# Simulation Study on Thermal Wake Characteristics of Underwater Vehicle under Rotary Motion
## 4 Analysis of Simulation Results
### Analysis of Water Surface Temperature Field  
By observing Figure 14f, it can be seen that when the underwater vehicle rotated in the water for 30 s, the thermal wake did not rise to the surface. Therefore, when studying the variation law of water surface temperature difference caused by the thermal wake, the working condition of 70 S of the underwater vehicle was adopted in this section. Figure 18 shows the surface temperature field of the underwater vehicle during 70 S slewing, in which three straight lines AB, CD and EF are selected. The temperature anomalous area is defined as the water surface area with temperature higher than 291.65 K. There is an arc-shaped temperature anomalous area on the water surface, which is composed of two high-temperature hot spots with an area of 2.627 m\({}^{2}\). By observing the temperature and width of the two hot spots near the three lines, it can be found that the characteristics of  
Figure 17: Line temperature distribution at different sections behind the underwater vehicle.  
the hot spots on the rotating side experience a process from weak to strong and then decay, and the hot spots on the deviating side gradually increase.  
Cross sections were taken along the YZ direction at the lines AB, CD and EF, and the temperature cloud and velocity vector diagrams of the three sections are shown. The color variable is temperature. In Figure 19, the widths of the hot spot on the rotating side are 0.25 m, 0.45 m and 0.12 m, respectively, and the widths of the hot spot on the deviating side are 0 m, 0.13 m and 0.19 m, respectively.  
The earliest abnormal area of water surface temperature is near line AB. Figure 20 shows the curve of the maximum difference \(\Delta\)T between the temperature on the line and the water surface with time. When the underwater vehicle is rotating, the development process of thermal wake is divided into three stages: thermal wake floating, mixing of thermal wake on the rotating side, and mixing of thermal wake on the deviating side. In the first stage, the variation range of \(\Delta\)T is small, and the thermal wake does not rise to the water surface to cause the abnormal area of water surface temperature. In the second stage, the thermal wake on the deviating side is still in the rising stage, while the thermal wake on the rotating side is undergoing intense heat transfer with the water surface, and \(\Delta\)T increases rapidly. At 55 s, the temperature anomaly characteristics of the thermal spot on the rotating side begin to decay, and \(\Delta\)T decreases. In the third stage, the deviation side thermal wake floats to the water surface in 60 s, and the deviation side hot spot begins to appear on the water surface. Since the temperature of the deviation side thermal wake is higher than that of the rotation side thermal wake, \(\Delta\)T continues to increase, and then the decay speed of \(\Delta\)T gradually decreases with time, and \(\Delta\)T decreases to 0 K after a long enough time.  
Figure 19: Temperature cloud diagram and velocity vector diagram of three sections.  
Figure 18: Surface temperature field of underwater vehicle turning 70 s.