PIN 2.2.1: Results hydro/vibro-acoustical modelling

Hydro/vibro-acoustical modelling in CAPEM is performed at Atlas Elektronik, FAU, FFI, and CNR-INSEAN. The goal is to estimate the vibro-acoustical response of flexible structures to turbulent boundary layer (TBL) excitation. Different modelling strategies are applied, and different numerical simulation tools are used. Combined, this provides a broad range of experiences towards the goal of the project, which is to improve modelling of conformal array performance. PIN 2.2.1 collects the work done on hydro/vibro-acoustic modelling in the different research groups to date. A full report with comparisons of results, conclusions, and recommendations will be issued at the end of the project. All the simulation approaches are so far based on the fundamental assumption that the vibration of the structure has negligible effect on the turbulent motion of the fluid surrounding it outside FLAME. This means that the structural computations can be fully decoupled from the simulation of the TBL, and only time-dependent data values at the surface are transferred to the structural simulations. FFI uses a structural code from MSC Software to do a coupled calculation of the flexible structure and the nearby water volume inside FLAME. As a first estimate, the details of the sensor box inside FLAME are omitted, and open boundary conditions are applied to the water volume inside FLAME. The first structure to be modelled in this approach is the silicone membrane used in Sea Trial 1, with material parameters estimated from experiments and assumptions. FAU use their own TBL simulation results, and the Finite Element solver CFS++ for the structural simulations. Eigenmodes have been calculated for the 0.8 mm steel plate used in Sea Trial 1, and the velocity at the inside of the plate is taken as a boundary condition for the linear wave equation, to describe the signal propagation inside FLAME. The first results show that the first eigenmode of the steel plate dominates. Atlas Elektronik solve the bending wave equation by a Finite Element method to find the eigenmodes of the 0.8 mm steel plate, the wave-frequency spectrum of the plate displacement induced by the TBL, and the corresponding wave-frequency spectrum of the acoustics inside FLAME. The geometry of the sensor box inside FLAME is included in this approach. It is also demonstrated, as indicated by FAU, that the wall shear stress from the TBL has negligible effect on the wall displacement and interior acoustics, compared with the TBL wall pressure. CNR-INSEAN present scaling laws that relate the power spectral density (PSD) of the surface displacement to the PSD of the wall pressure fluctuations from the TBL. Scaling laws based on dimensional analysis and energetic considerations are presented.