URANS analysis of a free-running destroyer sailing in irregular stern-quartering waves at sea state 7

Computational fluid dynamics (CFD) simulations and stochastic validation of free-running 5415M in irregular stern-quartering sea state 7 and Froude number 0.33 are presented. Unsteady Reynolds-averaged Navier-Stokes (URANS) computations are validated against experimental fluid dynamics (EFD) tests. EFD static stability, forward speed, wave direction and spectrum were set such that roll-motion resonance were induced, causing large roll angles, significant deck-edge immersion, and static instability. Prerequisite calm-water validation covers roll decay at zero speed and self-propulsion studies for propeller RPM. Subsequent CFD stochastic validation is achieved by statistical assessment of EFD data and CFD results (expected value, standard deviation, and probability distribution) for input waves and ship response: time series are assessed by their wave energy-spectra, autocovariance (waves only), and block-bootstrap analysis (waves and motions); mean-crossing amplitudes are studied by bootstrap method. Dynamic mode decomposition is used with EFD and CFD time histories to unveil the underlying ship dynamics and the motion correlation. Overall, CFD results are in agreement with EFD. Finally, a clustering approach is used to identify wave sequences causing large roll angles. As proof of concept, the characteristics of these sequences are used for the deterministic reconstruction of severe (large roll) and rare (capsize) events via regular wave computations.