Modal decomposition techniques are used to analyse the wake field past a marine propeller achieved by previous numerical simulations (Muscari et al. Comput. Fluids, vol. 73, 2013, pp. 65-79). In particular, proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) are used to identify the most energetic modes and those that play a dominant role in the inception of the destabilization mechanisms. Two different operating conditions, representative of light and high loading conditions, are considered. The analysis shows a strong dependence of temporal and spatial scales of the process on the propeller loading and correlates the spatial shape of the modes and the temporal scales with the evolution and destabilization mechanisms of the wake past the propeller. At light loading condition, due to the stable evolution of the wake, both POD and DMD describe the flow field by the non-interacting evolution of the tip and hub vortex. The flow is mainly associated with the ordered convection of the tip vortex and the corresponding dominant modes, identified by both decompositions, are characterized by spatial wavelengths and frequencies related to the blade passing frequency and its multiples, whereas the dynamic of the hub vortex has a negligible contribution. At high loading condition, POD and DMD identify a marked separation of the flow field close to the propeller and in the far field, as a consequence of wake breakdown. The tonal modes are prevalent only near to the propeller, where the flow is stable; on the contrary, in the transition region a number of spatial and temporal scales appear. In particular, the phenomenon of destabilization of the wake, originated by the coupling of consecutive tip vortices, and the mechanisms of hub-tip vortex interaction and wake meandering are identified by both POD and DMD.

For the effective operation of sonar systems mounted inside the bulb of fast ships, it is important to reduce all the possible noise and vibration sources that radiate noise and interfere with sonar sensor response. In particular, pressure fluctuations induced by turbulent boundary layers on the sonar dome surface represent the major source of self-noise for on-board sensors. Reliable calculations of structural vibrations and noise radiated inside the dome require valid statistical descriptions of wall pressure fluctuations beneath the turbulent boundary layer. Previous research about wall pressure fluctuations deals with equilibrium turbulent boundary layers on flat plates in zero pressure gradient flow, for which scaling laws for power spectral densities and empirical models for the cross spectral densities are well established. On the contrary, turbulent boundary layers on bulbous bow exhibit the combined effects of three-dimensionality, streamline and spanwise curvatures and pressure gradients. In order to collect information about realistic configurations, wall pressure fluctuations were measured in an experimental campaign performed in a towing tank; data were collected at two different locations along a large scale model of a ship bulb and their spectral characteristics were investigated in terms of auto and cross spectral densities. Mean flow parameters of the boundary layer, required in the analysis, were obtained by a finite volume code that solves the Reynolds Averaged Navier Stokes Equations. The applicability of classical scaling laws for pressure spectra on zero pressure gradient flat plate was investigated, together with the spatial characterization of the wall pressure fluctuations in the space-frequency domain; parameters of some semi-empirical models available in the scientific literature were tuned to fit the measured pressure field.

To comply with the more and more restrictive international standards and regulations for noise and vibration levels on board passenger ships, a renewed interest on secondary N&V sources, with respect to propeller and machinery sources, has been observed. In particular, the increase of ship performances in terms of velocity has been directed on study the hydrodynamic noise sources and among the others turbulent boundary layer (TBL). The great difficulties encountered in simulating the wall pressure fluctuations (WPF) due to TBL at high Reynolds numbers and for complex configurations typical of a real ship have pushed the research community to develop models for WPF based on theoretical considerations and model scale tests. In particular, scaling laws for pressure spectra have been established at least for simple geometries and flow conditions and models of cross spectral density for their spatial characterization have been obtained. Unfortunately, model scale tests do not allow reaching Reynolds number values comparable with full scale conditions. Therefore, to validate current models an experimental campaign devoted to WPF measurements have been performed on the hull of a Ro-Ro Pax vessel. Numerical simulations of the flow around the ship hull were performed to evaluate mean flow parameters.

The correct characterization of wall pressure fluctuations (WPF) and of the response of an elastic structure subjected to turbulent boundary layer (TBL) represents one of the most challenging problems in the fluid structure interaction field. This kind of excitation for an elastic structure is encountered on a number of different engineering applications: in naval field WPF acting along the ship hull impinge on comfort on board high speed vessels and they are also responsible for strong vibrations of the sonar dome, which can degrade the correct functioning of the sensors mounted inside the dome itself. Moreover, the sound pressure levels produced by TBL load acting along the aircraft fuselage can be intense enough to result in an unacceptable cabin noise and can cause a reduction of the lifetime of fuselage panels due to structural fatigue. The study of WPF induced by TBL load in the naval and aeronautical fields are characterized by important differences in terms of both flow and structural characteristics, which provide highly different dynamical responses of a typical naval and aeronautic panel. Nevertheless, the characterization of the TBL load using model scale tests of a ship and an aircraft or sections of them have also strong similarities and for a great number of problems can be analysed using parallel experimental approaches in towing tanks, water channels and wind tunnels. The base of this approach is given by the identification of the most appropriate scaling laws for wall pressure fluctuation spectra and spatial models in the frequency domain, which allow to obtain in principle the full scale spectra from the sole knowledge of few mean flow parameters. Unfortunately, these models are based on very restrictive hypotheses on the nature of the flow and the structure, basically canonical flat boundary layer. Aim of this work is to show how some of the typical perturbations from the canonical flat plate boundary layer, encountered when studying a real structure in naval and aeronautical fields, can interfere in the modelling of this load and to show possible solutions to these specific problems. To examine these features for complex boundary layer, the results of three different experimental campaigns performed at CNR-INSEAN towing tank and CIRA PT-1 transonic wind tunnel are here discussed.