MULTI-FIELD COUPLING EFFECT AND SIMILARITY LAW OF FLOATING ICE BREAK BY VEHICLE LAUNCHED UNDERWATER
摘要: 航行体出水破冰中的耦合效应及载荷特征, 是出水冰结构安全性评估的重要依据. 针对航行体出水破冰问题, 通过量纲分析, 获得了影响航行体动载荷及头部应力的主控参数和相似律. 基于LS-DYNA流固耦合计算方法, 得到了航行体在不同冲击速度、冰层厚度、冰层大小条件下的载荷特性. 计算结果表明, 航行体速度越大, 不同冰层对其过载和头部应力的影响差别越大, 这主要是因为航行体速度越大, 通过水介质对不同冰层的前期破坏程度不同. 对于无限大冰层, 当其厚度大于3倍航行体直径时, 航行体穿冰后期呈现稳定侵彻现象, 航行体的过载和头部应力只与航行体的速度和冰的动力学性能相关; 而对于薄冰, 航行体速度越大, 其头部应力反而越小, 这是因为航行体初速度越大, 其通过水的运动对冰的前期冲击破坏越严重, 冰层易开裂上鼓, 所以造成航行体头部应力较小. 对于径向尺寸为6倍航行体直径的碎冰, 当其厚度大于5倍航行体直径时, 碎冰对航行体运动特性的影响和无限大冰层几乎相同; 而当其厚度小于3倍的航行体直径时, 只有在初速度较低时, 碎冰的尺寸效应才可以忽略. 此外, 对比碎冰和无限冰层对航行体运动的影响可以看出, 越厚的冰受前期水的冲击破坏越小, 碎冰和无限冰层的影响规律基本一致; 而较薄的冰在前期水的冲击下破坏严重, 碎冰和无限冰层对航行体运动的影响都较小; 只有中等厚度的冰, 在较高冲击速度下碎冰和无限冰层才表现出径向尺寸效应相关的破坏程度, 如无量纲厚度为3的两种冰在航行体较高初速度40 m/s的条件下前期破坏差别较大, 导致后期对航行体运动特性的影响具有显著差异.Abstract: Fluid structure coupling effect and dynamic load characteristics are the important basis of the safety evaluation of the vehicle in the process of underwater launching and floating ice break. In this paper, the main control parameters which influence the dynamic load and the stress at the head of the vehicle, as well as the similarity law, are obtained by dimensional analysis. Based on the LS-DYNA fluid structure coupling method, the numerical simulations for underwater launched vehicle impacting with floating ice have been conducted, and the load characteristics of the vehicle under the conditions of different impact velocities, different ice thicknesses and different ice sizes are obtained. The simulated results show that, the higher the initial velocity of the vehicle is, the greater the distinction of the influence of different ice targets on the overload and the stress at the head of the vehicle is, which is mainly due to the different destruction of different ice targets under the strong water impact generated by the underwater motion of the vehicle in the early stage. For the infinite ice sheet, when its thickness is more than 3 times the vehicle diameter, the penetration into the ice appears stable at the later stage, the rigid body acceleration and the stress at the head of the vehicle are only related to the penetration velocity and the dynamic mechanical properties of the ice; for the thinner ice target, the higher the velocity of the vehicle is, the smaller the stress at its head is. This is because the early damage of the ice under the water impact generated by the higher speed motion of the vehicle is more serious, the ice layer is easy to crack and bulge, resulting in the smaller stress at the vehicle head. For the broken ice whose radial dimension is 6 times the vehicle diameter, when its thickness is also greater than 5 times the vehicle diameter, the influence on the vehicle motion is almost the same as that of the infinite ice sheet; when its thickness is less than 3 times the diameter of the vehicle, the ice size effect can be ignored only when the vehicle velocity is low. In addition, by comparing the influence of broken ice and infinite ice on the vehicle motion, it can be observed that, the thicker ice got less damage by the water impact at the early stage, thus the influence of broken ice and infinite ice are basically the same; while the thinner ice is seriously damaged by the water impact, resulting in little influence of broken ice and infinite ice on the vehicle motion; only for the ice targets with medium thickness, their damage degrees are related to the radial size under the condition of the higher vehicle velocity, for instance, when the initial vehicle velocity is 40 m/s, there is a great damage difference between the two kinds of ice targets with dimensionless thickness 3 at the early stage, which leads to a significant difference in the influence on the vehicle motion at the later stage.
Properties Values dimensions: infinite ice sheet
dimensions: broken ice
24 cm × 24 cm × hi
6 cm × 6 cm × hi
density 897 kg/m3 Young’s modulus 9.31 GPa maximum pressure 9.2 MPa minimum pressure −0.92 MPa element type *SECTION_SOLID (brick) typical element size 0.5 mm material properties *MAT_ELASTIC, *MAT_ADD_EROSION *hi is the height of the ice target, and the infinite ice sheet model uses the non-reflecting boundary.
表 2 航行体计算模型及参数
Table 2. Vehicle details
Properties Values dimensions 1 cm diameter, 10 cm length
and spherical head
initial vehicle-ice separation, s 5 cm density 7800 kg/m3 Young’s modulus 207 GPa yield strength 2.1 GPa element type *SECTION_SOLID (brick) typical element size 0.83 mm material properties *MAT_PLASTIC_KINEMATIC Properties Values dimensions 25 cm × 25 cm × 17 cm density 1000 kg/m3 element type *SECTION_SOLID (brick) typical element size 0.5 mm material properties *MAT_NULL, *EOS_GRUNEISEN Properties Values dimensions 25 cm × 25 cm × ha density 1.25 kg/m3 element type *SECTION_SOLID (brick) typical element size 0.5 mm, 0.75 mm material properties *MAT_NULL, *EOS_LINEAR_POLYNOMIAL *ha is the height of the air domain, and is bigger than the height of the ice target hi.
表 5 水的Gruneisen状态方程参数
Table 5. Gruneisen EOS parameters for water
C /(m·s−1) S1 S2 S3 γ a 1520 1.92 0 0 0.28 0
表 6 空气线性多项式状态方程参数
Table 6. Polynomial EOS parameters for air
C0 C1 C2 C3 C4 C5 C6 E/(J·m−3) 0 0 0 0 0.4 0.4 0 2.53×105
表 7 不同工况计算条件
Table 7. Calculation conditions for different cases
Parameters Value initial velocity of vehicle, v0/(m·s−1) 20, 30, 40 ice thickness, hi/cm 1, 3, 5 ice size/(cm×cm) infinite, 6 × 6
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