PhD opportunity in passive noise control of underwater structures

Project Description : A fluid flowing over the surface of a structure produces a turbulent boundary layer (TBL). The pressure field beneath a TBL excites the structure and causes the structure to vibrate, which generates a sound wave that radiates noise away from the structure. This effect is observed in many engineering applications, including over the surfaces of aircrafts, trains and marine vessels. Furthermore, many of these surfaces have a viscoelastic coating applied to them in order to control noise from adjacent machinery, as well as lowering noise from incident sound waves. The acoustic characteristics of these viscoelastic coatings do therefore play an important role in the overall performance of these structures. The examination of noise radiation from the outer surface of a marine vessel is challenging because of the complexity of the structures. For example, most shell structures need to be strengthened using ribs and this changes the vibration characteristics of the structure, as well as any viscoelastic material coating the structure. Moreover, the viscoelastic materials are often laminated to enable the absorption of incoming sound waves, as well as reducing machinery noise from the vessel. This project will investigate the radiation of sound from viscoelastic coatings excited by a TBL to improve our understanding of the interaction between turbulent boundary layers and acoustically coated structures through the development of new advanced theoretical models that provide a deep understanding of the underlying physical mechanisms associated with the vibroacoustic behaviour of coated structures induced by a TBL.

Research Fields: Mechanical engineering, Vibrations, Acoustics, Numerical and analytical modelling, Experimental analysis.

Project Description : The control of noise radiated from stiffened thin shells like the fuselage of an aircraft or the pressure hull of a submarine is of prime importance for industries. The thin shell is generally stiffened regularly by internal frames in order to resist to static loads keeping the structure as light as possible. However, it is well known that the periodic arrangement of these internal frames induces propagation of Bloch-Floquet waves in the structure which increases the radiation efficiency of the shell. In this thesis, we propose to study two innovative means to reduce the noise radiated by stiffened cylindrical shells: 1. Embedding acoustic black holes (ABHs) in the internal frames: An ABH is a passive and lightweight device for the control of noise and vibrations. It basically consists of a retarding wave guideline that slows down impinging waves and concentrates them at the ABH centre, where energy gets dissipated by means of viscoelastic materials. 2. Attaching locally resonant metamaterials (LRMs) to the internal frames: LRM involve periodically or randomly arranged resonators which are designed to manipulate waves in targeted frequency ranges, which are called band gaps. The two concepts can then be mixed to optimize the radiated noise reduction in the whole frequency range of interest. To date most researches on ABH and LRM have focused on the control of the vibration and sound radiation from academic structures like beams, plates and cylinders. This work will study the efficiency of these devices in the presence of a thin vibrating structure exhibiting Bloch-Floquet waves which are induced by the periodic arrangement of the stiffeners. In the first step, analytical and numerical models will be developed to analyse the physical phenomena involved when the stiffened shell is coupled to these devices. In the second step, parametric analyses will be performed to find the optimal parameters and design to reduce the radiated noise for realistic configurations. Finally, the most promising designs will be investigated experimentally in the lab.

Research Fields: Mechanical engineering, Vibrations, Acoustics, Numerical and analytical modelling, Experimental analysis.

Project Description : In industrial applications, noise and vibrations from naval structures, such as underwater vehicles or surface platforms, need to be controlled. Numerical models are developed to predict the vibro-acoustic behaviour of these structures, in order to assess their radiated noise at the early design stages. On the one hand, classical methods based on discretization approaches, such as the Finite Element Method or the Boundary Element Method are difficult to use in practice, for two main reasons: 1/ the strong coupling with the surrounding fluid needs to be taken into account, 2/ the structure is large in comparison with the wavelength of acoustic and structural waves in the mid-frequency range, leading to prohibitive calculation costs. On the other hand, analytical methods cannot account for the structural complexity of industrial systems. That is why sub-structuring methods as the CTF (Condensed Transfer Function) method have been developed to address the problem on a wide frequency range. The CTF approach consists in partitioning the global system in different subsystems, to characterize independently each subsystem by condensed impedances or admittances and finally, to assemble them to predict the vibroacoustic behaviour of the global system. This approach has been so far developed only for deterministic subsystems. However, deterministic approaches are not always best-suited for all subsystems, and as frequency increases, statistical approaches (like the Statistical Energy Analysis method for instance) can be more relevant. The aim of this PhD is to extend the formalism of the existing CTF method to be able to include uncertain subsystems described by statistic parameters. Applications of the developed model on naval structures will give us more insights on mitigation of noise and vibrations including the presence of uncertainties.

Research Fields: Mechanical engineering, Vibrations, Acoustics, Numerical and analytical modelling.