Michael Kingan

Relative importance of open rotor tone and broadband noise sources

A study is made of the noise levels and spectral characteristics of three contra-rotating propeller rigs: rig 140 tested in 1989, rig 145 build 1 tested in 2008, and rig 145 build 2 tested in 2010. We use tone deletion techniques, applied to the inflow microphone data, to show the relative importance of propeller broadband noise to propeller tones with increasing frequency and, in particular, that by the time we reach only moderate frequencies, the one third octave spectra become dominated by the broadband noise components. We also show that the broadband noise continues to be important as blade speed and rig thrust are varied and that these spectral characteristics are present on both modern and older contra-rotating propeller designs – even those with a profusion of tones and strong tone protusion. We also show how the tone and broadband noise levels have reduced with more recent, and aeroacoustically improved, blade designs

Effect of Centerbody Scattering on Advanced Open-Rotor Noise

Formulas for calculating the effect of centerbody scattering on the sound radiated from an advanced open rotor are presented. The effects of blade sweep and distributed blade loading are considered.Mach number effects are also implicitly included in the model. The work extends a previously published method and applies it to a practical situation in which scattering by the centerbody has a significant effect on the radiated sound field.

Broadband noise predictions from uninstalled contra-rotating open rotors

This paper presents broadband noise predictions from uninstalled contra-rotating open rotors (CRORs). CRORs are being increasingly investigated as an alternative to single propellers and turbofan engines for power plants on aircraft, since they yield a significant reduction in fuel burn for short-haul flights. However, there is the need to develop schemes by which CROR noise can be predicted and reduced. Using semi-analytical prediction schemes, the principal broadband noise source mechanisms (rotor trailing edge noise and rotor-rotor interaction noise) are investigated, leading to strategies for their mitigation. For the configuration considered, trailing-edge broadband noise is predicted to be dominant at low rotor loading conditions (typical of cruise and approach) whereas rotor-rotor interaction broadband noise is predicted to be dominant at high rotor loading conditions (typical of take-off). A systematic parameter study is also presented in which the dependence of CROR broadband noise on the rotor-rotor gap, rotor speeds and blade numbers are investigated at constant engine power, torque split and solidity.

Counter-rotation propeller tip vortex interaction noise

Counter-rotation propeller tip vortex interaction noise occurs when tip vortices, shed from each of the upstream propeller blades, interact with the blades on the downstream propeller. This paper describes two separate models for calculating the noise produced by this interaction. The first model approximates each tip vortex as a helical vortex tube of infinite extent while the second is based on a previously published study and uses a twodimensional approximation to model the tip vortex interaction and represents the velocity field induced by the tip vortices as a piecewise function for which the flow incident on the downstream blade row must be calculated numerically. The unsteady loading on the downstream propeller blades is determined from the incident flow predicted using either of the models and is used to calculate the radiated sound field using an analytic propeller noise formula. Nomenclature B = Number of blades on the propeller i C = Constant with value between 1 and 3 L C = Lift coefficient vtx K = Constant with value between 0.5 and 2 L = Streamwise length from upstream blade mid-chord to the downstream blade leading edge x

Prediction of sound transmission through, and radiation from, panels using a wave and finite element methoda)

This paper describes the extension of a wave and finite element (WFE) method to the prediction of noise transmission through, and radiation from, infinite panels. The WFE method starts with a conventional finite element model of a small segment of the panel. For a given frequency, the mass and stiffness matrices of the segment are used to form the structural dynamic stiffness matrix. The acoustic responses of the fluids surrounding the structure are modelled analytically. The dynamic stiffness matrix of the segment is post-processed using periodic structure theory, and coupled with those of the fluids. The total dynamic stiffness matrix is used to obtain the response of the medium to an incident acoustic pressure. Excitation of the structure by oblique plane waves and a diffuse sound field are considered. The response to structural excitation and the consequent radiation are determined. Since the size of the WFE model is small, computational times are small. Various example applications are presented to illustrate the approach, including a thin isotropic panel, an antisymmetric, cross-ply sandwich panel and a symmetric panel with an orthotropic core.

