Search

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]

Fan noise is one of the dominant sources of aircraft engine noise, both at approach and at takeoff. Improved attenuation of fan noise with acoustic liners and the reduction of fan noise at the source remain key technology challenges for civil aviation in the foreseeable future. Over-tip-rotor (OTR) acoustic treatments have been investigated experimentally during the last decade, and significant fan noise reductions have been measured, most recently using a single rotor and multiple lined circumferential grooves. This paper describes an analytical OTR prediction model in which the fan noise is modeled with point or distributed, static or rotating monopole and dipole sources based on Green's functions for infinite hard or lined cylindrical ducts containing uniform mean flow; these are combined with an anechoic or unflanged inlet termination and an embedded finite length lined section in which the rotor-alone source is located with its OTR liner. OTR liner insertion loss predictions are obtained for comparison with data from the W-8 NASA experimental rig. These yield peak broadband in-duct noise reductions of up to 4 dB, in line with the measurements. [ABSTRACT FROM AUTHOR]

The reduction of fan broadband noise in the next generation of ultrahigh-bypass-ratio engines remains a key technology challenge for the foreseeable future. The overtip-rotor liner concept has been studied as a technology with the potential to further reduce fan noise, and significant noise reductions have been measured. This paper describes a fundamental experimental evaluation that represents the overtip-rotor liner as a static airfoil with its tip located over a flat plate containing a flush-mounted lined insert and separated from the airfoil tip by a small gap. Differences in measured far-field sound spectra and in source power estimates derived from postprocessed spiral array data have shown broadband gap noise reductions of 5-10 dB, even in the absence of a tip gap. Overtip liners are found to suppress noise sources when located in the immediate vicinity, irrespective of the generation mechanism, mainly due to backreaction effects on the source. An analytical prediction model for the overtip liner noise reduction, based on a point source located over an infinite lined plane, is evaluated and compared with the experimental data. The model captures the backreaction effects and provides qualitative agreement with the measured data, including the variation of noise reduction with increasing gap size. [ABSTRACT FROM AUTHOR]

Experimental investigation of Over-Tip-Rotor circumferential groove liners has shown potential for fan noise suppression in turbofan engines whilst providing minimal penalty in fan aerodynamic performance. The validation of Over-Tip-Rotor liner analytical prediction models against published experimental data requires the modelling of an equivalent impedance for such acoustic treatments. This paper describes the formulation of two analytical groove impedance models as semi-locally reacting liners, that is locally reacting in the axial direction and non-locally reacting in the azimuthal direction. The models are cross-verified by comparison with high-order FEM simulations, and applied to a simplified Over-Tip-Rotor configuration consisting of multiple grooves excited by a monopole point source located close to the grooved surface. [ABSTRACT FROM AUTHOR]

This paper is a predominantly experimental study into the use of porosity located downstream of an aerofoil leading edge for the reduction of turbulence interaction noise. Locating the porosity downstream of the leading edge has been shown to be beneficial in reducing the aerodynamic performance penalty compared with locating it directly at the leading edge (Ocker et al., 2021), where most of the lift is generated. Noise measurements on a flat plate with downstream porosity are compared against the case of two flat plates in a tandem configuration. In both cases, the noise reduction spectra exhibit peaks of strong noise reduction at non-dimensional frequencies of f l d / U c = n , where l d is the distance between the leading edge and the downstream edge, U c is the convection velocity and n is an integer. To explain this behaviour requires a mechanism to be present in which a phase shift of 180 ∘ is introduced in the interaction process. In the paper we argue that the origin of this phase shift is due to secondary vorticity generated at the leading edge. Another key finding of this paper is that overall noise reductions are due to an effective shortening of the chord in which most of the radiation is produced by the section of the flat plate upstream of the porous section, leading to generally weaker radiation. Neither of these mechanisms have been reported previously in the literature. The paper concludes with noise measurements on a thin aerofoil with downstream porosity included, in which overall noise reductions of up to 2.8 dB are achieved. • Downstream porosity shows similar behaviour to two flat plates in a tandem configuration. • Up to 2.8 dB reductions in overall noise are observed on realistic aerofoil geometries. • Edge-to-edge interference is present with 180deg phase inversion between the edges. • The envelope of noise reductions is releted to an effective shorter chord. [ABSTRACT FROM AUTHOR]

This paper focuses on modelling and predicting the acoustic field generated by ducted point sources in close proximity to an acoustic liner. Two analytical models are presented. The first model comprises a point monopole or dipole source in an infinite lined duct based on an existing Green's function. The predictions are compared with classic solutions for a source over an infinite lined plane and with a high frequency asymptotic duct approximation. The second model extends the Green's function to include a liner section of finite length connected to hard-wall extensions by using mode-matching techniques. The new model features the inclusion of point sources in the vicinity of an impedance discontinuity. The accuracy of this model is demonstrated by comparison with reference FE solutions. Both models indicate a significant impact of the source proximity to the liner surface in the source power output. These models offer insight on the source modification effects beyond the conventional approach in the design of liners solely based on acoustic absorption. [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]

Propellers usually operate in non-uniform flow conditions, leading to significant levels of noise generation due to interaction with the leading edges of the propeller blades. Under these flow conditions, the noise generated can be substantially different from the rotor-alone self-noise generated under uniform mean flow conditions. This paper presents an experimental investigation aimed at studying the far-field noise due to a two-bladed propeller immersed in a controlled non-uniform flow with known characteristics. This paper presents the results of a parametric study with different degrees of flow non-uniformity generated from different geometric obstructions located in the jet nozzle, while maintaining constant thrust. Measurements of the radiated noise show that the incoming mean flow non-uniformities under investigation in this study cause a significant increase in tonal amplitudes of the blade passing frequencies of harmonic orders greater than 2. At the first two blade passing frequencies the tones are dominated by the rotor-alone 'Gutin-type' tones resulting from the steady blade loading. The measured noise spectra are compared with predictions from a simple frequency-domain flat plate model. The analytical model will be shown to capture the main features of the interaction tonal noise spectrum and directivity. • Propeller noise. • Interaction tones. • Tonal noise. • Broadband noise. [ABSTRACT FROM AUTHOR]