## 4 Simulation Result
### Streamline Global Route Planning  
According to the selected nautical chart, part of the coastline outline of the sea area chosen was simplified for the convenience of calculation. Using the numerical simulation software Space Claim from ANSYS FLUENT, a physical model of the surrounding environment of the sea area was constructed, as shown in Figure 12.  
This paper used the Fluent Meshing geometry processing function to mesh the established model. Unstructured grids are used for discretization during division, the minimum grid size is set to mm, and the grid elements mainly use tetrahedral grids. Meshing is a critical requirement for fluid simulation and its quality directly affects the accuracy of simulation results. An excessive mesh size will reduce simulation accuracy and lead to erroneous results. However, a larger mesh size increases the computation time and leads to easier-to-converge results. Therefore, it is crucial to choose the right mesh size. The flow field mesh division in this study is shown in Figure 13. The number of grids is 102,694, the number of nodes is 129,267, and the grid quality is above 0.3. No negative volume will affect the simulation, which meets the Fluent calculation requirements. The boundary conditions and initial physical quantities are set after creating the fluid analysis model. The final flow field solution is obtained using the iterative solution of the equations during the simulation. The fluid boundary conditions are set as velocity-inlet, velocity-outlet, and wall. The governing equation adopts the inviscid Euler equation and dimensions are in three dimensional double precision mode. The solver uses steady, pressure-based, and absolute velocity formulation. Set the operating pressure of the operating conditions to a standard atmospheric pressure and the operating density to the default value 1.225 kg/m\({}^{3}\). Check the gravity effect, since the y-axis is vertically upward  
Figure 12: Grid environment model. Figure 13: Flow field grid division diagrams. and set the y-direction acceleration to \(-9.81\) m/s\({}^{2}\), opposite to the gravity acceleration's direction. The simulation calculation model is shown in Figure 14.  
When generating the global route of the USV, it is necessary to post-process the data and analyze the simulation results. In this study, the results show convergence when the iteration is 200 steps. The simulation results of this paper are shown in Figure 15, including the velocity cloud diagram, velocity vector diagram, and streamline diagram. The velocity cloud diagram displays the velocity of the fluid flow using different colors, and the color scale demonstrates the spatial distribution of the fluid velocity, which is used to understand the overall velocity distribution of the fluid flow and the trend of velocity change; the velocity vector map shows the flow velocity and direction of the fluid in the form of arrows, which is suitable for detailed study of the velocity and direction of the fluid as well as local characteristics; and the streamline map presents the flow route of the fluid in the form of curves, in which the direction of each streamline tangent is consistent with the direction of the velocity vector, which helps us understand the path and overall trend of the fluid in the system. Velocity cloud and vector diagrams provide detailed flow velocity and directional information. In contrast, streamline diagrams are suitable for understanding the fluid's trajectory and overall flow pattern. All three are combined to provide a comprehensive visualization of fluid flow. By generating and displaying an appropriate number of streamlines from CFD simulation results, features such as fluid trajectories and flow patterns in the entire flow field can be demonstrated for design tuning and optimization. We used the streamline technique to obtain the visualized vector field from the three-dimensional CFD simulation results, and the generated global optimal paths are shown in Figure 16.  
Figure 14: Simulation calculation model diagram.