Rolling and Lifting Probabilities for Sediment Entrainment

This study addresses the rolling and lifting probabilities for sediment entrainment by incorporating the probabilistic features of the turbulent fluctuation and bed grain geometry. The lognormally distributed instantaneous velocity and uniformly distributed initial grain position, along with a relation between lift coefficient and particle Reynolds number, are used to extend the theoretical formulation of the entrainment probabilities in smooth-bed flows. The two threshold conditions identified herein enable us to precisely define the probabilities of entrainment in the rolling and lifting modes. The results obtained in this study coincide well with the published data. The lifting probability increases monotonously with the dimensionless shear stress θ, which is consistent with the earlier results yet displays improved agreement with the experimental data. The maximum value of rolling probability, with a magnitude of 0.25, occurs at θ ≈ 0.15. For θ<0.05 (or θ>0.6), the rolling (or lifting) probability makes up more than 90% of the total entrainment probability and thus can be used as an approximation to the total probability of entrainment. The proposed rolling and lifting probabilities are further linked to the two separate criteria for incipient motion to explore the critical entrainment probabilities. The results reveal that a consistent probability corresponding to the critical state of sediment entrainment cannot be found.     

对潭口新区工程方案编制与实践的认识

潭口新区建设方案编制以节能降耗为出发点,机采系统依靠井底流压和目标产量确定工作参数,杆柱组合采用能耗最低设计方法,根据斜井抽油杆侧向力分布状况设计抽油杆扶正器设置间距;注水系统优化地面管网布局,降低了高压注水单耗;集输系统对单井集油管线进行了优化设计,充分利用油井伴生气资源,实现了单管常温集输和单管油气密闭集输,减少了原油损耗,降低了区块的能耗水平。     

MEMS-based gas flow sensors

Micro-electro-mechanical system (MEMS) devices integrate various mechanical elements, sensors, actuators, and electronics on a single silicon substrate in order to accomplish a multitude of different tasks in a diverse range of fields. The potential for device miniaturization made possible by MEMS micro-fabrication techniques has facilitated the development of many new applications, such as highly compact, non-invasive pressure sensors, accelerometers, gas sensors, etc. Besides their small physical footprint, such devices possess many other advantages compared to their macro-scale counterparts, including greater precision, lower power consumption, more rapid response, and the potential for low-cost batch production. One area in which MEMS technology has attracted particular attention is that of flow measurement. Broadly speaking, existing micro-flow sensors can be categorized as either thermal or non-thermal, depending upon their mode of operation. This paper commences by providing a high level overview of the MEMS field and then describes some of the fundamental thermal and non-thermal micro-flow sensors presented in the literature over the past 30 years or so.     

330MW汽轮机组给水泵出力不足的热力性能分析

对某电站330MW汽轮机组水泵出力不足的问题，通过热力性能试验及系统分析，认为影响汽轮机汽动给水泵组出力不足的原因是小汽轮机轴功率输出不够，建议降低小汽轮机排汽压力，改造小汽轮机通流部分，以提高小汽轮机的输出轴功率。     

浅谈汽车发动机的电控可变配气机构

目前国外高级轿车普遍使用电控可变配气杌构，其功率性、经济性、安全排放性均达到欧洲四号标准，笔者对电控可变配气杌构的结构与工作原理进行论述，以期能尽快在国内普及。     

阀瓣类型对安全阀动作性能影响的试验研究

以升力传感器作为统一的衡量标准,对其进行静态标定,建立升力传感器输出信号与安全阀整定压力的关系;然后分析了安全阀全启排放现场实验的大量实测数据,通过对比传统刚性阀瓣与各种柔性阀瓣动作性能的差异,讨论了不同类型阀瓣对开启时间、回座时间、开启压力、排放压力、回座压力和排放稳定性等主要动作性能的影响,为合理地设计阀瓣提供了一些准则和依据。     

Transfer of the horizontal patient: The effect of a friction reducing assistive device on low back mechanics

Recognizing that the transfer of bedridden patients is associated with a high rate of low back injuries, various devices have been developed to assist with sparing the patient handlers. The purpose of this study was to quantify the friction-reducing ability of three different ‘sliding’ patient transfer devices together with the subsequent consequences on the low back loads of people performing the transfers. Coefficients of friction of the devices were determined by ‘transferring’ a standard object and a ‘patient’ over several surfaces common to a hospital setting. Then three participants performed controlled transfers with the various devices. Electromyography to measure muscle activation levels together with external forces and kinematic positional data were collected during push, pull and twist transfers. Spine loads were estimated with a three-dimensional biomechanical static link-segment model of the human body. Simply sliding a patient on a cotton sheet (control condition) produced a coefficient of friction of 0.45. The assistive devices substantially reduced friction by well over one-half (coefficients of 0.18?–?0.21). However, when using the devices the subjects adopted a variety of postures and techniques, such that there were no consistent influences on trunk inclination, low back compression or muscle activation profiles. Direct measurement of reduced friction between the bed and the patient with a friction-reducing device together with measurement of the back loads when actually transferring a patient formed a proof of principle. Specifically, while the device lowers friction, the transfer technique adopted by the lifter must be proper to reduce low back loading and any subsequent risks of back troubles associated with patient transfers. The direction of hand forces and torso position remains important.     

海底管道悬跨处理灌浆袋装置液压托举机构设计与实验

为了解决海底管道悬跨现象所带来的疲劳破坏问题,进行了灌浆袋处理装置的研究,提出了一次吊装可完成多处悬跨处理的装置总体方案,介绍了工作原理。运用三维建模、机构受力分析及有限元法进行了托盘托举机构的设计,确定了液压缸参数。针对两侧副液压缸同步性要求,进行了PID控制的液压缸的同步回路设计及仿真分析,采取试凑法确定了比例、积分与微分参数。最后,加工出样机进行了托举机构伸缩运动实验,结果验证了设计的合理性、可行性。该装置能够代替潜水员一次性下海进行海底多处悬跨管道的填充处理作业,提高了作业效率,因而,具有一定的海下工程应用意义。     

三峡升船机平衡重滑轮结构拓扑优化设计

为了解决三峡升船机平衡重滑轮腔体内焊接支撑块脱落导致异响问题,建立了以结构柔度最小为优化目标的拓扑优化模型,基于ANSYS对滑轮结构进行拓扑优化设计并对模型进行重构,得到了最终优化模型.对比分析了优化前后结果,表明优化后的滑轮总重与优化前的比值为0.91,所受最大应力比值0.852,所受最大位移比值0.94.通过优化结...     

单级入轨飞行器水平发射的优势

建立了可用于描述单级入轨飞行任务的动力学模型.选取了一种升力体外形的单级入轨飞行器作为算例,对水平发射的优势进行了论证.结合英国HOTOL的起飞参数对算例的水平发射过程进行了仿真,验证了地面助推发射对减少起飞段推进剂消耗的贡献.简要回顾并分析了美国X-33计划曾面临的技术困难,对算例在不同发射方式下所获得的入轨质量及其组成进行了比较,表明了降低起飞推重比对提高单级入轨飞行器运载效率的重要意义.选择相同的起飞参数,对采用不同发射方式时算例的飞行过程进行了对比.结果表明,水平起飞飞行器由于能够更好地利用其自身的升力体效应,在大气层内消耗的推进剂较垂直发射时少.