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An experimental proof-of-concept test was conducted to demonstrate rotor-stator interaction tone noise reduction through rotor trailing edge blowing. The velocity deficit from the viscous wake of the rotor blades was reduced by injecting air into the wake from a trailing edge slot. Composite hollow rotor blades with internal flow passages were designed based on Computational Fluid Dynamics codes modeling the internal flow. The hollow blade with interior guide vanes creates flow channels through which externally supplied air flows from the root of the blade to the trailing edge. The impact of the rotor wake-stator interaction on the acoustics was also predicted analytically. The Active Noise Control Fan, located at the NASA Glenn Research Center, was used as the proof-of-concept test bed. In-duct mode and farfield directivity acoustic data were acquired at blowing rates (defined as mass flow supplied to trailing edge blowing system divided by fan mass flow) ranging from 0.5% to 2.0%. The first three blade passing frequency harmonics at fan rotational speeds of 1700 to 1900 rpm were analyzed. The acoustic tone mode power levels (PWL) in the inlet and exhaust were reduced 11.5&–0.1, 7.2&11.4, 11.8&19.1 PWL dB, respectively. The farfield tone power levels at the first three harmonics were reduced 5.4, 10.6, & 12.4 dB PWL. At selected conditions, two-component hotwire and stator vane unsteady surface pressures were acquired. These measurements show the modification of the rotor wake due to trailing edge blowing and its effect on the stator vane to illustrate the physics behind the noise reduction.

An experimental investigation using trailing edge blowing for reducing fan rotor/guide vane wake interaction noise was completed in the NASA Glenn 9-by 15-foot Low Speed Wind Tunnel. Data were acquired to measure noise, aerodynamic performance, and flow features for a 22" tip diameter fan representative of modern turbofan technology. The fan was designed to use trailing edge blowing to reduce the fan blade wake momentum deficit. The test objective was to quantify noise reductions, measure impacts on fan aerodynamic performance, and document the flow field using hot-film anemometry. Measurements concentrated on approach, cutback, and takeoff rotational speeds as those are the primary conditions of acoustic interest. Data are presented for a 2% (relative to overall fan flow) trailing edge injection rate and show a 2 dB reduction in Overall Sound Power Level (OAPWL) at all fan test speeds. The reduction in broadband noise is nearly constant and is approximately 1.5 dB up to 20 kHz at all fan speeds. Measurements of tone noise show significant variation, as evidenced by reductions of up to 6 dB in the 2 BPF tone at 6700 rpm.: and increases of nearly 2 dB for the 4 BPF tone at approach speed. Aerodynamic performance measurements show the fan with 2 % injection has an overall efficiency that is comparable to the baseline fan and operates, as intended, with nearly the same pressure ratio and mass flow parameters. Hot-film measurements obtained at the approach operating condition indicate that mean blade wake filling in the tip region was not as significant as expected. This suggests that additional acoustic benefits could be realized if the trailing edge blowing could be modified to provide better filling of the wake momentum deficit. Nevertheless, the hot-film measurements indicate that the trailing edge blowing provided significant reductions in blade wake turbulence. Overall, these results indicate that further work may be required to fully understand the proper implementation of injecting flow at/near the trailing edge as a wake filling strategy. However, data do support the notion that noise reductions can be realized not only for tones but perhaps more importantly, also for broadband. Furthermore, the technique can be implemented without adversely effecting overall fan aerodynamic performance.Nomenclature

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