Numerical simulation of streamwise fluidelastic instability of tube bundles subjected to two-phase cross flow

This work aims to develop and validate a numerical model to simulate the flow-structure interaction in tube bundles subjected to two-phase flow. The model utilizes a mixture multiphase module in which a drift flux formulation is used to account for the slip between the phases. Two methods of numerical flow-structure interaction are used to predict the onset of fluidelastic instability (FEI) in the streamwise direction for a two-phase air–water flow mixture in parallel triangular tube bundles. These models are the hybrid analytical-flow field model and the direct numerical flow/structure coupling model. This work investigates the effects of void fractions in the range of 20% to 80% and several pitch-to-diameter ratios (P/D) in the range of 1.3 to 1.7. The results of the fluidelastic forces and the stability threshold are validated against the experimental data available in the literature and show an excellent agreement. The streamwise FEI threshold shows a significant dependency on the pitch-to-diameter ratio while the void fraction exhibits a lesser effect. Generally, the stability threshold increases as the pitch-to-diameter ratio increases. The model that was developed paves the way for devising of more reliable prediction tools for FEI in steam generators.     

薄膜拉伸褶皱失稳力学进展

薄膜拉伸起皱现象在自然界和现代工业中普遍存在，在过去二十年里，引发了学者们极大的研究兴趣．这种起皱失稳行为源自薄膜能与弯曲能之间的非线性竞争．我们回顾从本世纪初至今薄膜拉伸褶皱失稳力学研究进展，将其分为两个阶段：前十年的研究主要局限于薄膜小应变(~1 %)起皱现象，而后十年的兴趣主要集中在有限应变起皱-再稳定（孤立中心分岔）行为，在过度拉伸(~30 %)时褶皱最终消失．定量理解、预测和追踪这种强非线性力学行为的需求推进了有限应变板壳理论和数值计算方法的发展，不仅促进了对薄膜起皱-消皱机理的深入理解，也为无褶皱膜结构设计和薄膜表面形貌调控提供了新的思路．     

Stress distribution and surface instability of an inclined granular layer

Static granular materials may avalanche suddenly under continuous quasi-static drives. This phenomenon, which is important in many engineering applications, can be explained by analyzing the stability of the elastic solutions. We show this for a granular layer driven by its inclination angle in gravity, where the elastic problem can be solved generally and analytically. It is found that a loss of stability may occur only at the free surface of the layer. This result is considered to be relevant for understanding surface avalanches and the flows observed experimentally.     

Pattern formation in flame spread over thin solid fuels

The fingering char pattern emerging on the surface of thin cellulosic sheets burning against an oxidizing wind is discussed. Employing collocation-based averaging, the assumption of diffusive-thermal equilibrium, the strong temperature dependence of the reaction rate, and the strong disparity between the densities of the solid and gaseous phases, an elementary two-dimensional free-interface model for the flame spread is formulated. It is shown that the pattern-forming dynamics is functionally akin to the well-studied cellular instability occurring in low Lewis number premixed gas flames.     

A simple isotropic-distortional hardening model and its application in elastic–plastic analysis of localized necking in orthotropic sheet metals

In the present work an elastic–plastic constitutive model including mixed isotropic-distortional hardening is presented. The approach is very simple and requires only experimental data that are part of the standard characterization of sheet metals. It is shown that the distortional hardening contribution can be of considerable importance for localized necking prediction in orthotropic sheet metals.     

The effect of invalidity of Boussinesq approximation on the onset of natural convection in the electrochemical system

The effect of invalidity of Boussinesq approximation on the natural-convective stability of electrochemical system consisting of two plane horizontal electrodes and the binary electrolyte filling the space between the electrodes is considered. It is shown that the system stability against the rise of natural convection is higher under these conditions than under the Boussinesq approximation.     

SURFACE INSTABILITY OF A SWOLLEN CYLINDER HYDROGEL

Cylinder hydrogel is simple in geometry and easy to synthesize,therefore was widely used to investigate the swelling/shrinking instability of hydrogel and many instability patterns were accumulated in the literature.The mechanism of instability pattern formation of this unique configuration,nevertheless,is far from being fully understood.We applied and extended the recently developed nonlinear theory of polymer gels into cylindrical coordinates,and performed linear perturbation analysis of swelling-induced stability of a constrained cylinder hydrogel.We derived the incremental formulations of stresses and the associated equilibrium equations.We obtained the critical conditions for the onset of instability and probed in details the effects of various parameters on the stability diagram of the hydrogel.The physical meaning of the variation of stability diagram was also interpreted.     

The effect of the plasma density gradient on current filamentation instability

The filamentation instability is one of the basic beam-plasma instabilities that play a significant role in the energy deposition mechanism of the relativistic electrons generated by the laser-plasma interaction in the fast ignition scenario. In this paper, the effect of the density gradient into plasma on the filamentation instability was investigated in the Weibel unstable plasma, where the plasma temperature anisotropy can play an important role. Results indicated that the density gradient enhances the instability growth rate so that decreasing the density gradient from the critical surface to the core of fuel leads to instability for longer regions in k space. Also, investigations in the region close to the critical surface showed that for decreasing the beam number density n_b ≤ 0.01n₀, the instability occurs for while this can be different for higher values. Increasing the beam relativistic factor causes a decreasing peak of instability growth rate because of a reduction in beam current, whereas the initial thermal spread of plasma amplifies the filamentation instability.     

Ion velocity shear-induced drift wave and momentum transport in non-Maxwellian magnetoplasma flows

We have investigated the diamagnetic flow in a non-uniform partially ionized plasma with non-Maxwellian electron population to explain the dynamics of ion velocity shear-induced low-frequency drift mode and associated instabilities. The dispersion relations are found, and instability threshold conditions are pointed out along with ion-parallel momentum transport due to drift motion with relative phase shift of fluctuating quantities in a non-conservative system. The real frequencies and instability growth rates are studied numerically and illustrated for typical space and laboratory plasmas. This study should be useful in understanding some aspects of low-frequency time-delayed perturbations with sheared flow leading to drift instabilities and cross-field parallel ion momentum transport in nonuniform magnetoplasmas containing a non-Maxwellian electron population.