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Aerospace: Propulsion Noise

Noise from the aircraft jet engines is still the dominant source of community noise during take-off, and one of the dominant sources (together with airframe noise) during approach. Government regulations of community noise are getting more and more stringent in most countries, creating significant challenges for aircraft manufacturers forced to meet these ever increasing regulatory requirements.

The main causes of propulsion noise are jet noise–the sound created by the turbulent mixing of the jet and the surrounding air, fan noise–the interactions of turbulent flow in the turbine with rotating fans, compressors and turbine systems, and installation noise– the noise caused by interaction of the jet with other components of the airplane, in particular high-lift wings.

TECHNICAL CHALLENGES

The prediction of jet and fan noise is among the most challenging tasks for computational fluid mechanics. The noise generation mechanisms are not fully understood yet even on a theoretical level. Prediction methods based on traditional CFD technology require use of advanced LES turbulence models for which productive use for complex real-life engine geometries is still many years away. Therefore propulsion noise engineers have to rely almost exclusively on experimental tests today, with many significant limitations. Most tests cannot be performed at installation velocities and are generally limited to standalone configurations of the engine only. Installation noise can generally only be tested during late stage flight tests, leading to the requirement of high safety margins and overdesign to avoid noise related certification issues.

Visualization of turbulent flow structures in a three-dimensional jet

EXA SOLUTION

Exa’s innovative Lattice-Boltzmann based programs have the potential to overcome the issues that prevent broader use of CFD for engine noise prediction. Even with currently released versions for low-speed flows PowerFLOW® has been shown to have fundamental advantages in the prediction of the complex noise prediction mechanisms in turbulent jets. Together with the proven ability to represent complex geometries–such as modern chevron design – PowerFLOW opens completely new possibilities for jet and installation noise prediction.

Visualization of sound waves emanating from a two-dimensional jet

For the prediction of fan noise PowerFLOW’s easy-to-use rotating geometry capability enables the prediction of noise caused by the complex interaction of rotating fan and stator – taking into account all geometry details that can potentially impact noise generation mechanisms.

Visualization of stator-rotor interactions on the Advanced Noise Control Fan

Above: Band pass filtered pressure fluctuations in the fluid volume and on the surface of an experimental fan around the first blade passing frequency (480Hz), showing the propagation of the combined radial and circumferential modes.

EXA SOFTWARE USED FOR THIS APPLICATION

Simulation Preparation: 
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