Towing-tank measurement of unsteady wave elevation around two ships in head sea condition and comparison with computational results

Abstract

Many studies on ship wave elevation measurement have been performed focusing on steady waves in calm water and unsteady wave-cut analysis near ships, and a reliable experimental database of unsteady wave elevation with incident waves is not enough for the validation of numerical computations. In this study, a non-contact scheme for capturing the instantaneous free surface near two ships is adopted to avoid any disturbance caused by measuring equipment. The applicability of the newly established unsteady measurement technique is examined during a standard seakeeping test with various wave amplitudes. Heave and pitch motions are set free, while the soft spring system is used for surge motion. The instantaneous free surface elevation is obtained on equally spaced positions. The mean value and higher harmonic components of measured wave elevation clearly show nonlinear characteristics for various incident wave amplitudes. Snapshots at several time instants and the magnitude of the first harmonic components of unsteady wave elevation are compared with those of numerical computations based on a Rankine panel method. The overall trend shows fair correspondence, whereas the detailed local flows are somewhat different because of the nonlinearity of waves.