Michaela Herr

Experimental Investigations In Low-Noise Trailing-Edge Design

Within a parametric study on brush-type trailing-edge extensions, the noise reduction potential of several design concepts was determined. The obtained database represents the first phase of an ongoing project with the long-term objective to develop scaling laws for a future application of such devices as add-on solutions for todays aircraft components. The experiments comprised both acoustic and aerodynamic measurements on a zero-lift generic plate model (Re = 2.1 x 10 6 to 7.9 × 10 6 ) in DLRs open jet Aeroacoustic Wind Tunnel Braunschweig. Noise data were taken by means of a directional microphone system. Measurement results indicate a significant source noise reduction potential in excess of 10 dB, depending on the configuration. Two relevant noise reduction mechanisms were identified: 1) the suppression of narrowband bluntness noise, as well as 2) the reduction of broadband turbulent boundary-layer trailing-edge noise.

Design Criteria for Low-Noise Trailing-Edges

This paper summarizes the results of an experimental study on flow-permeable trailing-edge noise reduction means. Basic design rules are presented with respect to a future employment at current high-lift devices of passenger aircraft. The main focus is directed at the identification of the major design parameters of comb-type or slit edge-modifications. The achievable noise reduction capability was quantified by directional microphone measurements on a flat plate and on a two-dimensional NACA0012-like airfoil in the open-jet Aeroacoustic Wind Tunnel Braunschweig. It was found that flexibility of the comb material is beneficial, but not essential to achieve a noise reduction. Apart from a minimum device length the slit width was identified as the decisive design parameter. An almost zero-spacing of the comb fibers revealed the best results, leading to the assumption that the obtained noise reduction is mainly due to a viscous damping of turbulent flow pressure amplitudes in the comb area.

RANS/CAA based prediction of NACA 0012 broadband trailing edge noise and experimental validation

The prediction quality of a fast Computational Aeroacoustics (CAA) approach is studied for noise generated at a NACA 0012 trailing edge. Broadband spectra, the power law underlying their Mach number scaling, and the effect of Reynolds Number on the spectra are juxtaposed against published data from measurements and results from a semi-empirical prediction tool. The CAA method rests on the use of Reynolds Averaged Navier-Stokes (RANS) solutions to describe the turbulent flow problem around the airfoil. Acoustic Perturbation Equations (APE) are solved in the time domain, using a vortex source term, which is a function of turbulent field quantities. The acoustic approach was sccessfully validated in other works by utilizing turbulence data from Large Eddy Simulation (LES) to prescribe the unsteady sound sources. For the fast CAA approach applied in this work unsteady vortex sound sources are determined by a stochastic method, which generates 4D spatio-temporal synthetic turbulence that very accurately accompishes a local realization of all statistical and mean-flow features provided by steady RANS. Based on these prerequisites it becomes now feasible to make an assessment as to which acoustic accuracy can be achieved with such a hybrid RANS / CAA prediction method, bearing in mind the approximative nature and limited turbulence resolution of RANS:

Specifcation of Porous Materials for Low-Noise Trailing-Edge Applications

Systematic testing of the microstructural and aeroacoustic properties of porous metals applicable as low-noise trailing-edge (TE) treatments has been initiated within the Col- laborative Research Center SFB 880|Fundamentals of High-Lift for Future Civil Aircraft. Generic TE noise experiments were performed at Re = 0.8x10^6 to 1.2x10^6 in DLRs open-jet AWB facility. Complementary flow measurements in the closed test section MUB wind-tunnel of the TU Braunschweig served to quantify the induced aerodynamic effects. The presented database forms part of an ongoing cumulative effort, combining experimental and numerical methods, to gain a deeper understanding of the prevalent TE noise reduction mechanisms. For the large variety of porous materials tested herein a clear dependence of the achieved broadband noise reduction (reaching 2-6 dB at maximum) on the flow resistivity was identified. Basic design recommendations for material resistivity and pore sizes, the latter to minimize high-frequency self-noise contributions, were deduced for low-noise TE applications. An acoustic nearfield pressure release across the porous region, adversely coupled with a loss in lift performance for porous TE replacements, appears as the major noise-reduction requirement.

Trailing-Edge Noise Data Quality Assessment for CAA Validation

The paper provides a detailed reexamination of previously published trailing-edge (TE) noise experimental data as acquired in DLRs low-noise open-jet Acoustic Wind Tunnel Braunschweig (AWB). The objective is to set up a parametric reference data base to be later used for CAA validation purposes. A modular plate airfoil with variable chord length was used to investigate Reynolds number effects on both near field quantities and farfield sound. The original data base, including corrected farfield TE noise spectra as well as turbulent boundary-layer (TBL) characteristics, has been extended by unsteady surface pressure measurements in the TE region. Limitations and weaknesses of the applied measurement techniques are pinpointed by comparisons with independent data sets using alternative measurement approaches. Comparisons include also available semi-empirical prediction models for surface pressure spectra as well as for farfield TE sound emission. First RANS-based CAA prediction results using stochastic source models show essential agreement with the measurement data.

