Abstract
Unsteady pressure-sensitive paint (PSP) measurements were acquired on an articulated model helicopter rotor of 0.26 m diameter in edgewise flow to simulate forward flight conditions. The rotor was operated at advance ratios (free stream velocity normalized by hover tip speed) of 0.15 and 0.30 at a cycle-averaged tip chord Reynolds number of 1.1 × 105, with collective and longitudinal cyclic pitch inputs of 10° and 2.5°, respectively. A single-shot data acquisition technique allowed a camera to record the paint luminescence after a single pulse of high-energy laser excitation, yielding sufficient signal-to-noise ratio to avoid image averaging. Platinum tetra(pentafluorophenyl) porphyrin (PtTFPP) in a porous polymer/ceramic binder served as the PSP. To address errors caused by image blurring and temperature sensitivity, a previously reported motion deblurring algorithm was implemented and the temperature correction was made using temperature-sensitive paint measurements on a second rotor blade. Instantaneous, unsteady surface pressure maps at a rotation rate of 82 Hz captured different aerodynamic responses between the two sides of the rotor disk and were compared to the nominally steady hover case. Cycle-to-cycle variations in tip unsteadiness on the retreating blade were also observed, causing oblique pressure features which may be linked to three-dimensional stall.
Highlights
The main rotor blades of a helicopter in forward flight encounter time-varying pressure changes imposed by the difference in relative flow velocity between the advancing and retreating sides of the disk
Unsteady pressure-sensitive paint (PSP) measurements were acquired on an articulated model helicopter rotor of 0.26 m diameter in edgewise flow to simulate forward flight conditions
Platinum tetra(pentafluorophenyl) porphyrin (PtTFPP) in a porous polymer/ ceramic binder served as the PSP
Summary
The main rotor blades of a helicopter in forward flight encounter time-varying pressure changes imposed by the difference in relative flow velocity between the advancing and retreating sides of the disk. Lifetime-based PSP data acquisition is predicated upon the pressure sensitivity of the temporal decay of luminescence (with characteristic lifetime s) resulting from pulsed excitation This approach differs from intensity-based methods where the light source is flashed for each wind-off and wind-on image (Liu and Sullivan 2005); instead, the reference information is obtained from the same wind-on condition as the pressure information. Disotell and Gregory (2011) observed superimposed wave-like structures in the acoustic field of a resonance cavity operating at high sound pressure levels using the single-shot PSP technique This property is desirable for helicopter flows, where dynamic stall and tip vortex. Wong et al (2012) and Watkins et al (2012) utilized the single-shot lifetime technique to acquire unsteady PSP measurements near the tip of a scale helicopter blade in simulated forward flight at an advance ratio of 0.35. A complementary ACcoupled view of differential pressure changes measured by the PSP highlights cycle-to-cycle variations in pressure at the tip of the retreating blade
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