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VALIANT
VALidation and Improvement of Airframe Noise prediction Tools

Tags: Air

State of the Art - Background

The overall noise radiated by modern aircraft has two sources which are quite balanced at approach: the engine and the airframe. The airframe noise (AFN) has a broadband character and is mainly due to the interaction of the turbulent airflow with the high-lift devices (slats and flaps) and landing gears, and to a lesser extent to cavities, spoilers and to boundary layers developing along the fuselage.

One of the major ACARE objectives is a reduction in perceived noise level of fixed-wing aircraft by 50% by 2020 compared to 2001. In achieving this required breakthrough towards quieter aircraft, reducing AFN is very important now and will become even more important in the future, especially for large aircraft, due to the already anticipated development of quieter engines.

With the clearly expressed tendency of the modern airframe industry towards virtual prototyping and the increasing reliability of the design cycles on numerical simulations with the experimental verifications performed only at later stages of the design cycle, it is of utmost importance to increase the trust in noise predictions. However, the complexity and diversity of broadband turbulent AFN sources makes that prediction and subsequent reduction with present numerical tools extremely challenging.

Objectives

VALIANT is tackling this challenge by generating new experimental data, and validating and improving numerical tools for predicting AFN generated from landing gears, slats, flaps and local separation regions.

Due to the extremely complex physical nature of the phenomenon and the high cost of computing full aircraft configurations on the one hand, and the lack of a reliable experimental database on the other, VALIANT focuses on key generic test cases revealing the basic mechanisms of AFN generated by the most 'noise-dangerous' elements of a real aircraft:

- turbulent flow over a gap;

- flow past an airfoil with a flap;

- flow past an airfoil with a slat;

- flow past two struts (landing gear).

These four generic flows actually 'cover' the most important sources of AFN generated by a real aircraft and therefore their study provides a sufficient basis for evaluation and improvement of the Computational Aero Acoustic (CAA) tools aimed at predicting AFN. On the other hand, these flows are 'simple' in the sense that their accurate simulation and noise generation are computationally affordable.

For all these configurations, the components of the noise prediction chain (for the turbulent/source region, and near- and far-field propagation domains) and their mutual interactions are evaluated and avenues of improvement developed.

Description of Work

The project is divided into four technical work packages (WP).

WP1 focuses on generating a detailed and reliable experimental database for validation purposes for the four generic test cases listed above. Expected results are steady and unsteady aerodynamic data, as well as noise-source localisation and far-field noise spectra and directivities. Acoustic measurements will be performed in a cheap large aerodynamic wind tunnel.

WP2 is aimed at a thorough assessment of currently available CAA approaches in terms of turbulence and acoustics by comparing both with each other and with the experimental data. This systematic comparison will highlight both strong and weak points of the numerical approaches and suggest avenues of further improvements on the weak points within WP3.

Based on the remedies proposed in WP2, the CAA approaches will be improved in terms of turbulence representation and near-, mid- and far-field noise predictions. WP3 aims also at improving the analytical methods which are of significant importance to assess the noise around airports in term of EPNdB (Effective Perceived Noise in Decibels) and to help interpretation of the numerical results.

WP4 assesses the influence of the successive improvements in the numerical and analytical approaches and identifies the best-suited AFN prediction tools to be integrated into industrial processes in the future.

Expected Results

This project, being an essential step towards new efficient AFN reduction concepts and their optimisation, impacts the EU directly by:

- providing a high quality experimental database for validation on broadband noise associated with generic configurations representative of the most 'noise-dangerous' AFN mechanisms;

- validating and improving Computational Fluid Dynamics (CFD) / CAA tools for broadband AFN prediction within an expected accuracy of 1 dB, and generating a detailed numerical database;

- identifying the best suited AFN prediction tools which may have the potential to be integrated into industrial processes in the future, for designing efficient AFN reduction technologies expected to result in a further 3 - 5 dB overall AFN gain during approach (compared to 2000 state of the art).

VALIANT will also improve cost efficiency by reducing the design and development costs. This will be achieved by providing efficient AFN prediction tools in terms of CPU time reduction and by allowing a partial replacement of extremely expensive experiments aimed at testing and optimising AFN reduction technologies by reliable numerical predictions.

Finally, VALIANT will promote the participation of organisations from International Co-operation Partner Countries by building a strong collaboration with Russia, relying on their complementary expertise in aero-acoustics.

Example of AFN source localisations of a landing A340 with high-lift device and landing gears (from flight test campaign)
Example of AFN source localisations of a landing A340 with high-lift device and landing gears (from flight test campaign)
High-lift device broadband noise mechanisms to be investigated within VALIANT
High-lift device broadband noise mechanisms to be investigated within VALIANT

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