Ulf Michel

Flyover Noise Measurements on Landing Aircraft with a Microphone Array

The noise sources of landing commercial aircraft were examined with planar arrays consisting of 96 or 111 microphones mounted on an 8 m by 8 m plate under the glide path on the ground. It is shown that important airframe noise sources can be identified in spite of the presence of engine noise, i.e., landing-gear noise, flap side-edge noise, flap-gap noise, jet-flap interaction noise, slat-horn noise, slat-track noise. A surprising finding is a noise source near the wing tips of some aircraft which is tentatively called wake-vortex wing interaction noise. It is shown to be the by far strongest noise source (6 dB(A) louder than the engines) on a regional jet aircraft.

Experiments on the Transmission of Sound Through Jets

E on the superposition of a turbulent jet flow (0<A/<0.7) and pure tone sound coming from inside the nozzle are reported. A comparison between the transmitted sound power from the nozzle and the radiated sound power in the far field reveals a substantial attenuation at low frequencies. Therefore, a jet can be considered as a low-frequency muffler. It has been verified that this effect is independent of the broadband jet noise amplification previously reported by Bechert and Pfizenmaier.

Investigation of Entropy Noise in Aero-Engine Combustors

Strong evidence is presented that entropy noise is the major source of external noise in aero-engine combustion. Entropy noise is generated in the outlet nozzles of combustors. Low-frequency entropy noise, which was predicted earlier in theory and numerical simulations, was successfully detected in a generic aero-engine combustion chamber. It is shown that entropy noise dominates even in the case of thermo-acoustic resonances. In addition to this, a different noise generating mechanism was discovered that is presumably of even higher relevance to jet engines: There is strong evidence of broad band entropy noise at higher frequencies (I to 3 kHz in the reported tests). This unexpected effect can be explained by the interaction of small scale entropy perturbations (hot spots) with the strong pressure gradient in the outlet nozzle. The direct combustion noise of the flame zone seems to be of minor importance for the noise emission to the ambiance. The combustion experiments were supplemented by experiments with electrical heating. Two different methods for generating entropy waves were used, a pulse excitation and a sinusoidal excitation. In addition, high-frequency entropy noise was generated by steady electrical heating.

Localization of the acoustic sources of the A340 with a large phased microphone array during flight tests.

Flyover measurements with a phased array of microphones extending over an area of 16 m by 16 m are reported. The 161 microphone array was made possible by combining hardware from ONERA and DLR. In this investigation of the airframe noise of an Airbus A340, the yover altitudes were between 90 m and 165 m. The data reduction methods for moving objects of DLR and ONERA are compared. Some source maps are shown and discussed. It is demonstrated that nested arrays must be used for a study over a wide frequency range, and that comparisons of the noise maps between di erent arrays provide valuable information about the noise sources. The ONERA method is shown to be a powerful data reduction method based on a small number of microphones while the DLR method results in alias-free maps at the expense of a much larger number of microphones.

Azimuthal Sound Mode Propagation in Axisymmetric Flow Ducts

Assuming an axisymmetric mean flow and acoustic boundary conditions, a set of three-dimensional axisymmetric equations suitable for each azimuthal sound mode of fluctuation is derived based on the three-dimensional linearized Euler equations. The independent two-dimensional model for each azimuthal sound mode is deduced by a Fourier series decomposition. A computational aeroacoustics approach is then applied to solve each two-dimensional equation system. The dispersion-relation-preserving finite difference scheme is implemented for spatial discretization, whereas the 2N storage low-dissipation and low-dispersion Runge-Kutta scheme is applied for time integration. Appropriate boundary conditions are prescribed at the various boundary regions. The numerical procedure is first validated by cases of a straight circular pipe and an annular duct subjected to a subsonic uniform mean flow, the numerical results of which show very good agreement with the analytical solutions. A further numerical example is presented for an axisymmetric inlet duct with an aeroenginelike geometry including an internal spinner. The aeroacoustic computation is based on an inviscid irrotational mean flow calculated by a second-order computational fluid dynamics solver. The acoustic solutions agree well with the existing finite element and multiple-scales solutions in the literature. Finally, the cuton, cutoff transition phenomena are investigated based on the proposed numerical approach. The transition behavior is found to be highly dependent on the mean flowfield in addition to the known geometric effect. These results demonstrate the feasibility of the proposed model and solution procedure.

