# Performance Evaluation of a Novel Propulsion System for the Spherical Underwater Robot (SURIII)
## 4 CFD Simulation
### Results in the Post-Processing  
After solver execution, some results can be extracted from the post-processor of ANSYS CFX. In heave motion, there are two kinds of motions: up and down. Figure 15 shows the simulation results of the down motion. Figure 15a is the pressure contour for the down motion, the maximum pressure occurs in the bottom of the robot and near the edge of the robot. This confirms that the influence of  
Figure 14: Fluid boundary setting.  
Figure 12: Simulation model of SURIII.  
Figure 13: Flow field of SURIII.  
the edge in heave motion cannot be ignored. Figure 15b,c display the velocity vector and velocity streamline, respectively. As these two figures shown, the velocity of the down motion also affected by the edge of the robot. Therefore, the influence of the edge needs to be considered when calculating the drag coefficient. As well, the simulation results of up motion are shown in Figure 16. Figure 16a shows the maximum pressure occurs in the top and the edge of the robot. Figure 16b,c are the velocity vector and streamline in the up motion, respectively. As with the down motion, the up motion is also affected by the edge of the robot. For both up and down motions, the water will flow through these gaps and this may have influence on the robot in the simulation. Comparing the simulation results of these two motions, the robot has larger resistance force when it sinks. The resistance of the down motion in the simulation is 0.334 N. The maximum relative velocity in the post-processor is 0.1021 m/s and the maximum cross-sectional area is 0.1661 m\({}^{2}\). According to the Equation (14) and the drag coefficient can be calculated as 0.386. This drag coefficient has a 3.5% error compared to drag coefficient determined by Reynolds number for the sphere. It indicates that edges and gaps in the heave motion effect little.  
Figure 15: Down motion in heave. (**a**) Pressure contour; (**b**) Velocity vector; (**c**) Velocity streamline.  
Figure 17 shows the surge motion for the SURIII, the high-speed mode is analyzed in this research, both the gaps and the edge of the robot cannot be affected to this motion. Figure 17a shows the pressure contour and the maximum pressure occurs near the edge of the robot. Figure 17b,c are the velocity vector and streamline of the high-speed surge motion and the edge can be ignored because of its small impact for the velocity vector. The resistance of the forward motion is also extracted from the post-processor to verify the drag coefficient, the force acting on the robot is 0.988 N in the flow direction, the maximum relative velocity in the post-processor is 0.1998 m/s and the maximum cross-sectional area is 0.1256 m\({}^{2}\). The drag coefficient can be calculated by Equation (14) and is equals to 0.394, it has a 1.5% error compared to the drag coefficient determined by Reynolds number which indicates the robot can be similarly considered as a sphere in surge motion.  
Figure 16: Up motion in heave. (**a**) Pressure contour; (**b**) Velocity vector; (**c**) Velocity streamline.