An experimental investigation of unsteady surface pressure on an airfoil in turbulence—Part 1: Effects of mean loading

An experimental investigation into the response of an airfoil in turbulence is undertaken and the results are presented in a two part series of papers. The effects of mean loading on the airfoil response are investigated in this paper (Part 1) with the likely sources discussed in Part 2. Unsteady surface pressure measurements were made on a NACA 0015 immersed in grid turbulence (λ/c=13%) for angles of attack, α=0-20°, with a dense array of pressure transducers. These measurements reveal a reduction of up to 5 dB in pressure spectral level as the angle of attack is increased for reduced frequencies less than 5. This observed mean-loading effect has never before been measured or shown to occur theoretically. Lift spectra computed from pressure measurements show a similar result. Furthermore, the reduction in lift spectral level appears to have an α² dependence. Also, for small angles of attack (α<8°) Amiet's zero-mean-loading theory may be useful for predicting the airfoil response since the reduction in spectral level is less than 1 dB here. Based on comparisons at α=0°, Amiet's theory predicts with reasonable accuracy (within 4 dB at low frequency) pressure and lift spectral levels. This theory successfully predicts the shape of both pressure and lift spectra and the decrease in pressure spectral level moving away from the airfoil leading edge. Additionally, Reba and Kerschen's theory, which accounts for non-zero-mean loading using Rapid Distortion Theory, predicts large increases in pressure and lift spectral levels not shown to occur in the measurement. The predicted rise in spectral level appears to result from the flat-plate model with leading-edge singularity which does not fully account for the distortion of the inflow.