Roberto Camussi, and Alessandro Di Marco

Universita Roma Tre,

Dipartimento di Ingegneria Meccanica e Industriale,

Via della Vasca Navale 79, 00146, Roma, Italy

Abstract Boundary layer noise concerns the generation of acous­tic waves as an effect of the interaction of a fluid with a moving surface. Several issues are related to the noise generation mecha­nisms in such a configuration. In the present description we focalize mainly onto the case of an infinite flat plate and two main distinct situations are considered. The first one deals with the prediction of the far field noise as accomplished from the classical integral the­ories, and the main formulations, including Curle’s approach, are briefly reviewed. A novel approach based on the computation of the surface transpiration velocity is also presented. The second aspect concerns the interior noise problem and it is treated from the view point of the fluid dynamic effects rather than from that of the struc­tural dynamics. Attention is focused on the statistical properties of the wall pressure fluctuations and a review of the most effective theoretical models predicting statistical quantities is given. The discussion is completed by a short review of the pressure behavior in realistic situations, including the separated boundary layers in incompressible and compressible conditions and the effect of shock waves at transonic Mach numbers.

1 Introduction

Aerodynamic 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

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 weak coupling approximation and can be derived from an acoustic analogy analysis of the problem [see e. g. Dowling (1983) and Howe (1992)]. The hypothesis that the basic turbulence structure is unaffected by the acoustic motions is indeed the basis of the acoustic analogies and can be accepted if the acoustic velocities are small in comparison with the turbulence velocities. This position, even though not always satisfied, has become accepted as a standard method even at supersonic flow speed [see also Graham (1997)]. The main reason for this is that fully coupled computations are, at present, prohibitive for any length scale of practical relevance even with the most powerful computer resources. On the other hand, the engineering design still nowadays requires simple models which allows fast understanding and rapid computations.

In view of such considerations, in the following discussions the wall can be considered as a rigid plate and the panel vibrations considered apart.

The problem of the boundary layer noise can then be treated consider­ing two different aspects, namely the so-called community noise and the interior noise. The first term applies to the effect of the acoustic waves generated by the wall turbulence and evolving in the far field from the flow side of the surface. The second one pertains with the transmission of noise at the side of the surface in still air. In both cases, the attempt to predict the noise emission is based on the correct representation, in a statistical sense, of the random load acting on the surface. For this reason, most of the discussions that follow are concerned with the clarification of the prop­erties of the wall pressure field and the predictability of its main statistical properties. In Figure 1 an overall view of the mechanisms generating sound waves including the definitions adopted therein is reported. Figure 2 evi­dences the topics faced in the present chapter. The problem related to the interior noise is treated in more details in the second part of this chapter where the theoretical background regarding the noise transmission trough solid structures is presented.

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