Computation of Jet Noise using a Hybrid Approach

In this work, the subsonic jet flow through a short-cowl coaxial nozzle with both smooth and serrated lips has been investigated. The Mach numbers for the heated primary and cold secondary streams are 0.861 and 0.902, respectively. The Reynolds-averaged NavierStokes (RANS) equations are solved using a finite volume solver with a hybrid scheme employing a blending between upwind-biased high-order and full centred approximations. Detached-Eddy Simulation (DES) is used to treat the turbulence. The three-dimensional form of radiation non-reflective boundary conditions is applied in a modified formulation, which simplifies implementation and was found to suppress reflections successfully from the boundaries of the computational domain. The far-field noise is calculated using an aeroacoustic analogy of Ffowcs-Williams and Hawkings (FWH). The predicted Overall Sound Pressure Levels (OASPL) are generally in good agreement with the experimental data, within a maximum deviation of 3.0 dB for all observer positions. The dominant low frequency component is captured with the relatively coarse grid used. The numerical simulation has indicated the same trend of decreased sound intensity when using serrations as in the experiment, with an observed noise reduction of about 2 dB.

Impedance Deduction Based on Insertion Loss Measurements of Liners under Grazing Flow Conditions

This paper presents the results of insertion loss measurements and numerical impedance eduction of three dierent liner samples. An overview over the test rig and methodology is given and preprocessed results in terms of reection and transmission coecients as well as the energy dissipation are discussed. These coecients are calculated for discrete frequencies within the investigated frequency range. Subsequently, a numerical post processing is performed in the time domain and the educed impedance function for each sample and ow Mach number is presented. This post processing in the time domain uses an impedance model, which is based on the Extended Helmholtz Resonator with ve free parameters. The parameters of the model are tted via an optimization, which determines the whole frequency response by one optimization process. The comparison of measured and numerically evaluated energy coecients proves the usability of the tools for impedance evaluation under ow conditions. Finally the impedance results of the dierent samples are discussed, including a comparative study with Aermacchi data of the NLR ow tube and Aermacchi impedance tube experiments.

Airbus A319 Database from Dedicated Flyover Measurements to Investigate Noise Abatement Procedures

A series of flyover noise tests on the Airbus A319 performed in the framework of the German project ”Noise Optimized Approach and Departure Procedures (LAnAb)” took place at Parchim airport (Germany) in June 2004. A noise database was created that will be a support for the validation of aircraft noise prediction models dedicated to investigate noise abatement procedures. In all, 37 take-off and 82 approach conditions were simulated. Depending on the simulated flight phase, different values of engine power, airspeed, position of the high-lift devices, and also of the landing gears were tested. The aim of this paper is to show the different possibilities of using the signals recorded by a phased-array of microphones installed on the ground to analyse aircraft noise and confront some prediction models to the results.

Investigation of airframe and jet noise in high-speed flight with a microphone array

A Panavia Tornado aircraft was flown to investigate airframe and jet noise in high-speed flight. The lo- cations of the noise sources were examined with a line array consisting of 29 microphones. The flights were performed in an altitude of about 35 m above ground. The influences of various parameters were studied by flying with three different airspeeds (220, 250, 275 m/s) in unaccelerated flight and three dif- ferent engine power settings (flight idle, normal, max dry) with identical airspeeds (250 m/s) over the mea- suring position. The influence of external stores on airframe noise was studied by flying with stores first and repeating all tests with an operationally clean air- craft. Two surprising results were found, (i) airframe noise is louder with the aircraft in the operationally clean configuration, and (ii) a strong noise source ex- ists close to the nozzle exit plane that dominates the noise radiation toward the side and into the forward arc.

The Role of Source Interference in Jet Noise

It is shown that the acoustic analogy has the ability to explain several recent experimental findings that led to the development of a two-source model of jet mixing noise, (i) the directivity of the Mach number exponent, (ii) the enhanced sound radiation close to the Mach wave radiation angle, (iii) the dominance of the axisymmetric m = 0 component of jet mixing noise for angles close to the jet axis, and (iv) that jetnoise spectra scale on the Helmholtz number rather than Strouhal number for these angles. The influence of source interference on the sound radiation of the jet has to be included in the theory to achieve these results. The axial interference determines the directivity over almost all angles except for angles close to the downstream jet axis. The latter region is dominated by radial interference. In order to implement the recently proposed two-source model, the axial coherence of the sound sources is described by a small scale and a large scale component. Axial interference is first studied with a line model for the sources. This model is then extended to a source model on a circular cylinder to enable the investigation of axial and radial interference. The source cross-spectral density is expanded into a few azimuthal components. The radiation into small angles relative to the jet axis is dominated by the m = 0 component.