Broadband Trailing-Edge Noise Predictions - Overview of BANC-III Results

The Third Workshop on Benchmark Problems for Airframe Noise Computations, BANC-III, was held on 14-15 June 2014 in Atlanta, Georgia, USA. The objective of this workshop was to assess the present computational capability in the area of physics-based prediction of different types of airframe noise problems and to advance the state-of-the-art via a combined effort. This documentation summarizes the results from workshop category 1 (BANC-III-1) which focuses on the prediction of broadband turbulent boundary-layer trailing-edge noise and related source quantities. Since the forerunner BANC-II workshop identified some room for improvements in the achieved prediction quality, BANC-III-1 relies on the same test cases, namely 2D NACA0012 and DU96-W-180 airfoil sections in a uniform flow. Compared to BANC-II particularly the scatter among predictions for the DU96-W- 180 test case could be significantly reduced. However, proposed adaptations of previously applied computational methods did not systematically improve the prediction quality for all requested parameters. The category 1 workshop problem remains a challenging simulation task due to its high requirements on resolving and modeling of turbulent boundary-layer source quantities.

Characteristics of wall pressure fluctuations for a flat plate turbulent boundary layer with pressure gradients

The wall pressure fluctuations beneath a turbulent boundary layer with zero and non-zero pressure gradients were measured at a flat plate configuration in the Acoustic Windtunnel Braunschweig. The fluctuating pressure was measured by an array of subminiature pressure transducers. In addition, the mean flow velocity profiles within the turbulent boundary layer were obtained using hot wires. Adverse and favorable pressure gradients were realized by placing a rotatable NACA 0012 airfoil with a chord length of 40 cm above the flat plate. The one-point spectra and the two-point correlation properties are analysed. An empirical spectral model for the wall pressure fluctuations beneath an adverse pressure gradient boundary layer is developed based on the measured data. The effects of the pressure gradients on the characteristics of the wall pressure fluctuations are discussed.

Benchmarking of Trailing-Edge Noise Computations - Outcome for the BANC-II Workshop

The Second Workshop on Benchmark Problems for Airframe Noise Computations, BANC-II, was held on 7-8 June 2012 in Colorado Springs, CO, USA. The objective of this workshop was to assess the present computational capability in the area of physics-based prediction of dierent types of airframe noise problems and to advance the state-of-the-art via a combined effort. This paper summarizes the results from workshop category 1 which focuses on the prediction of broadband turbulent boundary-layer trailing-edge noise and related source quantities. 2D airfoil sections, namely a NACA0012 and DU-96-180, served as test cases. Code-to-code comparisons in this category were mainly restricted to relatively fast RANS-based methods applying statistical noise theory. Overall, the prediction capability was sufficient to capture the principal trailing-edge farfield noise scaling behavior in the mid-frequency range (about 1 kHz <= fc <= 5 kHz), i. e. the measured dependence of noise on angle-of-attack or free stream velocity. Differences in predicted trends appeared at lower and higher frequencies. Moreover, a comparatively large scatter among the DU-96-180 prediction results was observable, indicating individual room for improvement in the applied approaches.

In Search of Airworthy Trailing-Edge Noise Reduction Means

The EC co-financed project OPENAIR aims at a further development of both innovative and established aircraft noise reduction technologies. Among these activities an innovative two-component trailing-edge (TE) design, applicable at a slat TE, was evaluated through generic tests in DLRs Acoustic Wind-Tunnel Braunschweig(AWB). The proposed design combines a supporting frame structure (a coarse metal mesh or rib structure) with a thin, hydraulically smooth cover material (a fine metal mesh, micro-perforated sheet metal or cloth material) which are both permeable to the flow. The result is a mainly hollow, flow-transparent TE region. Variants of this concept were parametrically tested versus a solid reference TE at a 0.4-m chord generic airfoil. Material selection was based on airwor- thiness considerations. Detailed flow/acoustic resistance measurements were performed to characterise the used materials. TE noise was measured by a directional microphone system. The tested TE modifications achieve a broadband noise reduction of up to 4 dB, de-pendent of the configuration. However, the noise reduction effect diminishes with growing angle-of attack. This adverse behavior as well as geometrical dependences will have to be further studied prior to large scale testing. Pressure distribution measurements reveal a non-measurable to small effect of the TE modifications on the airfoils suction peak. Basic noise scaling laws for transposition of the results to full-scale applications are suggested.

Numerical Study of Wall Pressure Fluctuations for Zero and Non-Zero Pressure Gradient Turbulent Boundary Layers

Turbulent boundary layers on a flat plate configuration are simulated using synthetic turbulence generated by the Fast Random Particle-Mesh Method. The averaged turbulence statistics needed for the stochastic realization is provided by a Reynolds averaged Navier-Stokes calculation. Wall pressure fluctuations are obtained by calculating a Poisson equation including both the mean-shear turbulence interaction source term and the turbulence-turbulence interaction source term. The Poisson equation is solved by means of Hockney’s method. Wall pressure fluctuations for zero and adverse pressure gradient boundary layers are calculated. The adverse pressure gradient is realized by placing an airfoil above the flat plate. Simulated one-point spectra and two-point statistics are analyzed. The results are compared to the experimental results, which were measured in the Acoustic Windtunnel Braunschweig for the same configurations. Good agreement with the experimental results is obtained.