Michael Pott-Pollenske

Slat Noise Source Studies for Farfield Noise Prediction

Todays low-noise high-bypass-ratio engines have made airframe noise for large commercial aircraft in the approach configuration to be compatible to that of the engines. As a consequence, flow noise from landing gears and from high lift devices (HLD) on wings - and its control - will become ever more important. In a study, dedicated to HLD flow noise, parametric wind tunnel experiments were performed on scaled wing sections in the Aeroacoustic Wind Tunnel Braunschweig, on a complete scale model aircraft and a full scale wing-section in the German-Dutch Wind Tunnel to identify relevant airframe noise mechanisms and develop noise prediction schemes. As one essential result of these tests deployed slats were identified as prominent noise contributors. Based on an extensive set of farfield noise data and on first results from measurements of the unsteady local flow properties in the slat area the effects of flow velocity and aircraft angle of attack on slat noise radiation characteristics were determined. The test results support the assumption that slat noise originates from the upper slat trailing-edge and scales with the slat cove vortex dimension. As a consequence the transposition of slat noise data from scale model tests towards a full scale situation can approximately be based on the geometric scale factor. Based on the findings of this experimental study a simplified source model is considered for slat noise prediction.

Airframe noise characteristics from flyover measurements and prediction

Aircraft noise impact around airports will increase corresponding to the predicted growth in air-traffic if no measures for aircraft source noise reduction are taken or noise abatement flight procedures are developed. During the final approach phase engine noise and airframe noise are comparable in level, the latter being governed by flow noise originating from landing gears and high lift devices. Based on the results of dedicated wind tunnel studies semi-empirical/empirical airframe noise prediction schemes were developed for both high lift devices’ and landing gear noise to support the calculation of noise impact in the vicinity of airports. Within an ongoing German national research project on the development of noise abatement procedures, co-financed by the German Ministry of Education and Research (BMBF), flyover noise measurements were conducted on an Airbus A319 aiming at the validation of DLR’s airframe noise prediction schemes. In order to distinguish between airframe and engine noise sources flyovers were performed for different aircraft configurations and operational conditions.

An integrated design approach for low noise exposing high- lift devices

The DLR project LEISA combines and focuses activities in the research areas of high lift system design, flow control and aero-acoustic design methods, which have been carried out rather independently up to now. Furthermore, the competence in the fields of aerodynamics, aero-acoustics, structures and flight systems will be integrated to provide an interdisciplinary assessment of high lift system design for transport aircraft configurations. The project LEISA started at the beginning of 2005, so up to now only few results are available. This paper addresses the integrated design approach and first results for a noise reduced slat device and combined wind tunnel testing results for aerodynamics and aero-acoustics.

Enhanced Capabilities of the Aeroacoustic Wind Tunnel Braunschweig

The Aeroacoustic Wind Tunnel Braunschweig is DLR’s small, high-quality test facility for aero-acoustic noise measurements. After years of mainly pure acoustic measurements the actual and the future research work will focus on the combined acquisition of aerodynamic and acoustic data. In order to prepare the AWB for the next decades a modernization of this wind tunnel was initiated targeting the enhancement of the aerodynamic properties while the excellent acoustic properties should at least be preserved. Based on the assessment of AWB’s original acoustic and aerodynamic characteristics a new acoustic wall-treatment in combination with new sound absorbing turning vanes was installed in the flow circuit. In order to prevent the highly bended downwash flow caused by airfoils tested at high angles of attack from impinging on the test section floor a new collector was installed that can be moved upstream and vertically down. Finally the driven efforts resulted in an increase of the maximum flow velocity of about 8% while first the background noise levels for an empty test section were preserved and second the use of the adaptive collector lead to a significant background noise reduction for airfoil tests.

