ROUTE TO CHAOS IN CHEMICALLY ENHANCED THERMAL CONVECTION IN POROUS MEDIA

Exothermic chemical reactions can influence natural convection effects in a porous medium. Such phenomena may occur in tubular reactors, oxidation of solid materials in large containers, chemical vapor deposition systems, liquid explosives, and others. Experimental evidence indicates that the influence of natural convection in many chemically reacting systems cannot be neglected.

In the present work, transient effects of a two-dimensional convection generated and sustained by an exothermic chemical reaction and a constant boundary wall temperature are studied. The Darcy-Boussinesq equations are used to describe fluid flow through porous media with a zero-order chemical reaction. A recently developed method is used to compute the singular points (such as limit and symmetry breaking bifurcation points) precisely as a function of Rayleigh (Ra) and Frank-Kamenetskii (Fk) numbers. Fold curves are drawn as a function of Ra, and Fk. Flow behavior is governed by two natural parameters, namely, Ra and Fk. Multiple stationary solutions arise over an intermediate range of Ra. The solution structure, i.e., the interconnections between the various branches appear quite complicated. Determination of linear stability on the these solution branches reveal that all two-dimensional, stationary solutions develop some form of instability at higher values of Ra. Transient simulations reveal the emergence of time periodic solutions at higher Ra. The nature and frequency of these periodic solutions depend on the route followed in the parameter space Fk and Ra. The various routes to chaos are identified in this parameter space.     

On the lateral instability of a thin beam under periodic bending loads

The lateral instability of a thin beam under periodic bending loads was investigated. Physical evidences of the instability were observed previously by experiments. But an analytical study has not been reported. The object of this study is to demonstrate the nature and existence of dynamic lateral instability. The harmonic balance method is applied to bifurcation modes which result from the stability change of torsional mode of a beam and then compared with numerical simulations. It is found, in a certain frequency range, that a small bending load results in the lateral instability when damping is small. Inha University     

Instability in bending-under-tension of aged steel sheet

In combined bending and stretching of aged steel sheet, an instability of an unusual kind is identified. Analysis of the behaviour of a sheet using a simplified material model shows that under certain conditions the maximum tension is less than the fully yielding tension and springback in the opposite direction to that normally observed could occur.Some of the sheet used industrially for panels of small curvature may age and there is considerable current interest in the prediction of springback in panels formed by combined bending and stretching.     

Buckling behavior of cold-formed zed-purlins partially restrained by steel sheeting

This paper presents a study on the buckling behaviour of purlin-sheeting systems under wind uplift loading. The restraint provided by the sheeting to the purlin is modeled by using two springs representing the translational and rotational restraints. The analysis is performed using finite strip methods in which the pre-buckling stress is calculated based on the same model used for the buckling analysis rather than taken as the ‘pure bending’ stress. The results obtained from this study show that, for both local and distortional buckling, the restraints have significant influence on the critical loads through their influence on the pre-buckling stress rather than directly on the buckling modes. While for lateral-torsional buckling, the influence of the restraints on the critical loads is mainly due to their influence on the buckling modes rather than the pre-buckling stress.     

涡流室式柴油机油滴破碎和喷雾碰壁的三维数值模拟

根据不稳定性理论,将燃烧室中油滴破碎过程分为ＫＨ不稳定破碎和ＲＴ不稳定破碎;根据Ｗｅｂｅｒ数将喷雾碰壁过程分为粘附、反弹粘附和飞溅附壁射流３ 种模式,建立了油滴破碎和喷雾碰壁的数学模型。根据油滴破碎模型计算所得的喷雾贯穿距离和ＳＭＤ值同试验结果相一致,喷雾碰壁模型计算所得的油滴位置图形与高速摄影结果相一致。将已被试验验证后的模型应用于涡流室式柴油机中, 计算了涡流室内喷雾的发展过程,计算结果与高速摄影结果吻合良好。     

Disc brake squeal characterization through simplified test rigs

This paper presents a review of recent investigations on brake squeal noise carried out on simplified experimental rigs. The common theme of these works is that of approaching the study of squeal noise on experimental set-ups that are much simpler than commercial disc brakes, providing the possibility of repeatable measurements of squeal occurrence. As a consequence, it is possible to build consistent and robust models of the experimental apparatus to simulate the squeal events and to understand the physics behind squeal instabilities.     

离心力对旋叶式分离器壁面液膜界面不稳定性的影响

本文对旋转流场条件下离心力对旋叶式分离器壁面液膜界面不稳定性的影响规律进行了理论研究。首先利用势函数对汽液两相的动量方程和连续方程进行线性化处理。然后通过对无黏条件下液膜界面的受力分析,获得汽液界面的动力学边界条件和运动学边界条件,结合线性化方程组建立液膜界面波的色散方程。根据力平衡原理获得了液膜运动规律,并结合色散方程建立了界面不稳定发生的判定准则关系式。基于所获得的理论模型编写计算程序,对界面稳定性进行计算分析。研究发现,旋转流场条件下的离心力能够抑制旋叶式分离器壁面液膜界面不稳定的发生。     

On a Theory of Thermoelastic Materials with a Double Porosity Structure

In this article, we use the Nunziato–Cowin theory of materials with voids to derive a theory of thermoelastic solids, which have a double porosity structure. The new theory is not based on Darcy's law. In the case of equilibrium, in contrast with the classical theory of elastic materials with double porosity, the porosity structure of the body is influenced by the displacement field. We prove the uniqueness of solutions by means of the logarithmic convexity arguments as well as the instability of solutions whenever the internal energy is not positive definite. Later, we use semigroup arguments to prove the existence of solutions in the case that the internal energy is positive. The deformation of an elastic space with a spherical cavity is investigated.     

An analysis of flow and screech tone from supersonic axisymmetric jet at the initial stage

The screech tone from jet Mach number 1.18 is numerically calculated from the initial stage. A fourth-order optimized compact scheme and fourth-order Runge-Kutta method are used to solve the 2D axisymmetric Euler equation. Pulse jet problem with Mach number 1.56 is solved to validate the present method. Not only two important components of generating screech tones, shock cell structure and vortices, but also the transient behavior of the screech tone are investigated at the initial stage. Additional time is necessary to generate periodic screech tone after formation of shock cell structures. As time goes on, the location of the vortex generation becomes fixed near the nozzle exit, which is farther downstream initially, and the screech tone becomes periodic. The FFT results of three different periods are compared. It is observed that the component of lower frequency is dominant at the beginning, and the component of the screech tone becomes dominant as time increases. It can be concluded that the screech tones can be also numerically reproduced at the initial stage with the present inviscid method.