ON THE CHANGES OF MEANING OF “LI” AND “LIXUE” (“FORCE” AND “MECHANICS” IN CHINESE)1)

在古代中国,“力”字和“力学”一词早就出现了。“力”的含义最初是对“人的力量”的直观描述,后来发展到对一般物体的重力的描述,再到对运动状态原因的描述。本文对“力”字含义的变迁进行了辨析。中国现代“力学”是西学东渐的结果,学科名字的确定过程也有不小的 曲折。通过分析确定“力学”为学科名字的过程,揭示了中国古人对“力学”学科理解的变迁。     

A NEW EXPERIMENT TEACHING MODE FOR MACHANICS TO MEET THE NEED OF “NEW ENGINEERING” TALENT TRAINING

“新工科”是基于国家战略发展新需求、国际竞争新形势、创新人才培养新要求而提出的我国工程教育改革方向。“新工科”建设的主要目标即培养造就一批具有创新创业能力、跨界整合能力、高素质的学科交叉复合型卓越工程科技人才。本文针对“新工科”人才培养理念,分析了当前培养新型工科人才的背景和迫切性,探讨了适应新形势人才培养理念的力学示范中心建设思路和内涵,提出了建设实践教学新模式的途径和举措。     

INFLUENCE OF SHAMPOO ON THE MECHANICAL PROPERTIES OF HUMAN HAIRS1)

头发的弹性和抵抗意外拉断的能力是人们非常关注的指标。为了迎合这一需求,有些厂商推出了具有所谓“强韧”发质功效的洗发水,但其实并不清楚是否真能使头发变“强韧”。为此,针对市场上常见的一种标称具有“强韧”功效的洗发水,通过拉伸力学实验对头发洗护前后的力学性能进行了对比研究。结果表明,用洗发水处理后的头发,屈服应力下降,意味着头发更容易过早进入塑性变形;抗拉强度、最大应变减小,意味着头发抵抗外力拉断的能力下降;杨氏模量下降,说明头发抵抗弹性变形的能力变差。综上,所研究的洗发水对“强韧”发质没有帮助。 关键词: 头发,洗发水,力学性能,强度,韧性     

DISCUSSION ON THE APPLICATION OF NUMERICAL EXPERIMENT IN EXPERIMENTAL TEACHING OF FLUID MECHANICS

为强化学生对流体力学课程理论知识及实验教学环节的掌握,本文针对传统实验教学过程中受限于硬件设施、模型等问题,将数值模拟技术与流体力学实验教学相结合。以伯努利实验为例,应用数值模拟软件进行物理建模、网格划分、变参数数值模拟等实验内容,通过压力云图、流线等模拟结果对能量转换特性进行了分析,增强了学生对流体力学理论知识及相关参数的物理意义、变化特征等的理解,使学生在实验中产生强烈的“参与感”,有效地提高了教学水平和教学质量。     

Report on a symposium on “Computational approaches to disperse multiphase flow”

Some themes and developments emerged in the course of an IUTAM Symposium on “Computational Approaches to Disperse Multiphase Flow” held at Argonne National Laboratory on October 4–7 2004 are briefly summarized.     

Stability analysis of annular flow structure, using a discrete form of the “two-fluid model”

The differential form of the “two-fluid model” for annular flow, neglecting surface tension, is ill-posed, and it is not suited for examining the stability of the steady-state solutions with respect to the average film thickness. It is shown here that a discrete (difference) representation of the two-fluid model may lead to an appropriate criterion for the stability of the steady-state solutions. Exactly the same criterion is obtained from the requirement that the kinematic waves will propagate in the downstream direction. The suggested discrete form of the “two-fluid model” is used to perform transient simulation and for examining the system response to finite disturbances.     

