3.3.3. Results

The free pitch and heave decay curves of the light buoys are shown in Figure 10. As seen in the figure, the decay of the motions becomes faster when the appendages are adopted. The plate-type appendages are more effective than the conical-type ones for the reduction of pitch motions, while the effect of the porous cone appendage is almost the same as that of the plate-type one in reducing heave motions.

**Figure 10.** Free pitch (**a**) and (**b**) heave decay curves of the lightweight light buoys.

Table 2 shows the estimated non-dimensional damping coefficient and natural frequencies of the light buoys from the free decay tests. With the adopted appendages, the pitch natural frequencies of the buoys became higher than those of the Base model, resulting from the increase in the restoring moment due to the increments in metacentric height (GM) and draft. The reason the heave natural frequencies of the models with appendages become 10%–15% smaller might be related to the change in mass and added mass, and the viscous and radiation damping.


**Table 2.** Non-dimensional damping coefficients and natural frequencies of lightweight light buoys from free decay tests.

Figure 11 shows the vorticity distributions at the first and second peaks of motion, which were observed during the free decay simulations. Around the appendages, complicated flows and strong vortexes were observed, which led to high energy dissipations and strong viscous effects.

**Figure 11.** Comparisons of vorticity (j) distributions around the lightweight light buoys during (**a**) free pitch and (**b**) heave decay tests when the first and second peaks of motion occur.

#### *3.4. Potential-Based Motion Analysis*

## 3.4.1. Computational Conditions

Figure 12 shows the computational domain and the panels on the surfaces of the light buoys. Heading waves with frequencies of 0.1 to 6.0 rad/s were considered. The maximum panel size was approximately 1/7 times that of the shortest wavelength. The minimum and maximum numbers of the generated panels were approximately 4000 and 18,300 for the Base and Porous Cone models, respectively. The motion analyses of the light buoys were performed using ANSYS-AQWA with and without the application of the viscous damping coefficients, which were estimated using CFD simulations, as discussed in the previous section.

**Figure 12.** Computational domain and panels of the lightweight light buoys for potential-based simulations.
