Turbulent Noise Sources Reduction by Large-Eddy Breakup Devices and Riblets

An experimental study is presented on the application of large-eddy breakup (LEBU) on a flat plate as an outer layer source-targeting device to mitigate the wall pressure fluctuations and lateral coherence length scale of a turbulent boundary-layer developed at zero pressure gradient. 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 behavior against s ', which is a normalized 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 to achieve an overall reduction in the wall pressure fluctuations. What appears to be the optimal LEBU configuration for the mitigation of wall pressure fluctuations, however, will not be reciprocated in the lateral coherence length. The combined effect would render the LEBU rather ineffective to reduce the frequency-integrated, overall noise radiation. Our previous study has confirmed that riblets, a near-wall source-targeting device, is more effective in the mitigation of the lateral coherence length. Under the principle of non-interference, a combination of riblets and LEBU demonstrates that reduction of the frequency-integrated, overall noise radiation can be achieved, again at s '>3.