Characteristics of the fluctuating pressure field beneath the turbulent boundary layer attached to the hull of a high speed vessel

The fluctuating pressure field generated by the turbulent boundary layer is one of the main sources of excitation and radiated noise for aircraft as well as for marine vehicles when for instance the induced vibrations in the hull plate of high-speed vessels or in the sonar dome are considered. In this frame, the hydrodynamic load acting on an elastic structure is usually described by the wave-number frequency spectrum that is a quantity difficult to obtain by direct measurements and it is usually provided by mathematical models deduced from the behaviour of the normalised cross spectrum. However also this last analysis is not easy to perform because the finite size of the pressure sensors implies a poor spatial resolution and signal attenuation at high frequency. On the other side, the background noise of the facilities often causes contamination of the data at low frequency. The wall pressure fluctuations spectra, when considering a body completely immersed in a fluid, depend on Reynolds number while, in the case of a surface piercing body such as a ship, a contribution from the Froude number must be taken into account hence introducing a new variable in its description. In view of the evaluation of the noise radiated on board of civil high speed vessels generated by flow exciting vibrations, an experimental campaign aimed to measure the pressure fluctuations beneath the turbulent boundary layer attached to the hull of a ship model was performed in a towing tank. The chance to use this kind of facility, characterised by a very low background noise, allows also the description of the low frequency part of the pressure spectra. An array of pressure transducers, flush mounted on a ship hull, were used to derive general laws for the streamwise cross spectral density behaviour at model scale based on the Corcos model. A detailed discussion of the scaling laws for the power spectral density in the different frequency range is also provided.