R. Jeremy Astley

Validation and Application of a Hybrid Prediction Scheme for Bypass Duct Noise

A prediction scheme for noise propagation and radiation from aeroengine bypass ducts is validated against measured data and applied to a generic bypass duct to study the efiects of bypass duct geometry on modal scattering. The objective has been to develop a relatively simple scheme which can predict the efiects of bypass duct geometry and acoustic treatment at acceptable computational cost. To achieve this, the in-duct and radiation problems are uncoupled and separate methods applied to each. Two difierent radiation models are implemented and compared. The approach is validated by a comparison of predicted and measured data for the NASA ANCF (Active Noise Control Fan) ∞ow test rig. A well deflned interference tone is used as a source. The rig is of simple geometry but incorporates the efiects of mean ∞ow and refraction through the bypass shear layer. Numerical predictions for aft radiated noise are shown to be in a good agreement with measured data, and both radiation models are self-consistent except at small angles from the aft axis. This persists at higher frequencies for which measured data are not available. The model is also used to study in-duct geometry efiects on modal scattering and attenuation in a generic lined bypass ducts. I. Introduction The prediction of aft radiated fan noise from turbofan aero-engines is challenging for conventional numerical schemes, more so than for the corresponding intake problem. The major complicating factor is the presence of a shear layer between the bypass stream and the external ∞ow. This precludes the straightforward use of frequency-domain flnite and inflnite element (FE/IE) analysis, 1 an approach which has proven efiective for intake problems. 2 This approach, and others based on the solution of a convected wave equation for the acoustic potential, cannot readily be applied to exhaust problems due to the presence of irrotational mean ∞ow. 3,4 High order schemes based on the numerical solution of the Linearised Euler Equations (LEE) in the time domain have however been applied both to intake and exhaust problems . 5,6 The intrinsic instability of the shear layer gives rise however to unstable transient solutions in the linearised Euler solution for exhaust ∞ows which must be removed or flltered from the numerical solution. 7{9 Agarwal et al. 10 have demonstrated that this instability can be suppressed if the governing equations are solved by a direct solver in the frequency domain. Parabolic approximations, 11 have also been applied to both intake and exhaust problems, 12,13 but do not deal well with propagation at high mode angles. In all of the above numerical schemes, the discrete solution is terminated by a non-re∞ecting condition which must be imposed at a flnite boundary. This must be placed close to the near fleld if the overall problem size is to be contained within acceptable limits. This constraint is particularly demanding in cases where a full three-dimensional numerical model is required. In the current paper, a hybrid modal/numerical/analytic prediction scheme for the noise propagation and radiation from aeroengine bypass ducts is presented, validated against ∞ow data and applied to a generic lined bypass duct. The objective of the current approach has been to develop a scheme which can predict the efiects of bypass duct geometry and acoustic treatment at acceptable computational cost. To achieve this, the in-duct and radiation problems are uncoupled and treated separately. The in-duct problem is treated by a Finite Element (FE) scheme. The radiation problem is treated using two approximate methods. An outline of the method was presented at the 11th AIAA/CEAS Aeroacoustics conference 14 and numerical predictions

An integral formulation for wave propagation on weakly non-uniform potential flows

Abstract An integral formulation for acoustic radiation in moving flows is presented. It is based on a potential formulation for acoustic radiation on weakly non-uniform subsonic mean flows. This work is motivated by the absence of suitable kernels for wave propagation on non-uniform flow. The integral solution is formulated using a Green׳s function obtained by combining the Taylor and Lorentz transformations. Although most conventional approaches based on either transform solve the Helmholtz problem in a transformed domain, the current Green׳s function and associated integral equation are derived in the physical space. A dimensional error analysis is developed to identify the limitations of the current formulation. Numerical applications are performed to assess the accuracy of the integral solution. It is tested as a means of extrapolating a numerical solution available on the outer boundary of a domain to the far field, and as a means of solving scattering problems by rigid surfaces in non-uniform flows. The results show that the error associated with the physical model deteriorates with increasing frequency and mean flow Mach number. However, the error is generated only in the domain where mean flow non-uniformities are significant and is constant in regions where the flow is uniform.

