Effects of Degrees of Motion Freedom on Free-Fall of A Sphere in Fluid

Free-fall of a sphere in fluid is investigated at a Galileo number of 204 by direct numerical simulations(DNS). We mainly focus on the effects of different degrees-of-freedom(DOFs) of the sphere motion during free-fall. The characteristics of free-fall are compared with those of flow past a fixed sphere. Additional numerical tests are conducted with constraints placed on the translational or rotational DOFs of the sphere motion to analyze different DOFs of sphere motion. The transverse motion contributes significantly to the characteristics of free-fall; it results in the retardation of the vortex shedding, leading to the decrease of the Strouhal number. In addition, the transversal sphere motion exhibits the tendency to promote the sphere rotation. On the contrary, the effects of the sphere rotation and vertical oscillations during free-fall are negligible.

Optimal control of nonholonomic motion planning for a free-falling cat

The nonholonomic motion planning of a free-falling cat is investigated. Nonholonomicity arises in a free-falling cat subject to nonintegrable angle velocity constraints or nonintegrable conservation laws. When the total angular momentum is zero, the motion equation of a free-falling cat is established based on the model of two symmetric rigid bodies and conservation of angular momentum. The control of system can be converted to the problem of nonholonomic motion planning for a free-falling cat. Based on Ritz approximation theory, the Gauss-Newton method for motion planning by a falling cat is proposed. The effectiveness of the numerical algorithm is demonstrated through simulation on model of a free-falling cat.

Shape of free-fall arch in quasi-2D silo

In this study,the optical flow method is used to measure the velocity distribution of a granular flow in a rectangular quasi-two-dimensional silo.Using the velocity gradient,a free-fall arch(FFA)is obtained and its geometric characteristics are calculated.A parabola-shaped FFA structure is discovered above the orifice in the steady flow state.The shape of the FFA affects the flow rate through the orifice.Furthermore,as jamming begins to occur,the geometry of the FFA disappears gradually from both sides and then from the middle;finally,the FFA disappears completely in the state of jamming.As the boundary between finite-stress and stress-free regions,the FFA facilitates further studies regarding the discontinuity of the stress area above the orifice.

Study on laser pre-sowing treatment of rice seeds by free-falling transport method

Recently,laser pre-sowing treatment of seed is attracting a great attention of scientific community for its positive influence to enhance germination,seedling and growth of plants,vegetables and crops.In this study,effect of laser pre-sowing treatment of rice seeds was given,which was implemented by free-falling transport method.Test has been processed in Unzon district(DPRK)in 2017.Two kinds of Korean rice seeds(namely Pyongyang-53 and Dongsung-1)were irradiated by semiconductor laser with wavelength of 650 nm,which was controlled in PWM mode.Each group was irradiated using exposure time of 0.28 s and duty ratio of 50 percent.In result,a considerable improvement has been observed in germination,growth and final productivity.Using laser pre-sowing treatment of rice seed,total productivities have been increased by 7.7%and 21%for “Pyongyang-53”and “Dongsung-1”,respectively.Results show that laser pre-sowing treatment of seed can significantly enhance germination,seedling and growth,and thus can improve the productivity of rice.In future research,laser pre-sowing treatment by free-falling transport method should be processed for other kinds of plants,vegetables and cereal crops to find out appropriate parameters,such as mechanical structure of device,wave length and power of laser,frequency and duty cycle of PWM control signal and exposure time,etc.

Dimensionless variable groups for the free-fall grain dryer

Dimensionless variable groups(DGs)of free-fall grain dryer were developed by using the characteristic scaling method and engineering intuition.These DGs were intended for the analyses of drying characteristics and sizes of this dryer.Experiments of the free-fall rough rice dryer were carried out using different drying chambers(small and medium scales),drying air temperatures(100°C-150°C),and drying air speeds(1-3 m/s).Experimental data were used to study relationships and to validate similarities among these DGs.The results of the study showed the five appropriate dimensionless variables representing and describing the drying mechanism of the free-fall dryer.These DGs could be applied to design and analyze scaling up the dryer.In addition,the dimensionless correlation was established to predict the moisture contents of rough rice while being dried by the free-fall dryer.The predicted values from this correlation were in relatively good agreement with the experimental data under the proposed drying conditions(R2=0.989,MRD=1.82%and RMSE=0.0168).

