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DLR measurements reveal noise-generating structures in engine jets

Researchers at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) have, for the first time, been able to use laser measurement techniques and microphones to make highly precise, simultaneous measurements of the sources of noise in the air stream behind an aircraft engine.

Aircraft engine noise is a socially pressing issue with a wide range of causes. Until now, turbulent fluctuations in the exhaust gas stream have not been fully understood as one of the major sources of noise. The DLR team managed to make these turbulent flow structures in the engine exhaust gases visible using imaging the laser measurement technology, and measured the overall flow behind the engine with unprecedented quality.

On 23 September 2014, DLR presented the results of the SAMURAI (Synergy of Advanced Measurement techniques for Unsteady and high Reynolds number Aerodynamic Investigations) research project at Hamburg airport. From 23 to 28 September 2013, engine tests supported by Hamburg Airport and Lufthansa Technik (LHT) were conducted in the LHT soundproof hangar using DLR’s A320-ATRA research aircraft.

We were able to use this experiment to identify specific flow and density structures that cause some of the noise in the engine exhaust stream. The comprehensive data obtained will help with modern simulation methods that will predict the engine flow and the noise generated in it in greater detail.

—project leader Andreas Schröder from the DLR Institute of Aerodynamics and Flow Technology in Göttingen

To visualize the turbulent speed and density fluctuations, the scientists used optical measurement systems that work with special cameras and lasers and are synchronized with an array of microphones.

In the experiment, we used the microphones as observers that listened from a distance. The microphone signal acted as a filter for simultaneously identifying the measured flow structures that generate noise.

—Andreas Schröder

Very small light-scattering particles rendered the flow patterns visible in pulsed laser light. The Particle Image Velocimetry (PIV) measurement technique developed by DLR in Göttingen is a versatile tool for flow research.

ATRA_LHT_SAMURAI-528_breit
Laser measurements on the engine of the DLR ATRA research aircraft. Click to enlarge.

The researchers have used the combined visual and acoustic measurement technology to build a large library of data that significantly improves the understanding of the causes and distribution of jet noise.

The critical issue is how the flow in the engine can be better influenced to reduce the noise behind it. Options here include slowing down the engine jet and combining hot turbine exhaust gases with the cold air outside the rear of the engine faster.

—Lars Enghardt, who is Head of the Engine Acoustics Department at the DLR Institute of Propulsion Technology in Berlin

The first developments in this direction have already been made, such as the chevron nozzle with its serrated trailing edge. There is a wide range of options for reducing stream noise even more. The measurement results now available could be an important basis for helping engine manufacturers develop quieter engines with less jet noise, suggested Enghardt.

The DLR ATRA (Advanced Technology Research Aircraft) used in Project SAMURAI is an Airbus A320 airliner that is used by DLR for a wide range of aviation research projects. The medium-haul passenger jet, which has been converted into a flying test bed, is an ideal research subject for aircraft noise research. It has V2500 range engines, which were developed in a collaboration between MTU, Pratt & Whitney, Rolls Royce and Japanese Aero Engines Corporation; thousands of them are in use all over the world. The ATRA research aircraft is operated by DLR Flight Experiments. The DLR Institutes of Aerodynamics and Flow Technology, Propulsion Technology, Aeroelasticity, and Structures and Design are participating in DLR’s Project SAMURA.

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