Reduction of Flap Side-Edge Noise: Passive and Active Flow Control

Abstract

Microphone array and particle image velocimetry measurements have been performed to investigate the potential of passive and active flow control methods for flap noise reduction. The extended wing flap of the swept constant chord half-model was equipped with either a blowing facility as an active flow control device or with other flap side-edge modifications such as wing tip fences, microtabs and winglets. The flap side edge noise is reduced with the blowing configuration between 2 kHz and 5 kHz. The maximum noise reduction of 15:9 dB is achieved at 2:9 kHz. Varying the diameter of the blowing orifices shows that the noise reduction is governed by the momentum rather than the flow rate of the blowing. The winglets and the suction side fence are most effective in reducing the flap side-edge noise. The reduced level with these configurations is even lower than with the active blowing. The vortex generators and the pressure side fence turn out to be the least effective flow control devices. PIV-measurements at an isolated unswept flap model show that the main vortex on the suction side exhibits a very inhomogeneous vorticity distribution which can be attributed to the unstable shear layer that separates at the lower corner of the flap side-edge and to the negative vorticity production on the suction surface. With blowing the vorticity of the shear layer is concentrated in several small vortices. The distance between those vortices and the solid surface increases with higher blowing momentum and because of this the negative vorticity production due to recirculation is disappears. These distinct modifications of the side-edge flow field explain the noise reduction shown in the aeroacoustic measurements.