关键词： Blood flow   Cartesian grid generation   computational fluid dynamics   mechanical heart valve   non-Newtonian flow   shear stress  

摘要： In this paper, we perform a numerical analysis for simulating steady, two-dimensional, laminar blood flow through our proposed design, known as the Butterfly mechanical heart valve, where the leaflets are fully opened. Blood has been assumed to be Newtonian and non-Newtonian fluid using the Casson model for shear-thinning behavior. A non-uniform Cartesian grid generation technique is presented to generate a two-dimensional grid for the irregular geometry of the Butterfly valve. The governing Navier-Stokes equations of flow, written in a stream function-vorticity formulation, are solved by the finite difference method with hybrid differencing of the convective terms. The computed results show that the blood's non-Newtonian nature significantly affects the flow field with the existence of recirculation and consequently stagnation causing thrombus formation, as well as an increase of the shear stress along the wall, which contributes to hemolytic blood damage. The results demonstrate that the model is capable of predicting the hemodynamic features most interesting to physiologists. It can be used to assess thromboembolic problems occurring with heart valves and in the design of cardiac prostheses.

摘要： Background: The authors' previous studies have found that spinal protein kinase C gamma expressing neurons are involved in the feed-forward inhibitory circuit gating mechanical allodynia in the superficial dorsal horn. The authors hypothesize that nerve injury enhances the excitability of spinal protein kinase C gamma expressing interneurons due to disinhibition of the feed-forward inhibitory circuit, and enables A beta primary inputs to activate spinal protein kinase C gamma expressing interneurons. Methods: Prkcg-P2A-tdTomato mice were constructed using the clustered regularly interspaced short palindromic repeats and clustered regularly interspaced short palindromic repeats-associated nuclease 9 technology, and were used to analyze the electrophysiologic properties of spinal protein kinase C gamma expressing neurons in both normal conditions and pathologic conditions induced by chronic constriction injury of the sciatic nerve. Patch-clamp whole cell recordings were used to identify the nature of the dynamic synaptic drive to protein kinase C gamma expressing neurons. Results: A beta fiber stimulation evoked a biphasic synaptic response in 42% (31 of 73) of protein kinase C gamma expressing neurons. The inhibitory components of the biphasic synaptic response were blocked by both strychnine and bicuculline in 57% (16 of 28) of neurons. Toll-like receptor 5 immunoreactive fibers made close contact with protein kinase C gamma expressing neurons. After nerve injury, the percentage of neurons double-labeled for c-fos and Prkcg-P2A-tdTomato in animals walking on a rotarod was significantly higher than that in the nerve injury animals (4.1% vs. 9.9%, 22 of 539 vs. 54 of 548,P < 0.001). A beta fiber stimulation evoked burst action potentials in 25.8% (8 of 31) of protein kinase C gamma expressing neurons in control animals, while the proportion increased to 51.1% (23 of 45) in nerve injury animals (P = 0.027). Conclusions: The Prkcg-P2A-tdTomato mice the authors construct

关键词： Molecular dynamics simulations   alpha-graphyne   alpha(2)-graphyne   Young's modulus   Fracture strain   Maximum stress  

摘要： Molecular dynamics simulations are used to study the mechanical properties of alpha-graphyne and alpha(2)-graphyne. The square and rectangular alpha-graphyne and alpha(2)-graphyne with different dimensions are modelled. It is seen that for both of the considered nanosheets, Young's modulus is smaller than along the armchair direction than along the zigzag direction. Similarly, zigzag alpha-graphyne has larger fracture strain than the armchair one. For the alpha(2) graphyne, however, the fracture strain along the armchair direction is larger than zigzag direction. Besides, it is observed that Young's modulus of the alpha-graphyne is smaller than alpha(2)-graphyne for both of the armchair and zigzag structures. However, the alpha-graphyne possesses larger fracture strain than alpha(2)-graphyne. It is also seen that Young's modulus of the square alpha-graphyne and alpha-graphyne nanosheet is not dependent on the nanosheet dimension. However, fracture strain and maximum stress of these nanosheets decrease by increasing the nanosheet size. Finally, the Poisson's ratio of the alpha-graphyne and alpha(2)-graphyne is obtained as large as 0.9 for some sizes.

关键词： Ball screw system   Thermo-mechanical dynamics model   Dynamic thermal network model   Natural frequency  

摘要： The ball screw system operates under the coupling condition of external loads, preloads, and thermal loads, and its thermo-mechanical characteristics significantly affect its dynamics performance. Therefore, a thermal-mechanical real-time dynamics model is necessary. In this work, the screw systems thermo-mechanical coupling mechanism was first analysed. Second, a new dynamic thermal network model for the screw system with moving heat excitation was proposed using the position of the moving nut obtained by FANUC Open CNC API Specifications (FOCAS), which can rapidly determine the transient temperature field. Based on this model, a new thermo-mechanical coupling real-time dynamics model of the screw system was established. The influence of temperature on dynamics characteristics was analysed using the dynamics model, and the results showed that the system preload caused by thermal elongation of the screw decreases with an increase in temperature. The system preload caused by the thermal deformation of the bearing increases with an increase in temperature. The total preload of the system depends on the interaction between the thermal deformation of the screw and the axial thermal deformation of the bearing. Additionally, the system stiffness and natural frequency variation relative to the increase in temperature and position of the carriage were obtained. Thermo-mechanical dynamics performance experiments of the feed system were carried out to validate the effectiveness of the proposed model.

