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c noise from a turbulent boundary layer is a fundamental topic in flow-induced noise and is of interest for both fundamental studies and applied research. The action of the pressure fluctuations indeed provides the driving force to excite surface vibrations and produce acoustic radiation. Many engineering problems are connected with this topic. Fatigue loading on panels of an aircraft fuselage and the vibrational generation of acoustic radiation into an aircraft cabin enclosed by the boundary surface, R. Camussi (Ed.), Noise Sources in Turbulent Shear Flows: Fundamentals and Applications, CISM International Centre for Mechanical Sciences, DOI 10.1007/978-3-7091-1458-2_6, © CISM, Udine 2013

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R. Camussi and A. Di Marco

are two examples among many. Generally speaking, in high speed transport technology, the understanding of the physical mechanisms underlying the generation of pressure fluctuations at the wall, has received increasing attention in view of the use of lightweight and flexible structures. In the field of aerospace launch vehicles design, this problem is of great relevance since vibrations induced in the interior can cause costly damages to the payload while panel vibrations of the external surface must be avoided to prevent fatigue problems and structural damages. In the context of marine transportation, this topic has become quite important in the case e.g. of high-speed ships for passenger transportation where requirements of on board comfort have to be satisfied. This concern has become of great importance for ground transportation as well, notably for high speed trains design. In this case, the effect of pressure fluctuations induced by flow separations (e.g. due to the pantograph cavity) becomes the dominant noise producing mechanism, this situation being of relevance in the automotive industry in general, since large flow separations are unavoidable on cars. The vibration of a panel induced by a random pressure load leads to acoustic radiation into the flow as well. Also this problem is of relevance for many engineering applications including, for example, the generation of noise from piping systems or the transmission of pressure waves by underwater vehicles, the so–called acoustic–signature. Due to its importance, since the early 1960s, researchers have been studying this subject using different approaches including experimental investigations, numerical simulations and theoretical speculations. When a solid surface is overflown by a turbulent boundary layer, several relevant mechanisms contributing to the generation of sound waves, can be identified. To simplify the description, consider the case of a panel subject to a flow on one side. The pressure field on the surface flow side consists of the sum of the turbulence pressures which would be observed on a rigid wall and the acoustic pressures which would be generated by the plane motion in the absence of turbulence. At a first approximation, these two effects can be studied separately. This idea represents the so-called w eak coupling approximation and can be

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