## 4 Discussion  
The liquid sloshing-induced trends observed in Section 3 can be better understood in conjunction with an analysis based on the visually observed sloshing patterns. As explained in Section 2, two high frame rate video snapshots were used to observe the liquid sloshing from the side and top of the tank. Examples of video frames from one 50% filling level representative test are shown in Figure 16 at the selected time instances expressed as multiples of the period of oscillation of the first bending mode. Three guiding acceleration values are also reported for each snapshot, indicating the first component's amplitude at the corresponding location using the three tank accelerometers, as shown in Figure 1 (root, mid and tip). The tank is oriented here, with its outboard end towards the left. It was noted, in relation to Figures 12 and 13, that the amplitude \(A1\) is the same for the dry and wet cases in the first half-period of the transient motion. Figure 16 now offers a visual interpretation of this result. The liquid starts from a standstill configuration at \(t=0\), and then, following the release event, it starts moving together with the structure upwards. Only between \(t=T/2\) and \(t=T\) does the liquid start moving substantially upwards relative to the tank and starts impacting the ceiling of the tank. After this, the sloshing fully develops across all spanwise stations. The sloshing force acting on the tank was, in the past studies, isolated and shown to be correlated with the movement of the liquid relative to the tank in harmonically-excited systems [7]. A single degree-of-freedom system with the sloshing liquid undergoing a free transient vibration exhibits qualitatively and quantitatively different behaviours depending on the combined fluid-structure response characteristics. It was shown in [5] that the following vertical sloshing regimes and corresponding damping regions can be identified: R1--the most dissipative regime, characterised by the violent vertical sloshing with regular impacts between the liquid and the top/ceiling of the tank and R2--the region characterised by the well-defined fluid motion in one of its sloshing modal patterns with possible intermittent impacts with the top of the tank characterised by the dissipation rates notably higher than those of the underlying dry system. A transition region R2&R1 was also shown in [7], where both effects contribute depending on the amplitude of excitation. In regard to the vertical accelerations for these regimes, it was generally observed that at least 1g is needed to excite significant free-surface motion such that substantial and repeated impacts with the tank ceiling occur. Depending on the tank geometry, liquid sloshing modes can be excited at lower amplitudes as well, promoting more impacts with the top of the tank and thus more energy dissipation. All of these phenomena can be observed in the present case as well. The peak accelerations of component 1 at the root-end of the tank do not exceed 1 g. Small surface ripples in cells 1 and 2 are seen to develop gradually, but their amplitude is likely not large enough to contribute to any substantial dissipation in the system. There is a noticeable difference between cells 2 and 3 in what concerns the observed sloshing patterns since the cell geometry changes substantially as well (see bottom section of Figure 1 and also Figure 17 showing the geometry-dependent first few sloshing frequencies in each tank cell). Moving towards the middle section of the tank, cells 3 and 4, vertical accelerations of up to 3.3 g are reached there. After approximately two initial periods of motion, the sloshing in these cells develops to start hitting the top of the tank. Liquid motions are dominated by the (2,0) sloshing mode (mode shape with two nodes) at a frequency of \(F1/2\), as can be seen from the near-periodicity of the sloshing pattern in cells 3 and 4 corresponding to the two full tank periods.