悬停状态下小尺度旋翼的近场压力
The pressure field within the near field periphery of a small scale rotor blade is investigated by means ofclassical statistical analysis techniques and proper orthogonal decomposition. The signatures are acquired usinga circular arc array of dynamic pressure transducers, centered on the rotor tip at a distance of 1.5 chordlengths and below the tip path plane. The rotor is set to collective pitch angles ranging from 0. to 12. andis operated at 35Hz and 25Hz rotor speeds under hover conditions. Each blade from this two bladed rotoris investigated independently in order to isolate pressure signal differences existing between the blades. Theresults show that while the average inter-blade signatures are relatively constant, the variance of the fluctuationspossess noticeably different amplitudes. Given the scale of the rotor, these differences are attributedto the surface roughness effects. A low-dimensional analysis reveals that the first few most energetic modesproduced by each blade are relatively consistent in shape and so concerns about differences between the signaturesproduced by each blade are contracted. Two important features about the near-field signatures arethen revealed. The first is a low frequency, low wave-number type oscillation and is observed in all microphonesignals positioned between the tip path plane and 75. below. The second signature however comprises a highfrequency, high wave-number signature that manifests itself at shallow angles relative to the tip path plane ofthe rotor. The latter of these is believed to be the associated with the radiating component of the pressure fieldproduced by interactions of the rotor blade with the tip-vortex. A low-dimensional reconstruction of the rawpressure signal illustrates how each of these signatures contribute independently to the original raw signal.