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 of classical statistical analysis techniques and proper orthogonal decomposition. The signatures are acquired using a circular arc array of dynamic pressure transducers, centered on the rotor tip at a distance of 1.5 chord lengths and below the tip path plane. The rotor is set to collective pitch angles ranging from 0. to 12. and is operated at 35Hz and 25Hz rotor speeds under hover conditions. Each blade from this two bladed rotor is investigated independently in order to isolate pressure signal differences existing between the blades. The results show that while the average inter-blade signatures are relatively constant, the variance of the fluctuations possess noticeably different amplitudes. Given the scale of the rotor, these differences are attributed to the surface roughness effects. A low-dimensional analysis reveals that the first few most energetic modes produced by each blade are relatively consistent in shape and so concerns about differences between the signatures produced by each blade are contracted. Two important features about the near-field signatures are then revealed. The first is a low frequency, low wave-number type oscillation and is observed in all microphone signals positioned between the tip path plane and 75. below. The second signature however comprises a high frequency, high wave-number signature that manifests itself at shallow angles relative to the tip path plane of the rotor. The latter of these is believed to be the associated with the radiating component of the pressure field produced by interactions of the rotor blade with the tip-vortex. A low-dimensional reconstruction of the raw pressure signal illustrates how each of these signatures contribute independently to the original raw signal.