关键词： FIV   PIV   Gap flow   Pivoted cylinders   Mechanically coupled   Wake patterns  

摘要： Herein, experiments were carried out with two connected circular cylinders that are free to rotate around a pivot in different arrangements including both cylinders on the downstream, both on the upstream and a cylinder on each side of the pivot while the other cylinder is at the pivot point. The gap between the two cylinders is varied from 0 to 4.9D (D being the diameter of the cylinder) and the center of gravity (cg) is varied from -2.5D to +2.5D from the pivot point. The hydrodynamic forces on the cylinders are measured with a load cell and the surrounding flow is measured simultaneously using particle image velocimetry (P1V). The Reynolds number is varied in the range of 500 < Re < 2.4 x 10(4) during the test to find the maximum possible displacement amplitude for each configuration. Then, an spectral analysis is performed on the measured velocity field to have information about the vortex shedding frequency of the unsteady flow in the gap region and cylinder wake. When the gap between the cylinders (g) is zero, the cylinders behave like a single bluff body (SBB) in cross-flow. However, depending on the arrangement, the wake patterns are different. If both cylinders are at the downstream side of the pivot, the cylinders act as a single body with an extended surface and alternating vortexes shed behind the cylinders in each cycle forming a single vortex street on the wake side (SBBV). Sliding both cylinders to the upstream side shifts the vibration mechanism to the single body galloping region (SBBG). The shear layer behind the two cylinders is stretched along the fluid flow and vortexes are formed at a greater distance downstream comparing to the SBBV. Having one cylinder on the pivot changes the vibration to a typical wake induced vibration if the other cylinder is at the downstream. If the other cylinder is at the upstream, the vibration is similar to a single cylinder system but the cylinder on the pivot disturbs the wake and changes the stagnation point on t

关键词： digesta   grazing   modelling   ruminants  

摘要： Detailed representation of ingesta inflow to and digesta outflow from the rumen is critical for improving the modelling of rumen function and herbage intake of grazing ruminants. The objective of the current work was to extend a mechanistic model of a grazing ruminant, MINDY, to simulate the dynamic links between ingestive and digestive processes as affected by forage and sward features (e.g. sward structure, herbage chemical composition) as well as the internal state of the animal. The work integrates existing aspects of forage ingestion, oral physiology and rumen digestion that influence ingesta characteristics and digesta outflows from the rumen, respectively. The paper describes the structure and function of the new development, assessing the new model in terms of dynamic changes of oral processing of ingesta and rumen dilution rate under different grazing contexts. MINDY reproduces characteristics of ingesta inflow to and digesta outflow from the rumen of grazing ruminants, achieving temporal patterns of occurrence within and between meals, similar to those for grazing animals reported in the literature. The model realistically simulates changes in particle size distribution of the ingestive bolus, bolus weight and rumen dilution rate in response to contrasting grazing management regimes. The new concepts encoded in MINDY capture the underlying biological mechanisms that drive the dynamic link between ingestion and digestion patterns. This development advances in the understanding and modelling of grazing and digestive behaviour patterns of free-ranging ruminants.

关键词： load   posture   sensory encoding   walking  

摘要： Many sensory systems are tuned to specific parameters of behaviors and have effects that are task-specific. We have studied how force feedback contributes to activation of synergist muscles in serially homologous legs of stick insects. Forces were applied using conventional half-sine or ramp and hold functions. We also utilized waveforms of joint torques calculated from experiments in freely walking animals. In all legs, forces applied to either the tarsus (foot) or proximal leg segment (trochanter) activated synergist muscles that generate substrate grip and support, but coupling of the depressor muscle to tarsal forces was weak in the front legs. Activation of trochanteral receptors using ramp and hold functions generated positive feedback to the depressor muscle in all legs when animals were induced to seek substrate grip. However, discharges of the synergist flexor muscle showed adaptation at moderate force levels. In contrast, application of forces using torque waveforms, which do not have a static hold phase, produced sustained discharges in muscle synergies with little adaptation. Firing frequencies reflected the magnitude of ground reaction forces, were graded to changes in force amplitude. and could also be modulated by transient force perturbations added to the waveforms. Comparison of synergist activation by torques and ramp and hold functions revealed a strong influence of force dynamics (dF/dt). These studies support the idea that force receptors can act to tune muscle synergies synchronously to the range of force magnitudes and dynamics that occur in each leg according to their specific use in behavior. NEW & NOTEWORTHY The effects of force receptors (campaniform sensilla) on leg muscles and synergies were characterized in stick insects using both ramp and hold functions and waveforms of joint torques calculated by inverse dynamics. Motor responses were sustained and showed reduced adaptation to the more "natural" and nonlinear torque stimuli. Calculat

关键词： 机械工程   多体系统   动力学  

摘要： 多体系统动力学可以对产品进行虚拟的设计来预测产品的特点及性能,大幅度缩短产品研制时间,对市场做出快速响应。其在机械工程建设施工领域应用广泛、用途多样、成就巨大。因而,在机械工程领域,应当加大对多体动力学的研究,旨在机械工程领域能够得到更好的发展,为人们创造更大的价值,这对对于我国机械工程制造工业又好又快发展具有重要作用和深远意义。

