## 5 Experimental Study on Oval Discharge Ports  
The experimental equipment prepared for this experiment can be roughly divided into five four categories: the experimental pool, the underwater vehicle model, the electric drive track trailer, the infrared thermal imager and the photographic equipment. Figure 11 is the schematic diagram of this experiment. Figure 12 shows that the experimental pool is 7.5 m long, 1 m wide and 1 m high. At the bottom of the pool, there are two water inlets and one water outlet. The water inlet flow is 2084 cm\({}^{3}\)/s, and the water outlet drainage flow is 69 cm\({}^{3}\)/s. The controller of the trailer platform is shown in Figure 13.  
The geometric parameters are the same as the simulation model. The underwater vehicle model is shown in Figure 14. The model length of the underwater vehicle is 0.591 m, the maximum diameter of the underwater vehicle is 0.061 m and the discharge port is located 0.150 m behind the center of the underwater vehicle. The geometric dimensions of the model are obtained by scaling 200:1 according to the structural parameters of the actual underwater vehicle [25; 26]. The electric drive track trailer is located above the pool. The trailer adjusts the speed through the control box to drive the underwater vehicle to go straight in the pool. The speed of the trailer movement is the sailing speed of the underwater vehicle. Above the trailer, it is provided with a water tank, booster pump, a flow control valve, etc. The velocity of the water flow into the underwater vehicle model is adjusted by controlling the opening of the valve to adjust the velocity of the thermal jet discharge. Figure 14: Model of the underwater vehicle. Figure 12: Experimental pool. Figure 13: Controller of the trailer platform. The underwater vehicle model is placed 0.02 m below the water surface in the pool and is connected with the water tank through the iron pipe and the pipeline above the trailer, and the hot water in the water tank is discharged through the discharge port and is mixed with the room-temperature environmental water in the pool for mixing heat transfer. The temperature of the hot water is kept 40 \({}^{\circ}\)C higher than that of the environmental water. A black tracer is added into the water tank to reflect the motion state of the thermal jet. Moreover, the trailer is started by the control box to drive the underwater vehicle straight on the track to simulate the navigation state of the underwater vehicle. The infrared thermal imager is used to measure the temperature field of the water surface, and the infrared characteristics of the thermal jet water surface are measured. As shown in Figure 15, the thermal jet continuously diffuses in the environmental water over time. At the initial stage, the upward floating of the thermal jet is not obvious. After a period of movement, the thermal jet rises upward and reaches the water surface. The temperature difference between the thermal jet and the environmental water leads to different densities between the two types of fluids. The density of the thermal jet is small and rises upward under the action of floating. Under the influence of the thermal jet, a part of the rising temperature area is formed on the water surface, as shown in Figure 16. Different from the simulation results, the water surface temperature diffusion is uneven and discontinuous. However, the diffusion is mainly in the direction of motion, and the maximum temperature difference of the water surface is also low. Figure 15: Trajectory of the thermal jet from the circle discharge port. Under the same experimental conditions, Figure 17 shows the maximum surface temperature difference of the thermal jet affected by oval discharge ports with different radius ratios. Compared with the circle (Str 4) discharge port, when the radius ratio of the discharge port is less than one, the maximum water surface temperature difference decreases by 12.84%-35.80%.