Xiaoxian Chen

Computation of spinning modal radiation from an unflanged duct

The radiation of high-order spinning modes from an unflanged duct with or without a mean flow is studiednnumerically. The application is to noise radiation from the intake duct of an aircraft engine. The numericalnmethod is based on solutions of the linearized Euler equations (LEE) for propagation in the duct and near fieldnand the acoustic analogy for far-field radiation. A formulation of the LEE is used for a single azimuthal mode,nwhich offers an advantage in terms of computational efficiency: in the case of an axisymmetric mean flow-field,nthis model reduces computing costs connected with a three-dimensional model. In the solution process, acousticnwaves are admitted from upstream into the propagation area surrounding the exit of an axisymmetric duct andnthe region immediately downstream. The wave admission is realized through an absorbing nonreflecting boundaryntreatment, which admits incoming waves and damps spurious waves generated by the numerical solutions. Thenwave propagation is calculated using a high-order compact scheme. Far-field directivity is estimated by solvingnthe Ffowcs Williams–Hawkings equations. The far-field prediction is compared with analytic solutions, and goodnagreement is found.

Efficient Computation of Spinning Modal Radiation Through an Engine Bypass Duct

The aim of this work is to accurately and efficiently predict sound radiation out of a duct with flow. The soundnpropagation inside a generic engine bypass duct, refractions by the shear layer of the exhaust flow, and propagationnin the near field are the main focus of the study. The prediction uses either a modified form of linearized Eulernequations or an alternative model based on acoustic perturbation equations, which were extended to cylindricalncoordinates. The two models were compared on a canonical case of sound propagation out of a semi-infinite duct withnflow. Good agreements between the predictions were achieved. The more general case of a generic aircraft enginenbypass duct with flow was then investigated with the technique of adaptive mesh refinement to increase thencomputational efficiency. The results show that both linearized Euler equations and acoustic perturbation equationsnmodels can predict the near-field sound propagation and far-field directivity. The acoustic perturbation equationsnmodel, however, is more adaptive for its suitability to an arbitrary background mean flow.

Sound Radiation from a Bypass Duct with Bifurcations

The influence of bifurcations in an aeroengine bypass duct on noise radiation was investigated through high-orderaccurate three-dimensional numerical simulations. The physical process was described by a set of acoustic perturbation equations. Four bifurcation arrangements with an airfoil cross section were regularly placed in circumferential direction. Results were compared with those of an axisymmetric duct case without the installation of bifurcations. Both near-field propagation and far-field radiation were studied to show that the bifurcations scatter acoustic modes and redirect sound propagation levels. Compared to the axisymmetric duct case, there are a couple of decibel increases in sound pressure level on some angles due to the installation of bifurcations. The method is also applicable to other configurations with different bifurcations and modes

Acoustic radiation from a semi-infinite duct with a subsonic jet

The radiation of high-order spinning modes from a semi-infinite exhaust duct is studied numerically. The issues involved have applications to noise radiation from the exhaust duct of an aircraft engine. The numerical method is based on solutions of linearised Euler equations (LEE) for propagation in the duct and near field, and the acoustic analogy for far field radiation. A 2.5D formulation of a linearised Euler equation model is employed to accommodate a single spinning mode propagating over an axisymmetric mean flow field. In the solution process, acoustic waves are admitted into the propagation area surrounding the exit of an axisymmetric duct and its immediate downstream area. The wave admission is realised through an absorbing non-reflecting boundary treatment, which admits incoming waves and damps spurious waves generated by the numerical solutions. The wave propagation is calculated through solutions of linearised Euler equations, using an optimised prefactored compact scheme for spatial discretisation. Far field directivity is estimated by solving the Ffowcs Williams-Hawkings equations. The far field prediction is compared with analytic solutions with good agreement.

Computation of Fan Noise Radiation through an Engine Exhaust Geometry with Flow

This paper outlines a computational model of noise radiation from a realistic engine exhaust geometry with flow. The computational model described allows acoustic waves, propagating inside the bypass duct of a generic aircraft engine, to be admitted into a computational domain that includes the aft duct section, the exit plane of the duct, and the jet flow immediately downstream. The method has three parts: a matching process to admit acoustic waves into the induct propagation region; near field propagation inside the duct and diffraction at the lip of the exhaust duct; and an integral surface for far field directivity. In this model the near field propagation is determined by a numerical solution of a 2.5D form of the linearised Euler equations. The mean flow about which the equations are linearised is assumed to be axisymmetric. The proposed method is illustrated through a case study on the radiation of a typical fan assembly generated acoustic wave from a generic engine bypass duct. Inside the duct, an acoustic wave of circumferential order m = −13 and comprising five radial modes (n = 1 – 5) is admitted into the model as inputs on the boundary of the computation domain. The radiation of the acoustic wave through the exhaust geometry and mean flow is determined, with the effect of acoustic treatment through the inclusion of lined duct sections also examined.

