含气泡油滴撞击油膜壁面时气泡的变形与破裂

油--气润滑过程中润滑油液滴受高速气流扰动易形成含气泡油滴,微气泡将对油滴撞击壁面时的运动过程以及壁面油膜 层的形成质量产生重要影响. 基于耦合的水平集--体积分数 方法,对含气泡油滴撞击油膜壁面行为进行数值模拟研究, 考察含气泡油滴撞击油膜壁面时气泡的变形运动过程,探讨气泡破裂的动力学机制,分析气泡大小、碰撞速度和液体黏度等因素对含气 泡油滴撞壁过程中气泡变形特征参数的影响规律. 研究表明:含气泡油滴撞击油膜壁面后气泡会发生变形,并破裂形成膜液滴;气泡随同 液滴运动过程中,气泡内外压力和速度梯度变化是使气泡发生破裂的主要诱因. 气泡大小对气泡破裂方式影响较大,气泡较小时发生单 点破裂,而气泡较大时更容易发生多处破裂. 不同大小气泡受力差异较大,气泡大小与破裂发生时刻没有明显相关性. 碰撞速度和液体 黏度对气泡的变形、破裂和破裂发生时刻都具有一定的影响. 碰撞速度越大,油滴动能越大,更容易产生气泡变形和破裂现象. 液体黏 度增大,在油滴撞壁运动前期促进气泡变形,而在运动后期可以阻延气泡破裂行为发生.      

油滴撞击油膜层内气泡的变形与破裂过程的数值模拟

旋转工作的机械零部件和机械设备的润滑系统工作过程中普遍存在着油滴和油膜的碰撞行为,这一行为易引起气泡夹带现象.气泡将对油滴撞击油膜时的运动过程和附壁油膜层的形成质量造成不可忽视的影响.基于耦合的水平集-体积分数方法,对油滴撞击含气泡油膜的行为进行数值模拟研究,考察油膜层内气泡的变形运动过程,分析气泡大小和位置等因素对撞击过程中气泡变形特征参数的影响规律,并探讨气泡破裂的动力学机制.研究表明,随着气泡直径的增大,油滴撞击含气泡油膜后气泡会依次出现自由表面破裂、稳定变形以及油膜内部破裂等现象;直径d=20μm的气泡能较稳定地存在于油膜层内,同时该值也是气泡发生自由表面破裂和油膜内部破裂的临界值.此外,气泡所在位置同样对气泡变形历程有一定影响,气泡越接近油膜表面,其变形量越大;位于油膜底层的气泡会附着在壁面上.在自由表面破裂和油膜内部破裂过程中,气泡破裂是由气-液界面不稳定引起的,表面张力对这两种现象起重要作用;而黏性剪切力对油膜内部破裂现象也有着不可忽视的影响.     

Time to Creep Failure of Thin-Walled Cylindrical Pipes under Torsion

The time to creep failure is calculated for rectilinear thin-wall pipes subjected to pure torsion, torsion with uniaxial tension, and torsion with internal pressure. The problem is solved using the concept of equivalent stress. The equivalent stresses are found from the generalized mixed failure criterion whose form depends on the signs of the principal stresses. The criterion relates the maximum normal stress and the intensity of shear stress if the signs coincide, and the maximal shear stress and the octahedral shear stress if the signs are opposite. A technique for determining the material constants is developed. The calculated and experimental data are compared and found to be in satisfactory agreement     

Modeling the deformability of a material

The high-temperature extension of metals to fracture is modeled for creep at constant stress or at constant strain rate. The dependence of the ultimate fracture strain on the loading factor (stress or strain rate) is studied. The nonmonotonic nature of this dependence with an internal maximum is described using the Rabotnov kinetic theory with one and two damage parameters. Available experimental data are analyzed. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 5, pp. 183–188, September–October, 2007.     

Comportement mécanique et rupture de milieux granulaires cohésifsMechanical behaviour and rupture of cohesive granular media

The modelling of mechanical behaviour and rupture of cohesive granular media is carried out on a model medium made of aluminium cylinders and cohesive bonds playing the role of cohesion points. The mechanical behaviour of the cohesion point is studied under different loading situations (compression, traction, shearing, torque). The results are introduced into a code, which is based on a discrete element method extended to the cohesive case. The validation is achieved with compression tests of macroscopic samples. We notice a good correlation between simulation and experiment on the mechanical behaviour, on the appearance of fractures and their propagation. To cite this article: J.-Y. Delenne et al., C. R. Mecanique 330 (2002) 475–482.     

The drainage and rupture of a non-foaming liquid film formed upon bubble impact with a free surface

Experiments are performed to measure the thickness of a thin liquid film formed between a free surface and the apex of a bubble which is approaching at its terminal velocity. Measurements are made using bubbles of < 1 mm dia both in distilled water and alcoholic solutions (methanol, ethanol). The results of the experiments show that the models based on the lubrication approximation fail to predict the initial stage of drainage. A better agreement is obtained by modifying the model proposed by Chesters.     

