Propagation of propeller tone noise through a fuselage boundary layer

In earlier experimental and analytical studies, it was found that the boundary layer on an aircraft could provide significant shielding from propeller noise at typical transport airplane cruise Mach numbers. In this paper, a new three-dimensional theory is described that treats the combined effects of refraction and scattering by the fuselage and boundary layer. The complete wave field is solved by matching analytical expressions for the incident and scattered waves in the outer flow to a numerical solution in the boundary-layer flow. The model for the incident waves is a near-field frequency-domain propeller source theory developed previously for free-field studies. Calculations for an advanced turboprop (prop-fan) model flight test at a Mach number of 0.8 show a much smaller than expected pressure amplification at the noise directivity peak, strong boundary-layer shielding in the forward quadrant, and shadowing around the fuselage. Results are presented showing the difference between fuselage surface and free-space noise predictions as a function of frequency and Mach number. Comparison of calculated and measured effects obtained in a prop-fan model flight test shows good agreement, particularly near and aft of the plane of rotation at high cruise Mach number.