How Pressure Became a Scalar,Not a Vector

The gradual emergence of a science of hydrostatics during the course of the seventeenth century is testament to the fact that a technical concept of pressure that was up to the task was far from obvious. The first published version of a theory of hydrostatics containing the essentials of the modern theory appeared in book 2 of Isaac Newton’s Principia. Newton derived the propositions of hydrostatics from a definition of a fluid as a medium unable to withstand a distorting force. Newton’s reasoning required that pressure be understood as a force per unit area acting on either side of imaginary planes within the body of a fluid. For a fluid in equilibrium, the forces at some location within a fluid are independent of the orientation of such planes. As Newton came to realize, within the body of a liquid, pressure acts equally in all directions so that there is no resultant pressing in any direction. Pressure has an intensity but not a direction. In modern terms, it is a scalar, not a vector. Although earlier scholars such as Simon Stevin, Blaise Pascal, and Robert Boyle helped set the scene for Newton’s innovations, they were unable to transcend the common sense of pressure as a directed force acting on the solid surfaces bounding a fluid.     

定容燃烧弹内压力振荡机理研究

本文基于一种具有较好发展前景的醚类汽油添加剂异丙醚(DIPE),在定容燃烧弹内研究了该燃料在不同初始压力下随当量比变化的压力振荡特性。实验结果表明,压力振荡强度随当量比增加呈先增大后减小的趋势,且在当量比1.4附近达到最大值;随初始压力增大,压力振荡的强度也增强。本文通过利用He和CO₂以不同掺混比替代N₂作为稀释气体,验证了定容燃烧弹内的压力振荡主要由压力波与火焰相互影响引起,火焰自加速和混合气能量密度对压力振荡有至关重要的作用。     

空化单气泡外围压强分布

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Pressure and Stress Tensor in a Yukawa Fluid

Systems of particles interacting through a screened Coulomb potential of the Debye–Yukawa form are considered. The pressure is obtained from the stress tensor of the field corresponding to the Yukawa interaction, by a suitable statistical average. This approach is especially appropriate for systems living in a curved space. In a curved space, a self contribution to the pressure appears, and it is essential to take it into account for retrieving a correct pressure when the Yukawa interaction tends to the Coulomb interaction.     

The Effective Periphery of a High Pressure Arc

A brief account is given of ideas which are aimed at providing a common reference point for theoretical treatments of arc problems through the equations of continuum magne-to-fluid-dynamics. By unifying theoretical solutions in the manner described, their usefulness in analyzing the results of experimental investigations may be greatly enhanced. The ideas center on the concept of an isothermal arc periphery, the temperature of which is regarded as an artificial gas property to be derived from the measured voltage gradient-current characteristic of a wall-stabilized arc of low power gradient. Evidence in support of this concept is given for various types of arc in nitrogen at one atmosphere pressure. Some of the implications of the use of an effective periphery of a high pressure arc are examined, and a suggestion is made for the complete set of boundary conditions to be applied at the periphery.     

Pressure and Maxwell Tensor in a Coulomb Fluid

The pressure in a classical Coulomb fluid at equilibrium is obtained from the Maxwell tensor at some point inside the fluid, by a suitable statistical average. For fluids in a Euclidean space, this is a fresh look at known results. But for fluids in a curved space, a case which is of some interest, these unambiguous results from the Maxwell tensor approach have not been obtained by other methods.     

常温高压下水的拉曼谱峰标定压力的研究

 利用金刚石压腔测定了26 ℃高压下水的O—H伸缩振动拉曼谱峰的变化,并对其进行分峰处理,初步确定了水的拉曼拟合峰ν_{3 244}的峰位置与体系压力的关系,且论证了利用水的拉曼拟合峰ν_{3 244}的变化标定金刚石压腔压力的优点以及应用上的局限性。实验结果表明：26 ℃时,在实验的压力范围内,由水的拉曼谱峰拟合得到的ν_{3 244}峰位置随着体系压力的增加呈线性减小。其关系式为p (MPa)=32.9(ν_p)_{3 244}+200.7（3 215 cm^-1< ν_{3 244}<3 244 cm^-1）。     

The Thermodynamic Pressure of a Dilute Fermi Gas

We consider a gas of fermions with non-zero spin at temperature T and chemical potential μ. We show that if the range of the interparticle interaction is small compared to the mean particle distance, the thermodynamic pressure differs to leading order from the corresponding expression for non-interacting particles by a term proportional to the scattering length of the interparticle interaction. This is true for any repulsive interaction, including hard cores. The result is uniform in the temperature as long as T is of the same order as the Fermi temperature, or smaller.     

三维光子晶体压力传感器的研究

基于平面波展开法,分析聚苯乙烯三维光子晶体的能带结构和传输特性.研究聚苯乙烯三维光子晶体对压力的响应性质;计算结果表明:0~41.3 MPa的外加压力与归一化频率0.596~0.608之间具有良好的线性关系,通过测量归一化频率可以感知压力的大小;由于光子晶体的结构周期与光波长为一个数量级,也就有可能制造出一系列精巧的三维光子晶体压力传感器.     

