Markus Lummer

Maggi-Rubinowicz Diffraction Correction for Ray-Tracing Calculations of Engine Noise Shielding

For the assessment of geometrical shielding of engine noise already in the design phase of aircrafts, a ray-tracing code is under development at DLR. Since in the high frequency limit no sound signal reaches the shadow zone, diffraction effects have to be included using special approximations like, e.g., the geometrical theory of diffraction. An alternative approach is presented here which is based on the Maggi-Rubinowicz formulation of the Kirchhoff diffraction theory. Thereby, an approximation of the diffracted field is calculated by a line integral along the shadow boundary on the surface of the body. Results for generic geometries, like a disk, a sphere, a strip and a cylinder will be presented, as well as calculations for a full aircraft geometry. The results will be compared with a boundary element method. The coincidence of the shielding factor for the generic geometries is quite good. Although in case of the aircraft geometry essential features of the shielding are well described by the approximation, more accurate results could be obtained taking into account also the diffraction field of reflected rays.

Handling of Non-Periodic Contra Rotating Open Rotor Data

In this contribution, we present two different concepts to handle the non-periodic nature of Contra Rotating Open Rotor (CROR) if the front and the aft rotor rotate with a slightly different rotational speed. The first procedure that is presented consists of a correction matrix applied to the source data used in the DLR FWH-code APSIM+. For periodic data the correction matrix reduces to the identity matrix, thereby recovering the standard Fourier transformation. The second method is based on the Vanicek approximation, and consists of a successive least-square approximation of non-periodic data. The developed methods are tested with artificially generated data, illustrating the ability to accurately representing non-periodic data. A comparison between the two methods shows that the first method is more accurate than the Vanicek approximation. Preliminary results on actual non-periodic CROR data reveals the influence of the non-periodic correction as compared to uncorrected data, i.e., differences ranging up to 10 dB are seen for the considered cases.

Simulation of Sound Generation by Vortices passing the Trailing Edge of Airfoils

The interaction of vorticity with the trailing edge of airfoils is an important source of airframe noise. In order to study this process, in the present paper the interaction of a single vortex with the trailing edge of a symmetric Joukowsky airfoil under 0° angle of attack is calculated by the solution of nonlinear disturbance equations with a RANS mean flow in the Mach number range from 0.2 to 0.5. Nonlinearities are taken into account up to first order. The emphasis of the present study rests on the assessment of the influence of the Mach number and non-linearities on the generated sound field. It has been found that the sound intensity downstream of the profile scales approximately like M**4, which is near to the theoretical value. Taking into account non-linearities increases the sound pressure level in case of mathematically negative rotating initial vortices and decreases it in case of positive rotating ones. This should be kept in mind in the assessment of solutions obtained by the linearized Euler equations.

Validation of a Model for Open Rotor Noise Predictions and Calculation of Shielding Effects using a Fast BEM

For application in BEM/FMM shielding calculations a simple analytical model for the loading noise of contra rotating open rotors (CRORs) with different rotational speeds is derived. Following previous work of S.L.A. Glegg, the model is formulated in frequency domain and the pressure is approximated by a set of dipoles on circles on the propeller disk. The dipole strength depends on the blade loading function and can be obtained, e.g., by CFD calculations. For arbitrary rotational speeds, the blade loading function is not a perodic function on the propeller disk anymore and must be approximated, e.g., by a least-squares Fourier approximation. The CROR model is checked against a time domain solution using rotating dipoles and validated with data from a test of Rolls-Royce’s open rotor model rig 145 in DNW. The lowest three and most dominant peaks in the spectrum of the uninstalled rotor are well predicted by the method with errors less than 3dB with a correct directivity. For higher frequencies larger errors are observed. The CROR model has been developed for shielding calculations with the DLR BEM/FMM code. Some information about the code is given and the applicability of the model is demonstrated for a CROR installed at a modified DLR F6 aircraft geometry.

THE SIMULATION OF AIRFRAME NOISE APPLYING EULER-PERTURBATION AND ACOUSTIC ANALOGY APPROACHES

The capability of three different perturbation approaches to tackle airframe noise problems is studied. The three approaches represent different levels of complexity and are applied to trailing edge noise problems. In the Euler-perturbation approach the linearized Euler equations without sources are used as governing acoustic equations. The sound generation and propagation is studied for several trailing edge shapes (blunt, sharp, and round trailing edges) by injecting upstream of the trailing edge test vortices into the mean-flow field. The efficiency to generate noise is determined for the trailing edge shapes by comparing the different generated sound intensities due to an initial standard vortex. Mach number scaling laws are determined varying the mean-flow Mach number. In the second simulation approach an extended acoustic analogy based on acoustic perturbation equations (APEs) is applied to simulate trailing edge noise of a flat plate. The acoustic source terms are computed from a synthetic turbulent velocity model. Furthermore, the far field is computed via additional Kirchhoff extrapolation. In the third approach the sources of the extended acoustic analogy are computed from a Large Eddy Simulation (LES) of the compressible flow problem. The directivities due to a modeled and a LES based source, respectively, compare qualitatively well in the near field. In the far field the asymptotic directivities from the Kirchhoff extrapolation agree very well with the analytical solution of Howe. Furthermore, the sound pressure spectra can be shown to have similar shape and magnitude for the last two approaches.

