Wake Structure and Kinematics in the Vortex Ring State
- 1 July 2009
- journal article
- Published by AHS International dba Vertical Flight Society in Journal of the American Helicopter Society
- Vol. 54 (3), 32003-3200318
- https://doi.org/10.4050/jahs.54.032003
Abstract
High‐resolution computational simulations of the vortical wake of a rotor operating both near to and within the vortex ring state have been conducted using Brown's vorticity transport model. The nonlinear vortex kinematics of the wake is exposed using three‐dimensional visualizations of the simulated flow field. To reveal the vortex dynamics that underpin the highly unsteady flow within the vortex ring state, a rotor with just one blade was modeled. This blade was decoupled aerodynamically from the surrounding velocity field so that it acted merely as a source of trailed vorticity. The investigation identified a significant change in the dominant dynamics of the wake as it swapped from the tubular form that is characteristic of hover or very low speed descent into the toroidal geometry of the vortex ring state. Initial vortex “pairing” leads to rotation of vortex filaments away from their original attitude. This phenomenon plays an important role in regulating the downwash that the rotor can produce and thus in precipitating the onset of the vortex ring state. The considerable and persistent coherence of the vortical structure of the wake when in the vortex ring state is revealed, despite these disturbances, as are the mechanisms that lead to both small‐scale and large‐scale wake breakdown events. Simulations show the balance between the vortex pairing and short‐wave instability modes to be different in the vortex ring state at high descent speed, where the wake lies above the rotor, compared to in the vortex ring state at low descent speed when the wake lies predominantly below the rotor. This yields subtle differences to the kinematics and structure of the wake in the two cases.Keywords
This publication has 12 references indexed in Scilit:
- Efficient High-Resolution Wake Modeling Using the Vorticity Transport EquationAIAA Journal, 2005
- The flow field around a rotor in axial descentJournal of Fluid Mechanics, 2005
- Flow Visualizations and Extended Thrust Time Histories of Rotor Vortex Wakes in DescentJournal of the American Helicopter Society, 2005
- Predicting the Onset of Wake Breakdown for Rotors in Descending FlightJournal of the American Helicopter Society, 2003
- Rotor Wake Modeling for Flight Dynamic Simulation of HelicoptersAIAA Journal, 2000
- A weighted average flux method for hyperbolic conservation lawsProceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences, 1989
- The stability of a helical vortex filamentJournal of Fluid Mechanics, 1972
- Induced flow variation of the helicopter rotor operating in the vortex ring state.Journal of Aircraft, 1968
- Experiments on a model helicopter rotor operating in the vortex ringstate.Journal of Aircraft, 1966
- Airflow Patterns in the Neighbourhood of Helicopter RotorsAircraft Engineering and Aerospace Technology, 1951