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隐约响起的隆隆声如同打雷一般,将沉睡的我吵醒.我翻转身,望向窗外——夜空清澈又静谧,满是明亮的星星,视野之内见不到半片云朵.我叹息了一声,闭上眼睛,渴望能有一晚不受打搅的好眠. 一声古怪的轻鸣长驱直入,传进我的耳朵,开始变得越来越响,害得我背上的汗毛都竖立得笔直.窗外有个暗影在移动,随着它从夜空中逐渐下降,星星一颗又一...     

人类进化故事如何继续开枝散叶

我们人类不再是过去的模样.在过去20年里,基因组学、古DNA学和古人类学齐心协力,彻底修订了我们对于人类起源的理解.在最近几十万年里,人类进化的多样性和复杂性甚至超越了我们最无稽的想象.然而,我们对人类起源的了解越来越清楚,对于人类进化的叙述却越来越混乱,越来越不简练.事实证明,人类这个物种的身份纷繁复杂.     

科学和灵性 ——寻找火星上的生命

在西方长期的科学和宗教斗争中,某些角色并非一成不变.起初,宗教领袖坚信神创万物,他们谴责(经常是处死)那些质疑圣经中关于地球起源以及天堂美好故事之人.而科学家则不断对此提出挑战.如今,科学家反而成了确定论者,以理查德?道金斯(Richard Dawkins)为代表的新无神论者认为,宗教人士既软弱又愚蠢,因为他们无法接受...     

Sperm viability in the reproductive tract of females in a population of Sceloporus mucronatus exhibiting asynchronous reproduction

Asynchronous reproduction is a common phenomenon in high-elevation populations of lizards from Central México. Sperm storage in the reproductive tract of females is the mechanism for making oocyte fertilization possible. Our study addresses questions related to functional oviductal sperm storage of females mating on different dates throughout the reproductive season. A population of Sceloporus mucronatus with copulation in the summer and ovulation in the fall was chosen for this experiment. Eleven females that copulated in the field during June and 13 females that copulated in captivity during August were maintained in the laboratory until parturition. The number of pregnant females and the litter sizes produced in each experimental group were indicative of the viability of the stored sperm. Sperm stored in the reproductive tract of females were able to fertilize eggs after 4 months. No significant differences were found in the number of pregnant females between the 2 experimental groups nor in the litter sizes that they produced. We found that the amount of time sperm were held in the female reproductive tract (ca. 3 months) had no effect on the capacity of sperm to fertilize eggs. Histological examination of 8 oviducts collected before the mating season eliminated the possibility of sperm storage from one year to the next. In this system, sperm retention could have evolved as a response mechanism to deal with the asynchrony between sexes in the reproductive cycles. However, we cannot rule out alternative hypotheses.     

New testing methodology for the quantification of rock crushability: Compressive crushing value (CCV)

Crushing is a size reduction process that plays a key role in both mineral processing and crushing-screening plant design. Investigations on rock crushability have become an important issue in mining operations and the manufacture of industrial crusher equipment. The main objective of this research is to quantify the crushability of hard rocks based on their mineralogical and mechanical properties. For this purpose, the mineralogical, physical, and mechanical properties of various hard rocks were determined. A new compressive crushing value (CCV) testing methodology was proposed. The results obtained from CCV tests were compared with those from mineralogical inspections, rock strength as well as mechanical aggregate tests. Strong correlations were found between CCV and several rock and aggregate properties such as uniaxial compressive strength (UCS), the brittleness index (S₂₀), and aggregate impact value (AIV). Furthermore, the relationship between the mineralogical properties of the rocks and their CCVs were established. It is concluded that the proposed testing methodology is simple and highly repeatable and could be utilized as a pre-design tool in the design stage of the crushing process for rock quarries.     

