Werner Dobrzynski

Bio: Werner Dobrzynski is an academic researcher from German Aerospace Center. The author has contributed to research in topics: Noise & Airframe. The author has an hindex of 20, co-authored 37 publications receiving 1630 citations.

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

Abstract: 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.

Experimental investigations in low-noise trailing-edge design

Abstract: 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 today's 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 DLR's 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.

Airframe Noise Studies on Wings with Deployed High Lift Devices

Abstract: Employment of very quiet high-bypass-ratio engines to propel current and future "very large aircraft" has caused airframe noise to become a significant contributor to the overall radiated noise from an aircraft in landing approach. This has brought about a worldwide resurgence of airframe noise studies, to try and understand the aeroacoustics of, and to ultimately control the aerodynamically caused noise from aircraft components deployed during the final approach leg, such as landing gears and high-lift devices (HLD) on wings. In view of European aviation industry to design and build a very large commercial aeroplane, a substantial and dedicated German National Research Project was initiated, culminating in a series of model- und full-scale wind tunnel experiments on HLD. This paper discusses initial results from HLD-studies in the DLR Aeroacoustics Wind Tunnel Brauschweig (AWB) where an acoustic mirror was employed on a 1/10 scale-model wing section to identify the aeroacoustic source mechanisms of slat-noise and flap side-edge noise. Tests were performed for different flow velocities and wing angles-of-attack, indicating that the very slat tracks constitute sources of excessive flow noise. Moreover, both slat-noise and flap side-edge noise, respectively, were found to each be a result of specific combinations of different unsteady-aerodynamics mechanisms. Several, though still preliminary noise reduction techniques were tested, showing nevertheless significant promise to enable containment of otherwise excessive slat-noise and flap side-edge noise, such techniques being quite feasible for future full-scale application.

Slat Noise Source Studies for Farfield Noise Prediction

Abstract: 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.

Full-Scale Noise Testing on Airbus Landing Gears in the German-Dutch Wind Tunnel.

Abstract: The acoustic flyover signature of modern aircraft in their approach configuration - i.e. with slats, flaps and gears deployed - are often dominated by airframe noise contributions. To study relevant source and radiation characteristics, airframe noise tests were performed in the German-Dutch Wind Tunnel employing- for the first time ever - full-scale landing gears of an A320 aircraft. Farfield noise characteristics were determined for different gear configurations (starting form the base configuration and subsequently covering ever more gear elements with streamlined fairings) at wind speeds ranging from 40 to 78m/s. Aerodynamically generated noise from landing gears turnes out to be of broadband nature with constant levels (in 1/3-oct. bands) up to several kHz. One most essential result therefore is that landing gear noise, is not at all a "low-frequency phenomenon". Moreover, levels increase with the 6th power of flow velocity.

Reduction of Wind Turbine Noise using Optimized Airfoils and Trailing-Edge Serrations

Abstract: Acoustic field measurements were carried out on a 94-m-diam three-bladed wind turbine with one standard blade, one blade with trailing-edge serrations, and one blade with an optimized airfoil shape. A large horizontal microphone array, positioned at a distance of about one rotor diameter from the turbine, was used to locate and quantify the noise sources in the rotor plane and on the individual blades. The acoustic source maps show that for an observer at the array position, the dominant source for the baseline blade is trailing-edge noise from the blade outboard region. Because of convective amplification and directivity, practically all of this noise is produced during the downward movement of the blade, which causes the typical swishing noise during the passage of the blades. Both modified blades show a significant trailing-edge noise reduction at low frequencies, which is more prominent for the serrated blade. However, the modified blades also show tip noise at high frequencies, which is mainly radiated during the upward part of the revolution and is most important at low wind speeds due to high tip loading. Nevertheless, average overall noise reductions of 0.5 and 3.2 dB are obtained for the optimized blade and the serrated blade, respectively.

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

Abstract: 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.