The influence of the grooved fairing dimension on the AUVs for across-medium flight

The research object of this paper is the across medium (air and water) Autonomous Underwater Vehicles (AUVs) fairing. When the AUVs are flying in the air, for the purpose of protecting the detection equipment and reducing the air resistance, an ogive fairing is installed at the head of the aircraft. When the AUVs enter the water at a certain angle and speed after arriving at the destination, it is hoped that the fairing will be broken as much as possible, so that it does not affect the normal use of the detection equipment. Therefore, the influence of the dimension parameters of the fairing on the aerodynamic resistance is studied, and the influence of the groove and groove depth on the damage of the fairing after impact is also researched.


Introduction
The underwater environment is mysterious and dark, and the high-pressure environment can also threaten human life.AUVs are better suited to take the place of long-term underwater scientific investigations for researchers.AUVs play an increasingly important role in military application, resource exploration and scientific surveying.As a result, AUVs are usually equipped with a variety of sensors, which are located on the head or sides of the body.The launch modes of AUVs are various (torpedo is launched from submarine and ship, and rocket-propelled aircraft is launched from fixedwing aircraft) [1], air launch is the most effective launch method, which can quickly reach the target area, reduce energy consumption and increase range.But, in the air, flat head AUV can cause considerable air resistance, affecting launch range.The ogive fairing at the head of the AUV effectively reduces air resistance.However, the fairing will affect the performance of the detection equipment in the AUV head.A solution is to knock the fairing out by impact force when the AUV enters the water.Based on the research of Chaudhry, the axial impact load of ogive head is weaker than that of flat head and spherical head, but the radial impact load of ogive head is strongest [2].
Hence, how to design an ogive fairing is an urgent problem to be solved.When the AUV enters the water, the fairing is likely to suffer as much damage as possible, maintaining strength and obtaining low air resistance while the AUV fly in the air.Unfortunately, very few studies have paid attentions on this field.Many researchers have focused on the cavity evolution and trajectory of AUVs at different head shapes, inlet angles and velocities [1,[3][4][5].According to Shi's research, the hemispherical angle of AUV head and shell thickness have an impact on the impact stress response [6].The size of the fairing also affects its air resistance.In order to enhance the damage of the fairing, several grooves are opened on the inner surface of the fairing, but the strength of AUV cannot be reduced while in the air.Obviously, the orientation of present paper is to investigate the influence of grooved fairing dimension parameters on aerodynamic resistance and structural strength.

Influence of fairing dimension on aerodynamic drag
In order to facilitate the detection of underwater targets after the AUVs enter water and expand the search range, the underwater detection equipment is generally installed on the head of the AUV.When the AUV is flying in the air, in order to protect the detection equipment and reduce the air resistance, an ogive fairing is installed at the head of the aircraft.Therefore, the control variable method is used here to investigate the effect of dimension parameters (head radius (Rh), radius (Rb), thickness (bH) of fairing, as shown in Fig. 1) on aerodynamic resistance (the aerodynamic drag coefficient is used here to express the aerodynamic drag.The smaller the aerodynamic drag coefficient, the smaller the aerodynamic drag).For simulation calculation, the flying speed of AUV is 270m/s, the atmospheric pressure is 101325Pa, and the temperature is 288.15K.Taking the radius of the fairing as the research object, the aerodynamic drag coefficient is shown in the Table 1 when the head radius and thickness of fairing are fixed.When the radius of fairing tip is taken as the research object, the radius and the thickness of the fairing are fixed, the aerodynamic drag coefficient is shown in the Table 2.In the above three cases, the maximum stress of the fairing under aerodynamic influence is less than the yield strength of the material (Al7075-T651) [7,8].
In order to better illustrate the effect of fairing dimension parameters on aerodynamic drag coefficient, dimension parameters in the above three cases are normalized, and the results are shown in the Fig. 2. As can be seen from Fig. 2, curve Thickness has a higher slope, which means that the drag coefficient changes sharply, so the fairing thickness has the greatest influence on the drag coefficient.Similarly, fairing radius has the least effect on aerodynamic drag.

Influence of grooves dimension on fairing damage
When the AUV reaches the destination, it enters the water at a certain angle and speed to carry out underwater detection.During the process of entering the water, the fairing will be broken as much as possible so that it will not affect the normal use of detection equipment.In this part, the inner wall groove and groove size of fairing will be taken as the research object to study the damage degree of fairing during impact into water.
Firstly, the influence of the inner wall groove on the impact damage of the fairing is studied, which is compared with the situation without groove.Relevant dimensions are shown in Fig. 3.The process of entering water belongs to the fluid-structure coupling problem.ALE method is usually adopted to solve the problem.For the convenience of calculation, counterweight blocks are used to replace AUV, the angle between fairing axis and water surface is 30°, and the water entry velocity is 30 m/s, and Fig. 4 shows the simulation model.4. Equations of State describes the mechanical properties of air (EOS Linear Polynomial) and water (EOS Gruneisen), and relevant parameters are shown in Table 5 [2].The Elastic material model is adopted for the counterweight block, and the corresponding parameters are density 7850kg/m-3, Young's modulus 210GPa, and Poisson's ratio 0.Then, the fairing thickness is set to 9mm, so as to improve the strength of the fairing, and it is more convenient to investigate the influence of groove dimension on the impact damage of the fairing.The same groove structure as that in Fig. 5(a) will be adopted here, and the depth of the groove (7mm, 5mm) will be taken as the variable to carry out the research.The results are shown in Fig. 6.Groove depth has a certain influence on the damage for fairing, as illustrated in Fig. 6.

Conclusions
The fairing of across-medium unmanned aerial vehicle is studied in this paper.In order to reduce the air resistance of the fairing, the influence of the fairing body radius, the head radius and the fairing thickness on the aerodynamic resistance was investigated.The result is that fairing thickness has the greatest influence on aerodynamic drag compared with other dimension parameters.During the conversion from flight mode to underwater mode, the fairing is expected to be broken as much as possible.Therefore, the influence of groove and groove depth on the fairing damage in the water entry process is studied.Judging from the result, the groove enhances the ability to destroy and thickness is proportional to the damage effect.In the future research, a multi-objective optimization study can be carried out to find the appropriate fairing dimension with the aim of reducing drag, impact damage and light weight.

Figure 2 .
Figure 2. Influence of fairing dimension parameters on aerodynamic drag coefficient.

Figure 4 .
Figure 4. Water entry process.The fairing is made of Al7075-T651 material and material model is Johnson-Cook.The relevant parameters are shown in Table4.Equations of State describes the mechanical properties of air (EOS Linear Polynomial) and water (EOS Gruneisen), and relevant parameters are shown in Table 5[2].The Elastic material model is adopted for the counterweight block, and the corresponding parameters are density 7850kg/m-3, Young's modulus 210GPa, and Poisson's ratio 0.3.

Figure 5 .
Figure 5. Influence of groove or not on impact damage of fairing.

Figure 6 .
Figure 6.Influence of groove depth on impact damage of fairing.

Table 1 .
The effect of fairing radius on aerodynamic drag coefficient.

Table 2 .
The effect of fairing head radius on aerodynamic drag coefficient.When fairing thickness is taken as the research object, fairing radius and head radius are fixed, aerodynamic drag coefficient is shown in the Table3. 3

Table 3 .
The effect of fairing thickness on aerodynamic drag coefficient.