High-lift Noise Research Articles

SFB 880: aeroacoustic research for low noise take-off and landing

This paper gives an overview about prediction capabilities and the development of noise reduction technologies appropriate to reduce high lift noise and propeller noise radiation for future low noise transport aircraft with short take-off and landing capabilities. The work is embedded in the collaborative research centre SFB 880 in Braunschweig, Germany. Results are presented from all the acoustics related projects of SFB 880 which cover the aeroacoustic simulation of the effect of flow permeable materials, the characterization, development, manufacturing and operation of (porous) materials especially tailored to aeroacoustics, new propeller arrangements for minimum exterior noise due to acoustic shielding as well as the prediction of vibration excitation of aircraft structures, reduced by porous materials.

Conceptual study of micro-tab device in airframe noise reduction: (II) 3D computation

A three-dimensional numerical study is conducted to better understand noise reduction results seen in the previous two-dimensional investigation of the acoustic effects of micro-tab device on airframe noise reduction. Without sacrificing the aerodynamic performance, it is possible to achieve high-lift noise reduction with the application of the micro-tab device attached to the pressure side of the flap surface near its trailing-edge. This study was carried out by numerical hybrid method, which combines Computational Fluid Dynamics and acoustic analogy to predict the farfield noise spectrum. The near-full-scale computational results show that the micro-tab device with reduced deflection of the high-lift devices achieves noise reduction in mid-to-high frequency domain, in particular the range that human beings are most sensitive to. In addition, a parametric study in terms of geometric variation of the micro-tab was also investigated and reported. The three-dimensional results obtained thus far show reduction in noise levels with use of micro-tab.

Almost 40 Years of Airframe Noise Research: What Did We Achieve?

With the advent of low noise high bypass ratio turbofan engines airframe noise gained significant importance with respect to the overall aircraft noise impact around airports. Already around 1970 airframe noise, originating from flow around the landing gears and high-lift devices, was recognized as a potential “lower aircraft noise barrier” at approach and landing. Since then, the outcome of extensive acoustic flight tests and aeroacoustic wind tunnel experiments enabled a detailed description and ranking of the major airframe noise sources and the development of noise reduction means. In the last decade advances in numerical and experimental tools led to a better understanding of complex noise source mechanisms. Efficient noise reduction technologies were developed for landing gears while the benefits of high-lift noise reduction means were often compensated by a simultaneous degradation in aerodynamic performance. The focus of this paper is not on the historical sequence of airframe noise research but rather aims to provide a concise survey of the achievements in airframe noise source description and reduction over the last 40 years worldwide. Due to the vast amount of work focused on a variety of airframe noise problems, this review can only provide examples but does not claim to be complete.

Conceptual study of micro-tab device in airframe noise reduction: (I) 2D computation

A two-dimensional numerical study was performed to investigate the acoustic effects of micro-tab device on airframe noise reduction. As the noise generated by leading-edge slat and trailing-edge flap rise with their increased deflection angles, it is possible to mitigate such high-lift noise by using reduced settings without sacrificing the aerodynamic performance during approach. In this paper, micro-tab device attached to the pressure side of the flap surface is envisioned as a mean to achieve this goal. Hybrid method involving Computational Fluid Dynamics and acoustic analogy was used to predict the far-field noise spectrum. Results illustrate that the micro-tab device with reduced deflection angles of the high-lift settings provides lower noise signature at far-field positions, comparing to the baseline configuration, while the aerodynamic performance is maintained. In addition, two parametric studies which investigated the effects of micro-tab location and micro-tab height on acoustic spectra were also included.

