Traveling composition waves on isothermal catalyst surfaces

Recent experiments have indicated the presence of traveling composition waves along catalytic surfaces, waves that are accompanied by (and have come to be associated with) crystallographic changes in the catalyst itself during the course of reaction. We show, however, that ancillary phenomena of this kind are not essential to the existence of traveling composition waves. In particular, we use Conley index arguments to show that, for a large family of classical catalytic mechanisms (taken with Fick-type diffusion along the catalyst surface), the corresponding reaction-diffusion equations already have the capacity to admit stable traveling waves.     

Aerosol formation in an isothermal stagnation flow

An experiment has been performed in a laminar stagnation point flow in which two non-premixed reactants produced an aerosol of sub-micron particles. The reactants were NH₃ and HCl. The rate of mixing of the reactants was determined by the velocity gradient or strain rate of the flow; the response of the aerosol dynamics to the flow field was measured with a laser light scattering technique. Laser Doppler Spectroscopy was used to measure the particle size. It was found that the particle size was independent of the strain rate of the flow. On the other hand, the particle number density decreased as the strain rate increased. It is argued that the intensity of light scattered from the aerosol is, therefore, a measure of the amount of product of the relatively slow NH₃-HCl reaction.     

Numerical simulation of steady-state and transient natural convection in an isothermal open cubic cavity

In this work the numeric results, of the steady-state and transient heat transfer by natural convection in a horizontal isothermal open cubic cavity are presented. The most important assumptions in the mathematical formulation are two, the flow is laminar and the Boussinesq approximation is valid. The conservation equations in primitive variables are solved using the finite volume method and the SIMPLEC algorithm. The advective terms are approximated by the SMART scheme and the diffusive terms are approximated using the central differencing scheme. The results are obtained for a Rayleigh number range from 10⁴ to 10⁷.The numerical model predicted flow instabilities and Nusselt number oscillations for high Rayleigh numbers.     

The biological frontier of pattern formation

Morphogenetic patterns are highly sophisticated dissipative structures. Are they governed by the same general mechanisms as chemical and hydrodynamic patterns? Turing's symmetry breaking and Wolpert's signalling provide alternative mechanisms. The current evidence points out that the latter is more relevant, but reality is still far more complicated.     

Study of radiation interchange in an enclosure consisting of plane isothermal and adiabatic surfaces

This study examines the validity and accuracy of the commonly used diffuse, specular, and diffuse-specular constant property models for predicting radiant interchange among real surfaces by comparing analytical predictions with experimental data. The materials tested were sandblasted stainless steel, electropolished stainless steel, rough electroplated gold, and smooth electroplated gold. Measurements were made over the temperature range from 310.4 ?K to 644.0 ?K. The data indicates that the simple diffuse model yields reasonably accurate radiant heat exchange predictions. The variation in the radiative surface characteristics with direction should be accounted for in some manner, particularly for specular or nearly specularly reflecting surfaces.     

About macroscopic models of instability pattern formation

Various macroscopic models to describe instability pattern formation are discussed in this paper. They are similar to the Ginzburg–Landau envelope equation, but they can remain valid away from the bifurcation and are based on the technique of Fourier series with slowly varying coefficients. We focus on two questions: the need to take phase changes into account and the boundary conditions to be associated with macroscopic models. The analysis is carried out on the basis of numerical simulations for the problem of a compressed beam on a nonlinear foundation that is quite similar to the well known Swift–Hohenberg equation. The first macroscopic model involves a real envelope so that the phase is assumed to be constant. The second model is also macroscopic and it is a sort of Ginzburg–Landau equation with a complex envelope. The third one follows from a multi-scaled approach with a numerical bridging between the full model near the boundary and a macroscopic model in the bulk.     

Study of thermal flows from two-dimensional,upward-facing isothermal surfaces using a laser speckle photography technique

