CFD and Potential Flow Simulations of Fully Appended Free Running 5415M in Irregular Waves

The capabilities of CFD and PF methods are assessed for course keeping of 5415M advancing at a Froude number of 0.248 in irregular waves with JONSWAP spectrum for various wave headings. CFD simulations are conducted with N=32 wave components for the wave headings of 0, 45, 90, 105, 180 deg. An additional beam wave simulation is also conducted using N=1000 components. PF simulations are conducted using N=400 wave components for three wave headings (0, 45 and 90 deg). A PF beam wave simulation is also performed using a similar but different rudder controller. The results are validated against experimental data for the validation variables, i.e. 6DOF motions and wave elevation. The validation study focuses on the statistics of the time series (addressed as primary variables), as well as height and mean-crossing period (addressed as secondary variables). RAOs of motions are also investigated. Subseries and time series theories for primary variables and bootstrap method for secondary variables are applied in order to estimate uncertainties and evaluate validation. The rudder controller study showed the predictions do not depend strongly on the controller setting as all variables excluding the yaw motion vary by less than 4%. For wave elevations, the average CFD and PF uncertainties over all headings were close to EFD and errors were comparable. More wave components could reduce the CFD wave uncertainties for the beam wave case. For motions/velocities in beam waves, uncertainties for CFD with N=1000 and PF were also close to EFD. CFD and PF average error values for the primary variables were 26%D and 57%D, respectively. The errors were large for the secondary variables. The effect of number of wave components on the errors of motion/velocity was 13%D on average. The errors of the primary variables varied by <6.4%D. The largest effect of number of wave components on the primary variable errors was for the roll and yaw motion (~11%D) whereas the largest effect on the secondary variable errors was for the surge velocity. Considering all wave headings, average uncertainty values for both CFD and PF over all the variables were close to EFD such that average validation uncertainty is about 39%D. The overall average error values were similar for CFD and PF, at about 64%D. The trends versus wave headings for statistical results of the primary and secondary variables and RAOs were often well predicted by both CFD and PF. RAOs were similar with the ones from regular waves, available only at a single frequency for head and beam cases. Roll RAOs showed resonance in near beam wave conditions resulting in large roll motions, particularly for the case with passive fins at 105 deg wave heading.