Effect of Mictrotab on Reduction of Noise Due to Aircraft High-Lift Devices

D EFINED as the “nonpropulsive noise of an aircraft in flight” in [1], noise produced by airframe components becomes significant during the approach-to-land phase of aircraft operations. Research efforts on identifying and understanding the noise sources associated with the individual airframe components, in particular, high-lift systems, have been undertaken by many researchers [2]. It has been confirmed experimentally in [3–5], and analytically by using empirical methods [6], that both leading-edge slats and trailing-edge flap side edges are the most important noise contributors from the high-lift systems to overall noise levels of an aircraft in approach. The high-lift noise, intuitively thinking, is configuration dependent. Any increase in the deflection angle of slat and flap will lead to an increase in overall noise level. The larger the deflection angles are, the higher the noise level will be. This is due to the fact that the highlift components are more loaded when their deflection angles are increased, generatingwhat is referred to as the loading noise. Further, higher deflection angles may induce stronger vortex shedding and turbulent wake downstream which causes higher noise. Such intuitions were supported by experiments conducted at NASA Langley Research Center in 1998 and 2002 [7], which revealed that slat noise is generated due to its configuration setting angles. Khorrami et al. [8] hypothesized with 2-D unsteady computation that the vortex shedding at the slat trailing edge was responsible for the additional highfrequency spike observed only at higher slat angles. An acoustic whistling mechanism at the slat gap due to vortex shedding was later proposed by Tam and Pastouchenko [9] and Agarwal [10]. The theory supported that the simple vortex shedding and feedback mechanism resonating with gap tone frequency caused noise amplification and propagation. Similar observations were made for flap noise, as reported in [11,12]. Both references reported that the measured noise level at higher flap deflection angles exceeded the predicted noise level, which suggests that there might be an additional source contributor, most likely vortex instabilities, as a result of its interaction with the nearby flap upper surface. It may be noted that such phenomenon was not seen at lower settings of flap angles. These observations may infer that high-lift device noise level is directly affected by the configuration setting angles of slats and flaps. In general, high-lift device noise sources are present at wide frequency ranges of low-to-mid andmid-to-high. At a fixed observer location, under a constant flow speed, slat noise is dominant in the low-to-midfrequency range, and flap side-edge noise is dominant at the mid-to-high-frequency range as shown by both Guo and Joshi [13] and Guo et al. [14]. In this study, the proposed technique uses reduced deployment angles of both high-lift devices simultaneously to achieve the most effective noise reduction in different frequency domains. Aerodynamics performance also has to be carefully monitored to ensure no significant loss is present. The objective of the current Note is to examine the effectiveness of a microtab device in airframe noise reduction. Reducing deployment setting angles of both slats and flaps, the associated noise can be alleviated. Loss in aerodynamic lift, as a result of the reduced highlift settings, is compensated by use of a microtab device. The microtab device can be viewed as a small spanwise strip located at the pressure side of flap near the trailing edge, similar to a Gurney flap (Fig. 1). While deployed normal to local airfoil surface, the microtab device alters the local flowfield which leads to increased effective camber, and hence improves lift-to-drag ratio, similar to that of a Gurney flap introduced by Liebeck [15]. In the present work, a threedimensional numerical analysis is presented and compared to sound pressure level (SPL) of the proposed microtab configuration at a specified far-field location to that of the baseline, that is, conventional configuration in approach, to see if any reduction in noise can be achieved.

Reduction technologies and prediction of high lift noise

In the final approach phase airframe noise represents the ultimate aircraft noise barrier for future aircraft when equipped with quiet UHBR engines. This paper presents some of the main dependencies of high-lift noise sources on flow conditions as derived from extensive noise testing at DLR. Some successful reduction technologies for high-lift noise are discussed in view of their influence on the noise generation mechanisms. The remainder of the paper focusses on the numerical prediction of high-lift noise. The prediction concept as implemented in DLR's PIANO code is discussed. It is based on a two step approach, a) the computation of the time averaged turbulent flow field as a result of a CFD(=Computational Fluid Dynamics) simulation and b) the highly accurate CAA (=Computational Aeroacoustics) simulation of perturbations about this mean flow. The prediction relies on an advanced stochastic modelling of the turbulence related sources and thus avoids time consuming turbulence simulations. The method represents an accurate yet "low cost" prediction of high-lift noise. This is particularly important for real applications at flight (very high) Reynolds numbers and corresponding design trade-off studies, which typically require the computation of large numbers of configurations. Examples for such kind of applications are given.