# Analysis of a New Twin Hybrid Autonomous Underwater Vehicle
## 4 Hydrodynamic Analysis  
When underwater equipment works in the water, it is mainly affected by buoyancy and the hydrodynamic force generated by the relative movement of the water around the body in the process of movement. Compared with the buoyancy, the calculation of hydrodynamic force is not easy; however, it is very critical. Currently, three methods are usually used to calculate the hydrodynamic force of water-related equipment, i.e., for the target with a regular shape, the empirical formula is mostly used to predict the hydrodynamic force; for the equipment with a simple structure and low manufacturing cost, the method of the proportional model experiment is often used to obtain the hydrodynamic parameters directly; however, for the equipment with a complex structure and high manufacturing cost, the above method cannot accurately and effectively calculate the hydrodynamic force of the target. For such cases, ANSYS Fluent [14] software is used to calculate the hydrodynamic parameters of the target. The hydrodynamic drag force of the THAUV was simulated using Fluent in this paper. The simulation domain is a rectangular body and the dimensions of the simulation domain are: 14 m in length, 6m in both width and height, and the inlet of the simulation domain is 5 m away from the floating center of the THAUV. The k-omega SST model [15] is selected as the turbulence model for this simulation and the SST model is widely used for underwater equipment simulation analysis. Due to the complexity of the THAUV tail structure, a non-structural grid is used for hydrodynamic simulation in this paper, with a total grid number of about 3,790,000 and a Yplus less than 1 [16]. The created grid is shown in Figure 5. \begin{table}
\begin{tabular}{c c c c} \hline
**Term** & **Case 01** & **Case 02** & **Case 03** \\ \hline \(m\) (\(kg\)) & 67.37 & 67.02 & 66.3 \\ \hline \(m_{p}\) (\(kg\)) & 1.022 & 1.022 & 1.022 \\ \hline \(m_{s}\) (\(kg\)) & 65.33 & 64.98 & 64.256 \\ \hline \(r\) (m) & 0.05 & 0.05 & 0.05 \\ \hline \end{tabular}
\end{table} 
Table 2: Values of the THAUV. In order to ensure that the number of meshes can meet the accuracy of the simulation results, the comparison of the simulation results of different meshes in the state of the THAUV with an angle of attack of +2 degrees and a gliding speed of 0.5 m/s can be obtained from the verification results of the mesh independence [17] shown in Figure 6. The grid numbers used in the figure are 2,250,000, 3,330,000, 3,790,000, and 75,200,000. In view of the simulation results, simulation time, and computer performance, the number of meshes used in this paper is about 3,790,000. Figure 5: Non-structural grid of the CFD simulation. Figure 6: Grid independence verification. A comparison of hydrodynamic lift and drag relative to cruising velocity of the THAUV when it is in the AUV mode of horizontal cruise is shown in the figure below. Figures 11 and 12 show the relationship between the hydrodynamic lift and drag of the THAUV in the horizontal plane relative to the cruising speed, respectively. It can be concluded from the plot of drag that the wing area has a relatively small effect on drag as it decreases, since the buoyant body is the main factor in drag generation. The simulation results of the THAUV for the three cases in two working cycles (dive and then surfacing, then dive and surfacing again) are shown in Figures 13-15. During the first stage of diving and surfacing, the displacement of the piston is plus or minus 0.03 m, i.e., the change in the mass of the ballast water is plus or minus 483 g. The positive sign indicates that the buoyancy engine absorbs water inward and the buoyancy of the THAUV decreases. During the second stage of diving and surfacing at the time of 1060 s and 1430s, the displacement of the piston is plus or minus 0.05m. Figure 16 presents the comparison of the angle of attack (AoA) for the three cases. Figure 14: Comparison of horizontal displacement. Figure 13: Comparison of gliding velocity.