Acoustic propagation on irrotational mean flows using time-domain finite and infinite elements

A formulation is presented for the computation of transient wave fields propagating on unbounded, irrotational mean flows. The numerical scheme is based on an inner mesh of conventional finite elements, which is matched to high-order multipole infinite elements in the far field. The scheme is validated against simple transient test solutions and shown to be effective for large three-dimensional problems provided that iterative solvers are used to advance the solution at each time step.

Acoustic Effects of Liner Damage on Zero-Splice Turbofan Intake Liners: Computational Study

Traditional installations of turbofan intake liners include acoustically “hard” axial splices between liner segments for ease of fabrication and assembly. The splices scatter energy from strong rotor-locked tones into adjacent azimuthal orders for which the liner is less effective, thereby degrading the liner performance. The significance of this “splice effect” has led to the adoption of “zero-splice” liners in recent turbofan nacelles. However, damage can occur to such liners in service, and the extent to which local liner repairs reduce the effectiveness of the zero-splice design then becomes an issue. In the current paper, the acoustic effect of damage and repair in a zero-splice liner is simulated numerically. The effects of the extent and the location of a hard patch, representing the liner damage and repair, on the overall performance of the liner are predicted. A close agreement is demonstrated with results from an asymptotic analytical model valid for small patch widths. An approximate method is ...

Integrating CFD source predictions with time-domain CAA for intake fan noise prediction

A method is proposed for integrating a source prediction obtained from a Computational Fluid Dynamics (CFD) model for the fan stage of a turbofan engine with a Computational Aero-Acoustics (CAA) propagation code to predict tonal noise radiation in the far field. The Reynolds-Averaged Navier–Stokes equations are used to model the generation of the tones. Their propagation through the intake is simulated by applying the Discontinuous Galerkin Method to solve the linearized Euler equations in the time domain. The CFD and the CAA solutions are matched in a region where both solutions overlap and where non-linear effects, important close to the fan, can be considered to be less significant. An equivalent modal source on a notional source plane behind the fan is used to duplicate the sound field in this matching region and is then to drive a fully three-dimensional CAA radiation model for a near-field acoustic solution. The far-field sound pressure is obtained by applying the Ffowcs Williams-Hawkings formulation on a porous surface within the CAA domain. The accuracy and efficiency of this approach are investigated and results obtained are compared to measured data from a fan rig.

On the Prediction of the Effect of Interstage Liners in Turbofan Engines: A Parametric Study

