Reduction of tip-leakage noise by using over-tip liners

Abstract

Over-Tip-Rotor (OTR) liners have been investigated over the last decades as a technology to further reduce fan broadband noise in turbofan engines. It has been found that Over-Tip-Rotor liners suppress noise by conventional attenuation of acoustic waves and source modification effects. This paper describes a fundamental experiment to determine the balance between these two mechanisms and to establish whether the source modification is purely due to acoustic back-reactions or also changes the source itself. The Over-Tip-Rotor liner configuration is approximated by a static aerofoil 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 surface pressure fluctuations on the aerofoil tip have shown that the modification of the source near-field by the liner is the dominant noise reduction mechanism on the pressure side of the aerofoil. It is shown that over-tip liners modify the unsteady pressure fluctuations on the aerofoil tip and that those are correlated with the far-field noise reductions. Over-tip liners are also found to alter the unsteady flow field in the gap region by generally reducing the turbulent intensity close to the liner surface. This study has shown that, although far-field noise predictions based on analytical models are useful to estimate the performance of over-tip liners, a complete assessment must also include the impact of the liner on the tip-leakage flow and hence the tip noise sources.

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Identifiers

ORCID for Sergi Palleja Cabre: orcid.org/0000-0002-5414-7716

ORCID for Suresh Palani: orcid.org/0000-0003-0123-7635

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