Blade-Vortex Interaction Noise Reduction by Active Trailing-Edge Flaps

The feasibility of utilizing active trailing-edge flaps to minimize the unsteady loading at selected spanwise control points, and thus the resulting acoustics, is examined for isolated blade-vortex interactions generated by a separate vortex generator. Unsteady indicial aerodynamics is used in conjunction with the acoustic analogy. The prediction tool is validated against experiment and computational fluid dynamics for a simple rectangular blade undergoing a parallel interaction at 180 degrees of azimuth. Good agreement is obtained for the surface airloads; while excellent agreement is obtained for the acoustic time histories. Similar comparisons with computational fluid dynamics are obtained for an oblique interaction. The aspect ratio of the blade and the effective miss distance of the vortex are key parameters in determining the required flap actuation frequencies and rates. Although discrete finite-span flaps result in only partial cancellation of the unsteady loads away from the control points, significant noise reduction can be obtained for both parallel and oblique interactions if multiple spanwise flaps are utilized (7-10 dB). For close interactions, complete cancellation may not be feasible due to excessive flap rates and extreme sensitivity to phasing errors. However, partial cancellation and reduced sensitivity can be obtained for reduced flap rates. The chordwise extent of the flap should be chosen as a compromise between the pitch rates and the resulting pitching moments. Additional experimental and/or computational work is warranted to validate finite-span trailing-edge flap aerodynamics and acoustics for high-frequency actuation.