## 1 Introduction
[16] proposed a unified formula for thermal jet flow based on grid difference, derived the pressure correction equation from the energy equation and simplified the Poisson equation as a continuity constraint. This method can better simulate the fluid flow process with a large density change. [PERSON] et al. [17] discussed the mixing characteristics of the vertical round thermal jet mean flow in a weak cross-flow by an integral method and established Reynolds mean partial differential equation with continuous tracer mass and momentum conservation in a curvilinear-cylindrical coordinate system to describe the phenomenon of thermal jet flow. Based on the above research, the study of thermal jet characteristics and its changing process is relatively mature, but the influence of the discharge port structure on the characteristics of thermal jets has not been deeply studied and discussed. The thermal jet discharge port of an underwater vehicle generally has a circular structure. However, it is not sufficient to study whether different discharge port structures affect the thermal jet properties. In addition, compared with the single discharge port, the multi-discharge port structure can change the single thermal jet into multiple thermal jets and can strengthen the heat transfer effect of mixing thermal jets with environmental fluids. [PERSON] et al. [18] conducted numerical simulations on the discharge process of cross-flow thermal jets through circular and square ports of equal area based on an RSM model. The calculation results show that the velocity loss near the discharge port was large. Compared with the square port jet, the round port jet had a stronger impact, stronger mixing with environmental fluid and faster temperature attenuation. [PERSON] et al. [19] studied the flow field and temperature field of dual jets. The analysis shows that, when the distance between the discharge ports is larger, the interaction between sub-jets is smaller, but the mixing degree between the rear sub-jets and environmental water is greater in the flow direction of the environmental water. [PERSON] et al. [20] calculated the thermal jet flow field from one discharge port to five discharge ports by using the open-source computing platform and compared it with relevant experimental data. The calculations show that the near field flow characteristics of porous jet are nonlinear. The author of [21] carried out relevant studies on the pressure fields, velocity fields and temperature fields of different numbers of porous jets. The biggest difference between porous jets and single-hole jets was that there was an adsorption effect between adjacent thermal jets. At the same section distance from the discharge port, as the number of ports increased, the correlation characteristic value became greater. [PERSON] conducted a numerical analysis on the heat transfer process between multiple jets and wall surfaces [22]. When the ratio of jet velocity to environmental velocity was large, the Nussel number increased sharply. [PERSON] et al. [23] conducted relevant research on porous turbulent jets in the CFD-FLUENT software (Fluent 2020R1, Wuhan, China) and analyzed variations in the flow-related heat transfer coefficients, temperature and turbulent kinetic energy under different discharge port structures and distributions. However, there are still some deficiencies in the study of discharge port structures. This paper mainly focuses on the structure of discharge ports, taking into account oval discharge ports with different radius ratios and the number and distribution of discharge ports, to obtain the discharge port structure which is most beneficial for restraining the infrared characteristics of the thermal jet, which can improve the thermal stealth performance of underwater vehicles, can reduce the risk of underwater vehicles being detected by space-based or air-based infrared equipment and can provide theoretical research ideas for industrial design and manufacturing.