Modelling the wetting of a solid occlusion by a liquid film

We investigate in this work how the presence of an occlusion affects the dynamics of the wetting front of a liquid film draining down a vertical surface. This numerical study is developed in the context of the lubrication approximation. Through a parametric study, we show that depending on the asymptotic film thickness and the fluid properties, there exists a critical substrate contact angle below which separation of the contact line from the occlusion wall is observed which results in the appearance of a dry zone in the wake of the occlusion. In analogy with external aerodynamics, we also show that a sharp corner in the occlusion can induce this contact line separation. Our numerical results also highlight the importance of the occlusion wettability on the morphology of the wetting front suggesting a possible mechanism to control and mitigate the often undesirable fingering instability.     

Chain rupture and tensile deformation of polyamide 6 fibers

Summary The tensile deformation of polyamide 6 fibers gives rise to irreversible changes of its tensile properties, which apparently arise from chain rupture. This molecular failure process is usually analyzed by means of the ESR-spectroscopy. The number of ruptured chains, as obtained by this method, is, however, much too small to account for the changes of the tensile properties and for the observed strong decrease of the average molecular weight. Based on studies on energy dissipation, on structural as well on spectroscopical investigations we come to the conclusion that chain rupture is mainly responsible for the irreversible changes of the tensile properties. The dissipation of energy can be accounted for in terms of the energy release on chain rupture and energetic interactions with neighboring segments, in agreement with the data on the molecular weight decrease. The spectroscopically determined number of ruptured chains is governed by a fast recombination of the free radicals.

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Zusammenfassung Eine Deformation von Polyamid 6 Fasern führt zu irreversiblen Veränderungen in den mechanischen Eigenschaften, die offensichtlich durch Kettenbrüche verursacht werden. Dieser molekulare Versagensprozeß wird gewöhnlich mittels der ESR-Spektroskopie analysiert. Die hierdurch erhaltene Anzahl an Kettenbrüchen ist jedoch viel zu gering, um sowohl die Veränderungen in den mechanischen Eigenschaften als auch die starke Abnahme des mittleren Molekulargewichtes erklären zu können. Auf der Grundlage unserer Untersuchungen zur Energiedissipation, aus Streuexperimenten und spektroskopischen Untersuchungen kommen wir zu dem Schluß, daß Kettenbrüche für die Veränderungen der Eigenschaften allein verantwortlich sind. Die Energiedissipation läßt sich vollständig auf die Energiefreisetzung beim Kettenbruch und energetische Wechselwirkungen mit umgebenden Segmenten zurückführen, in Einklang mit den Ergebnissen zur Molekulargewichtserniedrigung. Die spektroskopisch bestimmte Anzahl an Kettenbrüchen wird durch eine schnelle Rekombination der freien Radikale beeinträchtigt.

     

Surfactant accumulation within the top foam layer due to rupture of external foam films

The analysis of processes taking place in a steady pneumatic (dynamic) foam shows the possibility of different modes of surfactant accumulation within the top layers of bubbles due to rupture of external foam films. An increasing surfactant concentration within the top layers promotes the stabilisation of bubbles and the foam as a whole. Considering the balance of surfactant and water during the bursting of films it is possible to estimate the accumulated surfactant loss caused by a downwards flow through the Plateau borders of the subsurface bubble layer. This effect depends on the particular conditions, especially on the surfactant activity and concentration of the surfactant, water volume fraction in the foam and size of foam bubbles. The process of surfactant accumulation in the top foam bubble layer can be complicated due to the removal of part of the accumulated surfactant through transport with droplets spread out during bubble bursting.     

Diaphragm rupture. Impingement by a conically-nosed, ram-accelerator projectile

The effect of diaphragm rupture by a conically-nosed projectile on the gasdynamics related to ram accelerator operation was experimentally and numerically studied. The experiments were conducted using a 25-mm-bore ram accelerator. Either air or nitrogen was used as the test gas. Using a high-speed image converter camera, it was observed that during the process of the diaphragm rupture a region with strong radiation developed between the diaphragm and the approaching projectile/sabot. This radiating region corresponds to the shock-heated gas which is originated from a precursory shock wave driven by the accelerating projectile/sabot. The flow around the projectile upon entering the test section by rupturing the diaphragm was visualized by holographic interferometry. During the diaphragm rupture, the system of oblique shock waves around the conical nose of the projectile was seen undisturbed on the downstream side of the diaphragm. Under the same condition as the experiment, numerical simulation was conducted using GRP (Generalized Riemann Problem) scheme which was extended to the computation of compressible flow fields bounded by moving surfaces. Two diaphragm rupture models were examined: (1) the diaphragm deformed by wrapping tightly around the moving projectile; (2) the diaphragm was ruptured instantly at the moment the projectile touched the diaphragm. Comparing these models with the experimentally visualized flow, the former was found to express the diaphragm rupture process much better than the latter. Received 9 February 1998 / Accepted 9 September 1998     

Three‐point bending tests. Part II: An improved formula for the modulus of rupture and numerical simulations

The goal of this paper is to analyze analytical and numerically, from several perspectives, the modulus of rupture (MOR) for brittle materials, studying the bending test of three points which is normally used in laboratory to calculate it. In particular, we will give four different approaches to the MOR: through the classical theory of beams; by means of the one‐ and three‐dimensional numerical simulations; and by using an improved expression to the MOR obtained through its asymptotic analysis. Finally, we will present these methodologies for cylindrical and rectangular beams made of porcelain. Copyright © 2011 John Wiley & Sons, Ltd.     

Point defect generation,nano-void formation and growth. II. Criterion for ductile failure

Using the derived relation for point defect generation according to a new constitutive relation, the notion of nano-void formation at grown-in nano-particles is examined and its consequences deduced as the nano-voids grow in size with continued deformation. Assuming that void growth is due only to point defect accumulation, the analysis of fracture strains in tension of natural-aged AA6111 suggests that coalescence by micro-plastic activity occurs when the void diameter becomes about one third of the evolving inter-void spacing. Hence, the derived limit strain to incipient void-coalescence is inversely proportional to the square root of point defect generation as determined from the stress–strain data. Using this criterion, failure prediction maps can be constructed for strain modes of plane-strain and balanced bi-axial to result in the outer bounds of the forming limit diagram. Trial examinations with AA5754 and AA3003 show great promise.     

网络结构的边毁裂度

在毁裂度的基础上,研究图的边的毁裂度.通过优化组合、归纳假设的方法界定了图的边毁裂度的值,如笛卡尔积图:Pm×Pn,Pm×Cn,Cm×Cn,Km×Kn,并界定了G=G1×G2的边毁裂度的界.最后给出了一些基本图,如路、圈、星图、完全二部图Km,n的线图边毁裂度.     

An improved nonlocal Gurson model for plastic porous solids,with an application to the simulation of ductile rupture tests

In recent work, the author and co-workers have developed a new methodology to delocalize the damage in Gurson model for porous ductile materials. The motivation was to rectify the difficulties connected to the excessive damage smoothing arising in the practical use of the original damage delocalization method. The new approach consists of delocalizing the logarithm of the damage instead of the damage itself. The relevance of the new method to avoid mesh size effects and satisfactorily reproduce typical ductile fracture experiments are explored in this work.     

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.