Simulation study on flow and response temperature of armored vehicle in low temperature environment test

In this paper, three-dimensional CFD simulation and vehicle’s environmental test were combined, according to the size and operating parameters of the actual environmental test chamber, a model of the low-temperature environmental test chamber was constructed by using 3D modeling software; by removing the structure that has little influence on the process of environmental experiment, a three-dimensional vehicle model suitable for simulation calculation is built, and by means of 3D model assembly, the combination of vehicle and environmental chamber model is realized. According to the low temperature environment test conditions, CFD numerical simulation method was used to analyze the flow velocity distribution in the test chamber and the response trend of each chamber to the low temperature environment, the results were also compared with the experimental data of low temperature environment, which provides a theoretical basis for carrying out simulation and prediction research based on digital environment test.

Armored vehicle equipment is the core force of land combat, its ability to adapt to various natural environments is an important guarantee to exert its effective force [1].In the extremely low temperature and ice and snow environment in the northeast border areas of China, vehicles are prone to start difficulty, heat loss increase, engine undercooling and other conditions, which seriously affect the combat effectiveness of equipment.Therefore, vehicle performance under low temperature environment is an important part of vehicle environmental suitability [2].
The current test evaluation method is the main method and means to evaluate the environmental suitability of armored vehicles, including natural environment test, laboratory environment test and operational environment test, but it also has some shortcomings such as long test period, high cost [3].Thanks to the improvement of computer computing power, the continuous improvement of thermodynamics, fluid mechanics and other disciplines, environmental adaptability simulation technology has gradually become the focus of the research on environmental adaptability of armored vehicles [4].
The simulation of environmental adaptability is generally based on actual combat requirements.The structure is drawn by computer software, and relevant simulation models are established for the environment of various equipment.Finally, the analysis and prediction are carried out through numerical calculation, which can greatly shorten the test period and provide a reasonable evaluation mechanism.Although the research on environmental test simulation technology has been carried out for many years, and the relevant achievements have been gradually applied in practice [5][6][7], the simulation research on flow and vehicle temperature response in low-temperature environment test chamber is relatively limited.Therefore, this paper carried out CFD simulation research of vehicle environmental test based on low temperature environment, so as to provide basis for simulation and prediction of digital low temperature environment test.

Environmental test chamber system
Before building the simulation model of the low-temperature environment chamber, it is necessary to have an in-depth understanding of the structure of the test chamber.The main design parameters of the low-temperature test chamber in this paper are shown in the  As shown in Fig. 1, the environmental test system is mainly composed of pressure test chamber body, air duct, gate, transition chamber, dynamometer and control system.The pressure chamber body is the main frame of the test chamber system; the chamber body is designed as a rectangular structure to ensure sufficient strength and stability; Air flow is mainly realized by the axial fan in the power section.

Simplification and construction of test chamber model and vehicle
As can be seen from the above, simulation model building of environmental test involves model combination from vehicle components, key components, vehicle model and environmental experiment chamber, so model building is relatively complicated.If we blindly strive to completely restore the environment chamber and vehicle structure for simulation research, it will greatly increase the simulation calculation time, so it is necessary to carry out a certain degree of structural simplification before simulation.

Simplification and construction of test chamber model
Firstly, the reinforcement bar of the test chamber is installed outside the shell, so it can be simplified.Secondly, the air-tight door, observation window, dynamometer and other structures of the test chamber can also be ignored in the simulation, while the air duct, the shell of the experiment chamber and the relevant baffle structure need to be retained.To sum up, the structure of the simplified test chamber is shown in Fig. 2.

Simplification and construction of vehicle
The structural model of the armored vehicle studied in this paper is shown in Fig. 3.In order to effectively reduce the complexity of the model, the surface structure of the vehicle was partially smooened, and the internal structure of the vehicle was simplified.The turret, the body, the power output assembly, the cooling grille and other structures that have a great influence on the convection heat transfer were retained, which can reflect the situation of the vehicle in the environmental experiment chamber to a certain extent.

The construction of CFD grid model
In this paper, Ansys Fluent is used for CFD simulation of environmental test.In order to better simulate the temperature response of different areas of armored vehicles under low temperature environment, the structure of armored vehicles is divided into four areas, namely pilot compartment, passenger compartment, turret and engine compartment, as shown in Fig. 5.The total number of grids of the final model is about 6.04 million, as shown in Fig. 6.

Determination of computational model and simulation boundary parameters
Realizable k-ε model was selected as the turbulence model, and boundary conditions were set in a given region, including flow boundary conditions, pressure boundary conditions, wall boundary conditions, etc.The temperature variation curve in the environmental chamber during the low-temperature test is shown in Fig. 7. Fig. 7 The temperature variation curve in the environmental chamber As can be seen from the temperature curve, the whole low temperature test process is mainly composed of low temperature preparation stage, cooling stage and insulation stage.In this paper, temperature curve fitting was used to generate fitting functions for each of the three stages of environment temperature, which were input as inflow temperature parameters.As the air power source of the test chamber, the fan is defined as the flow inlet and outlet, the flow velocity is set to 1.7m/s; The chamber body of the test chamber and the body of the armored vehicle were set as wall conditions, and the initial temperature of the chamber body was consistent with that of the fan inlet.The initial temperature of the armored vehicle was set according to the test data, as shown in Table 3. Vehicle surface no sliding wall initial temperature: 18.8°C

