Numerical study on motion responses and added resistance for surge-free KCS in head and oblique waves based on the functional decomposition method

Abstract

A hybrid approach of potential and viscous flows is developed, based on the functional decomposition model SWENSE (Spectral Wave Explicit Navier-Stokes Equations), to investigate the seakeeping performance of the surge-free ship models accurately and efficiently. In the SWENSE method, the total physical field, including velocity, pressure and the signed distance function of the free surface, are decomposed into the incident field and the complementary field. For the incident wave field, the linear wave model is used and the physical field is extended to the flows up to the air zone using the Wheeler stretching method. With respect to the complementary field, the variables are governed by SWENSE. The adapted single-phase level-set model is used to capture the free surface. Based on this hybrid approach, an in-house code HUST-SWENSE is developed. The spring-mass-damper module, 4DOF (Degrees of Freedom) motion equations of the rigid body, three coordinate systems and the dynamic structured grid with the overset technology are applied to deal with the 4DOF ship motions. Firstly, the size of the computational domain for the simulations of the oblique waves is discussed. The numerical results can be convergent for HUST-SWNESE when the radius R/L of the cylindrical computational domain is larger than 2.0. After determining the grid system and the time step based on the verification and validation study, simulations are conducted using the HUST-SWENSE to predict the surge, heave, roll, and pitch motions of the KCS in regular oblique waves. Wave conditions are based on the benchmark cases provided by the Tokyo2015 CFD workshop. The simulated results are then compared with experimental data and other numerical results. The comparison shows that the respective differences were less than 2% for incident regular waves, 10% for heave and pitch motion responses, 5% for roll motions and 20% for added resistances. Therefore, the HUST-SWENSE model can be considered reliable for the prediction of ship motions and added resistance in head and oblique waves. Finally, the influences of different DOFs, wavelengths and wave heights on the motions and added resistance for the surge-free ship model in oblique waves are investigated in detail. It is found that the importance of surge motion in the seakeeping performance prediction of the ship in astern seas, especially for the added resistance. Moreover, the nonlinearity of wave heights is analyzed, and the results indicate that the nonlinearity of the large-amplitude roll is mainly induced by the roll restoring moments, while the nonlinear features of the incident, radiation and diffraction wave systems were responsible for the nonlinearity of added resistance and surge motions.