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关键词： nucleosome clutches   mesoscale modeling   nucleosome positions   linker histone density   histone acetylation levels  

摘要： Nucleosomes cluster together when chromatin folds in the cell to form heterogeneous groups termed "clutches". These structural units add another level of chromatin regulation, for example during cell differentiation. Yet, the mechanisms that regulate their size and compaction remain obscure. Here, using our chromatin mesoscale model, we dissect clutch patterns in fibers with different combinations of nucleosome positions, linker histone density, and acetylation levels to investigate their role in clutch regulation. First, we isolate the effect of each chromatin parameter by studying systems with regular nucleosome spacing;second, we design systems with naturally-occurring linker lengths that fold onto specific clutch patterns;third, we model gene-encoding fibers to understand how these combined factors contribute to gene structure. Our results show how these chromatin parameters act together to produce differentsized nucleosome clutches. The length of nucleosome free regions (NFRs) profoundly affects clutch size, while the length of linker DNA has a moderate effect. In general, higher linker histone densities produce larger clutches by a chromatin compaction mechanism, while higher acetylation levels produce smaller clutches by a chromatin unfolding mechanism. We also show that it is possible to design fibers with naturally-occurring DNA linkers and NFRs that fold onto specific clutch patterns. Finally, in gene-encoding systems, a complex combination of variables dictates a gene-specific clutch pattern. Together, these results shed light into the mechanisms that regulate nucleosome clutches and suggest a new epigenetic mechanism by which chromatin parameters regulate transcriptional activity via the three-dimensional folded state of the genome at a nucleosome level. (C) 2020 Elsevier Ltd. All rights reserved.

关键词： Tidal current energy   Vertical axis twin-rotor turbine   Hydrodynamic performance   Forced oscillation  

摘要： For a floating tidal current turbine power station, the motion response of the floating carrier would affect the hydrodynamic performance of the tidal current turbine. To study the turbine loads and power output characteristics of the proposed twin-rotor turbine in moving condition, the hydrodynamic performance of a vertical axis twin-rotor turbine in surge motion is thoroughly discussed in this manuscript. Based on the moving mesh and sliding mesh technique, the rotary motion and surge motion of the turbine are combined to simulate the turbine operation conditions in waves and currents. The twin-rotor turbine working at different surge frequencies, surge amplitudes, and tip speed ratios is simulated and the simulation results are compared with the fixed turbine in an uniform flow. The change characteristics of thrust force, lateral force, and flow field are analyzed. The results show that the twin-rotor can improve the power output efficiency better than a stand-alone turbine. Surge frequency and surge amplitude have very little influence on the average power output of the turbine. The fluctuation amplitude of the thrust and lateral forces of a twin-rotor turbine is larger than the stand-alone turbine, while the fluctuation amplitude of thrust and lateral forces increase with surge amplitude and frequency. Spectra of turbine loads and power output show that surge and rotation frequencies have different impacts on the dynamics and performances of the turbine. The research results can provide a reference for the development of floating tidal current power stations in the future.

关键词： Styrene  

摘要： A novel method by a one-step introduction of axial chirality and sterically hindered group has been developed for facile synthesis of axially chiral styrene-type carboxylic acids. With the palladium-catalyzed C-H arylation and olefination of readily available cinnamic acid established, this transformation demonstrated excellent yield, excellent stereocontrol (up to 99% yield and 99% ee), and broad substrate scope under mild conditions. The axially chiral styrene-type carboxylic acids produced have been successfully applied to Cp*Co-III-catalyzed asymmetric C-H activation reactions, indicating their potential as chiral ligands or catalysts in asymmetric synthesis.