A laser specklegram or speckle photography technique allows a direct measurement of surface temperature gradients and provides a full field interrogation with an extremely high resolution from a single data taking. The specklegram technique has been successfully applied to investigate the natural convection heat transfer from an upward-facing isothermal plate. For a plate with a large aspect ratio of 15, both local and global Nusselt numbers have been determined from the direct measurement of local temperature gradients. The Rayleigh number, based on the length scale equivalent to the ratio of the surface area to the perimeter, has been varied from 9.0 × 10³ to 4.0 × 10⁴. The present result for the global heat transfer has shown that a 1/5-power law, i.e., Nu = C₁ Ra ^1/5, correlates the data more properly whilst previously published results showed a large scatter in the exponent, ranging from 1/8-power to 1/4-power. The proportional constant, C₁ has been determined to be 0.56 which shows a fairly good agreement with previously published theoretical results. The laser specklegram technique has shown a strong potential as a powerful and convenient method for an experimental assessment of natural convection heat transfer problems. The specklegram technique at the same time has eliminated the deficiencies of both the mass transfer analogy technique and the classical heat transfer measurement technique.List of symbols a characteristic length scale defined as a = A/P where A is the surface area and P is the perimeter of the plate edge [mm] - AR aspect ratio [L/H] - c defocusing distance [mm] - d image distance of Young's fringes from speckle negative - h thermal convection coefficient [W/m² · K] - average thermal convection coefficient [W/m² · °C] - H width of the test section measured perpendicular to the optic axis [mm] - k thermal conductivity [W/m · K] - L length of the test section measured parallel to the optical axis [mm] - n index of refraction - Nu local Nusselt number [ha/k] - global Nusselt number - Pr Prandtl number [v/] - q heat flux per unit area [W/m² · s] - Ra Rayleigh number - s fringe spacing [mm] - Sc Schmidt number [v/D] - T temperature [K] Greek symbols thermal diffusivity [m²/s] - volumetric coefficient of expansion (1/T) - v kinematic viscosity of air [m²/s] - wavelength of helium-neon laser [632.8 nm] - amount of speckle dislocation     

Tire tread pattern design trigger on the stress distribution over rigid surfaces and soil compaction

Studies comparing the structural differences of tires have not qualitatively or quantitatively considered the effects of tread geometry on tire behaviour or the interactions of the tire with the surface. Therefore, to determine the effects of different tire tread patterns on the stress distribution of the tire and soil compaction, we compared the structural behaviours of a high-flotation tractive-tread (TT) tire and a smooth-tread (ST) tire. The experiments were conducted over a rigid and over a deformable surface. The results from the rigid surface shows the influences of the tread pattern and sidewalls is dependent of the loads. Over the deformable surface, the contact area of the TT tire was larger than that of the ST tire. The inflation pressure (IP) was mainly responsible for the load support before the soil reached its maximum deformation. Next, the tread and sidewalls exhibited the same behaviour as observed on the rigid surface. In addition, we observed alterations in the balloon point with the tread geometry and the type of surface due to changes in the contact pressure. With carcass deformation, the volume of the tire was visibly reduced, which indicated that the IP could increase.     

Impact of noise on pattern formation in a predator–prey model

In this paper, we investigated the influence of color noise on pattern formation in a predator–prey model. When the model has no noise, it exhibits wave dynamics. A series of numerical simulations showed that break-up of waves will emerge when noise is added. Furthermore, stationary pattern can be induced by noise. The obtained results may point out that noise can have great effect on spatial complexity of ecosystems.     

Bridging techniques in a multi-scale modeling of pattern formation

Bridging techniques between microscopic and macroscopic models are discussed in the case of wrinkling analysis. The considered macroscopic models are related to envelope equations of Ginzburg–Landau type, but generally, they are not valid up to the boundary. To this end, a multi-scale approach is considered: the reduced model is implemented in the bulk while the full model is applied near the boundary and these two models are coupled with the Arlequin method (Ben Dhia, 1998). This paper focuses on the definition of the coupling model and the transition between two scales. Especially, a new nonlocal bridging technique is presented and compared with another recent one (Hu et al., 2011). The present method can also be seen as a guide for coupling techniques involving other reduced order models.     

Experimental investigation of splash and crown formation during single drop impact on wetted surfaces

Experimental results concerning crown formation during liquid drop impact on wetted surfaces are reported. Different liquids and numerous impact conditions are investigated. In particular, crown-splash (C-S) and deposition-crown (D-C) limits are determined on the basis of the experimental observations. These limits converge for dimensionless film thickness thinner than 0.03, leaving the outcome of crown formation unobserved. The sole Weber number and dimensionless film thickness cannot explain the phenomenon. It appears that all these data can be described using a combination of Weber and Ohnesorge numbers versus dimensionless film thickness.     

