The onset and development of the vortex ring state (VRS) on a helicopter rotor is hypothesized to be the result of a developing spatial and temporal wake instability. The onset of VRS occurs when instabilities cause perturbations in the vortex wake to reach sufficient amplitudes that the wake breaks down from its regular helicoidal form, and eventually causes wake induced flow disturbances to reach the rotor disk. An analytical wake stability analysis is used to confirm that the flight conditions that lead to the onset of wake instabilities will be both rotor configuration (e.g., number of blades, blade twist etc.) and operating state specific. In descending flight, the wake is shown to be more susceptible to the onset of instabilities and persistence of wake perturbations than in hovering flight, mainly because of the reduced helicoidal spacing between individual tip vortex filaments. Time-accurate free-vortex wake results show that even after wake instabilities are initiated, equilibrium descending flight conditions can still be reached when wake perturbations are being formed as quickly as they are being convected away from the rotor. However, the imposition of increasingly higher rates of descent takes the rotor into the beginnings of the VRS, the onset of which occurs when the wake breaks down substantially from its helicoidal form. This is accompanied by an increase in rotor thrust fluctuations. Deeply in the VRS, the rotor wake is shown to engulf the rotor and can recirculate in the rotor plane as individual tip vortices that form tight bundles of vorticity resembling vortex rings. These rings are found to be spatially and temporally unstable, and break away from the rotor disk, leading to the low-frequency, aperiodic fluctuations in rotor airloads that are a known characteristic of flight operations in the VRS.
The vortex ring state as a spatially and temporally developing wake instability
Der Wirbelringzustand als räumliche und zeitliche Nachlaufinstabilität
Journal of the American Helicopter Society ; 49 , 2 ; 160-175
2004
16 Seiten, 19 Bilder, 43 Quellen
Aufsatz (Zeitschrift)
Englisch
The Vortex Ring State as a Spatially and Temporally Developing Wake Instability
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