Improved rotorcraft aeroacoustic and performance
analyses are highly dependent on the accurate
modeling of the rotor wake structure, in particular,
the strength and size of the blade-tip vortex. Particle
image velocimetry (PIV) can provide these structural
wake measurements very efficiently. The present
investigation represents the first three-component PIV
measurements acquired in a rotor wake by NASA.
The objective was to acquire three-component
velocity fields in the wake of a hovering, two-bladed,
untwisted rotor and determine the vortex size and
strength as a function of wake age. A 7.5-foot-diameter
rotor with a constant chord of 7.5 inches
was selected for the study. The rotor was operated at
a tip speed of 342 feet per second with the rotor
thrusting downward (wake up) to minimize recirculation;
500 PIV stereo image pairs were acquired for
each wake age, which ranged from 0 to 270 degrees.
Figure 1 shows an instantaneous in-plane velocity
field with associated contours of vorticity. The
vorticity map clearly identifies the location of the
vortices shed from the rotor blade tip. The wake age
of the vortex in the lower left corner is approximately
30 degrees. The upper vortex was generated by the
preceding blade and has a wake age of approximately
210 degrees. For clarity, only alternate rows
and columns of velocity vectors are shown (1/4 the
total number of vectors). Figure 2 shows the same
instantaneous in-plane velocity field with associated
contours of out-of-plane velocity. The out-of-plane
velocity map helps identify the wake trailed from the
inboard part of the blade. The direction of positive
out-of-plane velocity is out of the paper. This is also
the direction of blade motion.
Vortex-wander effects necessarily contaminate
measurements of vortex wakes that rely on point
measurement techniques. Whole flow-field techniques
like PIV allow us to mitigate such effects by
averaging in a coordinate system fixed with respect to
the vortex. Such an average retains the vortex
structure despite vortex wander and provides a better
estimate for the mean vortex structure.
These results clearly demonstrate the feasibility of
using three-component PIV for rotor wake measurements
in hover, and this technique will be extended
to the study of rotors in forward flight.
Point of Contact: A. Wadcock
(650) 604-4573
awadcock@mail.arc.nasa.gov
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