龙门山地震带的地质背景与汶川地震的地表破裂

龙门山位于青藏高原与扬子地台之间, 系由一系列大致平行的叠瓦状冲断带构成, 自西向东发育汶川—茂汶断裂、映秀—北川断裂和彭县—灌县断裂,并将龙门山划分为3个构造地层带,分别为变形变质构造地层带（主要由志留系—泥盆系浅变质岩和前寒武系杂岩构成）、变形变位构造地层带（主要由上古生界—三叠系沉积岩构成）、变形构造地层带（主要由侏罗系至第三系红层和第四纪松散堆积构成）。
龙门山断裂带属地震危险区,3条主干断裂皆具备发生7级左右地震的能力,其中映秀—北川断裂是引发地震的最主要断层,据对彭县—灌县断裂青石坪探槽场地的研究结果表明,在该断裂带上最晚的一次强震发生在930±40a.B.P.左右,据此,可以初步判定,这3条主干断裂的单条断裂上的强震复发间隔至少应在1000a左右,表明龙门山构造带及其内部断裂属于地震活动频度低但具有发生超强地震的潜在危险的特殊断裂,以逆冲-右行走滑为其主要运动方式。
汶川地震属于逆冲—走滑型的地震,地表破裂分布于映秀—北川断裂带和彭县—灌县断裂带上。根据近南北向的断裂（小鱼洞断层、擂鼓断层和邓家坝断层）和地表断距可将映秀—北川断层的地表破裂带划分为两个高值区和两个低值区,两个高值区分别位于南段的映秀-虹口一带和位于中北段的擂鼓—北川县城—邓家坝一带;两个低值区分别位于中南段的白水河—茶坪一带和北段的北川黄家坝至平武石坎子一带,两个高值区分别与小鱼洞断层和擂鼓断层相关。根据保存于破裂面上的擦痕,可将该地震破裂过程划分为两个阶段,早期为逆冲作用,晚期为斜向走滑作用,其与地壳增厚构造模式和侧向挤出摸式在青藏高原东缘的推论具有不吻合性。鉴于龙门山的表层运动速率与深部构造运动速率具有不一致性,初步探讨了龙门山地区的地表过程与下地壳流之间的地质动力模型,认为下地壳物质在龙门山近垂向挤出和垂向运动,从而造成导致龙门山向东的逆冲运动、龙门山构造带抬升和汶川特大地震。在此基础上,根据汶川地震所引发的地质灾害,对地震灾后重建提出了的几点建议。     

Comparative estimation of high-temperature creep and rupture of structural materials

The possibility of comparing the rates of creep processes and times to rupture of materials is shown by the example of several structural alloys of different types. The creep equation is written in dimensionless form, which allows one to compare creep processes up to rupture in normalized quantities, to estimate their difference, and, in the case of coincidence of the normalized quantities for some materials under corresponding temperature-load conditions, to model the indicated processes using experimental data for one of these materials. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 2, pp. 123–130, March–April, 2008.     

Probing the Interdigitated Phase of a DPPC Lipid Bilayer by Micropipette Aspiration

We used micropipette aspiration of giant unilamellar vesicles to directly measure the areal expansion of gel (L_β′) phase 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid bilayers induced by exposure to ethanol/water mixtures. Areal expansion began in 7 vol% ethanol and increased monotonically as the concentration of ethanol was increased to 15 vol% at which point areal expansion reached a plateau of 50%. This ethanol concentration range is in good agreement with that of the interdigitated phase (L_βI) of DPPC, therefore, we believe that this is the first direct measurement of the areal expansion accompanying interdigitation of gel-phase lipids. Our observations are consistent with the presence of coexisting L_βI and L_β′ phases in ethanol concentrations between 7% and 15 vol% and 100% L_βI phase in 15 vol% ethanol and higher. We observed a bimodal distribution of areal expansion (0% and 20%) induced by 7 vol% ethanol indicating that at the threshold concentration, interdigitation is induced in only a portion of DPPC vesicles. Areal expansion could not be easily reversed, consistent with kinetic trapping of the L_βI phase. DPPC vesicles exposed to butanol at the known threshold and plateau concentrations for the L_βI phase displayed areal expansion behavior consistent with our ethanol observations. However, the area expanded significantly faster for DPPC bilayers exposed to butanol vs. ethanol, which we attribute to enhanced partitioning of the longer-chained butanol into the lipid headgroups. Ethanol-induced areal expansion of DPPC bilayers was inhibited by inclusion of 10 mol% and 25 mol% cholesterol in the bilayer. However, areal expansion could be induced by application of tensions (∼8 mN/m) similar to the phenomena of interdigitation induced by high pressure. The presence of 20 vol% ethanol significantly decreased surface cohesion of DPPC bilayers containing 25 mol% cholesterol as evidenced by a decreased area compressibility modulus and lysis tension.     

RUPTURE OF THIN METAL TUBES BY NORMAL AND OBLIQUE IMPACT OF BLUNT CONICALNOSED MISSILES: EXPERIMENTS

Impact tests at both normal and oblique angles of incidence were conducted on thin mild tubes using a moderate size of 90° conical-nosed missiles. The minimum impact speed that generated cracks through the thickness of the wall, termed the speed for rupture, was measured, and various modes of rupture were identified. For a thin tube hit by a missile at a normal angle of obliquity at the speed for rupture, the contact region spreads across the nose of the missile, and the transverse shear deformation is predominant in the final failure process. If the angle of obliquity is 30°, the missile pierces a hole through the wall of the tube. At the speed for rupture, the kinetic energy of the missile for oblique angle 30° is only about 45% that required for plugging at a normal angle of obliquity. The project sponsored by National Natural Science Foundation of China(No. 19842001, 19872048) and Scientific Research Foundation for Returned Overseas Chinese Scholars of State Education Commission and Shanxi Province of China.