关键词： 多体动力学   机械工程领域   应用  

摘要： 多体系统是由多个不同的部件连接而成的机械系统,其机械系统创建主要是实现运动以及机械功能的实现,其中的各个机械部件会出现用力、位移、速率等多种参数的变化。当下多体动力学在航空航天、机械制造、机器臂等多种机械工程领域有重要应用,并取得了一定的成果。所以文章也对多体动力学进行了分析,继而谈到它在机械工程领域上的运用。

关键词： Flexoelectricity   Finite size effect   Core-shell model   Young's modulus   Phase transition  

摘要： This study focuses on the size dependent flexoelectric effect of Barium Titanate Oxide (BaTiO3) and its mechanical property. The an-harmonic core-shell model is employed to study the cross-sectional size dependent properties of the BaTiO3 nanobelt. Flexoelectricity describes the relationship between the induced electrical polarization and the applied strain gradient. Molecular dynamics involved core-shell interatomic potential model predicts the electrical polarization by allowing the ion shell to react freely to the electrostatic environment induced polarizability. The competing and opposite trends of longitudinal and transverse shear flexoelectric parameters with increased sizes is found in the present study. The elastic modulus decreases with the increase in cross-section size. In both cases, the material parameters gradually converge to the existing bulk values.

关键词： Metamaterials  

摘要： Entwining-induced robust natural biosystems show superior mechanical performances over their counterparts. However, the role played by topological entwinement in the mechanical properties of artificial nanohelixes remains unknown. Here, the tensile characteristics of nano-entwined carbon nanocoil (ECNC) metamaterials are explored by atomistic simulations. The simulation results show that ECNCs exhibit heterogeneous pre-stress distribution along the spiral surfaces. The predicted stretching stress-strain responses correlate with the topological nano-entwining and dimensionality. Topological analysis reveals that the collective stretching of the bond and bond angle on the inner hexagon edge of the coils characterizes both early and final elastic extensions, whereas the intermediate elasticity is exclusively attributed to the inner-edged hexagon-angular deformation. The ECNCs impart pronounced tensile stiffnesses to the native structures, surprisingly with a maximum of over 13-fold higher stiffness for one triple-helix, beyond the scalability of mechanical springs in parallel, originating from the nano-entwining mechanism and increase in bulkiness. However, the reinforcement in strengths is restricted by the elastic strain limits that are degraded in ECNCs owing to the steric hindrance effect. All metastructures show superelongation-at-break due to a successive break-vs.-arrest process. Upon plastic deformation, the localized reduction in the radii of ECNCs leads to the formation of carbyne-based networks.

关键词： 动力机械   喷气式发动机   火箭发动机   机械装置   能量转换   燃气轮机   航天事业   力学知识  

摘要： 说起动力机械,那可是在人类进程中充当非常重要的角色,可以说它直接影响了人类生产力的发展。动力机械说白了就是一种能将自然界中的能量转换为机械能的机械装置,比如风车将风能转化成机械能,通过水的流动发明了水车水磨等等,这些都是最早的动力机械,随着发展到后来发明了蒸汽机、内燃机,有了火车、汽车。后来又有了汽轮机、燃气轮机、喷气式发动机、火箭发动机,航天事业也得以开拓。对于现在的小朋友来说,能够形象地对通过动力机械了解最基本的结构动力学知识,增加一些物理的兴趣,不管在后面的学习或生活中都是非常有益的。

关键词： GCr15轴承钢   渗硼   微观组织   机械性能   生长动力学  

摘要： 对GCr15轴承钢表面渗硼层的生长动力学与机械性能进行了研究.采用固体渗硼的方法,在1123、1173、1223和1323 K温度条件下,分别保温处理2、4、6和8 h,进行渗硼层制备.采用光学显微镜、扫描电镜、X射线衍射仪、维氏硬度计等对制备的渗硼层进行组织观察与性能分析,并通过试验数据对渗硼层的生长动力学特性进行了研究.研究结果表明：试样表面获得了均匀致密的渗硼层,渗硼层的相成分主要是Fe B和Fe2B;渗硼层的厚度随处理温度与保温时间的增加而增厚,变化范围为33.4-318.5μm;渗硼层的表面硬度随处理温度及保温时间的增加而增大,主要是由于随着渗硼层厚度的增加,高硬度Fe B相的含量上升,低硬度Fe2B相的含量下降,表面硬度HV0.1变化范围为1630-1950,与基体组织相比,提高了5-6倍;渗层截面硬度测试结果表明,渗层与基体之间有较宽的硬度梯度过渡;通过Arrhenius公式,对渗硼层的生长动力学方程进行了推导,可知B元素在GCr15轴承钢中的扩散激活能为188.595 k J·mol-1,对推导的动力学方程进行了试验验证,结果表明最大误差仅4.93%,可有效的实现对渗层厚度的预测.

关键词： 矿用轮式装载机   动力舱热分析   虚拟风洞   热源隔离  

摘要： 针对由于矿用轮式装载机冷却系统散热效率较低,造成整机行走困难以及工作装置提升无力的问题,基于虚拟风洞的仿真模型,对装载机动力舱热环境进行了三维CFD仿真。预测了排气管和消声器等高温热源对热环境的影响规律,得到了分别在吹风式风扇和吸风式风扇工况下的动力舱内部流场和温度场分布,并对高温热源进行了隔离。对装载机在典型作业工况下进行了试验测试,得到了结构改进后的整车热平衡测试结果,表明高温热源进行隔离后,整车热平衡得到明显改善。