# Simulation Study on Thermal Wake Characteristics of Underwater Vehicle under Rotary Motion
## 4 Analysis of Simulation Results
### Spatial Evolution of Far-Field Thermal Wake  
When the underwater vehicle is rotating, the cooling water moves asymmetrically near the body and gradually develops into a strong asymmetric thermal wake after it breaks away from the body. The thermal wake on the rotating side is above the thermal wake on the deviating side. In this section, the coupling effect of thermal wake and turbulent wake is analyzed by taking the underwater vehicle rotating 90\({}^{\circ}\) as an example, and the spatial evolution of thermal wake after it is separated from the body is discussed. Dz is defined as the horizontal distance after leaving the underwater vehicle in the negative direction of the Z-axis, the thermal wake center is defined as the position with the highest temperature of the thermal wake, and the height coordinate Y of the underwater vehicle axis is 0. As shown in Figure 14, cross sections were taken along XY direction at distances of 0.5 m, 1.0 m, 1.5 m, 2.0 m, 2.5 m and 3.0 m from the rear of the body, and temperature and velocity vector diagrams at different cross sections were observed. Figure 15 is the variation diagram of the height of thermal wake center with the distance of DZ from the rear of the body. Combined with Figures 14 and 15, it can be seen that the thermal wake center on the revolving side has experienced the process of rising, sinking and then rising, and the cross-section shape in XY direction has successively developed into oval, kidney and oval. The center of the thermal wake on the deviating side went through the process of sinking and rising successively, and the shape of the cross section in the XY direction changed from circular to elliptical. The diffusive area and central rising velocity of the deviating side thermal wake in the XY direction are smaller than those of the rotating side thermal wake. Figure 14: Temperature cloud and velocity vector at different sections behind the underwater vehicle. There is a great difference in the spatial evolution of the thermal wake on both sides because the buoyancy of the thermal wake is mainly affected by the vortex and the buoyancy caused by the temperature difference. When the horizontal distance from the body is more than 1.5 m, the fluid velocity in the vortex is large, and the thermal wake on both sides follows the vortex. When the horizontal distance from the body is less than 1.5 m, the strength of the vortex gradually decreases with the distance from the underwater vehicle, and the thermal wake floats under the dual influence of the vortex and the buoyancy force, and the buoyancy speed decreases nonlinearly. Figure 16 shows the variation of the center temperature of the thermal wake as a function of the distance behind the body. It can be found that from the discharge to the beginning of leaving the body, the heat wake has a drastic energy exchange with the surrounding seawater, and the temperature decreases rapidly. In the process of spatial evolution of the thermal wake, the temperature of the thermal wake on the rotating side is always lower than that on the deviating side, which is related to the different intensity of the vortices on both sides. In the rotating side vortex, the fluid velocity is high, the vortex intensity is high, and the heat transfer between the thermal wake on the rotating side and the surrounding sea water is strong. With the distance from the underwater vehicle, the temperature of the thermal wake center further attenuated and finally tended to be slightly higher than that of the water body. Figure 16: Variation of center temperature of thermal wake with distance DZ away from the rear body. Figure 15: Variation of center height of thermal wake with distance DZ away from the rear of the body. As shown in Figure 14, five horizontal lines are taken from bottom to top in the six sections behind the underwater vehicle, which are, respectively, 0.5 m, 0.4 m, 0.3 m, 0.2 m and 0.1 m away from the water surface, and named Lines1-5. Figure 17 shows the distribution of line temperature.