液压互联悬架参数全局灵敏度分析与多目标优化

液压互联悬架（Hydraulically Interconnected Suspension,HIS）系统的参数匹配直接影响着其动态性能. 为提升HIS系统的综合性能,对抗俯仰抗侧倾HIS系统的主要参数进行了全局灵敏度分析和多目标优化. 建立7自由度“机-液”耦合整车频域动力学模型,以四轮随机路面为输入,以反映车辆平顺性、稳定性和抗俯仰抗侧倾性能的各项性能指标为目标函数,采用Sobol指数法对HIS系统参数进行了全局灵敏度分析,获得影响HIS系统性能的关键参数. 基于参数灵敏度分析结果,使用NSGA-II算法对HIS系统进行了多目标优化. 结果表明：管路阻尼阀和蓄能器阻尼阀线性损失系数对车辆各性能有较大影响,参数间的交互效应对车辆抗侧倾性能影响明显.优化后,通过权重系数法选取的优化结果表明,车身质心处加权加速度均方根值降低20.95%,俯仰角加速度均方根值降低12.95%,侧倾角加速度均方根值降低8.05%,轮胎动载荷均方根值均值降低11.17%. 通过参数灵敏度分析和多目标优化,可以显著提升HIS系统的综合性能.     

哥西定理之推廣

本文主要系构造一辅助函数,从而将哥西中值定理推广到n个函数。茲先讨论三个函数的情形。定理1 设函数f(x),φ(x),ψ(x)在闭区间[a,b]上连续,在开区间[a,b]上可微,则一定有这样—点c(a相似文献   

石灰石粉对水泥浆体水化特性及流变性能的影响

利用微量热仪和旋转黏度计,从掺量和细度两方面研究了石灰石粉对水泥浆体水化特性和流变性能的影响.从水化放热速率和放热量角度分析了石灰石粉对水化特性的影响,从紧密堆积理论和固体颗粒体积分数两个角度分析石灰石粉对流变性能的影响.结果表明:石灰石粉可以促进体系的水化进程,且石灰石粉细度越大,促进作用越明显.石灰石粉掺量增大导致水泥含量减少,所以体系第二放热峰峰值和总放热量随石灰石粉掺量的增大而减小.随着石灰石粉掺量或细度的增加,复合体系中固体颗粒的体积分数逐渐增大,粒径分布模数减小,且体系的粒度分布曲线逐渐接近于最密堆积的理想分布曲线.复合体系的屈服应力和塑性黏度随石灰石粉掺量的增大而减小,随石灰石粉细度的增大而增大.     

分离式微通道热管低风量下换热性能模拟研究

为了降低数据中心的运行能耗，分离式热管冷却系统会根据实际负载降低风扇转速，造成换热性能的衰减. 为了对低风量和低负载下分离式微通道热管的换热性能进行研究和分析，建立了适用于预测低风量下分离式微通道热管换热性能的一维稳态模型，并与试验结果对比，验证了模型的准确性，其最大预测平均偏差为6.3%. 利用该模型研究了运行参数对分离式微通道热管的换热性能和数据中心热安全的影响. 不同风量下，服务器排风温度从27 ℃上升至39 ℃时，系统换热量均有超过60%的提升；冷冻水供水温度从6 ℃提升至18 ℃会导致系统换热量最高下降41.8%. 在200～1 400 m3/h风量下，增加蒸发器和冷凝器的高度差可有效提升系统换热量和制冷剂质量流量，风量越大，换热量增长率越高. 但在相同风量下，随着高度差的逐渐增大，换热量增长率逐渐降低. 研究结果对分离式微通道热管在数据中心的设计优化和节能运行有一定促进作用.     

基于深度神经网络的高环境 适应性水声通信系统研究

深度神经网络中的自动编码器(Autoencoder,AE)通过收发端两个神经网络模块进行全局优化,利用端到端的训练方式以提高通信系统的可靠性.然而,现有对AE的研究未针对信道进行特殊设计,尤其对于时变的水声信道的多径效应,难以进行灵活调整,降低了该方法的实用性.本文提出一种提高水声通信系统信道环境适应性的Attention-Autoencoder网络模型,基于Attention网络可以高效地从大量信息中筛选出关键信息的特点,设计了一种针对水声信道的Attention机制,该机制能够增加网络提取水声信道特征的能力,使系统的适应性大大提高.仿真验证和湖试实验结果表明,基于Attention-Autoencoder网络模型的通信系统与基于文献中AE模型和没有引入神经网络的水声通信系统相比,具有更高的信道环境适应性.