Transient Dynamics of Helicopter Rotor Wakes Using a Time-Accurate Free-Vortex Method
Transient Dynamics of Helicopter Rotor Wakes Using a Time-Accurate Free-Vortex Method
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Date
2001
Authors
Bhagwat, Mahendra J.
Advisor
Leishman, J. Gordon
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Abstract
A second-order accurate predictor-corrector type algorithm has been developed to
obtain a time-accurate solution of the vortical wake generated by a helicopter rotor.
The rotor blade flapping solution was fully integrated with the wake geometry solution
using the same time-marching algorithm. The analysis was used to predict the locations
of wake vortex filaments under transient flight conditions, where the rotor wake
may not be periodic at the rotational frequency. Applications of this analysis include
prediction of the rotor induced velocity field and blade airloads during transient flight
and maneuvers.
The stability of the rotor wake structure is important from the perspective of free-vortex
wake models. The wake stability was examined using a linearized stability
analysis, and the rotor wake was shown to be physically unstable. Therefore, the
stability of the numerical algorithm is an important consideration in developing robust wake methodologies. Both the stability and accuracy of the numerical wake solutions
algorithms was rigorously examined. The straight-line vortex segmentation used in
the present analysis was shown to be second-order accurate. The overall numerical
solution was also demonstrated to converge with a second-order accuracy. A technique
for increasing the order of accuracy for high resolution solutions is also described.
Along with a formal (mathematical) verification of solution accuracy, the numerical
solution for the rotor wake problem was compared with experimental results for
both steady-state and transient operating conditions. The steady-state wake model was
shown to give good predictions of rotor wake geometry, induced inflow distribution
as well as performance trends. Under transient conditions, such as those following a
pitch input during a maneuver, the time-accurate wake model was shown to correctly
model the dynamic response of rotor wake. In axial descent passing through the vortex
ring state, the present analysis was shown to properly model the associated power
losses as shown by experimental results. The present analysis was also shown to give
improved predictions of wake distortions during simulated maneuvering flight with
various imposed angular rates of the rotor.