关键词： Uranium silicides   U3Si2   Nuclear fuel   DFT plus U   BTE   Thermal conductivity  

摘要： Triuranium diSilicide (U3Si2) has been identified as an accident reSistant alternative to uranium dioxide (UO2) based fuels. While experimental studies of U3Si2 are not new, only few studies on the thermal conductivity has been reported. Moreover, theoretical studies of the thermal properties of U3Si2 are scarce. We found that inaccuracies in experimental data were a source of discrepancies in recently reported theoretical results, as such need to be addressed. In this work, the structural, thermal, and mechanical properties of, the non-magnetic and magnetic phases, of U3Si2 were investigated uSing Hubbard corrected denSity functional theory (DFT+U) and molecular dynamics (MD) Simulations. The lattice and electronic thermal properties were determined based on Boltzmann transport equation (BTE) approaches. A conSiderable anisotropy was determined the lattice and electronic contributions to the thermal conductivity from DFT + U/BTE results, in which the electronic contribution is predominant. A relative good agreement between the lattice contributions to the thermal conductivity, predicted from DFT+U and MD computations, was associated to Similarities in the acoustic phonon modes. The present computational results Significantly enhance the understanding of the thermal properties of U3Si2. The good agreement between DFT + U/BTE results with experimental data confirm the accident tolerant characteristic of U3Si2, but also further demonstrating the accuracy of the DFT( + U)/BTE approach for the investigation of thermal properties of advanced nuclear fuels. Crown Copyright (c) 2020 Published by Elsevier B.V. All rights reserved.

摘要： Radical pair recombination reactions are known to be sensitive to extremely weak magnetic fields and can therefore be said to function as molecular magnetoreceptors. The classic example is a carotenoid-porphyrin-fullerene (C center dot+PF center dot-) radical pair that has been shown to provide a "proof-of-principle" for the operation of a chemical compass [K. Maeda et al., Nature 453, 387 (2008)]. Previous simulations of this radical pair have employed semiclassical approximations, which are routinely applicable to its 47 coupled electronic and nuclear spins. However, calculating the exact quantum mechanical spin dynamics presents a significant challenge and has not been possible until now. Here, we use a recently developed method to perform numerically converged simulations of the C center dot+PF center dot- quantum mechanical spin dynamics, including all coupled spins. A comparison of these quantum mechanical simulations with various semiclassical approximations reveals that, while it is not perfect, the best semiclassical approximation does capture essentially all of the relevant physics in this problem.

标准号: GB/T 19725.2-2020

摘要： GB/T 19725的本部分规定了以内燃机为动力源并将机械动力传递至切割附件的便携式背负动力割灌机和割草机的设计、结构和使用方面的安全要求及其试验方法。本部分提出了消除或减少使用整机所产生的各种危害的方法，规定了由生产厂家提供的安全操作方面有关资料的要求。本部分和第1部分的相关章节（见4.1）涉及了在正常使用以及合理的、制造商可预见的误用整机时所有与整机相关的严重危险、危险情况及严重危害事件，背负式动力装置引起的全身振动危害除外。

标准号: GB/T 20888.1-2020

摘要： GB/T 20888的本部分规定了以内燃机为动力源经传动轴传递动力至切割附件的杆式侧挂式动力修枝锯（以下简称侧挂式修枝锯）的设计、结构和使用方面的安全要求及其试验方法。本部分中切割附件是指锯链及做往复运动或旋转运动的外径不大于205 mm的锯片。本部分提出了消除或减少使用以内燃机为动力的侧挂式修枝锯所产生的各种危害的方法，规定了由生产厂家提供的有关安全操作方面的资料要求。本部分涉及了与侧挂式修枝锯相关的，正常使用时以及合理的、可预见误用时的所有的重要危害，但没有涉及在操作手册中已经提出警告的架空电缆引起的电击危害。

标准号: GB/T 19725.1-2020

摘要： GB/T 19725的本部分规定了以内燃机为动力源并将机械动力传递至切割附件的便携式侧挂动力割灌机和割草机（以下简称整机）的设计、结构和使用方面的安全要求及其试验方法。本部分提出了消除或减少在使用割灌机和割草机过程中所产生的各种危害的方法，规定了由生产厂商提供的有关安全操作方面资料的要求。本部分涉及了在正常使用以及合理的、生产厂家可预见的误用整机时所有与整机相关的严重危险、危险情况及严重危害事件。

标准号: GB/T 20888.2-2020

摘要： GB/T 20888的本部分规定了具有背负式动力装置且动力源经传动轴传递动力至切割部件的杆式背负式动力修枝锯（以下简称“背负式修枝锯”）的设计和结构方面的安全要求及其试验方法。本部分中切割部件是指锯链及做往复运动或旋转运动的锯片。本部分提出了消除或减少使用背负式修枝锯所产生的各种危害的方法，规定了由生产厂家提供的有关安全操作方面的资料要求。本部分和GB/T 20888的第1部分的相关章节涉及了与背负式修枝锯相关的，正常使用时以及合理的、可预见误用时的所有的严重危害，但没有涉及接触架空电缆引起的电击危害（使用说明书中包含的警告和建议除外）和背负式动力装置引起的全身振动危害。