MIT-WAVE – Realistic Nonlinear Wave Environment Inputs

n the performance analysis and design evaluation of surface ships using modern nonlinear time-domain computational tools such as the potential-flow-based LAMP (Large Amplitude Motion Program) and the viscous-flow-based CFDShip-Iowa, an appropriate phase-resolved (i.e. deterministic) specification of the velocity and pressure field of ambient surface wavefields for design sea states is essential. Proper deterministic specification of the instantaneous boundary conditions is also necessary for the detailed specification and repeatable generation of irregular wave events in a wave tank/basin where ship dynamic stability and performance is tested.

At present, these specifications are obtained based on linear Airy (or low order Stokes) wave theory for given wave-energy spectra or field (or laboratory) wave-elevation data. This linear approach is valid only for small amplitude waves/motions. For moderate to steep waves, it is known that this linearized approach significantly under-predicts the occurrence of extreme wave events and inadequately describes the velocity and pressure fields. The inclusion of nonlinear effects is critical to the reliable prediction of ship motion performance under rough sea conditions where severe motions are encountered.

The purpose of this study is to generate a collection of synthetic nonlinear ocean wavefields, which is to be called MIT-WAVE, from specified wave spectra and directional spreading using a direct deterministic wave simulation tool, SNOW (Simulation of Nonlinear Ocean Wavefields). MIT-WAVE will provide the nonlinear environmental wave inputs essential for nonlinear episodic ship design and performance analyses. MIT-WAVE will ultimately contain O(100) unidirectional nonlinear irregular wavefields for different frequecy spectra; and O(30) directional (three-dimensional short-crested) wavefields for different combinations of wave frequency spectra and directional spreading functions. The domain size and evolution time of each three-dimensional wavefield are respectively O(102~3)km2 and O(hours) to ensure that key nonlinear wave-wave interaction effects are captured and represented. The wave spectra considered will represent a variety of sea states of interest, from mild to severe wave conditions (up to sea state 8). From MIT-WAVE, representative episodic wave events each covering a domain size of O(1) km2 and evolution time of O(minutes) are extracted. In this two-year effort, wind forcing will not be considered nor would effects associated with other environmental factors such as currents or finite depth. The detail wave kinematics and dynamics description of each wave event are obtained, (re-)represented, and output in the space-time regions and formats required for different ship motion programs.