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Semi-buried engines are part of of many future aircraft designs. From acoustic’s point of view the increased noise shielding is one of the potential benefits. But at the same time the fan will experience a disturbed inflow which can lead to additional noise. In order to assess such new concepts numerical predictions can help, but it at the same time it is important to validate the methods against experimental data. The current paper describes a wind tunnel experiment which can serve as a validation case. The setup consists of a duct which is mounted on a flat plate. The whole model is placed inside a wind tunnel and tested with different flow speeds. Upstream the duct, a boundary layer develops which enters the duct, imitating boundary layer ingestion. The sound is generated by a specially designed source that it can be easily translated to computational methods. All important design details are given which makes it possible to use it as a benchmark case for numerical methods. The sound radiation is measured upstream the duct. Since the sound source operates at dedicated frequencies, clear interference patterns become visible. Such patterns can help to make small configuration changes visible (e.g. different flow speeds). The results serve as a starting point for detailed studies of flow and boundary layer effects on sound radiation from a duct. Numerical methods can be validated against the given result and extend the parameter range e.g. to much thicker boundary layers as well as recording results at much larger evaluation planes. However, such numerical studies are beyond the scope of this paper.

Propeller Noise reduction for E-drives, noise reduction potential E-driven propulsors are indirect

Because of the growing interest in noise abatement through the shielding of aircraft component noise (including engine noise), there has been, in the past few years, an increased necessity for the development of appropriate evaluation methodologies for the shielding properties of new aircraft designs. Progress in this domain is necessary to develop technologies which allow the accurate prediction of the noise reduction potential of an aircraft design. Part of DLR's effort, in this respect, is focused on the development and application of a laser-generated reference sound source. While most applications of this type of sound source deal with very high intensity, non-linear, shock-like phenomena, here the focus is on generating an isentropic sound source as a valuable tool to realize dedicated shielding experiments in a non-intrusive manner. Due to its small size, its non-intrusiveness and its monopole character, the laser source is also very well suited for the assessment of sensors transfer function in a wind tunnel environment. Its application in hybrid anechoic wind tunnels also provides appealing prospects. The laser sound source was shown to display the characteristics of a moving monopole i.e. a sound source convecting with the surrounding medium [1]. Experimental evidences presented revealing that the laser sound source displays, in fact, the characteristics of a moving monopole i.e. a sound source convecting with the surrounding medium are presented. A solution to the convected wave equation for a moving point heat source in a uniform subsonic flow is derived, providing i) means for an appropriate scaling of experimental results and ii) an appropriate closed-form sound source easily reproducible in numerical simulations. In order to establish a sound aeroacoustic shielding database for a firsthand validation of acoustic prediction codes, a set of interrelated aeroacoustic shielding tests were planned and executed [2]. The emphasis was put on an extensive 2D diffraction study of NACA 0012 airfoil. These studies laid the groundwork for which the characterization of acoustic shielding properties of actual aircraft geometries was done. The 2D shielding tests presented herein were conducted in DLR's Acoustic Wind Tunnel Braunschweig (AWB), ONERA's F2 Low-Speed Wind Tunnel and NASA's Quiet Flow Facility (QFF)[3] for nominally the same test conditions in order to learn about the measurement scatter across different facilities. A major objective of this study was to determine how consistent the measured data would be across the three facilities, since each has a different test section configuration. The AWB and QFF have, respectively, a fully open and partially open test section, while the F2 tunnel has a closed hard-wall test-section. The aim of this oral presentation is thus to give an overview of the current development status of the laser sound source methodology developed at DLR. Aspects of numerical and experimental realizations as well as analytical representation will be discussed. The content of this oral contribution was already, in part, presented at the 21st and 24th AIAA/CEAS Aeroacoustics Conferences [1][2] and at the STAB symposium 2018 [4]

First, results of experimental and numerical investigations on generic engine noise shielding by the SACCON UCAV configuration are presented. The results suggest that propulsion system intake noise is much less attenuated compared to exhaust noise. This is a consequence of the position of the intake on the symmetry plane of the UCAV as well as to the relatively large radius of curvature of the aircraft wings leading edge. This result emphasizes the potential necessity to achieve an acoustically optimized integration of both intake and exhaust altogether to maximize acoustic attenuation. The use of a generic, monopole-like, sound source in the experimental and numerical assessment of SACCON (Stability And Control CONfiguration) leaves many open questions with respect to the acoustic signature of a real UCAV configuration. For real highly-integrated propulsion systems, noise radiation from the intake and exhaust is critical to the acoustic detection of an aircraft, especially to low-observable configurations. Aiming at an assessment of the noise signature of a realistic UCAV configuration at an early design stage, experimental and numerical methodologies are presented which are able to account for mean flow and airframe interaction effects on noise generation and propagation. DLR-F24 MULDICON UCAV (MULti-Disciplinary CONfiguration) test bed is introduced, along with numerical simulations results providing first insights into the aeroacoustics of a realistic agile UCAV configuration.

The integration of advanced propulsion systems into the aircraft, and the assessment of their benefits related to e.g. fuel burn, noise emission and potential further measures, is one of the major activities of the Large Passenger Aircraft (LPA) Platform 1 of the Clean Sky 2 initiative. In order to optimize and assess the potential of advanced propulsion integration, it is necessary to establish suitable design, evaluation, and measurement tools. The so-called Cross-Demonstrator-Capabilities (XDC-) activities have been set up to develop and demonstrate powerful numerical and experimental methods for aerodynamic, aeroacoustics and aeroelastic simulation and measurement tasks, supporting the dedicated demonstrators of LPA. The paper will give an overview of the XDC activities and some examples of the developments related to this demonstrator.

In a two-step CFD-CAA simulation framework with DLR's CAA code PIANO enhanced by the treatment for porous materials simulations of a NACA0012 airfoil with a trailing edge made of porous materials starting at 88.75% chord length were run. The beneficial influence of porous materials was shown to be in good accordance with the experimental results. Furthermore, the effect of anisotropic as well as non-uniform materials was studied. Particularly, it was found that the application of nonuniform materials, as shown in Fig. 31, yields a notable noise reduction potential.

Der Beitrag gibt einen Überblick über Lärmminderungstechnologien für bestehende Verkehrsflugzeuge und einen Ausblick auf Entsprechendes in der Zukunft.

Future challenges in aircraft noise are discussed and requirements are derived for future aeroacoustic testing in wind tunnels. Concepts for a more synergistic use of acoustic measurements and numerical simulations are presented with first examples.

The effect of installation on propeller noise was investigated. The chosen configuration consists of a nine bladed propeller in tractor configuration with a high-lift wing. Tests in the 3m acoustic wind tunnel DNW-NWB provided a valuable dataset for validation purpose of numerical codes. The CAA computations were based on a new highly efficient concept to represent the propeller as a perturbation consisting of rotating mass and momentum source distributions fed by CFD RANS data on the propeller blades, thus avoiding rotating grids. This approach will allow quantifying the installation effect at propeller driven aircraft especially to assess design changes with moderate computational effort. The validation showed good agreement. The particular tractor propeller/ high-lift wing arrangement showed excess noise due to the impingement of the propeller tip vortices on the (swept) wing nose and trailing edge flap.