PhD student
Elina
CROS
LMFA PhD student
Additional informations
Home laboratory :
LMFA - Thesis director(s) : Michel Roger
- Thesis supervisor(s) : Gilles Serre
- Start date of the thesis : 15/04/2019
- End date of the thesis : 31/08/2022
- Date of defence : 24/11/2022
Scattering of the tonal noise of a marine propeller by the hull of a ship at very low frequencies: an analytical, experimental and numerical study
This work is devoted to the prediction of the tonal noise of a marine propeller at low frequency and its diffraction by the hull of a ship. The production of sound and its diffraction are inherent to the constraints of installation in industrial applications. Firstly, the propellers of marine vehicles are mounted in the rear part of the hull and operate in the boundary layer developing along the whole length of the hull. This is referred to as the hydrodynamic installation effect. The induced lift fluctuations on the blades radiate as equivalent dipoles. Secondly, the mounting of propellers in close vicinity of the hull makes the sound radiated from the blades scattered by the hull, in such a way that the radiating properties of the sources are strongly modified. This is called the acoustic installation effect. The present work is an in-depth study of these two effects. The emphasis is on the tonal noise at the blade passing frequency and its harmonics. Each tone corresponds to a series of spinning radiation modes. Each mode is exactly reproduced by a circle of phased stationary dipoles, called source-mode. The source-mode formalism is well suited to calculate the diffraction by surrounding obstacles. To this end, the exact Green's function of the cylinder is used in this thesis as a generic configuration of installed marine propellers. The particularity of marine propellers gives rise to a compact regime. The set constituted by the source circle (propeller) and the cross section of the diffracting body (hull) is significantly smaller than the wavelength. An amplification phenomenon is expected in this regime, thoroughly investigated in the thesis. An experiment in air confirms that, with Helmholtz numbers typical of marine applications, the amplification is mainly due to the diffracting effect of the rigid cylinder on the source-modes. These results correspond to key features of the asymptotic cylinder Green's function. The analytical model is also extended by taking into account the free surface. In this configuration the amplification combines with surface reflection to produce a different wave pattern. Finally, an extension is proposed by numerically determining the tailored Green's function for a realistic to hull, and taking into account the real distortions of a flow. The results show that predicting propeller noise in free field is meaningless and that the installation on a ship is a crucial aspect.
https://celya.universite-lyon.fr/members/elina-cros--195953.kjsp?RH=1524471175521