压缩真空中梯形三能级原子的辐射压力

研究了压缩真空中梯形三能级原子在单色行波场中的辐射压力。从系统的哈密顿量出发,利用玻因-马尔可夫近似,推导出了原子的光学布洛赫方程。此时用数值方法求得布洛赫方程的稳态解,然后用图示法考察了原子的辐射压力随双光子失谐、拉比频率、自发发射率等参量的依赖关系。结果表明：单光子跃迁和双光子跃迁可导致辐射压力出现各自的多普勒位移共振峰;辐射压力表现出较宽的失谐范围且强烈依赖于压缩参量以及压缩真空和相干光之间的相位关系。当相位满足匹配条件时,辐射压力减小。     

对转超音压气机低压转子尾迹对高压转子的作用研究

本文采用数值方法针对对转超音压气机转子叶片排间的相互干涉效应进行了研究,分析了超音转子在两级转子叶片排间具有三种不同轴向间距情况下的性能和前排转子叶片尾迹影响下的超音转子非定常特征。研究结果表明,轴向间距的增大使得尾迹在较长的间距内经过了较为充分的掺混,超音转子最大压比和最高效率随轴向间距的变大而增大;前排叶片尾迹的影响范围从超音转子叶片压力面直至吸力面,横贯整个叶片通道,较高叶展处的尾迹强于较低叶展处;横贯超音转子通道内的低压动叶尾迹先后与斜激波和正激波相互作用,尾迹区内熵进一步增大,并且由于尾迹区的传播速度由主流速度决定,尾迹在通道内继续向下游传播且传播显著延迟于通道外。     

Molecular Emission Spectrohetry ih a Low Pressure Electrical Discharge

A low pressure electrical discharge for excitation of polycyclic aromatic hydrocarbons, PAHs, without fragmentation is described. The discharge characteristics as well as the plasma background spectra are given. The application of the discharge for the excitation of molecular spectra of PAHs is described. Difficulties with correction of the analyte emission spectrum for the background emission spectrum of the gas (He) are mentioned. Analytical use of the discharge for detection of PAH3 introduced by gas chromatography is described and is shown to be of marginal analytical use because of the rather poor detection limits of about 1 μg.

Jurgensen and Winefordner¹ have reviewed both reduced pressure and atmospheric pressure nitrogen discharges for detection of atomic and molecular species. The same authors² have also described an active nitrogen-induced chemiluminescence system for detection of polycyclic aromatic hydrocarbons, PAHs, and achieved detection limits in the range of 0.01 - 2000 ng. The intriguing aspect of the active nitrogen—induced chemiluminescence approach² for detection of PAHs was that the discharge was gentle resulting in little fragmentation of the PAHs and in the appearance of emission spectra characteristic of the parent molecules. Because of the poor understanding of the mechanism of excitation of the large organic molecules and the minimal fragmentation and because of the lack of control of the direct discharge as opposed to the afterglow in the studies of Jurgensen and Winefordner², the present authors began a careful study of the physical and analytical characteristics of this discharge in the fall of 1984. The physical characteristics were reported by Yu and Winefordner³ and indicated that the previous workers², were using the direct discharge plasma as a result of llpoorlv grounding. In addition, the same workers³ showed that the active nitrogen afterglow was not capable of exciting PAHs. In this note, we report on the analytical capabilities of an improved active gas (nitrogen or helium) electrical discharge system for gaseous PAHs.     

Generation of a Low Flow Atmospheric Pressure Neon ICP

Abstract

A 27.12 MHz low flow (3 1/min), laminar flow, atmospheric pressure neon ICP has been generated. The forward power used is 500 W with a reflected power of less than 5 W. Using higher powers caused the plasma to either extinguish or form numerous filaments. The Hß line is used to determine an electron number density of 8 × 10¹³cm^?3. The N₂ ⁺(0, 0) and OH(0, 0) transitions did not readily emit. This fact, coupled with the low electron density and low input power, indicates a relatively cool plasma.     

Conditions of Pressure Wave Focusing in a Bubbly Wedge

Acoustical Physics - Dynamics of pressure waves in a flat channel with a sloping boundary between bubbly and pure liquids is investigated. It is shown that, at the boundary between water–air...     

不对称垂直裂缝压力动态特征

针对压裂过程中出现的不对称垂直裂缝问题,基于渗流力学原理,根据点源函数和Green函数基本理论建立三种不同外边界条件下不对称有限导流垂直裂缝试井解释数学模型.采用Laplace积分变换和Stehfest数值反演获得典型特征曲线.研究表明：典型特征曲线分为四个流动阶段,井储阶段、压力导数曲线斜率为1/4的双线性流阶段、压力导数曲线斜率为1/2的线性流阶段及0.5水平线的径向流阶段;不对称因子主要影响双线性流阶段结束的时间,不对称因子越大,双线性流阶段结束的越早;裂缝的导流能力越大,不对称因子对特征曲线的影响越不明显;不对称因子越大,裂缝流量分布越不对称.为不对称有限导流垂直裂缝的试井分析提供理论依据.     

Pressure induced transitions in calcium: a tight-binding approach

A tight-binding (TB) hamiltonian for calcium is built with a high precision parametrization technique based on density functional calculations of the energy bands and the total energy at various lattice volumes. The new set of TB parameters is appropriate to study phenomena under pressures as high as 20 GPa. Specifically, both the metal to nonmetal transition at 4 GPa and the structural transition fcc to bcc at 19 GPa are well reproduced. These transitions and several static properties are in excellent agreement with experiments. Phonon frequencies, plasmon energy, melting temperature and the coefficient of thermal expansion were calculated with a molecular dynamics scheme of this TB hamiltonian.