标准号: QC/T 1147-2021

摘要： 本标准规定了汽车发动机电控硅油风扇离合器的术语和定义、技术要求、试验方法、检验规则，以及标识、包装、运输和贮存。本标准适用于汽车发动机，工程机械、拖拉机、小型船舶以及其它固定、移动式内燃机可参照执行。

标准号: JB/T 10611-2021

摘要： 本标准规定了?DLY0?系列、DLY3?系列、DLY5?系列牙嵌式电磁离合器的术语和定义、分类、特性、产品的有关资料正常的使用、安装和运输条件、结构和性能要求、试验等。本标准适用于?DLY0?系列、DLY3?系列、DLY5?系列牙嵌式电磁离合器，该离合器在机械传动系统中，作从动部分与主动部分结合或分离之用。

关键词： 操纵机构   压盘   从动盘   总泵   分泵   离合器支架  

摘要： 目前,我部使用载重和通用汽车离合器操重机构普遍采用机械式和电动式。其结构简单,维修方便,但长时间使用容易出现操纵机构沉重。例如:解放1122、东风EQ2082等系列,在城市道路上使用半联动容易出现此故障,甚至沉重导致部分司机对此型号车出现抵触心里,修理人员多次维修和更换各零件,但效果不是很乐观。笔者通过观察吊车吊臂得到启发,对此类车型的离合器配件系统进行改进,通过不断的验证,最终达到要求。

关键词： 混合动力汽车   模型预测控制   离合器接合  

摘要： AMT换挡后的离合器结合过程控制好坏对系统冲击有较大的影响,针对P2型单轴并联混合动力汽车离合器的结合过程中,离合器接合阶段离合器尽快接合和冲击度尽可能小这两个控制目标,分析了不同驱动模式下换挡离合器接合阶段动力学过程,设计了基于模型预测控制的离合器接合控制策略,搭建了Amesim-Simulink联合仿真模型对控制策略进行仿真分析,并将模型预测控制的效果与模糊PID控制进行了比较。结果表明,模型预测控制方法虽然会使离合器接合时间在可接受的范围内略微增加,但滑磨功和冲击度会不同程度的减少。其中,冲击度的减少尤为明显,在发动机单独驱动时冲击度减少43%,混合驱动时冲击度减少35%。

关键词： 离合器   黏滑振动   摩擦   显式动力学  

摘要： 针对摩擦离合器在接合过程中出现的黏滑振动问题,建立了某车型离合器三维有限元模型,通过采用有限元软件ABAQUS内嵌的显式动力学分析算法,对离合器系统在接合过程中产生的黏滑振动特性进行研究,通过改变法向载荷和摩擦片厚度,探讨了两者对离合器系统黏滑振动行为的影响。研究结果表明,利用ABAQUS显式动力学分析能有效地模拟出离合器接合过程中产生的黏滑振动现象。黏滑振动发生过程中,系统位移信号呈现出明显的锯齿状波动,飞轮盘和摩擦片会在一定时刻形成相互咬合的状态。离合器黏滑振动的频率和系统的自然频率非常接近,受到系统结构的影响。当法向载荷从1.0 MPa增大到1.5 MPa时,系统振动位移信号的波动幅值增大,黏滑振动周期减小,系统颤振现象更加明显。但当法向载荷进一步增大到3.0 MPa时,黏滑振动逐渐演变为持续的摩擦自激振动现象。通过合理地调控离合器接合的法向载荷,有利于改善黏滑振动现象。摩擦片厚度对系统黏滑振动特性的影响显著,当摩擦片厚度从2 mm逐渐增加为4 mm时,系统的结构频率发生变化,从而使得系统黏滑振动的频率发生变化。此外,增大摩擦片厚度能够改善系统黏滑振动行为,降低系统黏滑振动强度,因此适当地增大摩擦片厚度,是改善黏滑振动强度的有效手段。

关键词： 间歇移动   烟苗移栽机   成穴机构   凸轮摆杆式   运动特性  

摘要： 为提高烟苗杯罩式井窖移栽技术的作业效率,设计了一种凸轮摆杆式的间歇移动烟苗移栽机成穴机构。通过分析凸轮摆杆式成穴机构的结构和工作原理,基于解析法建立了成穴机构运动数学模型;再利用RecurDyn多体动力学软件建立成穴机构虚拟样机模型,获得了成穴钻头顶点位移、速度及加速度运动特性。结果表明:成穴钻头y方向位移行程约309mm,成穴深度可达209mm,速度和加速度特性均呈周期性波动,且幅度较小,均在合理范围内;由此表明:成穴机构具有稳定良好的成穴运动特性,且几何参数设计满足井窖移栽技术要求,可为后续研制间歇移动式烟苗移栽机提供重要参考依据。