Weakly nonlocal envelope solitary waves: numerical calculations for the Klein-Gordon (φ) equation

“Weakly nonlocal” solitary waves differ from ordinary solitary waves by possessing small amplitude, oscillatory “wings” that extend indefinitely from the large amplitude “core”. Such generalized solitary waves have been discovered in capillarygravity water waves, particle physics models, and geophysical Rossby waves. In this work, we present explicit calculations of weakly nonlocal envelope solitary waves. Each is a sine wave modulated by a slowly-varying “envelope” that itself propagates at the group velocity. Our example is the cubically nonlinear Klein-Gordon equation, which is a model in particle physics (φ⁴ theory) and in electrical engineering (with a different sign). Both cases have weakly nonlocal“breather” solitons. Via the Lorentz invariance, each breather generates a one-parameter family of nonlocal envelope solitary waves. The φ⁴ breather was described and calculated in earlier work. This generates envelope solitons which have “wings” that are (mostly) proportional to the second harmonic of the sinusoidal factor. In this article, we calculate breathers and envelope solitary waves for the second, electrical engineering case. Since these, unlike the φ⁴ waves, contain only odd harmonics, the envelope solitary waves are nonlocal only via the third harmonic.     

The moving-load problem in soil dynamics—the vertical displacement approximation

The moving point load problem in soil dynamics is analyzed in the vertical particle displacement approximation. Prior to its motion, the load is stationary. From the instant at which it is set into motion it moves, with constant speed, along a straight path on the (horizontal) planar surface of a semi-infinite elastic medium. The modified Cagniard method for solving transient wave problems is used to determine closed-form expressions for the vertical component of the particle displacement from the elastodynamic wave equation of which only the vertical component is taken into account. The relevant approximation is standard in soil dynamics. Both the cases of “subsonic” and “supersonic” surface load speeds are considered. Methods to include losses in the model are briefly discussed. The study has been initiated with a view to the application of the results to the analysis of the ground motion generated by high-speed trains traveling on a poorly consolidated soil.     

ON THE ROTATION-RELEASE METHOD IN STRUCTURAL MECHANICS1)

换铰法是结构力学中用于判定独立线位移未知量的一种方法,但经典的换铰法对约束的考虑并不全面,从而导致该方法在使用时存在一定的局限性。针对换铰法的局限性,提出了修正的换铰法,该方法考虑了各种约束以及无穷刚性杆的影响,从而在原理上更全面地体现了“铰化”和受弯直杆假定。算例表明,修正的换铰法可正确应用于复杂情形下独立线位移未知量的判定,并且在考虑参与质量后,该方法还可应用于较复杂的振动自由度判定问题。     

Observations of “granular jump” in the pneumatic conveying system

This paper presents a preliminary study of a previously unreported phenomenon of the “gas driven granular jump”, observed in the gas–solids flow within the pneumatic conveying system. From the phenomenological point of view, it resembles the already known processes such as hydraulic jumps in shallow water or granular jumps in granular flows in chutes or avalanches (although it seems most appropriate to explain it by analogy to a propagating granular bore). Clearly, unlike in classical phenomena of this type, the flow itself is driven by the aerodynamic forces related to the gas flow and the behaviour of the front of the “jump” is modified significantly by their presence. A series of high-speed camera visualisations are presented, which focus on this unusual behaviour of the flow on the border-line between cluster and stratified flow regimes in a horizontal pipe. Some similarities are drawn between the observed phenomenon and the broader class of problems exhibiting transition between super- and sub-critical flows. The fluid dynamical aspects and possible mechanisms behind the new phenomenon are discussed and the results obtained are compared quantitatively with simple theoretical models.     

Soot distribution in turbulent nonpremixed chloromethane — Air flames

The results of an experimental study on soot and temperature distribution in turbulent, nonpremixed chloromethane-air jet flames are presented. Transient measurements of soot volume fraction and temperature are made using a three-line optical pyrometry method. This method enables measurement of the “total” (i.e., absorption-related), and “hot” (i.e., emission-related) soot. Significant amounts of cold soot with hot soot is observed to coexist for all of the measurements made in these flames. Images of soot presence using a gated camera provide information about the mixing phenomena. The effects due to differential probe lengths are also discussed.     

