VALidation and Improvement of Airframe Noise prediction Tools
The overall noise radiated by modern aircraft has two sources: the engine and the airframe. The airframe noise has a broadband character and is mainly due to the interaction of the turbulent airflow with high lift devices (slats and flaps) and landing gears. And to a second extent to cavities, spoilers and boundary layers developing along the fuselage.
One of the major ACARE objectives was the reduction of perceived noise level of fixed-wing aircraft by 50% by 2020 compared to 2001. In achieving this, reduction of airframe noise is very important especially for large aircraft, as the development of quieter engines continues.
The focus of the project was on broadband noise generated by the turbulent flow around the airframe, as it is one of the most important components of aircraft generated acoustic nuisances. Its prediction and subsequent reduction was essential for achieving the ACARE2020 objectives of noise level reductions.
However, the complexity and diversity of broadband turbulent noise sources makes that prediction with present numerical tools extremely challenging and far from mature.
The objectives of the VALIANT project were to generate new experimental data and to validate and improve numerical tools for prediction of airframe noise generated from landing gears, slats, flaps and local separation regions.
The VALIANT project was an upstream research-oriented project, with the objective to tackle this challenge by generating new experimental data and validating and improving numerical tools for prediction of airframe noise generated from landing gears, slats, flaps and local separation regions.
Due to the extremely complex physical nature of the phenomenon and the high computational cost of computing full aircraft configurations on the one hand, and a lack of a reliable experimental database on the other hand, VALIANT focused on key generic test cases representing the major broadband airframe noise mechanisms associated with multiple body interactions: flow past two-struts (landing gear), flow past air foil with flap, flow past air foil with slat and turbulent flow past a gap. For all these configurations, the components of the noise prediction chain (for turbulent/source region, near- and far-field propagation domains) and their mutual interactions are evaluated and avenues of improvement developed.
It was expected that by validating and improving the predictive tools, a deeper insight into the mechanisms behind airframe noise would be obtained, which is an essential step towards new efficient airframe noise reduction concepts and their optimisation to achieve the required breakthrough towards quieter aircraft.
The consortium was formed by 12 European and Russian partners: 2 universities and 7 research establishments including the most important technology providers to the airframe industry, and 3 companies (2 SMEs) providing dedicated engineering services and software tools in aero acoustics to transport industries with emphasis on aeronautics.
- Provision of a benchmark database for the computations.
- Obtaining more insight in the basic noise generation mechanisms which enabled enable a thorough assessment of the numerical results NLR performed both far-field and microphone array measurements in order to determine the noise directivity and the source localisation for different regimes of speeds and angles of attack of the two-struts alignment.
Besides these acoustic measurements NLR obtained pressure data on the surfaces of the struts and VKI performed PIV measurements.
CIMNE, IMM, NUMECA and TUB performed numerical simulations on three distinct configurations of this test case, while NLR has been leading the experimental campaign. CIMNE has performed CFD simulations using a variational multi-scale technique. The inhomogeneous Helmholtz equation has been solved to compute the acoustic pressure in the far-field. IMM uses a hybrid Detached Eddy Simulation (DES) method for the numerical representation of the compressible viscous flow while the aeroacoustic analysis is based on the FW-H formulation. NUMECA has performed LES simulations and completed the aeroacoustic analysis based on the FW-H formulation. TUB performed the CFD simulations using a DDES technique and completed the aeroacoustic analysis based on the FW-H formulation.
Near-field and far-field results obtained by the different numerical partners have been extensively compared with experimental data. The computed time-averaged flow-fields are overall in good agreement with the experimental data. The discrepancies that are observed for some quantities (i.e. Cp values on the downstream side of the second strut, turbulent intensities between the two struts for the inclined configurations) are similar for all LES-based results (IMM, NUMECA TUB).
The same post-processing approach has been used to evaluate near-field and far-field spectra from unsteady pressure signals. The dominant peaks in the solid-wall spectra are well captured for both struts with maximum discrepancies of about 5 dB in level and about 20 Hz in frequency.
Again, the same discrepancies with respect to the experimental data are generally observed for all LES-based results. Since the simulated spanwise length is not the same as in the experiments, a correction had to be applied to the computed far-field noise. This correction allows for some freedom in the choice of the numerical values for certain parameters. Even though more uncertainties are added through this correcti
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