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TEENI
Turboshaft Engine Exhaust Noise Identification

Tags: Air

State of the Art - Background

Helicopters can generate a large amount of external noise, which can be perceived as aggressive by citizens; a helicopter's traditional missions - rescue, medical, law enforcement - are normally very close to populated areas. As emphasised in the ACARE Strategic Research Agenda 2 (SRA2), the tendency to increase rotorcraft missions in the public vicinity should not lead to an increase of public disturbance.

The main exterior noise sources on helicopters include the main rotor, tail rotor and engine. The turboshaft engine is known to be a major contributor to exterior noise at take-off.

Former projects (Hortia, Silence(R)), or ongoing ones (Friendcopter) have managed to reduce significantly the noise coming out of the exhaust of the engine, and show the industrial evidence that a liner can viably be installed on the exhaust.

Though, in order to comply with ACARE SRA2 objectives, this attenuation has to be maximised for the most dominant engine noise source in flight requiring a better knowledge of the exhaust sound-source balance. Broadband noise at a turboshaft exhaust - generally called core noise - is assumed to be a mix between combustion and turbine noise.

Objectives

TEENI deals with understanding aeronautical noise, in particular helicopter noise, with the goal of noise reduction. As engine noise is a main contributor to the entire exterior noise at take-off, and because exhaust noise still needs an increased attenuation, this project focuses on understanding exhaust noise sources.

The main aim is to determine on which noise source a turboshaft exhaust liner should be optimised. Four objectives can be summarised as follows:

- Discriminate the origin of the sound field radiated from a turboshaft engine's exhaust and highlight the priorities for exhaust liner optimisation on the most prominent engine noise source in flight;

- Develop and test sensors, adapted to fluctuating quantities (such as acoustic pressure, acoustic velocity, temperature fluctuations, etc.) and resistant to the engine environment (temperature, grazing flow);

- Understand how the broadband noise propagates through turbine blade rows;

- Develop and apply noise-source breakdown techniques which are able to locate the noise origin from inside the engine casings.

Tackling this broadband noise identification problem is an ambitious goal, due to:

- The complexity of the physics involved;

- The severe environmental conditions in the exhaust which prevent using standard instrumentation;

- The small space available on this kind of engine.

Description of Work

These important challenges need to be answered in order to maximise liner attenuation. Therefore, to achieve the previously mentioned four objectives, TEENI's work plan includes:

- Innovative sensor development, from design to laboratory test in representative engine conditions;

- New noise-source breakdown techniques, to help discriminate the far-field noise origin with respect to each engine module. This includes the use of both acoustic near-field and far-field arrays, with newly developed algorithms that should provide an insight into noise generation from the exterior;

- Basic studies, including rig experiments, to understand the propagation effects of broadband noise through blade rows. These studies will also give an opportunity to verify noise-breakdown techniques and algorithms;

- New instrumentation and source-breakdown techniques will be applied to a full-scale engine test;

- Study the development in HELENA (numerical platform developed in Friendcopter) of the source-breakdown capability;

- Using this new tool, and the TEENI and Friendcopter outputs, estimate the engine-noise source, which should be reduced.

Expected Results

The most important TEENI deliverables will be:

- A set of sensors for measuring unstationnary quantities, adequate for full-scale engine testing (650°<T< 1000°C);

- A noise breakdown technique selected out of a panel of methods due to an adapted treatment of the engine-noise database;

- A thorough understanding of noise generation, propagation and radiation through the exhaust;

- A comprehensive full-scale engine-test database;

- A ranking of exhaust-noise sources, with a recommendation on the noise source to be reduced.

Teeni

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