Correlations predicting frictional pressure drop and liquid holdup during horizontal gas-liquid pipe flow with a small liquid holdup

Experimental data and correlations available in the literature for the liquid holdup ε_L and the pressure gradient ΔP_TP/L for gas-liquid pipe flow, generally, do not cover the domain 0 < ε_L < 0.06. Reliable pressure-drop correlations for this holdup range are important for calculating flow rates of natural gas, containing traces of condensate. In the present paper attention is focused on reliable measurements of ε_L and ΔP_TPIL values and on the development of a phenomenological model for the liquid-holdup range 0 < ε_L < 0.06. This model is called the “apparent rough surface” model and is referred to as the ARS model. The experimental results presented in this paper refer to air-water and air-water + ethyleneglycol systems with varying transport properties in horizontal straight smooth glass tubes under steady-state conditions. The holdup and pressure gradient values predicted with the ARS model agree satisfactorily with both our experimental results and data obtained from the literature referring to small liquid-holdup values 0 < ε_L < 0.06. Further, it has been shown that in the domain 38 < < 72 mPa m the interfacial tension of the gas-liquid system has no significant effect on the liquid holdup. The pressure gradient, however, increases slightly with decreasing surface tension values.     

Interaction and “apparent” reconnection of 3D vortex tubes via direct numerical simulations

We present evidence of: “binding” of anti-parallel vortex tube segments; strong noncircular core development; evolution of new secondary finger-like vortex structures: and finally “apparent” vortex reconnections due to entanglement. The latter three processes are not present in Biot-Savart filament simulations.     

A probabilistic linearization method for non-linear systems subjected to additive and multiplicative excitations

A general method to obtain approximate solutions for the random response of non-linear systems subjected to both additive and multiplicative Gaussian white noises is presented. Starting from the concept of linearization, the proposed method of “Probabilistic Linearization” (PL) is based on the replacement of the Fokker–Planck equation of the original non-linear system with an equivalent one relative to a linear system subjected to additive excitation only. By means of the general scheme of the weighted residuals, the unknown coefficients of the equivalent system are determined. Assuming a Gaussian probability density function of the response process and by choosing the weighting functions in a suitable way, the equivalence of the proposed method, called “Gaussian Probabilistic Linearization” (GPL), with the “Gaussian Stochastic Linearization” (GSL) applied to the coefficients of the Itô differential rule is evidenced. In addition, the generalization of the proposed method, called “Generalized Gaussian Probabilistic Linearization” (GGPL), is presented. Numerical applications show as, varying the choice of the weighting functions, it is possible to obtain different linearizations, with a variable degree of accuracy. For the two examples considered, different suitable combinations of the weighting functions lead to different equivalent linear systems, all characterized by the exact solution in terms of variance.     

Equations differentielles non lineaires: Differentiabilite des solutions par rapport aux conditions initiales et problemes de la stabilite

In this paper, we examine the problem of the asymptotic stability of the solutions of a differential equation (E), Y = X(Y, t), from the point of view of the differentiability of the solutions with respect to the initial conditions. The method allows us to deal with cases in which the mapping X is not differentiable everywhere, and in which the variational equation of (E) is not defined in the usual meaning. This method can be carried on for the research of periodic solutions. We give two examples.

Zusammenfassung

In diesem Bericht wird das Problem der asymptotischen Stabilität der Lösungen einer Differential Gleichung (E), Y = X(Y, t), vom Standpunkt der Differenzierbarkeit der Lösungen in Bezug auf die Anfangsbedingungen untersucht. Das Verfahren erlaubt die Fälle zu behandeln, worin die Abbildung X nicht überall differenzierbar ist, und worin auch die Variations Gleichung nicht im üblichen Sinne definiert ist. Das Verfahren kann erweitert werden zur Untersuchung periodischer Lösungen. Zwei Beispiele werden vorgeführt.