Synthetic turbulence methods for leading edge noise predictions

An advanced digital filter method to generate synthetic turbulence is presented for efficient two- and three-dimensional leading edge noise predictions. The technique, which is based on the Random Particle-Mesh method, produces a turbulent inflow that matches a target isotropic energy spectrum. The discretized equations for the synthetic eddies, and the input parameters needed to recover the desired turbulence statistics, are presented. Moreover, a simple and fast implementation strategy, which does not require an additional boundary condition, is presented under the frozen turbulence assumption. The method is used in a linearized Euler solver to predict turbulence-airfoil interaction noise from a number of configurations, including variations in airfoil thickness, angle of attack and Mach number. For the first time, noise predictions from a digital filter method are directly compared to those provided by synthetic turbulence based on a summation of Fourier modes. The comparison indicates that the advanced digital filter method gives enhanced performance in terms of computational cost and simulation accuracy. In addition, initial tests show that this method is capable of reproducing experimental noise measurements within 3 dB accuracy.

Prediction of Contra-Rotating Open Rotor broadband noise in isolated and installed configurations

Broadband noise is a significant part of the noise emitted by contra-rotating open rotors. Several noise sources can contribute to the total broadband sound field, with the most dominant ones probably being trailing edge noise, rotor-wake interaction noise and pylon-wake interaction noise. This paper addresses the prediction of these noise sources using analytical models based on Amiet’s flat plate airfoil theory and also to empirical turbulence models, fed by input data extracted from steady and unsteady CFD RANS simulations. The models are assessed against wind tunnel tests of Rolls-Royce’s rig 145 (build 1) conducted at the DNW anechoic open jet test facility using Rolls-Royce blades and Airbus pylons. The study showed promising results in terms of the ability of the models to predict acoustic power spectrum shapes, peak frequencies and absolute levels. The effects of changes in thrust on broadband wake-interaction noise are well reproduced. However, the models significantly underestimate the effect of thrust on trailing edge noise and the effect of rotational velocity on pylon interaction noise.

Parallel computation of 3D acoustic radiation from an engine intake

Tonal noise from an engine intake is generally dominated by only a few cut-on modes. For the case of an axisymmetric intake geometry, the three-dimensional (3D) problem can be reduced to a more convenient set of 2D problems through a modal decomposition. However, for the more general case, the 3D formulation must be maintained. In this paper the radiation of 3D acoustic modes from a realistic engine intake duct with background mean flow is studied numerically. The method is based upon a parallel implementation of a computational scheme which allows acoustic modes, propagating inside the intake duct of a generic aircraft engine, to be admitted into a computational domain that includes the duct section, the exit plane of the duct, and the surrounding flow. The method comprises three elements: a matching process to admit acoustic waves into the in-duct propagation region; near-field propagation inside the duct and diffraction at the lip of the duct; and an integral surface for far-field directivity. The wave admission is realised through an absorbing non-reflecting boundary treatment which admits incoming waves and damps spurious waves generated by the numerical solutions. The wave propagation and diffraction are calculated by solving the linearised Euler equations, using high-order compact schemes. Far-field directivity is estimated via an integral surface solution of the Ffowcs Williams - Hawkings equation. The 3D parallel solver is used to determine multi-mode propagation from a realistic engine intake geometry with background mean flow. The solver can account for the effect of a swirling flow. Validation of the solver is performed by comparing 3D propagation results with a previously developed 2.5D formulation. The simulations include the effect on an acoustic lining modeled using a time-domain impedance boundary condition. The 3D method is extended to the case of multi-mode calculations.

Leading edge noise predictions using anisotropic synthetic turbulence

An advanced digital filter method is presented to generate divergence-free synthetic turbulence with homogeneous anisotropic velocity spectra. The resulting fluctuating velocity field is obtained through a superposition of anisotropic Gaussian eddies. This method is used to generate a two-dimensional turbulent flow with the key statistics of homogeneous axisymmetric turbulence. This type of turbulence has been reported in aero-engine intakes, fan wakes and open-jet wind tunnel experiments. The advanced digital filter method is implemented in a linearized Euler solver in order to investigate potential effects of anisotropic turbulence on leading edge noise. Computational aeroacoustic simulations are performed for anisotropic turbulence with streamwise-to-transverse length scale ratios ranging from 0.33 to 3 on a number of isolated airfoil configurations, including variations in mean flow Mach number, airfoil thickness and angle of attack. Noise reduction due to airfoil thickness is assessed on a NACA 0012 airfoil at zero angle of attack, showing similar trends for both isotropic and moderately anisotropic turbulent flows. Effects of anisotropic turbulence on noise become evident for airfoil configurations at non-zero angle of attack.

Gradient Term Filtering for Stable Sound Propagation with Linearized Euler Equations

A new, stable gradient term filtering (GTF) method is applied to time-domain linearised Euler equations (LEE) to compute sound propagation problems. The method employs a Laplace operator as a filter to obtain acoustic wave components. Through the filtering process, both the Kelvin-Helmholtz and the Rayleigh–Taylor instabilities can be removed from the solution process. Stability analysis confirms the stable behaviour of the solution in the presence of a sheared background mean flow, as against the conditional stability of LEE and gradient term suppression (GTS) methods. In accounting for vortical wave propagation, a curl operator can be conventionally utilised to obtain two-dimensional vortical wave components. Several benchmark test cases are studied to validate the proposed methods. Tests show that the proposed method can obtain stable solutions for acoustic wave propagation and is capable of modeling vortical interactions.