Analysis of Advanced Open Rotor Noise Measurements

This paper presents an analysis of the acoustic data collected during a series of tests of a model-scale advanced open rotor installed within the DNW low-speed wind tunnel. There is an overview of the tests and a description of the computational, numerical and analytical methods for predicting both tone and broadband noise levels of advanced open rotors. The measured data is discussed and assessed via comparisons with predictions and through analyses of tone radiation efficiencies. Comparisons between predictions and measured data are analysed in terms of the narrowband and one third octave noise levels, and in terms of both the tonal and the broadband contributions. It is shown that fast, analytical prediction methods are sufficiently accurate for both rotor-alone and interaction tones provided that the required inputs are obtained from a high-fidelity steady aerodynamic calculation. The effects of centerbody scattering and operation at angle of attack on measurements and predictions are also investigated. The paper concludes with a discussion of broadband noise showing, first, that broadband noise dominates the sound field at mid to high frequencies and, second, that there is some disagreement between current low-fidelity analytical methods for broadband noise and the measured broadband noise levels.

Speech enhancement using a microphone array mounted on an unmanned aerial vehicle

An attempt to record speech signals using an array of directional microphones carried by an unmanned aerial vehicle (UAV) is reported. The speech enhancement algorithm estimates the power spectral densities (PSD) of the target speech and rotor noise which are used to calculate a Wiener filter for extracting the target speech. The calculated Wiener filter is then applied to the output of a beamforming implemented on the microphone array signals. The proposed algorithm was tested on a prototype UAV equipped with directional microphones each of which was orientated in different directions. The experimental results revealed that the proposed method was able to significantly reduce both the UAVs rotor noise and an interfering speech in a recording.

Sound radiation of fan tones from an installed turbofan aero-engine: fuselage boundary-layer refraction effects

A distributed source model to predict fan tone noise levels of an installed turbofan aero-engine is extended to include the refraction effects caused by the fuselage boundary layer. The model is a simple representation of an installed turbofan, where fan tones are represented in terms of spinning modes radiated from a semi-infinite circular duct, and the aircrafts fuselage is represented by an infinitely long, rigid cylinder. The distributed source is a disc, formed by integrating infinitesimal volume sources located on the intake duct termination. The cylinder is located adjacent to the disc. There is uniform axial flow, aligned with the axis of the cylinder, everywhere except close to the cylinder where there is a constant thickness boundary layer. The aim is to predict the near-field acoustic pressure, and to quantify the effect of the boundary layer on the sound pressure levels on the cylinders surface. Thus no far-field approximations are included in the modelling. The sound propagation through the boundary layer is calculated by solving the Pridmore-Brown equation. Results from the theoretical method show that the boundary layer has a significant, nuanced effect on the sound pressure levels on the cylindrical fuselage, owing to sound radiation of fan tones from an installed turbofan aero-engine.

Near-field sound radiation of fan tones from an installed turbofan aero-engine

The development of a distributed source model to predict fan tone noise levels of an installed turbofan aero-engine is reported. The key objective is to examine a canonical problem: how to predict the pressure field due to a distributed source located near an infinite, rigid cylinder. This canonical problem is a simple representation of an installed turbofan, where the distributed source is based on the pressure pattern generated by a spinning duct mode, and the rigid cylinder represents an aircraft fuselage. The radiation of fan tones can be modelled in terms of spinning modes. In this analysis, based on duct modes, theoretical expressions for the near-field acoustic pressures on the cylinder, or at the same locations without the cylinder, have been formulated. Simulations of the near-field acoustic pressures are compared against measurements obtained from a fan rig test. Also, the installation effect is quantified by calculating the difference in the sound pressure levels with and without the adjacent cylindrical fuselage. Results are shown for the blade passing frequency fan tone radiated at a supersonic fan operating condition.

A Method for Predicting the Tone Noise Produced by an Open Rotor in a Rectangular Wind Tunnel

This paper describes an analytical model for predicting the tonal noise produced by an open rotor situated in a rectangular wind tunnel with rigid walls. The mathematical derivation of the model makes use of Graf’s theorem to transform from Cartesian coordinates, which are useful for describing the duct geometry, to cylindrical coordinates, which are useful for describing the distribution of ‘sources’ on the rotating blades. This transformation greatly simplifies the analysis and provides a final expression which is relatively straightforward to evaluate. An interesting validation of the model is also presented where it is shown that the final expression is equivalent to that produced by an infinite series of ‘image sources’. The model is then used to compare sound pressure levels produced by an open rotor operating within a rigid walled wind tunnel to those which would be produced if the rotor was operating in a free-field environment. The motivation for this work was to assess how measurements made in a closed test section wind tunnel could be used to infer noise levels in a free-field environment.