ACOUSTIC RADIATION FROM A THIN AIRFOIL IN NONUNIFORM SUBSONIC FLOWS

Noise resulting from the interaction of impinging unsteady vortical disturbances on a thin airfoil in subsonic flow is directly calculated using unsteady aerodynamic theory. Exact expressions for the far-field acoustic pressure, intensity, and total acoustic energy radiated are derived in terms of the Fourier transform of the unsteady pressure along the airfoil surface and account for both dipole and quadrupole effects. This paper provides benchmark results for the case of a thin airfoil for comparison with future calculations for a loaded airfoil. It is shown that the acoustic pressure pattern strongly depends on the value of the reduced frequency and the mean now Mach number. The effects of an oblique gust are also examined and shown to significantly reduce the radiated acoustic power, which exhibits a maximum as the reduced frequency is increased. High frequency and noncompact source effects are also investigated and show multilobe directivity patterns that seem to recur at certain multiples of the reduced frequency.