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Over-Tip-Rotor (OTR) liners have been investigated over the last decades as a technology to further reduce fan broadband noise in turbofan engines. The suppression of noise with OTR liners is attributed to conventional attenuation of acoustic waves and source modification effects. This paper describes a fundamental experiment to gain a better understanding of the source modification effects and establish whether they are purely due to acoustic back-reactions or also due to hydrodynamic changes on the source. The OTR liner configuration is approximated by a static airfoil with its tip located over a flat plate containing a flush-mounted liner insert and separated from the airfoil tip by a small gap. Synchronous measurements of the far-field noise and wall pressure fluctuations on the airfoil tip have shown that the reduction of wall pressure at the airfoil tip by the liner is the dominant mechanism of noise reduction. The reduction in unsteady pressure fluctuations on the airfoil tip by the liner is mainly caused by back-reaction effects at high frequencies and hydrodynamic modifications at low- and mid-frequencies. Over-tip liners are found to alter the unsteady flow field in the gap region and weaken the flow structures responsible for the generation of tip noise. This study has shown that far-field noise predictions based on analytical models are useful to estimate the performance of over-tip liners, but a complete assessment should also include the impact of the liner on the tip-leakage flow, and, consequently, the sources of tip noise. [ABSTRACT FROM AUTHOR]

The application of acoustic liners near/directly over a sound source has gained significant interest for the excess noise reduction achieved with Over-the-Rotor (OTR) liners compared to the conventional liner installations at the intake of an aero-engine. However, the mechanism of noise reduction achieved in the OTR liners is not clearly understood. This paper aims to explain this mechanism by considering a static monopole source placed over a finite liner insert with a zero background mean flow. This has been investigated numerically using COMSOL Multiphysics in a half-space domain and compared with reference analytical solutions for infinite lined walls. One of the key findings of the paper is the underlying physics of the source modification mechanism, which has been found to be the interference between the primary noise source and a secondary noise source forming on the liner surface. It is identified through an optimal impedance study that this back-reaction mechanism is dominant when the source is located for a normalised tip gap, e / λ < 0.25 , where e is the distance between the source and the liner surface and λ is the acoustic wavelength. Within this region, there exists an optimum normalised liner length, L / e providing a maximum insertion loss. [ABSTRACT FROM AUTHOR]