A New dynamic model for study of dislocation pattern formation

Based on the principle given in nonlinear diffusion-reaction dynamics, a new dynamic model for dislocation patterning is proposed by introducing a relaxation time to the relation between dislocation density and dislocation flux. The so-called chemical potential like quantities, which appear in the model can be derived from variation principle for free energy functional of dislocated media, where the free energy density function is expessed in terms of not only the dislocation density itself but also their spatial gradients. The linear stability analysis on the governing equations of a simple dislocation density shows that there exists an intrinsic wave number leading to bifurcation of space structure of dislocation density. At the same time, the numerical results also demonstrate the coexistence and transition between different dislocation patterns. The project supported by the National Natural Science Foundation of China, Grant No.19392300     

河型发育过程的二维数值模拟研究

改进已有二维平面水沙模型,进一步考虑河床粗化、弯道二次流及河床结构对泥沙输运的影响;并针对河湾形态对崩岸的影响提出新的非粘性土崩岸模拟方法,使其能模拟边滩的形成。利用改进后模型分别对Yen弯道水槽泥沙冲淤试验及Friendkin室内弯曲小河塑造试验进行模拟;模拟结果与试验观测资料吻合较好,表明考虑弯道对泥沙输运及河岸崩塌影响的模型能够更准确反映天然河流演化过程,为研究河型演化机理及各控制因素变化规律提供新手段。     

A perspective on regression and Bayesian approaches for system identification of pattern formation dynamics

We present two approaches to system identification, i.e. the identification of partial differential equations(PDEs) from measurement data. The first is a regression-based variational system identification procedure that is advantageous in not requiring repeated forward model solves and has good scalability to large number of differential operators. However it has strict data type requirements needing the ability to directly represent the operators through the available data.The second is a Bayesian inference framework highly valuable for providing uncertainty quantification, and flexible for accommodating sparse and noisy data that may also be indirect quantities of interest. However, it also requires repeated forward solutions of the PDE models which is expensive and hinders scalability. We provide illustrations of results on a model problem for pattern formation dynamics, and discuss merits of the presented methods.     

A note on the steady-state advection-diffusion equation

In this note we show that the numerical solution of the advection-diffusion equation can be improved by considering the asymptotic behaviour of its analytical solution. This is accomplished by including a correction term based on the numerical differentiation of the asymptotic (Pe » 1, Pe being the Peclet number) solution. This correction forces the usual oscillations associated with centred schemes to disappear.     

刻槽式MEFP的成形及侵彻钢靶模式

多爆炸成形弹丸(MEFP)能有效提高炸药利用率和命中概率。基于LS-DYNA软件对刻槽式MEFP战斗部成形及侵彻双层无间隔钢靶模式进行了研究,得到了刻槽式MEFP战斗部成形及对靶板侵彻的数值计算结果。结果表明:刻槽式MEFP成形过程要经过翻转、头部挤压断裂、尾部拉伸断裂、交叉飞散、稳定飞行5个阶段, 聚能爆轰波对药型罩的径向力是其交叉飞散的主要原因;侵彻过程要经过开坑、联合侵彻、贯穿3个阶段。模拟结果与实验结果基本吻合。     

Mechanism for the promotional formation of NH4+ by SO2 on different mineral dust surfaces

Ammonium is an important atmospheric particulate component that dictates many environmental processes. The promotion of the heterogeneous conversion of NH₃ to NH₄⁺ by SO₂ on different mineral dust surfaces displays remarkable discrepancies, especially on MgO and α-Fe₂O₃ surfaces, however, the underlying mechanisms are not well known. Here, using periodic density functional theory (DFT) calculation and Born-Oppenheimer molecular dynamics (BOMD) simulation, we explored the heterogeneous adsorption of NH₃ on MgO (110) and α-Fe₂O₃ (001) surfaces in the presence and absence of SO₂. The results show that on MgO (110) surface, hydrogen-bonding interactions of NH₃ on both adsorbed hydroxyl or bisulfite/bisulfate sites are observed no matter whether SO₂ is present or not. While, on the α-Fe₂O₃ (001) surface, significant conversion of NH₃ to NH₄⁺ occurs with the coexistence of SO₂, which is due to the hydrogen transfer reaction from surface HSO₄ to N in NH₃. The fundamental reason may be that the stronger electron affinity of Fe³⁺ than Mg²⁺ results in adsorbed bisulfate and/or bisulfite with greater acidity on α-Fe₂O₃ surface than MgO surface. Our results give a molecular-level explanation for the heterogeneous conversion of NH₃ to NH₄⁺ on different mineral dust surfaces under complex air pollution conditions. Considering the fact that ammonium is abundant in secondary particulates, this work would help in understanding the rapid conversion of ammonia to ammonium and in developing classification governance policies for the key precursor pollutants in China.