Karl-Stéphane Rossignol

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.

Flow Field Measurements to Characterize Flap Side-Edge Noise Generation

n this paper, the relation between noise generated at flap side-edges (FSE) and mean flow characteristics in the vicinity of the FSE solid surface is investigated. Of interest is (1) the identification of mean flow features near the flap tip which are important to FSE noise generation, (2) the extraction of quantitative parameters relating flap settings to the emitted noise levels and, finally, (3) the formulation of a prediction scheme based on these observations. The results of the investigation demonstrate that FSE noise can be properly represented by the summed contributions of two independent source mechanisms. Moreover, this new formulation necessitates only a limited knowledge of the mean flow parameters in the vicinity of the FSE to model the noise emission. It is assumed that unsteady vorticity fluctuations, originating from the unstable pressure side shear layer, sweeping over the forward half of the upper tip edge, lead to midto- high frequency noise production. Flow unsteadiness in the proximity of the suction side surface near the FSE aft half, are assumed to be the main contributor in the low-to-mid frequency range. A prediction scheme is formulated and validated for a wide range of experimental measurements at small scale wing configurations. This new formulation only necessitates knowledge of the flap chord length and lift coefficient to provide accurate estimates of the far field noise radiation.

Development of an empirical prediction model for flap side-edge noise

The ultimate goal of the present study is the elaboration of an empirical noise prediction model applicable in the early development stage of low-noise aircraft. To this end, a series of aeroacoustic tests were performed to investigate the characteristics of flap side-edge noise. Two generic models, an isolated flap and a high-lift wing were selected for the experiments. Measurements of sound pressure levels as well as sound directivity were done using dedicated microphone arrays. Moreover, the acoustic investigation was complemented by flow measurements done with a 7-hole probe. Further analysis of the data revealed the effectiveness of the flap tip cross-flow velocity in scaling noise spectra. The main characteristics of the noise directivity were also determined. These results allow the formulation of a preliminary empirical prediction scheme.

Analysis of the Noise Shielding Characteristics of a NACA0012 2D Wing

A laser-generated non-intrusive impulsive sound source is used to perform shield- ing experiments in the acoustic wind tunnel Braunschweig at a generic 2D wing with NACA0012 profile. The sound source was already shown in previous reports to have a monopole-like character with a nearly omni-directional sound radiation pattern, both in stagnant medium and in a flow field. In the current report, emphasis is first put on the analysis of the data post-processing steps required for the obtention of correct spectral and time domain data for the laser sound source. In particular, the subject of microphone corrections is reconsidered in details. A missing free-field correction of the results in,1 is corrected herein to reveal the correct time domain evolution of the generated pressure pulses. Further, non-linear propagation effects are considered. It is found that sound attenuation due to atmospheric absorption does have a non-negligible broadband impact on the far-field source noise levels, increasing in importance with mi- crophone distance. Other non-linear effects related to high-amplitude wave propagation i.e. wave steepening and higher rates of attenuation with far-field distance, could not be observed herein. Noise shielding results are presented for two Mach number values and three angles of attack. Except for a source location in the proximity of the trailing edge, the flow is found to have no measurable influence on the shielding characteristics of the NACA0012 wing for the experimental setup considered. For a source near the trailing edge, a reduced noise shielding is measured upstream of the trailing edge. It is postulated, that noise is generated through the interaction of the source sound wave with the trailing edge, and that it is amplified in the forward arc according to the mean flow field velocity.

On the Relevance of Convection Effects for a Laser-Generated Sound Source

A laser-generated sound source is used to generate a nearly omni-directional sound field. This sound field is investigated experimentally to reveal its characteristics as a function of the surrounding medium mean velocity. It is found that the sound source approximately has monopole character when generated in a stagnant medium. Increas- ing the surrounding flow velocity leads to a positive Doppler-shift combined with a convective amplification for an observer downstream of the source while the reverse is observed upstream of the source. The laser sound source behaves therefore as a convected point heat source in a uniform flow. A solution of the convected wave equa- tion for a moving point heat source in a uniform subsonic flow is derived, providing a closed-form description of the source and appropriate scaling parameters for exper- imental results. The derived theoretical model enables the determination of the heat release function resulting from the laser energy deposition in the medium and associ- ated to the measured far-field pressure pulses. An implementation of the heat release function in CAA codes is straigthforward, making numerical replication of shielding tests with the laser sound source possible.

