Coastal vegetation is efficient in damping incident waves even in storm events, thus providing valuable protections to coastal communities. However, large uncertainties lie in determining vegetation drag coefficients (C D), which are directly related to the wave damping capacity of a certain vegetated area. One major uncertainty is related to the different methods used in deriving C D. Currently, two methods are available, i.e. the conventional calibration approach and the new direct measurement approach. Comparative studies of these two methods are lacking to reveal their respective strengths and reduce the uncertainty. Additional uncertainty stems from the dependence of C D on flow conditions (i.e. wave-only or wave-current) and indicative parameters, i.e. Reynolds number (Re) and Keulegan-Carpenter number (KC). Recent studies have obtained C D-Re relations for combined wave-current flows, whereas C D-KC relations in such flow condition remain unexplored. Thus, this study conducts a thorough comparison between two existing methods and explores the C D-KC relations in combined wave-current flows. By a unique revisiting procedure, we show that C D derived by the direct measurement approach have a better overall performance in reproducing both acting force and the resulting wave dissipation. Therefore, a generic C D-KC relation for both wave-only and wave-current flows is proposed using direct measurement approach. Finally, a detailed comparison of these two approaches are given. The comprehensive method comparison and the obtained new C D-KC relation may lead to improved understanding and modelling of wave-vegetation interaction.

The reliability of machine tool is getting more and more attention. Reasonable reliability allocation is beneficial to improve the inherent reliability of machine tool. However, the existing reliability allocation methods for machine tool have some limitations. For example, static part is selected as the reliability allocation object of machine tool, which cannot reflect the characteristics "function realized by motion"; factors affecting reliability allocation are not considered comprehensively, weights of experts are treated as the same, and the allocation results are not optimized or the impact of time on enterprise is neglected in the optimization. To solve these problems, a new multiobjective optimization reliability allocation method for machine tool is proposed in this paper. Firstly, the latest achievements about meta-action are given, and meta-action is set as the reliability allocation object. Secondly, more reasonable and comprehensive factors affecting reliability allocation are extracted. Thirdly, expert weight coefficient is brought to reduce the subjective impact of expert scoring. Fourthly, time factor is brought to make the optimized allocation results more reasonable and accurate. Finally, a numerical control (NC) machine tool made in China is taken as an example, with a comparison on the reliability allocation results of current methods and proposed method. The results verify the applicability, rationality, and accuracy of the proposed method, which lays a foundation for the subsequent study on the quality characteristics of machine tool based on meta-action.

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