## 5 Conclusions  
A novel flat-face coupling system (NFFCS) design incorporating an instantaneous pressure relief device based on [PERSON]'s law was presented in this research. The mechanism and kinematics of the NFFCS were explained and compared with a similar sized CFFCS. The flow characteristics and axial force during the opening process of the internal valve of the coupling systems were simulated using the CFD dynamic mesh algorithm. Combined with the Ansys Mosaic meshing technology, a series of numerical investigations were carried out on the flow characteristics of the entire system. Based on the distribution of turbulent kinetic energy, the energy-consuming locations of the initial design of NFFCS were determined and optimized. Thus, a design with optimal flow characteristics and hydraulic power transmission efficiency was obtained. The conclusions drawn from these investigations are summarized as follows:  
The kinematics of the CFFCS and NFFCS are explained through the establishment of theoretical models of their coupling processes. In the NFFCS design, there is no major design change in the socket connector, while a high-pressure relief pin valve based on [PERSON]'s law is incorporated in the plug connector. During coupling, the pin valve connects the two chambers by overcoming a relatively small resistance force, which equalizes the pressures on both sides of the main valve so that it can be easily opened. In order to verify the correctness of the NFFCS design, comparative simulations of the NFFCS and CFFCS coupling processes based on the novel CFD dynamic mesh technology were carried out. Analysis of the transient simulation results showed that the opening of the pin valve in the NFFCS has a balancing effect on the pressure difference between the neighboring chambers. Monitoring of the axial resistance during the opening of the valves of the two systems showed that the resistance generated by the NFFCS pin valve is 34% less than that of the CFFCS. This observation verifies the correctness of the NFFCS design. Comparison of the steady-state flow characteristics of the CFFCS and the initial NFFCS design revealed that although the internal geometry of the NFFCS is more complicated due to the incorporation of the instantaneous pressure relief device, its pressure drop is  
Figure 15: Flow characteristics of the optimized NFFCS design, (**a**) Pressure distribution and (**b**) Velocity and streamline distribution. about 2.5% less than that of the CFFCS. This confirms the feasibility of replacing CFFCS with NFFCS in future hydraulic power transmission applications. To reduce further the energy consumed during the process of hydraulic power transmission through the NFFCS, the distribution of turbulent kinetic energy in the initial NFFCS design was observed. A series of geometric modifications and CFD simulations were carried out to devise the optimized NFFCS design with excellent hydraulic power transmission efficiency. The pressure drop of the optimized design is 42.79 kPa, which is 13.5% less than that of the initial design. In summary, the easier installation (lesser thrust requirement) characteristics and smaller hydraulic power transmission loss of the proposed optimal NFFCS have been verified through a series of CFD numerical investigations. Additionally, the design and CFD-based investigation methodologies used in this research can serve as guides for future works related to the design of high-pressure hydraulic power transmission system components. Y.-T.W. and Z.Q. contributed equally. Conceptualization, Y.-T.W. and Z.Q. ; methodology, A.E. ; software, Y.-T.W. ; validation, Z.Q. and S.-K.L. ; formal analysis, S.-K.L. ; investigation, Y.-T.W. ; resources, A.E. and S.-K.L. ; data curation, Y.-T.W. and Z.Q. ; writing--original draft preparation, Y.-T.W. and Z.Q. ; writing--review and editing, A.E. and S.-K.L. ; visualization, Z.Q. ; supervision, Y.-T.W. ; project administration, S.-K.L. ; funding acquisition, S.-K.L. All authors have read and agreed to the published version of the manuscript.