Abstract

B aдннoй paбaтe иccлeдyeтcя вoпoc oб accимeтpиХecкoй ycтoйХибocти peшeний диффepeHциaлънoгo ypaвнeния (E), Y = X(Y, t), c тoХки зpeния диффepeнциpyeмocти peшeний пo oтнoшeнию к нaХaлъным ycлoвиям. Иcпoлъзyeмый мeтoд дaeт вoзмoжнocтъ иccлeдoвaтъ cлyХaи, кoгдa пpeoбpaзoвaниe X нe вcюдy диффepeнциaлнoe. Bapиaциoннoe ypaвнeниe для oпpeдeляeтcя нecкoлъкo инaХe Хeм oбыХнo. Дaнный мeтoд мoжeт иcпoлъзoвaтъcя для иccлeдoвa*ncy;ия пepиoдиХecкиx peшeний. Дaютcя двa пpимepa.     

SIMILARITY LAWS OF HYPERVELOCITY PENETRATION EFFECTS OF CONCRETE1)

为了研究混凝土超高速侵彻效应的相似规律,总结相似关系基本理论并开展1.4~3.9 km/s 的混凝土侵彻实验,综合本文实验和相关文献实验结果确定关键相似准数并得到归一化侵彻深度的变化规律。结果表明,归一化侵彻深度主要取决于Johnson 数、弹靶密度比和弹体长径比,弹靶强度比和弹头形状系数等因素影响不大;归一化侵彻深度随归一化撞击速度增加而先增后减;当撞击速度接近混凝土纵波声速时,存在以侵彻深度突降为主要现象的“跨声速效应”。     

Acoustic scattering from baffled membranes that are backed by elastic cavities

An elastic membrane backed by a fluid-filled cavity in an elastic body is set into an infinite plane baffle. A time harmonic wave propagating in the acoustic fluid in the upper half-space is incident on the plane. It is assumed that the densities of this fluid and the fluid inside the cavity are small compared with the densities of the membrane and of the elastic walls of the cavity, thus defining a small parameter . Asymptotic expansions of the solution of this scattering problem as →0, that are uniform in the wave number k of the incident wave, are obtained using the method of matched asymptotic expansions. When the frequency of the incident wave is bounded away from the resonant frequencies of the membrane, the cavity fluid, and the elastic body, the resultant wave is a small perturbation (the “outer expansion”) of the specularly reflected wave from a completely rigid plane. However, when the incident wave frequency is near a resonant frequency (the “inner expansion”) then the scattered wave results from the interaction of the acoustic fluid with the membrane, the membrane with the cavity fluid, and finally the cavity fluid with the elastic body, and the resulting scattered field may be “large”. The cavity backed membrane (CBM) was previously analyzed for a rigid cavity wall. In this paper, we study the effects of the elastic cavity walls on modifying the response of the CBM. For incident frequencies near the membrane resonant frequencies, the elasticity of the cavity gives only a higher order (in ) correction to the scattered field. However, near a cavity fluid resonant frequency, and, of course, near an elastic body resonant frequency the elasticity contributes to the scattered field. The method is applied to the two dimensional problem of an infinite strip membrane backed by an infinitely long rectangular cavity. The cavity is formed by two infinitely long rectangular elastic solids. We speculate on the possible significance of the results with respect to viscoelastic membranes and viscoelastic instead of elastic cavity walls for surface sound absorbers.     

A SURFACE SUBSIDENCE MODEL OF COAL SEAM MINING IN MOUNTAIN AREA BASED ON PLATE BENDING DEFORMATION1)

分析了长壁工作面采煤上覆岩层破坏、移动的 “三带” 理论,认为地表的下沉变形受控于弯曲下沉带,且地表的最大下沉量远小于弯曲带岩层的厚度,弯曲带岩层的变形符合板的小挠度弯曲问题。在此基础上,根据山区地形地貌的特征及山体坡向与煤层倾向的关系,将地表山体简化为顺向坡、反向坡、沟槽、山岭四种情况,用薄板理论中的半逆解法分别建立了四种地貌情形下的地表下沉盆地模型。模型虽然作了一些简化假设,但体现了煤层倾角、埋深、岩层物理力学性质、地貌形状等影响开采沉陷的主要因素。