A Testbed for large scale and high Reynolds number Airframe Noise Research

Airframe noise testing in open jet wind tunnels is and will be an indispensable means to (i) identify airframe noise sources and develop noise reduction technologies and (ii) to provide validation data in order to support the development of numerical acoustic methods like e.g. CAA codes. Usually such wind tunnel experiments were conducted in small research facilities like DLRs AWB (nozzle size 1.2 m x 0.8 m) using 2-dimensional wind tunnel models featuring a retracted chord length of about 300 to 400 mm. In very rare cases full aircraft models of 1:7.5 to 1:11 scale were tested in the Large Low-Speed Facility of DNW (DNW-LLF), which provides a nozzle size of 8 x 6 m2 and a maximum flow speed of 78 m/s. Even though these full aircraft models provide almost realistic 3-dimensional flow conditions the mean retracted chord length is pretty similar to the 2D test cases. Therefore most airframe noise tests are restricted to a Reynolds number regime between Re=1.0*106 and Re=2.0*106. This finally means that the aerodynamic conditions during most airframe noise wind tunnel experiments do not comply with realistic operating conditions. In order to overcome these deficiencies DLR, Airbus and EADS-IW decided to design a new large scale test bed for airframe noise testing. This plan could be realized in the mainframe of the German national founded research project FTEG (Flight Physics Technologies for Green Aircraft).The finally designed high lift system features a retracted chord length of 1200 mm and a wing span of 7200 mm. It was tested for representative approach conditions and Reynolds numbers up to 5.0*106. By means of this wind tunnel model high Reynolds number slat noise data were acquired that later serve for the validation of the DLR CAA code PIANO. It reveals the slat noise spectra did not contain any relevant tonal slat noise components that are well known from small scale experiments. Dedicated Reynolds number variations gave evidence that (i) low Reynolds number tests on high lift airfoils do not completely represent the broadband sound radiation typical for respective full scale components and that (ii) the tonal components are related to more than one generation mechanism. The new large scale F15LS high lift system successfully proved its ability to enable the assessment of noise reduction technology and the acquisition of validation data for numerical methods under almost realistic flow conditions.

CAA-RPM prediction and validation of slat setting influence on broadband high-lift noise generation

Broadband sound generated at the slat of a three-element high-lift wing is simulated with a CAA method. Especially, the effect of different slat and gap settings is studied and the results are validated with measurements. The applied method rests on the use of steady Reynolds Averaged Navier-Stokes (RANS) simulation to prescribe the time-averaged motion of turbulent flow. By means of synthetic turbulence generated with the Random Particle-Mesh (RPM) method the steady one-point statistics (e.g. turbulent kinetic energy) and turbulent length- and time-scales of RANS are translated into fluctuations with statistics that very accurately reproduce the spatial target distributions of RANS. The synthetic fluctuations are used to prescribe sound sources which drive linear acoustic perturbation equations. The whole approach represents a methodology to solve statistical noise theory with state-of-the-art Computational Aeroacoustics (CAA) tools in the timedomain. The comparison with experiments are conducted for four selected settings, chosen from a matrix comprising in total 43 individual slat-gap and overlap combinations. CAA simulations are performed for all matrix positions. The CAA computations are obtained as blind predictions prior to the measurements conducted in the AWB wind tunnel. Good agreement of the noise trends are found between CAA and experiments. The difference in level between the selected configurations is obtained qualitatively and quantitatively by CAA.

Installation Effects of a Propeller Mounted on a High-Lift Wing with a Coanda Flap. Part I: Aeroacoustic Experiments

In this contribution, we present aeroacoustic experiments concerning installation effects of propellers. Such installation effects are important as they can significantly alter the sound radiation as compared to an isolated propeller. For this purpose, detailed experiments have been conducted in the NWB aeroacoustic wind tunnel in Braunschweig, Germany. The considered geometry is a nine-bladed propeller installed in front of a high-lift wing (employing a Coanda flap). The results illustrate the influence of propeller rotational speed, blade pitch angle, wind tunnel velocity, and angle of attack variations on the sound radiation. Furthermore, with a source localisation technique insight is gained in the dominant sound sources, and reveals the importance of periodic as well as broadband noise for the considered geometry.

