present unstructured mesh flow solver for solving the formation and the behavior of the vortex ring in the vicinity of the rotor.
