A hybrid RANSE – strip theory method for prediction of ship motions

In the context of a linear seakeeping theory, radiation forces are the most affected by viscous effects and free surface non linearity. This paper presents an innovative ship motions prediction method aimed to recover these effects addressing radiation forces calculations to the solution of Navier-Stokes equations. The novelty is brought by the solution of the radiation problem for 2D ship sections forced to oscillate in calm water. This problem is solved using a fully viscous method with non-linear free surface based on the solution of RANS equations coupled with a VOF technique for the prediction of the free surface flow. The numerical grid is automatically generated around ship sections offsets using algorithms specifically designed for the solution of radiation problems. The viscous numerical solver is based on OpenFOAM libraries. A Fourier transformation technique finally leads to the definition of radiation forces in the frequency domain. A novel technique based on energy equivalence has been formulated and implemented in the method presented in this paper with the goal to recover higher harmonics of radiation forces captured with the fully-viscous non-linear free surface prediction model. The coupled numerical method is described in details in the paper. Also, the method has been verified on a series of successively refined grids and validated against experimental results on a classic mono-hull.