Research on the Impact Behavior of Molten Metal on the Mold Shell in Gravity Casting of Large Titanium alloy Castings

The impact behavior of molten metal on the mold shell in gravity casting of large titanium alloy castings by investment precision castings was studied.The physical and mechanical models of the impact of molten metal on the mold shell during the pouring process were constructed using numerical simulation.The effects of molten metal pouring rate and pouring weight on the maximum impact force of the mold shell were studied.The research results indicated that during the entire pouring process,the impact force of the first molten metal contacting the mold shell was higher than subsequent molten metal.The maximum impact force increased with the increase of pouring rate and pouring weight.The total impact force of the molten metal on the mold shell was composed of the instantaneous impact force converted from instantaneous impulse and itself gravity.The instantaneous impact force of the molten metal that first impacts the mold shell was much greater than its own gravity,while the impact force of the molten metal at the end of pouring was much less than its own gravity.The maximum impact force on the mold shell of a large casting with a pouring weight of 800kg was about three times higher than that of a medium-sized casting with a pouring weight of 80kg.The difference in the total impact force on the mold shell between them mainly comes from the instantaneous impact force converted from instantaneous impulse.

Comparison of Four Methods for Handing Missing Data in Longitudinal Data Analysis through a Simulation Study

Missing data can frequently occur in a longitudinal data analysis. In the literature, many methods have been proposed to handle such an issue. Complete case (CC), mean substitution (MS), last observation carried forward (LOCF), and multiple imputation (MI) are the four most frequently used methods in practice. In a real-world data analysis, the missing data can be MCAR, MAR, or MNAR depending on the reasons that lead to data missing. In this paper, simulations under various situations (including missing mechanisms, missing rates, and slope sizes) were conducted to evaluate the performance of the four methods considered using bias, RMSE, and 95% coverage probability as evaluation criteria. The results showed that LOCF has the largest bias and the poorest 95% coverage probability in most cases under both MAR and MCAR missing mechanisms. Hence, LOCF should not be used in a longitudinal data analysis. Under MCAR missing mechanism, CC and MI method are performed equally well. Under MAR missing mechanism, MI has the smallest bias, smallest RMSE, and best 95% coverage probability. Therefore, CC or MI method is the appropriate method to be used under MCAR while MI method is a more reliable and a better grounded statistical method to be used under MAR.

Deep-reinforcement-learning-based UAV autonomous navigation and collision avoidance in unknown environments

In some military application scenarios,Unmanned Aerial Vehicles(UAVs)need to perform missions with the assistance of on-board cameras when radar is not available and communication is interrupted,which brings challenges for UAV autonomous navigation and collision avoidance.In this paper,an improved deep-reinforcement-learning algorithm,Deep Q-Network with a Faster R-CNN model and a Data Deposit Mechanism(FRDDM-DQN),is proposed.A Faster R-CNN model(FR)is introduced and optimized to obtain the ability to extract obstacle information from images,and a new replay memory Data Deposit Mechanism(DDM)is designed to train an agent with a better performance.During training,a two-part training approach is used to reduce the time spent on training as well as retraining when the scenario changes.In order to verify the performance of the proposed method,a series of experiments,including training experiments,test experiments,and typical episodes experiments,is conducted in a 3D simulation environment.Experimental results show that the agent trained by the proposed FRDDM-DQN has the ability to navigate autonomously and avoid collisions,and performs better compared to the FRDQN,FR-DDQN,FR-Dueling DQN,YOLO-based YDDM-DQN,and original FR outputbased FR-ODQN.

ICU患者身体约束循证护理审查指标的制订及障碍因素分析

目的开展ICU成人患者规范化身体约束的循证护理,制订护理质量审查指标,并分析循证护理过程中的障碍因素和促进因素。方法以Johns Hopkins循证护理实践模式为理论指导,组建团队,系统检索、评价并汇总证据,确立审查指标及审查方法,逐条进行质量审查,并根据审查指标及基线审查结果分析障碍因素及促进因素,制订行动策略。结果本研究共纳入23条最佳证据,依据最佳证据制订了12条质量审查指标,障碍因素主要为系统层面流程和工具缺乏、实践者层面护士知信行水平不高等。结论本研究基于循证并结合临床专业人员的判断,制订的质量审查指标具有科学性、可操作性和实用性,可为循证护理奠定基础。

考虑同时攻击约束的多异构无人机实时任务分配

针对侦察/攻击型多无人机协同搜索和攻击作战想定,考虑其中相互耦合的任务分配和航迹规划问题,提出了一种获得最大系统效能的任务分配算法以及同时攻击目标的航迹规划算法.该任务分配算法基于合同网协议,考虑目标的资源需求和无人机的资源约束关系,其目标函数中同时包含执行目标的收益和代价.航迹规划中,考虑无人机的运动学约束和同时攻击目标的需求,提出了基于协同粒子群算法和协同函数、协同变量相结合的算法.仿真结果表明,所提出的任务分配方法较之多项式时间联盟构建算法(PTCFA),能够获得至少8%的效能提升.

