Vincent P. Blandeau

Broadband Noise Due to Rotor-Wake/Rotor Interaction in Contra-Rotating Open Rotors

A semi-analytical model for the prediction of the broadband noise due to the interaction between turbulent rotor wakesandarotorincontra-rotatingopenrotorsispresented.Theunsteadyloadingoftherearrotorismodeledusing classicalisolated flat-platetheory.Striptheoryisusedtotreatthespanwisevariationsofaerodynamicquantitiesand blade geometry. The turbulent wake is assumed to be homogeneous and isotropic turbulence that is modulated by a train of wake profiles. The model is presented in detail and insight into its modal behavior is provided. A parameter study is conducted to investigate the effects of blade number, rotor–rotor gap and rotor speeds on broadband noise emissions due to rotor–wake/rotor interaction in contra-rotating open rotors. Scaling laws for sound power levels have been established analytically and show good agreement with the results of the parameter study. Nomenclature a = empirical wake parameter Bi = blade number of theith rotor bW = half-wake width, m Cd = drag coefficient of front airfoils c0 = speed of sound, m:s � 1 ci = blade chord, m cl

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

Validity of Amiet's Model for Propeller Trailing-Edge Noise

This paper presents a fundamental investigation into the validity of the classical model by Amiet for predicting the broadband trailing-edge noise due to rotating blades. This approximate model is compared analytically against a model in which the effects of rotation are treated exactly. Low- and high-frequency limits are identified, within which Amiets model is in excellent agreement with the exact solution. Estimates of the error of Amiets model outside of these limits are also provided. The method is illustrated by application to an open propeller, a model cooling fan, and a wind turbine. The use of Amiets model for treating the effects of rotation can provide substantial reductions in computation time when the unsteady blade response is computed from computationally demanding methods, such as computational fluid dynamics.

Comparison of sound power radiation from isolated airfoils and cascades in a turbulent flow

An analytical model of the sound power radiated from a flat plate airfoil of infinite span in a 2D turbulent flow is presented. The effects of stagger angle on the radiated sound power are included so that the sound power radiated upstream and downstream relative to the fan axis can be predicted. Closed-form asymptotic expressions, valid at low and high frequencies, are provided for the upstream, downstream, and total sound power. A study of the effects of chord length on the total sound power at all reduced frequencies is presented. Excellent agreement for frequencies above a critical frequency is shown between the fast analytical isolated airfoil model presented in this paper and an existing, computationally demanding, cascade model, in which the unsteady loading of the cascade is computed numerically. Reasonable agreement is also observed at low frequencies for low solidity cascade configurations.

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.

Camber effects in cascade-gust interaction noise through a simple extension of analytical models

This paper introduces a two-stagger angles method to take into account some effects of camber in cascade-gust interaction noise through a simple extension of a flat-plate based analytical model. The emphasis is put here on cascade-gust interaction tonal noise, for which camber effects are known to be significant, but the present method can be similarly used for broadband noise predictions. Numerical results from a 2D and a 3D test-cases are compared with analytical predictions with and without camber effects, and a clear improvement of the agreement is observed when the method for camber effects is used.

On the Validity of Amiet's Model for Propeller Trailing-Edge Noise

A study of the validity of the model due to Amiet for predicting the broadband trailingedge noise emitted by blade sections in rotation is presented. This widely used approximate model is analytically compared to an equivalent exact model. Conditions de ning the frequency range of validity of Amiets model are identi ed, and estimates of its error in the low and high frequency limits are provided. Using these criteria, the validity of Amiets model is tested on three practical applications : an open propeller, a model cooling fan and a wind turbine. It is shown that, despite some fundamental theoretical di erences between Amiets model and the exact model, the agreement between the two models is generally excellent at frequencies of interest.

Broadband Noise Prediction From Rotor-Wake Interaction In Contra-Rotating Propfans

A semi-analytical model for the prediction of the broadband noise due to the interaction between turbulent rotor wakes and a rotor in contra-rotating propfans (CRPs) is presented. The unsteady loading of the rear rotor is modelled using classical isolated flat-plate theory. Strip theory is used to treat the spanwise variations of aerodynamic quantities and blade geometry. The turbulent wake is assumed to be homogeneous and isotropic turbulence that is modulated by a train of wake profiles and is modelled using an analytical 3D turbulent velocity spectrum (Von Karman’s model). The model is presented in detail and insight into its modal behaviour is provided. Predictions and measurements dier by no better than 14dB. This may indicate the presence of other broadband noise sources or measurement error due to the facility.

Low and high frequency models for the prediction of noise due to cascade-turbulence interaction

Cascade theory is often used to predict the sound power radiation from a fan subject to incident turbulent flow, but in the high frequency regime it is slow due to the large number of Fourier modes required in the calculations. We demonstrate that the sound power spectrum due to a single isolated airfoil subject to the identical turbulent flow closely matches the equivalent cascade sound power spectrum at suciently high frequency. Several important dierences are apparent in the results and an understanding of these allows the identification of agreement frequencies above which the fast single airfoil theory may be substituted for the computationally expensive cascade theory with negligible dierence to the result.

Sound power due to an airfoil of arbitrary stagger angle in a turbulent flow

An analytical model of the sound power radiated from a at plate airfoil of in nite span in a 2D turbulent ow is presented. The e ects of stagger angle on the radiated sound power are included so that the sound power radiated upstream and downstream of the fan axis can be predicted, for application to the broadband noise due to turbomachinery. Closed-form asymptotic expressions, valid at low and high frequencies, are derived for the upstream, downstream and total sound power. A study of the e ects of chord length on the total sound power at all reduced frequencies is presented. The single-airfoil model presented in this paper is compared to an equivalent cascade model in a companion paper.