# Design and Analysis of the High-Speed Underwater Glider with a Bladder-Type Buoyancy Engine
## 2 Analysis of UG
### Hull Resistance Analysis  
Using computational fluid dynamics (CFD), the parameters related to the steering fluid force acting on the underwater glider were identified and used to estimate the kinetic performance of the UG at the design stage. The actual UG model designed for this purpose has a cylindrical shape with wings at a sweep angle of 20\({}^{\circ}\) attached to the oval regions at the fore and aft. The k-\(\omega\) SST model was applied to analyze turbulence. The k-\(\omega\) SST model is known to be a turbulence model developed for flow calculations with a broader range of vortices than the k-\(\varepsilon\) model and is widely used in the ship field, so it was applied as the k-\(\omega\) SST model [15]. All the results mentioned in this study used the solver provided by STAR-CCM+.  
The grid for performing the analysis was set up as shown in Figure 2. The inlet of the grid was set at a position equal to the length (L) of the hull from the bow, and the outlet part was set at a position 3 L from the stern. Both sides, top and bottom, were placed at 1.5 L from the hull. The coordinate system was set so that the bow direction of UG was a positive (+) on the x-axis, the right direction was a positive (+) on the y-axis, and the upward direction was a positive (+) on the z-axis. The distance from the surface of the UG to the first grid was determined using a value where y+ is approximately 20 mm or less. The reason for selecting about 20 mm or less is that the UG has a shape close to an elongated body, so the total resistance is. It was expected that friction resistance would account for the majority rather than pressure resistance, and this was to calculate the development of the boundary layer more precisely. Figure 3 shows the flow speed, pressure distribution, and streamline for the speed of the moving object viewed at regular intervals from near the rear of the UG when the line speed is 1.0 m/s. It can be seen that the larger the angle, the more a vortex occurs from the center of the moving UG to the back.  
\begin{table}
\begin{tabular}{c c c} \hline
**Parameters** & **Description** & **Units** \\ \hline \(\theta\) & Included tail angle & radians \\ \(d\) & Maximum hull diameter & m \\ \(l\) & Vehicle total length & m \\ \(l_{f}\) & Vehicle forward length & m \\ \hline \end{tabular}
\end{table}
Table 1: _Cont._  
Figure 1: Myring equation parameters model.  
Figure 2: Grid design for computational fluid analysis.  
The flow velocity was analyzed with increasing speed from approximately 0.2 m/s to 1.1 m/s. The flow velocity distribution around the UG decreases in the nose and curvature parts. The rear antenna and rudder were installed vertically based on these analysis results. The results of the resistance calculation for each speed obtained through the analysis are shown in Figure 4.  
A resistance of approximately 5.4 N acts when the UG moves straight at 1.0 m/s. The resistance increases nonlinearly with speed, and the average drag coefficient calculated according to the obtained drag result is 0.29. Considering that the drag coefficient of an AUV with a torpedo-type hull is approximately 0.3, the calculated values are similar to those previously reported [23].  
Figure 4: Relationship between speed and drag force.  
Figure 3: Computational flow analysis results.