Correlation analysis of three influencing factors and the dust production rate for a free-falling particle stream

The effects of three factors （i.e., drop height h, hopper outlet diameter do, and material temperature T] on the dust generation rate derived from a free falling particle stream were investigated via filll factorial experiments. The correlation between the three factors and dust generation rate was also analysed. Results show that Tand h affect the first fugitive dust rate largely, whereas the second fugitive dust rate is mainly dominated by h and do. Through analysing the first fugitive dust percentage data, it is found that h and T should be considered first for higher temperatures and lower flow rates, whereas h and do can be considered under contrasting conditions, and h should be controlled in the remaining two sets of conditions. Relationships between the influencing factors and total and first fugitive dust rates were developed via multiple regression to quantify the dust emission rates for different contact surfaces （rigid or water）.

明渠自由跌水水力特性及其在流量计量上的应用

【目的】明渠跌水作为衔接渠道上下游水流的有效设施,可应用于明渠流量计量,通过简单测量跌坎断面水深即可实现渠道流量的精确计量。为研究明渠自由跌水水力特性及其在流量计上的应用,【方法】采用分析对比法将国内外明渠自由跌水的研究成果进行整理,从自由跌水水力特性、跌水量水试验研究及理论研究3个方面进行了对比分析,【结果】发现明渠自由跌水水力特性的研究多集中在跌坎断面压力分布;明渠跌水量水试验研究集中在he/hc的影响因素(渠道底坡S、糙率n、渠道宽度、渠道断面形式、弗劳德数等),该值可基于能量方程、动量方程、N-S方程及其他数值模拟方法得出。【结论】结果表明:试验中he/hc值在矩形渠道中基本是一个常数,但其他断面形式的渠道中该结论未得到验证;该值与明渠底坡S和糙率n有关,底坡方面的研究主要集中在缓坡(S=0.002 6~0.097 3)渠道,陡坡研究较少而平坡研究更少,糙率n的研究范围在0.009 3~0.045 0,但实际工程中n值变化较大。因此有必要在其他形式渠道上对he/hc值做进一步验证,并在更大范围内研究he/hc值与底坡S和糙率n的关系,这样更有利于明渠跌水量水技术的推广和应用。

基于全非线性边界元法的楔形体入水冲击载荷特性研究

在兵器入水、飞行器迫降水面等结构物入水冲击问题属于强非线性流固耦合问题,计算时网格的划分重构、数值稳定与传递,以及射流根部的数值准确求解,通常制约着计算模拟的成败。本文中建立了一套满足全非线性边界条件的冲击载荷理论预报方法,通过引入拓展坐标系,有效解决了非线性自由液面上网格重构和数值传递的问题;通过引入辅助函数解耦了时域下的运动方程,准确求解了每个时间步下楔形体的速度、加速度和冲击压力,以二维楔形体为研究对象,通过开展落体试验验证了本理论方法的有效性。研究结果表明:流体重力对冲击压力具有显著影响,45°底升角楔形体的入水冲击压力主要分布在1.6倍入水深度的半宽区域内,压力峰值和次峰值一般发生在楔形体的尖端和射流根部;楔形体的冲击压力对较小底升角极为敏感,定量描述了二者之间的非线性关系;壁面效应与入水初速度无显著相关性,但底升角越小壁面效应越显著,45°底升角楔形体入水深度小于水域深度的1/3时,壁面效应尤其明显。研究结果为冲击载荷的理论研究和工程应用提供了参考。

自流充填系统的管道压力与流量特征研究

以某高浓度自流充填矿山为例,采用理论分析、室内试验及实测验证相结合的方式,研究了自流充填系统管网参数与阻力特征。结果表明,自流充填系统工序多,正常充填的流量波动范围可达100~240 m^(3)/h,压力1.2~2.2 MPa。管道具体点位的流量呈似正态分布,均值为190 m^(3)/h;压力具有2个区间特征峰,分别为1.86 MPa和3.04 MPa,单一点位的管道参数不能用于分析与判断堵管、泄漏等工况。管道区间压力差和流量差具有显著的正态分布特征,统计数学期望值分别为4.62 m^(3)/h和0.64 MPa,可用瞬时流量差和压力差判断管道堵泄状态。用Swamee-Aggarwal公式预测层流-紊流过渡区的管流阻力为2.08 MPa/km,与实测值2.15 MPa/km相吻合。

智能化动力触探试验设备的研发

研发了一款实现便携、智能的动力触探设备替换当前手动触探设备。研发成果重点解决3个问题:1)落锤自由落体运动要排除任何的结构、零件接触到落锤,采用电磁铁吸附落锤,释放彻底并与落锤完全分离,不影响落锤的自由落体运动。2)500 mm落距采用传感器控制,准确控制落距。3)轻型动力触探试验落锤重量为10 kg,提升和释放时,落锤的势能比较大,采用稳定的大底盘和三脚架结构增加了整个设备的稳定性。

