面对面呼吸飞沫传播和防护的流体力学初步分析

针对SARS-CoV-2疫情,围绕病人和戴防飞沫面罩的健康人面对面呼吸状态下飞沫的流动传播问题,利用计算流体力学方法进行了数值模拟。首先采用二维非稳态不可压缩雷诺平均N-S方程描述平均流场,计算区域采用H型结构网格拓扑结构,将多相流的欧拉模型用于描述空气-飞沫流动,湍流脉动采用RNG k-ε湍流模型。计算获得了呼吸飞沫喷射速度、粒径和入口含量对飞沫流动传播的影响规律,分析了水平传播线和垂直传播线的飞沫浓度分布。研究表明:呼吸飞沫喷射速度、入口含量和流动传播速度、浓度分布呈正相关性,粒径主要影响飞沫流动传播的浓度峰值大小;呼吸飞沫跟随空气流动先以水平传播为主,后转变为以垂直扩散为主,扩散过程会形成"猫眼"状飞沫云团;呼吸飞沫在气流作用下侵入到防飞沫面罩内形成旋涡,导致防飞沫面罩的失效,因此,防飞沫面罩必须结合口罩共同使用。     

Modelling of grinding mechanics: A review

Grinding is one of the most widely used material removal methods at the end of many process chains. Grinding force is related to almost all grinding parameters, which has a great influence on material removal rate, dimensional and shape accuracy, surface and subsurface integrity, thermodynamics, dynamics, wheel durability, and machining system deformation. Considering that grinding force is related to almost all grinding parameters, grinding force can be used to detect grinding wheel wear, energy calculation, chatter suppression, force control and grinding process simulation. Accurate prediction of grinding forces is important for optimizing grinding parameters and the structure of grinding machines and fixtures. Although there are substantial research papers on grinding mechanics, a comprehensive review on the modeling of grinding mechanics is still absent from the literature. To fill this gap, this work reviews and introduces theoretical methods and applications of mechanics in grinding from the aspects of modeling principles, limitations and possible future trendencies.     

爆炸破片作用下 舰载导弹战斗部的安全性数值模拟

文章主要针对爆炸破片作用下,垂直发射系统中舰载导弹战斗部的安全性进行分析,分别讨论了破片的质量、速度、侵彻角度和破片的数量等因素作用下,舰载导弹外防护设置以及战斗部的响应特性。结果表明:在临界起爆条件附近,炸药的点火增长对撞击速度的变化很敏感。同时,入射角对其是否引爆威胁战斗部的影响也很大;当两破片间距小于破片直径时,破片撞击战斗部形成的冲击波会相互叠加,压力峰值增大,更容易引爆战斗部,而当两破片间距大于1.7倍的破片直径时,此时两破片产生的冲击波叠加效应降低。     

基于LS-DYNA对于舰艇舱室焊缝模拟方法

探讨了3种方法等效模拟焊缝结构：材料赋值法，等效接触法，等效约束法；对比分析了3种方法建模的简便性、计算的耗时性、结果的准确性。材料赋值法可以准确反映舱室焊缝结构最先出现破口的位置以及破口延伸过程；等效接触法建模与网格划分过程简单，计算时间适中，准确度较高；等效约束法网格划分要求节点不重合，计算时间较短，但焊点个数设置不易把握。     

Influence of lunar regolith compressibility on sampling performance of thick wall spiral drills

A 2 m class robotic drill was sent to the Moon and successfully collected and returned regolith samples in late 2020 by China. It was a typical thick wall spiral drill (TWSD) with a hollow auger containing a complex coring system to retain subsurface regolith samples. Before the robotic drill was launched, a series of laboratory tests were carried out to investigate and predict the possible drilling loads it may encounter in the lunar environment. This work presents how the sampling performance of the TWSD is affected by the regolith compressibility. Experiments and analysis during the drilling and sampling process in a simulated lunar regolith environment were conducted. The compressibility of a typical lunar regolith simulant (LRS) was measured through uni-directional compression tests to study the relationship between its inner regolith stress and bulk density. A theoretical model was established to elucidate the cutting discharge behavior by auger flights based on the aforementioned relationship. Experiments were conducted with the LRS, and the results show that the sampling performance is greatly affected by the flux of the drilled cuttings into the spiral flight channels. This work helped in scheduling reasonable drilling parameters to promote the sampling performance of the robotic drill in the Chinese Chang'E 5 mission.