Sources of Noise from Boundary Layers over vehicles – SONOBL

The project addresses the interior noise generated by the turbulent boundary layer around modern transportation vehicle such as aircrafts, high speed trains and automotives. The excitation by the wall turbulence usually constitutes a major internal noise contribution during cruising trip. The random character of the pressure distribution induced by the boundary layer has a major effect on the noise transmitted through the fuselage and its cross-spectral density plays a major role in determining the effective force causing motion to the structure.

The objectives of the project are twofold: from a more fundamental point of view, the question of the contribution of the acoustic part of the pressure fluctuations will be raised. In particular, the comparison between a well-controlled experimental database and simulations (using Direct Noise Computations) should help to clarify some issues. One of the most important goals is to provide a non-ambiguous quantification of pressure levels in the acoustic domain. The role of shear stresses and the behaviour at very low wave numbers are also fundamental issues considered in the present project. Hypotheses made in semi-empirical modelling will be investigated and improved with regard to experimental and numerical databases. From a more industrial point of view, the reliability of existing semi-empirical models used for design is uncertain, notably for high subsonic Mach numbers characteristic of aeronautic applications (M=0.8), and for non-equilibrium boundary layers, where the spatial homogeneity assumption is broken by a pressure gradient or a surface discontinuity. Simulation tools on academic cases can be used to help understanding the modifications of the excitation sources. A validation step is crucial where experiments and DNC are compared in a similar flow configuration. DNC simulations can then be used to tackle M=0.8 configuration, the effect of a pressure gradient, and the presence of a forward or backward facing step. The analysis of the results and improvement of semi-empirical model should provide the designer with predictive tools for the boundary layer induced noise over representative structures and realistic Mach conditions.

The project will thus deliver enhanced models (numerical and semi-empirical) for the pressure fluctuations beneath a turbulent boundary layer (notably in the acoustic domain and for the effect of pressure gradients) over complex structures. Guidelines for the industrial use will be derived.