20℃的静止空气中直径5 mm的雨滴的自由下落运动研究

文章采用球形雨滴模型,利用空气阻力系数C_(D)的较严格的表达式即Wallis公式,计算出直径5 mm的雨滴在20℃的静止空气中自静止开始自由下落时的v~t图像.文章又在C_(D)统一取作0.44的近似下,解出解析的、普适的v(t)函数,并作出相应的v~t图像.结果显示这两条v~t曲线几乎完全重合,这就意味着近似处理是完全合适的.文章解释了其中的原因,并指出实测极限速率比球形雨滴模型下的理论值小2.00 m/s的原因在于实际雨滴的形状呈现汉堡状,其实际的空气阻力系数经计算约为0.52.文章认为球形雨滴模型从总体上看是比较实用的.

煤岩体突水通道中骨料沉积运移规律研究

先行灌注骨料后注浆可有效控制煤岩体突水。骨料在煤岩体突水通道中沉积运移规律的研究是确定骨料粒径、骨料灌注量以及骨料灌注孔间距的重要前提,但目前在该方面的理论和试验研究甚少,实际施工主要依赖经验,带有较大的盲目性。鉴于此,基于浆体管道输送理论和泥沙运动力学,结合牛顿第二定律,将骨料从灌注孔进入突水通道的全运移过程划分为:自由落体,类平抛以及滑移3个运动阶段,并分别对其进行受力分析,建立由单颗骨料到骨料颗粒群的沉积运移扩散模型,得出表征骨料颗粒群在水平方向上的沉积运移规律;自行设计煤岩体突水通道骨料沉积运移可视化的模型试验平台,借助该平台开展在不同动水流速、骨料粒径、灌注高度等多因素影响下的骨料沉积运移模型试验,分析骨料在突水通道内的沉积运移规律,得到在不同试验方案下骨料沉积运移区的总长度。试验结果表明:骨料实际运移过程与理论3个运动阶段的假设吻合,骨料沉积运移距离的试验值与理论计算值平均误差为7.34%;在控制投注高度与动水条件一致的前提下,相较细骨料(粒径≤5 mm)而言,理论公式对于粗骨料(粒径>5 mm)更具适用性;在工程中解决煤岩体突水问题时,若骨料粒径>5 mm,利用本文理论公式可计算得到骨料颗粒群在水平方向上的运移距离,从而确定骨料灌注孔的间距,降低实际施工过程中的盲目性。

Experimental observation on water entry of a sphere in regular wave

This paper presents a novel experiment to observe the whole water entry process of a free-falling sphere into a regular wave.A time-accurate synchronizing system modulates the moment elaborately to ensure the sphere impacting onto the water surface at the desirable wave phase.Four high-speed cameras focus locally to measure the high-precision size of the cavity evolution.Meanwhile,the aggregated field view of the camera array covers both the splash above the free surface and the entire cavity in the wave.The detailed methodologies are described and verified for the hardware set-up and the image post-processing.The theoretical maximum deviation is 1.7%on the space scale.The integral morphology of the cavity is captured precisely in the coordinate system during the sphere penetrates through the water at four representative wave phases and the still water.The result shows that the horizontal velocity of the fluid particle in the wave impels the cavity and changes the shape distinctly.Notably,the wave motion causes the cavity to pinch offearlier at the wave trough phase and later at the wave crest phase than in the still water.The wave motion influences the falling process of the sphere slightly in the present parameters.

自由降落救生艇降落试验分析

通过实艇降落试验,结合相似理论建立缩比模型进行降落试验,对典型工况进行对比,验证模型的可靠性,并以此模型进行多工况试验,验证某型艇的安全性。对试验过程中测点位置的选择、传感器的固定方式、人员重量的模拟及布设、试验环境的要求进行分析,并给出建议。

自由落体状态下二维结构物砰击载荷计算

将各类砰击入水的解析理论模型与时域上的精细积分法相结合,研究了在自由落体状态下具有任意对称形状的二维结构物的砰击过程.通过对解析理论模型的数学表达式进行分析,物体所受的砰击力可以分解为两项,分别为速度相关项和加速度相关项.将本文提出的模型与试验和其他数值模拟方法进行对比,验证结果良好.此外,针对轻质结构物或者具有较大入水速度的物体,可以合理地忽略自身的重力,那么对于指定形状和质量的二维物体,自由落体状态下其在砰击过程中的最大加速度或砰击力峰值总是发生在同一个浸没深度处,而与初始的入水速度无关.

