Perturbation of Turbulent Boundary Layer by Outer Layer and Inner Layer Devices for Turbulent Noise Source Reduction

Abstract

An application of the Large Eddy BreakUp (LEBU) as a source-targeting device to mitigate the wall pressure fluctuations and lateral coherence length scale of a turbulent boundary layer is investigated. Both represent the prominent noise sources for the trailing edge noise radiation. When a LEBU is placed strategically at the outer part of a turbulent boundary layer, the wall pressure spectra can establish a self-similar behaviour against s′, which is a normalised separation distance between the LEBU’s trailing edge and the targeted location for noise mitigation. It is found that s′ has to be greater than 3 in order to achieve an overall reduction in the wall pressure fluctuations. However, this criterion does not apply to the lateral coherence length. The combined effect would reduce the effectiveness of the LEBU to abate the frequency-integrated, overall noise radiation. Under the principle of non-interference, a joint-implementation of RIblets and LEBU (RIBU) can produce promising results where reduction of the frequency-integrated, overall noise radiation can be achieved, again at s′ > 3.