Aerodynamic and Acoustic Design of Silent Leading Edge Devices

The high lift system noise of current transport aircraft is dominated by slat noise under certain operating conditions. Suitable means to reduce the noise impact in the vicinity of airports are (i) to increase the distance between the source and the observer and (ii) to reduce source noise levels. Both objectives can only be achieved by means of a multi-disciplinary aerodynamic and acoustic development since the slat is at the same time a very important element to achieve the necessary high lift performance and the dominant noise source of a high lift system1. First attempts to reduce slat noise by means of a slat setting optimization were conducted at DLR in the mainframe of the project Leiser Flugverkehr2. This purely acoustically driven study revealed that a slat gap reduction results in a local flow speed decrease at the slat trailing edge and thus to remarkable noise reductions of up to 10 dB, the latter of course depending on the magnitude of the slat gap reduction. The drawback of this approach was that at the same time the aerodynamic performance of the high lift system was degraded by a non-acceptable level. However, this study was the starting point of the DLR project LEISA (Low noise exposing integrated design for start and approach) that combined activities in the research areas of high lift system design and aero-acoustic design, which were carried out rather independently up to this point in time. In the project LEISA different types of high lift configurations were addressed and investigated in a 2-dimensional approach. The first one is a long chord slat that provided a source noise reduction of about 6 dB while maintaining the aerodynamic performance of the reference slat system. The second, and more radical concept was to omit the slat and apply a droop nose system in order to reduce the aerodynamic losses as much as possible. The finally achieved source noise reduction with the droop nose system was about 8 dB while from the aerodynamic point of view about 50% of the losses were recovered. Based on these promising results the transposition of these high lift systems to a real 3-dimensional wing was carried out in the follow-up project SLED (Silent Leading Edge Devices). The final outcome of the project SLED can be summarized as follows. From the aerodynamic point of view the performance of the 3-dimensional long chord slat compares very well to the reference slat system. The final droop nose design was capable to recover about 40% of the lift loss due the omitted slat. The final acoustic results in terms of source noise levels are an overall 4 dB noise reduction for the long chord slat and about 6 dB noise reduction in case of the droop nose. The obtained aerodynamic and acoustic characteristics were finally transposed to flight in order to assess the effect on community noise which can be expressed in terms of noise iso-contour areas. Regarding the 60 dB(A) and the 65 dB(A) noise iso contour areas the achieved benefit is a reduction of up to 40% of the respective area’s size.

Large-Scale Studies on Slat Noise Reduction

Slat noise research activity within the EC co-financed project OPENAIR involved both experimental and numerical studies at the new large (1:3.3-scaled) swept high-lift wing model F15LS. Experiments were performed in the DNW-LLF (Large Low-speed Facility) to verify the noise reduction benefit of selected slat noise reduction concepts under more realistic test conditions than in precursor projects. Moreover, the gained test data served to extend current slat noise validation datasets towards larger Reynolds numbers, i.e. up to Re = 5.1 Mio. in the current experiment. Slat noise reduction concepts under review were 1) slat gap/overlap setting variations, and 2) an adaptive slat with the potential to reduce the gap width for noise reduction. Both concepts were proven highly e�fficient: Sealing of the gap leads to a maximum 5-dB noise reduction at wing level, equivalent to a full elimination of the slat noise source. Optimized slat settings or an adaptive slat with partially closed gap are suited to reduce slat noise by about 2-3 dB at wing level while producing negligible aerodynamic impact at the operative test angles of attack within the linear polar region. When transposing these results to overall aircraft flight conditions, optimized slat settings might bring about a 0.5-EPNdB reduction of approach certification noise levels, provided all other relevant noise sources than the slat remain untreated. CAA (Computational Aeroacoustics) prediction results derived with DLRs PIANO and DISCO codes coupled with RANS-based stochastic source models revealed a generally good reproduction of the measured trends.

LEISA2: an experimental database for the validation of numerical predictions of slat unsteady flow and noise

In the LEISA2 project (Silent Take-Off and Landing), ONERA and DLR have collaborated on an common program which main motivation was to build an experimental database for the validation of numerical codes devoted to the simulation of unsteady flows and noise generation by a high-lift wing configuration, data which are globally missing to the aeroacoustic community. Such database may also help explaining the physical mechanism of high lift airfoil noise generation and answer the general need for resolving uncertainties in high lift noise testing and code validation. The activity is based on testing the existing DLR model F16 in two test campaigns. In a first step, the model has been adapted by DLR to F2 aerodynamic windtunnel located in Onera-Le Fauga and intensive aerodynamic measurements were achieved using optical devices such as PIV and LDV, but also acoustic measurements using a wall microphone array. Then acoustic tests were conducted in AWB, an anechoic open-jet windtunnel located in DLR-Braunschweig. The present paper gives a detailed description of this database, with several comparisons of data measured in both windtunnels. The formatted and documented database is now available to the worldwide aeroacoustic community in the framework of the Benchmark for Airframe Noise Computations (Category 6) organized by NASA LaRC and sponsored by AIAA.