Two-phase guidance law for impact time control under physical constraints

An impact-time-control guidance law is required for the simultaneous attack of suicide attack unmanned aerial vehicles.Based on the nonlinear model,a two-phase guidance strategy is proposed.The impact time is derived in a simple analytical form of initial states and switching states,and it can be controlled by switching at an appropriate range.Firstly,a two-phase guidance law is designed to make the magnitude of the heading error decrease monotonically from its initial value to zero.And then,the feasible interval of the switching ranges and of the impact times under the acceleration constraint are given analytically in sequence.Furthermore,a general form of twophase guidance law is proposed,which allows the magnitude of the heading error to increase in the first phase,to improve the applicability of the methodology.Having the same structure as proportional navigation guidance with a time-varying gain,the proposed algorithms are simple and easy to implement.The corresponding feedback form is presented for realistic implementation.When a predefined impact time is taken within its permissible set,the constraints on the acceleration and fieldof-view will not be violated during the interception.Finally,simulations validate the effectiveness of the methodology in impact time control and salvo attack.

Solubility of RE_2O_3(RE=La and Nd) in light rare earth fluoride molten salts

The chemical reaction and dissolution processes of RE2 O3 in REF3-LiF（RE = La and Nd） molten salts were studied by X-ray diffraction, differential thermal analysis and chemical analysis. Firstly, RE2 O3 disperses in molten salts and reacts with REF3 and LiF, which lead to the generation of a new phase REOF, resulting in the dissociation or formation of complex ions by part of REOF, and the sedimentation of excessive REOF at the bottom. The RE contents in molten salts were used to determine the solubility of RE2 O3. The results show that with 20 mol%-40 mol% of REF3, the solubility ranges from 1.79 to 3.05 g RE2 O3 per100 g electrolyte and the corresponding mole concentration is 0.33%-0.87%. RE2 O3 solubility increases with increasing temperature and REF3 concentration. The natural logarithm of the RE2 O3 solubility is plotted against 1/T and can be interpreted by linear relationship. The fitting parameters can be used to estimate the solubility of RE2 O3 with minimized experimental efforts and difficult experiment conditions.

Electrical conductivity of REF3-LiF(RE=La and Nd) molten salts

The electrical conductivity of REF3-LiF(RE=La and Nd) molten salts(xREF3=5 mol%-40 mol%) was systematically measured over the temperature range from 1223 to 1423 K by using continuously varying cell constant(CVCC) method.Electrical conductivity values of LaF3-LiF and NdF3-LiF molten salts within the studied ranges are 4.11-9.39 and 3.62-9.51 S/cm,respectively.The composition and temperature dependences of electrical conductivity and the factor of RE2 O3 on electrical conductivity were investigated.The electrical conductivity nonlinearly decreases with the increasing mole percent of REF3 for the changing of structural ion in molten salts.A good linear relationship between the natural logarithm of electrical conductivity(lnκ) and the reciprocal of the absolute temperature(T-1) can be interpreted by the trend that electrical conductivity linearly increases with increasing temperature.Although it is an important factor of RE2 O3 on electrical conductivity,the influence of RE2 O3 on conductivity is small for the very limited solubility of RE2 O3 in REF3-LiF molten salts.The results of this work supplement the present electrical conductivity data of rare earth fluoride electrolyte.

Detection and Initial Response to a Type 2 Vaccine-Derived Poliovirus—Sichuan Province,China,2019

What is already known about this topic?After the type 2 strain of the live,attenuated poliovirus vaccine was withdrawn globally in 2016,any identification of a type 2 poliovirus is a Public Health Emergency of International Concern.A vaccinederived type 2 poliovirus(VDPV2)was identified in Sichuan,prompting an urgent,comprehensive investigation and response.What is added by this report?Type 2 monovalent,live,attenuated oral poliovirus vaccine(mOPV2)is being used to respond to the numerous VDPV2 outbreaks seen around the world.In contrast,the response in Sichuan used Sabin strain inactivated poliovirus(sIPV)to stop circulation of the VDPV2.In the 6 months following the vaccination response,there have been no VDPV2s detected in Sichuan,despite extensive search.What are the implications for public health practices?Further search for the VDPV2 must continue in order to determine whether transmission has been stopped.The ongoing investigation and response to the Sichuan VDPV2 is providing evidence to the Global Polio Eradication Initiative on managing VDPV2 outbreaks.

Identification of a SARS-CoV-2 virus-encoded small non-coding RNA in association with the neurological disorders in COVID-19 patients

Dear Editor,Coronavirus disease 2019(covid-19),caused by severe acute respiratory syndrome coronavirus 2(SARS-CoV-2),has led to 460 million cases confirmed and 6.1 million deaths across almost 200 countries until March 2022.In addition to the vaccine-based protection against viral infection,it is urgently needed to accurately diagnose covid-19 patients as early as possible after infection.