## 5 Discussion and Conclusions
Finally, it was assumed that, by developing the third validation cycle, the towline estimation should be improved even further. Besides this, a second assumption was that the CFD model should capture the physics of the problem, including the shielding effect, and therefore, it could be used to verify the AS drag coefficients and the AS shielding effect formula proposed by [PERSON] et al. (1966). Following the third validation cycle (Section 4.3), the first assumption was proven correct for a nominal span and a wide span. The CFD model produced drag coefficients in cross flow that were close to the AS values, but they led to an even more-accurate towline tension in the nominal span, in comparison with the second validation cycle's results (the mean deviation improved from 14% to 7%). For the narrow span, the drag coefficients derived from the CFD model generated an overestimation of the towline tension of about 97%. Therefore, the first assumption was not validated for the narrow span, but improvement is expected after including physical phenomena currently missing in the CFD model, for example the net's flexibility. The outcome was similar for the second assumption. The AS calculation method was verified for nominal and wide spans, but the AS formula in Equation (8) reduced the drag coefficient for low \(\theta\) more than what was estimated by the CFD model and actually slightly more than what the GPGP data indicated. Therefore, further investigation is necessary to verify what is actually happening in the narrow span. In the meantime, and only for engineering and design purposes, the average between the 2 nd and 3 rd towline tension curves can be used to obtain results in the narrow span that deviate on average 43% from reality. There were also general assumptions, meaning that they applied to all three validation cycles. For example, it was assumed that the drag force over other wing components (fenders, ropes, chains, etc.) was correctly modeled in OrcaFlex and/or that the drag force produced by these components was small in comparison with the net's drag. Although neither of these assumptions have been proven yet, the accuracy of the results indicates that both of them are probably true. A second general assumption is related to the OF and the CFD models. It was assumed that modeling the system's net as explained in Sections 3.2 and 3.5 was sufficient to obtain accurate results for the towline tension. In light of the results from the validation process, presented in Figure 19a-c, this assumption was proven correct. A third general assumption is related to the CFD model and AS calculation and the quantification of the shielding effect. It was expected that the estimated reduction of the cross-flow drag coefficient when moving from perpendicular flow to parallel flow would be similar for both AS and CFD. The 2D DNS with Basilisk, in nominal operation with \(u_{\mathrm{stw}}=0.75\) m/s and Reynolds number around of 1000, estimated a reduction of about 70%. AS estimated a reduction of 100%. The consequence of that difference was a 3 rd cycle towline tension curve in the narrow span that doubled the 2 nd cycle's curve (cf. Figure 19b,c). Since none of these curves were enough to validate the CFD model or the AS calculation, a twofold future investigation needs to be developed. On the one hand, the CFD model needs to be analyzed and improved adding possible missing physics. As an example, the facts that the net is flexible and that it deforms in parallel flow have not been considered yet. On the other hand, the assumptions and supporting data behind the AS formula need to be investigated and validated. Based on the previous discussion, two types of implications or conclusions can be established. The first type comprises the conclusions directly related to the objective of the present study. The second type comprises conclusions that align with a wider perspective or context and that have an impact on the research or industry sectors.