Trailing-Edge Noise Modeling and Validation for Separated Flow Conditions

The framework for a semi-empirical model for the prediction of aerodynamically generated trailing edge sound from subsonic low Mach number 2D airfoils with mildly separated turbulent boundary layers is presented. Aerodynamic and acoustic measurements were performed and results indicate that the separated trailing edge flow noise can be modeled in a similar fashion to trailing edge noise generated by attached turbulent boundary layers. Comparisons with aeroacoustic measurements show agreement between prediction and results.

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.

Investigating Noise Shielding by Unconventional Aircraft Con[|#28#|]gurations

First experimental results on investigations of the shielding characteristics of three unconventional aircraft configurations are presented. DLRs Low Noise Aircraft, a scaled-down version of NASAs Hybrid Wing Body configuration and DLRs F17E Unmanned Combat Air Vehicle configuration are all investigated in the acoustic plenum of the DNW-NWB wind tunnel at DLR in Braunschweig. Using a laser-based non- intrusive impulsive point source, various effective engine placements were investigated for selected model settings. Shielding factors are calculated from in-flow measurements made over a large spatial extent in the fly-over plane. For the present application of the laser-based point source, a custom optical setup was developed to achieve a focal length large enough for its use in the DNW-NWB. In this report emphasis is, therefore, put on a presentation of the optical design and on the characterization of the resulting sound surce. The shielding results presented herein provide a first look into the acquired database. Future work will be focussed on a finer analysis of the acquired database as well as on the realization of companion numerical simulations aimed at the validation of in-house simulation tools. The work presented herein will also serve as a basis for the elaboration of guidelines for the design of low-noise UCAV configurations.

Empirical prediction of flap tip noise.

In this paper, DLR’s empirical prediction model for flap tip noise is presented and discussed in details. The prediction scheme is based on a comprehensive acoustic and aerodynamic database acquired in the Acoustic Wind Tunnel Braunschweig. It was verified, through successful scaling of the measured noise spectra, that the cross-flow velocity at the flap tip is an important parameter characterizing the flow mechanism(s) responsible for the noise production. This finding led to the definition of a universal flap tip noise spectral shape in terms of a linear least-squares fit of the corresponding measurement data. Using a similar approach, a model for the flap tip noise directivity was formulated. The prediction model was compared against full-scale fly-over measurement data (B747-400 and A319) and an acceptable agreement of the overall predictions was found. A slight underprediction of the noise levels at high frequencies suggests that additional airframe noise sources might be needed in the complete aircraft noise prediction scheme to get a better agreement between measured and predicted noise levels. It is also found that, for large flap deflection angles, flap tip noise dominates the high frequency part of the predicted complete aircraft high-lift noise spectra. Knowledge of the flap tip noise peak frequency and high-frequency decay are therefore sufficient to account for this noise source in the total aircraft noise prediction. Finally, the limitations of the prediction scheme are discussed and research needs are identified.

Numerical and experimental insights into the noise generation of a circulation control airfoil

With the advances in reduction of propulsion related noise from aircraft, airframe noise gets more and more into focus. During approach and landing, the high-lift system of the wings becomes one major acoustic source region contributing to the overall emitted noise. One promising approach to reduce this airframe noise is to change the complete high-lift system from a classic three element slat-wing-flap configuration to a slot-less system with active blowing and droop nose. Preceding experimental investigations have shown, that such a configuration may provide a noise reduction above 2 kHz on the model scale. In the present paper both numerical and experimental investigations concerning the acoustics of a high-lift wing with droop nose and active blowing are presented. Thereby, an insight into the acoustic source mechanisms for different aerodynamic setups is provided that in the future will serve as a basis for the design of a low-noise high-lift configuration. It was found, that in principle three source mechanisms are to be considered. In the low to mid frequency domain, mostly turbulence-geometry interaction noise such as trailing edge noise, jet-nozzle interaction noise and curvature noise from the flow being bent around the flap are supposed to be the driving mechanisms. Moreover, the high frequency domain is found to be dominated by mixing noise from the high speed jet.