Identification and attenuation of slat noise

Abstract

In the case of civil transport aircraft, engines were the dominant noise source until the advent of the high-bypass ratio engines in the early 1970s. Since then, airframe noise has become more important, particularly during the approach-to-landing stage of aircraft operations. The main components of airframe noise are the flap side edge, leading edge slat, and the landing gear. Experiments in both the wind tunnel and via fly-over measurements have shown that the slat noise is a major contributor to the overall airframe noise during the landing approach for a commercial aircraft.
To achieve the goal of reducing slat noise significantly without adversely affecting the aerodynamic performance of the wing, it is obligatory to improve the understanding of the mechanism of slat noise generation. Experiments and numerical simulations were performed to investigate the phenomena of slat noise. It was found that the slat broadband noise generation is governed by two kinds of mechanism. At a low angle of attack of the wing, the typical circulation region is not formed in the slat cove and the slat noise level is low. As the angle of attack increases to a certain value, vortical structures are intermittently generated due to flow interaction occurring between the shear layer originating from the slat cusp and the flow convected from the stagnation line on the main element. Intense slat noise is produced as the vortical structures approach the slat cove surface. With the angle of attack increasing further, the slat noise becomes weak again. The interaction effect tends to become weaker as the shear layer deviates away from the surface of the main element.
Two approaches with the aim of attenuating the slat noise were experimentally and numerically studied. The first approach was to reduce the slat noise using air blown on the suction surface of the slat near its trailing edge. A numerical simulation showed that the slat noise levels over most of the frequencies, especially above a St number of 7, were obviously attenuated. In the second approach, a strip mounted on the pressure surface of the main element model was experimentally proven to be an effective method for reducing the broadband slat noise at an angle of attack of 8 degrees and a freestream velocity of 25 m/s. The position and height of the strip also influenced the level of the reduction.
Several tonal noise components appear in the slat noise spectrum at an angle of attack of 4 degrees and a freestream velocity of 25 m/s. The dominant tone is associated with the vortex shedding off the slat cusp through the Kelvin-Helmholtz instability. This tone was successfully suppressed using a plasma actuator employing an open-loop control. A maximum reduction of 11 dB was achieved at a St number of approximately 19.7. A quasi-static feedback control system was also developed, wherein a controller is responsible for calculating the control inputs in terms of feedback signals. The experimental results show that the controller can work effectively to suppress the slat noise.

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