Although proven successful for steady pressure
measurements, extension of the pressure-sensitive
paint (PSP) technique to unsteady flows, especially
for rotorcraft, is considered quite difficult. This is
partly because of the complex chemistry of a
pressure-sensitive paint which has both fast time-response
(greater than 1 kilohertz (kHz) as opposed
to a typical PSP time response of 1 hertz (Hz)) and a
self-referencing capability (often provided by adding
a second pressure-insensitive luminescent dye to
produce a biluminophore paint). Also, there is the
problem of developing a camera coupled to a flash
illumination system that is fast enough to stop the
motion of the rotor model. Unsteady PSP would
allow full pressure measurements on helicopter
rotors, which would be of great benefit because
rotors are extremely difficult to instrument with
conventional pressure sensors.
The objective is to set up an oscillating-wing
wind tunnel facility for PSP measurements using flash
illumination and to use it to test candidate fast
biluminophore paints. The oscillating-wing rig is
simpler and more convenient than a rotor model, but
it contains most of the model's essential elements: a
moving wing and the requirement for measurements
over a large surface area.
Two candidate fast biluminophore paints were
tested. One was provided by researchers from TsAGI.
The second paint was developed under NASA
contract by ISSI, Inc. Only the results from the ISSI
paint are discussed here. Figure 1 shows a PSP image
of the wing upper surface at an 8 degree (deg) pitch
angle, showing the suction peak produced by the
NACA 0012 airfoil. Figure 2 shows the pressure level
along a chordwise line at midspan, again at an 8 deg
pitch angle, for the static case as well as for two
different oscillation rates. The suction values reported
by the PSP decrease significantly in the dynamic
cases. This is in conflict with theory, which predicts
only a slight change in pressure at the 2-Hz and 5-Hz
oscillation rates. These data suggest that the PSP's
time response is not sufficient to capture the fluctuating
pressures over the airfoil.
Reformulated versions of the ISSI paint are
currently being prepared. A new oscillating airfoil is
being fabricated which contains unsteady pressure
transducers for direct comparison with the PSP data.
New flash units have been obtained which will
provide higher brightness with a much shorter flash
duration. A second round of testing is to be
conducted.
Point of Contact: E. Schairer
(650) 604-6925
eschairer@mail.arc.nasa.gov
Back To Top
Previous Paper
Return to Revolutionary Technology
Next Paper 
