validation of caa prediction of noise radi- ated from turbofan intakes

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16 th International Congress on Sound and Vibration, Kraków, Poland, 5–9 July 2009 As fan noise propagates through the intake, it is attenuated by acoustic liners in the nacelle and scattered by the intake geometry, by liner discontinuities and by the non-uniform mean flow. Scattering effects are most pronounced in the tones. Numerical predictions of the far-field Sound Pressure Level (SPL) field shapes for a given source distribution are computationally challenging. The acoustic wavelengths are generally an order of magnitude smaller than the geometric scales, and methods are needed which exhibit low dispersion and low dissipation. Conventional low order CFD is unsuitable on both counts. Methods that have been applied to this problem include; Boundary Element (BE) methods 1 , Finite and Infinite Element (FE/IE) models 2 and Computational Aeroacoustics (CAA) methods based on the solution of the full or Linearised Euler Equations (LEE). BE and FE/IE models solve a convected wave equation in the frequency domain. They are applicable only when the mean flow is irrotational. More general CAA methods which are applied to the LEE do not suffer from this limitation. They are applied most effectively in the time-domain but this brings additional challenges such as controlling non-physical shear instablities and modelling the impedance of acoustic liners in the time-domain. The assumption of irrotational flow is however not a major limitation in the intake given that the mean flow originates in a region of uniform flow upstream of the nacelle and that the boundary layers on the inner surfaces of the nacelle are relatively small on a wavelength scale. FE/IE methods are therefore an attractive option for intake predictions and are widely used for this purpose. In the case of axisymmetric problems (such as those presented in this paper) FE/IE predictions of intake noise can be computed for an acceptable frequency range and for a complete set of sources in times which are acceptable for industrial application (minutes or hours rather than days or weeks of serial rather than parallel computation). One major limitation both of Helmholtz FE/IE methods and of more general CAA methods based on LEE is that they are unable to deal with non-linear propagation effects which can be significant close to the fan when the blade tip speed is supersonic. In the current article these will be dealt separately using a different technique. In this paper, predictions are made by using ACTRAN/TM 3 , a commercial code developed by Free Field Technologies SA. ACTRAN/TM uses a Finite/Infinite Element approach 5,2 . In the current application, it is executed within a shell programme ANPRORAD, developed at ISVR within a collaborative partnership with IHI Corporation. Tone predictions are presented at BPF for realistic hard-walled and lined intakes, and compared to data from a fan test rig. Broadband noise predictions have also been made, but these results are not presented here. The BPF tone source levels used in the prediction are calibrated by using in-duct mode detection measurements. At supersonic fan tip speeds a non-linear propagation code ‘Frequency Domain Numerical Solution’ (FDNS) developed by McAlpine and Fisher 4 is used as a correction to the linear ANPRORAD/ACTRAN predictions. 2 Forward arc noise Intake liner Non-linear decay of rotor-locked tones Fan Scattering by liners, internal geometry and non uniform mean flow OGV Figure 1. Propagation of fan noise through the intake and radiation into the far-field.
16 th International Congress on Sound and Vibration, Kraków, Poland, 5–9 July 2009 Baffle Duct intake Spinner 2. The Prediction Scheme ANPRORAD is a shell program that generates and executes a sequence of ACTRAN/TM computations for an axisymmetric turbofan intake with subsonic mean flow. ANPRORAD reads key geometric data, mean flow data and frequency data and generates a sequence of finite element (FE) meshes for the intake and a surrounding near field region, each with an appropriate resolution for a given frequency range of interest. Fan source data is defined on the fan plane in terms of prescribed set of incident modal intensities for hard-walled annular duct modes and anechoic conditions are assumed for noise propagating back into the fan. Prior to the acoustic computation, the compressible Euler equations are solved within ANPRORAD to define the mean flow in the intake and in the surrounding region. A coarse version of the acoustic mesh is used and the results are interpolated onto the acoustic mesh for a sequence of ACTRAN/TM acoustic analyses at prescribed frequency intervals. A coarse low-frequency ANPRORAD FE mesh is shown in Fig. 2 for the rig geometry shown in Fig. 3. The ACTRAN model contains a conical baffle which is treated as an extended nacelle lip. The FE mesh itself is extended to the back of what would be the nacelle in a flight configuration, to model the anechoic nature of the chamber into which the test rig protrudes. The inner domain shown in Fig. 2 is discretised by quadratic elements giving fourth order accuracy in the usual sense. The outer region is modelled by a single layer of high order infinite elements which are compatibly matched to the FE mesh at the FE/IE interface. The acoustic pressure within each infinite element is modelled by an n th order multipole expansion. The order of the multipole expansion used for the results presented here is typically 15. The treatment of acoustic liners within ACTRAN/TM is based on a slip condition at the wall and continuity of pressure and particle displacement over an infinitely thin vortex sheet above the impedance surface. This is modelled by Eversman’s implementation of the Myers 6 boundary condition. In the far-field, acoustic pressure amplitude is computed on an arc with its centre point on the duct axis. This defines a field shape for the sound pressure level (SPL) which can be compared with measured SPL data. 3. Experimental Set-up FE/IE interface Fan plane Figure 2. An example of a low-frequency ANPRORAD FE mesh for a fan rig intake incorporating the baffle as an extension of the nacelle intake geometry. Noise measurements were taken on the 1/3-scale model fan rig, shown in Fig. 3. The rig was mounted so that the intake radiated into an Anechoic Test Cell . The configuration tested had a cylindrical intake barrel, and a 24-bladed fan. The general set up is shown in Fig. 3. A ring of pressure transducers is located at the entrance the lined intake barrel. Two intake configurations are considered: a hard-walled intake barrel with a hard-walled fan case, and a lined intake barrel with a lined fan case. Far field SPL measurements were taken at an array of far-field microphones located at the rig centreline height, between 0 and 120 degrees relative to the intake axis with a 5 degrees spacing. 3
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validation of caa prediction of noise radi- ated from turbofan intakes

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