Analysis of response temperature variation in each compartment of armored vehicle
Fig. 8 (a) shows the response temperature curves of each area of armored vehicle under low temperature environment.It can be seen that in the early stage, due to the slow decline of the environment temperature, there is little difference between the response temperature and the environment temperature in each area of the vehicle, and the temperature difference between each area is basically the same.When entering the rapid cooling stage, the environment temperature decreased from 10°C to -43°C in less than 200 minutes, and the response temperature changes in each area of the armored vehicle gradually began to differ.From the pilot compartment to the engine compartment, the response temperature drop rate gradually slows down, indicating that the flow of armored vehicles in different areas will increase the variation trend difference of response temperature during the temperature drop.As the environment temperature of the low-temperature test gradually stabilized into the insulation stage, the temperature difference between the crew chamber and the power chamber gradually decreased until it basically reduced to the environment temperature level together with the cockpit, indicating that in addition to the effect of external low-temperature flow, the heat exchange process was also taking place between different structures of the armored vehicle.
According to the comparison curves of simulation and test response temperature data in Fig. 8 (b), errors between simulation results and tests are mainly reflected in: in the low temperature preparation stage, the response temperature drop rate of the engine compartment is significantly faster than the test data; in the rapid cooling stage, the response temperature drop rate is slower; in the low temperature insulation stage, the final response temperature level is slightly higher than the test value.The main reason is that the chamber structure and the fan are simplified, and the flow generated by the fan inlet is different from the actual test.No matter the vehicle surface or internal structure, the heat transfer model also has some idealization treatment.The simplified treatment of the power chamber structure by the simulation model will also affect the response temperature trend.But in general, the simulation results can still well reflect the dynamic variation trend of the temperature in each area of the vehicle under the low temperature environment.

Simulation analysis of velocity field in test chamber
The velocity distribution in the low-temperature chamber is shown in Fig. 9.It can be seen that the flow velocity near the fan outlet is relatively high and uniform.When it passes through the diffusion section of the air duct, the flow non-uniformity begins to strengthen.The flow velocity decreases along the radial direction of the air duct from top to bottom, but the flow velocity is relatively stable.Subsequently, under the action of the deflector and the corner of the air duct, the flow heterogeneity is further enhanced.Especially, when passing through the deflector structure, there is a backflow vortex in the opposite direction of the fan flow, which has a certain influence on the flow in the chamber, resulting the distribution of high velocity in the upper part of the diffusion section and low velocity in the lower part.But then, under the action of the shrinking section of the test chamber, the flow gradually gathers and the velocity gradually increases.When flowing through the surface of the armored vehicle, there is no obvious stratification of velocity, which indicates that the test chamber can achieve a uniform and controllable wind speed environment.
When flowing through the armored vehicle, the upper turret and cockpit structure are affected by strong flow heat transfer due to their large windward area, and the temperature drops quickly.After passing through the front of the vehicle, the flow pattern changes greatly.Whether it is up and down or on the side, the flow velocity around the passenger compartment and the engine compartment at the rear of the vehicle decreases greatly, so the temperature drops slowly.In addition to the flow at the top of the turret, under the action of the flow at the bottom of the vehicle, a small range of flow around the rear of the vehicle is formed, and because the contact area of the upper and lower and side flow of the passenger compartment is larger than that of the engine compartment, the temperature drops slightly faster than that of the engine compartment.

Analysis of temperature field variation in vehicle interior
In this section, slices of the temperature field of the armored vehicle were intercepted to analyze the temperature variation process inside the vehicle, as shown in Fig. 10.As can be seen from the figures, the internal temperature of the vehicle is basically consistent with the environment temperature in the first two hours because the environment temperature dropped slowly.During the rapid cooling phase, the vehicle began to lower its temperature from the outside to the inside.Due to the higher initial temperature and smaller fluid contact area, the temperature of the engine compartment and the passenger compartment dropped slower than that of the turret; In 720min, that is, the low-temperature insulation stage, the internal temperature of the vehicle is basically the same as the environment temperature, which is consistent with the response temperature curve in Fig. 8. 1) Vehicle working condition and body structure have great influence on the flow and heat dissipation of vehicle body and power compartment.
2) In the process of temperature drop in the low-temperature environment test, the flow condition of the armored vehicle in each area will increase the variation trend difference of the response temperature.
3) The thermal insulation performance of the power chamber and crew chamber is better than that of the cockpit and turret in low temperature environment.

Fig. 1
Fig. 1 Overall structure of low temperature environmental test chamber (a) body structure (b) heat sink and power assembly structure

Fig. 3
Fig. 3 Armored vehicle model On this basis, the environmental test chamber model and vehicle model were combined to complete the construction of the integrated structure model of environmental test chamber and vehicle.The model diagram is shown in Fig. 4.

Fig. 4
Fig. 4 Environmental test chamber-vehicle model 3. Numerical simulation of low temperature environment test

Fig. 5
Fig. 5 Schematic diagram of the zone division of armored vehicle

Fig. 6
Fig. 6 Environmental test chamber-vehicle grid model Fig. 8 Response temperature curves of each area of the vehicle (a) velocity vector diagram (b) velocity nephogram

Fig. 9
Fig. 9 Distribution of velocity in chamber

Fig. 10
Fig. 10 Variation of temperature field distribution inside vehicle4.ConclusionIn this paper, combined with the research needs of vehicle adaptability to low temperature environment and relevant CFD theory, reasonable selection of turbulence model, mesh division mode, boundary conditions and other simulation parameters, the low temperature environment test was numerically simulated.Through the comparative analysis of the response temperature change and the test data, and the analysis of the flow field in the test chamber and the temperature field inside the vehicle, the dynamic

Table 1 .
1: Main parameters of low temperature test chamber

Table 2 .
Boundary and initial condition setting