Experimental and numerical approaches for the study of the stochastic response of naval structures to turbulent flow

In this work experimental and numerical methodologies for the calculation of vibrations induced by turbulent boundary layer (TBL) on ship structures are presented. In particular, analytical expressions for the hydrodynamic load are derived on the basis of direct pressure measurements on ship models, in a towing tank and, to avoid the limitations of experimental data in the low wavenumber range, by an inverse method based on the use of the structural response. The models are discussed for the case of hull form where the assumptions of stationary and zero pressure gradient flow over flat surface are often not applicable. Moreover, taking into account the characteristics of the fluid-structure interaction for naval applications, formulations for WPF spectra representation that highly reduce the computational time needed for the calculation of the structural response are provided. However, the possibility to correctly identify the pressure load passes through the accurate identification of boundary conditions and structural properties in terms of wet modal parameters. The problem is here solved applying classical modal analysis and an output-only identification technique based on an appropriate formulation of POD (Proper Ortogonal Decomposition) and on the use of the TBL load as ambient load. The application of this method provides an accurate calculation of wet natural frequencies and mode shapes as well as a robust estimation of the total damping coefficient. Moreover, the methodology is particularly fruitful for full scale size problems for which the use of a classical modal analysis is clearly unfeasible. Examples of application of the developed numerical and experimental methodologies are given considering the response of ship structures, at model and full scale size, to TBL excitation.