Towards high-lift noise from Fast Multipole BEM with anisotropic synthetic turbulence sources

The present paper reports about recent developments to establish a novel broadband noise prediction approach for complete 3-D full scale high-lift wings. The approach combines stochastic noise source modeling with a Fast Multipole Boundary Element Method (FM-BEM) as an ultra-efficient acoustic propagation tool. The first principle based simulation process rests upon Reynolds Averaged Navier-Stokes (RANS) simulation which provides mean flow parameters and turbulence statistics. A stochastic method generates synthetic time resolved turbulence in form of fluctuating vorticity, which fulfills the RANS derived turbulence statistics if averaged over time. The flow around the slat exhibits complex structures which involve a shear layer, reattachment and strong anisotropy. For a numerical efficient description of the slat noise sources a stochastic turbulence model for 3-dimensional anisotropic solenoidal fluctuating vorticity is developed. An anisotropic scaling related to the Reynolds-stresses is validated with results from Large Eddy Simulation (LES) for a flat plate turbulent boundary layer with zero pressure gradient. Furthermore, the unsteady vorticity is transformed into acoustic surface sources applied as a Neumann boundary condition in the FM-BEM problem. Broadband slat noise of a 3-dimensional high-lift wing section of the DLR F15LS airfoil is simulated and compared to experimental data. A good agreement with the acoustic spectra predicted by the novel method is observed. Finally, first results of simulations for the noise radiated from a 3-D model scale high-lift are reported.

Validation of DLR's sound shielding prediction tool using a novel sound source

This paper is concerned with an experimental validation methodology for DLR’s acoustic shielding prediction boundary element code BEMPAR. This code in turn will be integrated into the overall aircraft noise prediction tool PANAM of DLR. The presented validation concept is based on a novel laser-based sound source. Almost perfect monopole-type test signals may be produced with a frequency content up to roughly 100kHz in combination with a very small source extension. These characteristics make this technique especially attractive for shielding/installation tests, which typically have to be performed at relativey small scale. BEMPAR is a boundary element code (BEM) which solves for the scattered pressure field. Three generic test cases are evaluated, a circular plate, a long cylinder and DLR’s low noise aircraft (LNA-1) nacelle model. The outcome of this investigation clearly demonstrate the potential of BEMPAR for the prediction of installation eects.

Simulating the Interaction of a Three-Dimensional Vortex with Airfoils Using CAA

The noise generated by the interaction of localized three-dimensional vorticity with the leading edge of Joukowski-type airfoils is calculated numerically. As governing equations the linearized Euler-equations are solved in curvilinear coordinates. For spatial discretization the well known DRP-scheme is used and for time marching the classical explicit 4th order Runge-Kutta scheme is employed. The mean-flow is pre-calculated analytically. Calculations are performed for three-dimensional vorticity spots striking the leading edge of the airfoil. The influence of the principal fluctuation component on noise generation is assessed by different rotational axes of the vorticity. The investigated airfoils have a thickness of 6, 12 and 18%. As limiting case the simulations are also performed for an infinite thin flat plate. Directivity patterns are evaluated and compared.

Aircraft and technology for low noise short take-off and landing

This paper discusses characteristic multi-disciplinary issues related to quiet short take-off and landing for civil transport aircraft with a typical short to medium range mission. The work reported here is focussing on the noise aspects and is embedded in the collaborative research centre CRC880 in Braunschweig, Germany. This long term aircraft Research initiative focusses on a new transport aircraft segment for operation on airports with shorter runway length in commercial air transport. This calls for a community-friendly aircraft designed for operations much closer to the home of its passengers than today. This Scenario sets challenging, seemingly contradictory aircraft technology requirements, namely those for extreme lift augmentation at low noise. The Research Centre CRC880 has therefore devised a range of technology projects that aim at significant noise reductions and at the generation of e�cffient and flexible high lift. The research also addresses flight Dynamics of aircraft at takeoff and landing. It is envisaged that in general significant noise reduction -compared to a reference turbofan driven aircraft of year 2000 technology- necessarily requires component noise reduction in combination with a low noise a/c concept. Results are presented from all the acoustics related projects of CRC880 which cover the aeroacoustic simulation of the source noise reduction by flow permeable materials, the characterization, development, manufacturing and operation of (porous) materials especially tailored to aeroacoustics, new UHBR turbofan arrangements for minimum exterior noise due to acoustic shielding as well as the prediction of jet noise vibration excitation of cabin noise by UHBR engines compared to conventional turbofans at cruise.

Computation and Validation of Acoustic Shielding at Realistic Aircraft Configurations

Acoustic shielding calculations were performed for a generic test object (NACA 0012) and two realistic aircraft configurations, an Unmanned Aerial Vehicle (UAV) and a Hybrid Wing Body (HWB), and compared with experimental data, where the monopole sound source was realized using laser pulses. The agreement between calculation and experiment is good. Due to wind tunnel size limits experimental data are often restricted to the acoustic nearfield and its usability for farfield shielding predictions must be assessed numerically. In the present paper, shielding patterns were defined and its evolution with increasing distance from the geometry was studied using a simple shifting and scaling procedure. For higher frequencies, a linear scaling of the patterns with increasing distance could be established.