土壤原位触探技术与贯入机理研究进展

土壤原位触探技术是一种获取土壤物理力学参数的重要方法,具有准确、快速、可连续测试等优点,在海洋岩土勘探、行星表面地质探测、军用车辆越野路面机动性评估等领域具有广阔的应用背景。本文系统地概述了土壤原位触探技术的发展历程、仪器组成与基本原理,并介绍了其在民用与国防领域的应用。基于圆锥静力触探和自落式动力触探这两种典型的原位触探技术,总结了理论分析方法、数值模拟方法和试验方法在贯入机理研究方面的进展。最后,对土壤原位触探技术进行了展望,为土壤原位触探技术未来的发展提供了理论依据和工程技术参考。

自由落体式球形贯入仪在硬-软黏土中的贯入响应

自由落体贯入仪(free falling penetrometer,简称FFP)通过自由下落贯入土层中,由于其高效便捷的特性,越来越广泛地被应用于海底浅层土体原位勘察。采用耦合欧拉-拉格朗日(Coupled Eulerian Lagrangian,简称CEL)方法,进行了球形FFP在硬-软双层黏土中贯入的大变形有限元分析,考虑了土体的应变率和应变软化效应。与离心机试验、现场试验和数值模拟结果进行了对比,验证了模型的可靠性。分析了成层土对贯入过程的影响,发现FFP在贯入过程中上部硬黏土层形成空腔,且底部会伴随着土塞。通过广泛的变参数分析结果,拟合了球形FFP最终贯入深度与总能量之间的归一化表达式,建立了土体不排水抗剪强度、FFP直径和贯入速度与FFP在硬-软黏土层中最终贯入深度的关系。

“‘科学有曰’之再现经典实验:比萨斜塔实验”教案

本教案以“比萨斜塔实验”为素材,将“科学思维”与“探究实践”的科学方法及相应的科学思想作为重点,以“像科学家探究科学一样学习科学”为教学理念,在“5E教学法”的基础上强化了二次“探究—解释”,以探究式学习与情境教学为主要教学方法,以动手实验、交流讨论、角色扮演相结合的活动形式,引导学生进行“质疑→思辨→实证”的科学探究,实现“具象→抽象”的科学认知,初步了解和运用“思辨的科学”与“实证的科学”。

基于轨迹的低速传感器冲击速度测量方法

落锤试验机的冲击速度是一个瞬时量,基于冲击前一刻的距离和时间计算平均速度进行近似是目前常用的测量方法。但该方法需传感器具有较高的响应速度,才能准确捕捉落锤冲击瞬间的位置信息,导致研发成本较高。为了摆脱测量原理对传感器响应速度的依赖,本文基于落锤的下落特性及理论分析,提出一种基于轨迹的冲击速度测量方法和轨迹函数的约束条件。实验证明:与传统测量方法相比,该方法测量精度和测量不确定度均有大幅提升。

轻中度闭合型创伤性脑损伤小鼠模型建立及评价

目的 建立稳定的轻中度闭合型创伤性脑损伤小鼠模型并对其进行评价。方法 将170只SPF级ICR小鼠分为假手术组(Sham组,n=34)和模型组(TBI组,n=136),模型组分为TBI-A组、TBI-B组、TBI-C组和TBI-D组,其致伤高度均为20 cm,下落砝码重量分别为60 g、80 g、100 g和120 g。按照以上条件对小鼠进行脑损伤敲击,每组34只。造模后通过mNSS评分、疲劳转棒实验、Morris水迷宫实验评估小鼠平衡运动能力和学习记忆能力;脑含水量实验、伊文思蓝实验、苏木精-伊红(HE)染色、GFAP免疫组化以及TUNEL免疫荧光分析脑组织病理改变。结果 与Sham组相比,TBI组的mNSS评分值和水迷宫逃避潜伏期出现不同程度的升高,持续在转棒上行走的时间明显下降,表明TBI组小鼠出现严重的神经功能障碍。TBI组小鼠脑组织内的含水量和伊文思蓝含量明显高于Sham组,提示该组小鼠血脑屏障受损并出现脑水肿。组织病理学检测显示,TBI组小鼠脑组织神经元出现明显变性皱缩,星形胶质细胞显著增生且凋亡细胞比例显著增加,损伤程度随打击物重量的增加而加重。综合分析表明,在下落高度为20 cm,打击重量为60 g和80 g时表现轻度颅脑损伤,打击重量为100 g和120 g时可模拟出为中度颅脑损伤。结论 基于自由落体原理建立了稳定的轻中度闭合型小鼠脑损伤模型,为轻中度创伤性脑损伤的机制和治疗研究奠定了基础。