The current trends for next generation turbofan engines are towards shorter nacelles and increased distances between the fan and the outlet guide vanes. This leads to an overall reduction in lined surface areas as well as an increase in the relative importance of the interstage liner, which is the liner placed between the rotor blades and the stator vanes. The interstage is different in that the liner is subject to a mean flow with a strong swirl component and shear. This project will contribute to understanding and predicting the effect of the swirl on liner attenuation and consists of 4 steps: To model an eigenvalue problem that includes sheared and swirling mean flows and acoustic absorption, to develop a code based on this eigenvalue problem and to validate it, to compare results from this code with experimental results and to carry out a parametric study to evaluate how the swirling flow afects liner attenuation and optimum impedance. Two models were developed. The first one considers a ducted sheared mean flow and is based on the Pridmore-Brown equation and the second one takes into consideration a mean flow with swirl and shear and is based on the Linearized Euler Equations. For both cases an eigenvalue problem was obtained by applying the normal mode analysis to the governing equations together with the impedance boundary condition. Both models were discretized using a Finite Difference Method. The codes were exhaustively validated against predicted values obtained by other methods for uniform, sheared and swirling mean flows and hard-walled and lined ducts. The swirling mean flow, when present, is a combination of rigid body and vortex swirl. A cross-validation between the Finite difference code based on the Linearized Euler Equation and the JM66 code from Rolls-Royce was carried out for a more realistic case. Axial wavenumbers and pressure and velocity eigenvectors obtained with the JM66 code were compared with the current predictions. A comparison has been conducted of predictions from the current Finite Difference code with measured data at a single frequency for a range of spinning mode numbers. Qualitative agreement is obtained for the measured Power Transmission loss (TL) but the low Mach numbers and modest TL levels meant that the effect of swirl was small nad it was difficult to validate the accuracy at the Finite Difference code specifically for the swirl case. Finally, a parametric study was undertaken for hard-walled and lined ducts for realistic interstage conditions to evaluate the effect sound propagation in swirling flows. This confirmed that the effect of swirl is higher for radial modes near cut-off and tends to vanish for higher radial mode orders. The swirl strongly changes the modal content. When swirl is included, the modal distribution for positive and negative azimuthal mode orders is no longer symmetrical. The higher the swirling flow magnitude, the more the modal content is shifted to negative circumferential mode orders. Co-rotating modes become more cut-on and contra-rotating modes become more cut-off. When acoustic absorptive liners are considered, the swirl changes the liner optimum resistance and reactance and affects the optimum insertion loss. The optimum resistance becomes considerably higher and the change in optimal liner reactance is not as pronounced. The swirling flow also reduces attenuation; the insertion loss is lower when swirl is considered. As a conclusion, swirling flow should be considered when designing liners.

Folded cavity liners for turbofan engine intakes

Reducing aircraft noise is crucial to the growth of air transport and quality of people’s lives. Fan noise propagates through engine intake and bypass ducts and is radiated to the far eld. It is a major contributor to aircraft noise at take-o and landing. Acoustic liners are installed on the internal walls in the engine ducts to attenuate the fan noise. They are typically single or double degree of freedom liners, and their acoustic performance is strongly dependent on the cell depth. Typical cell depths are selected to attenuate noise in the frequency range which is critical to community noise. Increasing the attenuation at low frequencies requires deeper cells, which is often prohibited by mechanical design constraints. A potential remedy would be to fold the cells to t in a shallow space. In this study, the design and performance of folded cavity liners for turbofan engine intakes are investigated. A nite element model was developed for predicting acoustic impedance of such liners. The numerical model was used for parametric studies and validated against test data. The acoustic performance of folded cavity liners when installed in the nacelle of a rig intake was predicted by using a nite/innite element analysis with the predicted acoustic impedance of a folded cavity liner and compared to the noise reduction by an optimised community noise liner. It is demonstrated that folded cavity liners work as a combination of a deep and shallow liners corresponding to the centreline length and the hight to the fold. The performance can be tuned and optimised to be eective over wide frequency range by selecting the geometry and properties of resistive layers, i.e., facing and septum sheets. Folded cavity liners have a great potential to provide a solution for reducing both mid-high frequency community noise as well as low-frequency noise.

A numerical study on multimode sound propagation in lined ducts and radiation to the far field

In previous articles, the authors developed a hybrid scheme for analysing bypass duct noise, in which a numerical analysis using finite element method for in‐duct propagation and an analytic radiation code with fully represented effects of bypass shear layer are coupled. Such procedure permits detailed study on the interaction between duct configurations, such as geometry and acoustic liner impedances, and modal propagation and attenuation, and also the effects on the radiation pattern, within practical timescale and at modest computational cost. The scheme has been applied to realistic aero‐engine bypass ducts and has been integrated with an optimisation programme. The numerical results obtained so far have revealed that for ducts with acoustic liners highly attenuated modes are not necessarily those with high mode angles, which is contrary to general anticipation. The aim of the study in the current paper is to understand the physics behind this phenomenon and its effect on the radiation to the far fiel...