Aeroacoustic Research at German Aerospace Center (DLR)

Abstract

The German Aerospace Center (DLR) is the largest publically funded research establishment in Europe. DLR is closely collaborating with Universities and industrial partners around the world on Aeroacoustic problems, mainly focusing its engagement on applications in the area of civil aviation. Aero-engines of large transport aircraft are still the most significant noise sources on modern aircraft. The DLR works on all known major contributors to engine noise, i.e. the fan, the jet, the combustor and the LP turbine. Both experimental, analytical, and numerical investigations are conducted to assess the noise sources, to understand the physical noise generation processes, and to provide minor geometrical changes to retrofit actual designs or whole new low-noise designs for the next generation of aero-engines. For the turbomachinery noise components, the tone excitation process is well documented in literature, whereas the broadband noise components gain more and more attention, as the attenuation of this important noise source is still inhibited by a limited understanding of the dominant source mechanisms. Consequently, DLR was increasing their research activities on broadband noise in recent years. In the field of combustion noise, indirect noise sources like entropy or vortex sound were taken into account in actual research. Moreover, a lot of effort is spent on liner research, mainly concentrating on hot stream liners and novel liner concepts. Recently established experimental facilities enable the enhancement of existing liner designs and the optimization for the application in hot environments. The sound radiation from transport aircraft is not entirely dominated by the engines and propellers alone. The presence of the flying aircraft as such is responsible for a) sound generated by the turbulent flow past its airframe (“airframe noise”) and b) acoustic installation effects, which may modify the acoustic radiation from the propulsion source. The airframe noise typically originates from the landing gear and the high lift system. Landing gear noise typically dominates for long range aircraft, while high lift noise becomes an issue for short range aircraft. As a result of long term research, partly with European partners DLR has been able to develop flyable technologies to reduce landing gear noise on the order of 6dB. The reduction of high lift noise is considerably more complex since the aero sound is generated at an aerodynamically highly optimized component, making any changes in favor of less sound generation a complicated design compromise. While the silencing of the flap side edges as important high lift sources could be demonstrated using flow permeable tips, the leading edge slat remains a very challenging noise reduction task. DLR has developed and tested concepts, capable of reducing high lift noise by some 3-4dB without compromising the high lift requirements on the aircraft. There is a multitude of sources due to the fact that the various aircraft components aerodynamically influence one another once they are installed at the aircraft. The excess flap sound generated by the turbulent wake of a landing gear or a turbulent jet has been studied as well, including noise reduction technology.