Dynamics of trailing vortices in the wake of a generic high-speed train

The three-dimensional dynamics of a pair of counter-rotating streamwise vortices that are present in the wake of an ICE3 high-speed train typical of modern, streamlined vehicles in operation, is investigated in a 1/10th-scale wind-tunnel experiment. Velocity mapping, frequency analysis, phase-averaging and proper orthogonal decomposition of data from high-frequency multi-hole dynamic pressure probes, two-dimensional total pressure arrays and one-dimensional multi-hole arrays was performed. Sinusoidal, antisymmetric motion of the pair of counter-rotating streamwise vortices in the wake is observed. These unsteady characteristics are proposed to be representative of full-scale operational high-speed trains, in spite of the experimental limitations: static floor, reduced model length and reduced Reynolds number. This conclusion is drawn from favourable comparisons with numerical literature, and the ability of the identified characteristics to explain phenomena established in full-scale and scaled moving-model experiments.     

修正的Leslie-Gower捕食者-食饵扩散模型解的整体性态

讨论带有一般扩散的修正的Leslie-Gower捕食者-食饵模型解的整体性态。发现仅带有一般扩散的系统与相应的常微分系统解的动力学行为相似。所以，一般扩散不能导致Turing不稳定现象发生。     

Viscous flow in a mixed compression intake

Numerical simulation of the flow in a two‐dimensional mixed compression intake is carried out by solving unsteady viscous compressible equations using a stabilized finite element method. The effect of bleed in starting/unstarting of the intake and controlling the buzz instability is investigated in detail. Higher bleed leads to an increase in the ability of the intake to sustain larger back‐pressure for stable operation. The amount of bleed and its location is varied to understand its effect on the performance of the intake. Two kinds of unsteady oscillations are observed: ‘little’ and ‘big’ buzz. The frequency of the both kinds of buzz oscillations is found to be super‐harmonic of the fundamental acoustic frequency of intake modeled as an open‐closed organ pipe. The frequency as well as amplitudes of the big buzz cycles is larger than those of the little buzz. The little‐ and big‐buzz are found to occur for low‐ and high‐subcritical state of the intake and are very similar to Ferri and Dailey criteria, respectively. Buzz is eliminated when relatively high bleed is implemented, both, upstream and downstream of the throat. The effect of rate of change of back‐pressure on the start/unstart of the intake is investigated. Two situations are considered. The first case is that of an intake where the back‐pressure remains below the critical value. It is found that the intake remains started if the change in back‐pressure is gradual. However, it unstarts if the back‐pressure is changed relatively rapidly. The second set of simulations is an attempt to model the situation where the back‐pressure at the exit of the intake exceeds the critical value and a logic is incorporated in the feed back loop of the fuel modulation to start the intake. Low rate of change of pressure is unsuccessful in starting the intake. Relatively high rates result in either a quick starting of the intake or a slow unstart. Copyright © 2010 John Wiley & Sons, Ltd.     

Formation of Concentrated Nanoemulsions by Extreme Shear

Abstract

We have systematically investigated the production of “nanoemulsions,” droplets of one liquid phase in another immiscible liquid phase that have diameters less than 100 nm. Our approach relies on a combination of extreme shear due to multipass, high‐pressure microfluidic injection and systematic control of the emulsion's composition. By repeatedly shearing a silicone oil‐in‐water emulsion in an inhomogeneous extensional shear flow, the multipass approach enables us to reduce the droplet polydispersity and average radius. Using dynamic light scattering, we study the changes in the average radius, ?a?, as a function of the number of passes, driving injection pressure (i.e., shear rate), droplet volume fraction, surfactant concentration, and droplet oil viscosity. The smallest nanoemulsion that we obtain has ?a?=18 nm. At large droplet volume fractions φ≥0.65, we observe phase inversion, rather than a reduction in the droplet size. This provides evidence that droplet coalescence can occur during extreme shear, even when a significant excess of a strongly stabilizing surfactant is present.     

Bifurcation analysis of a diffusive predator–prey system with a herd behavior and quadratic mortality

In this paper, a diffusive predator–prey system, in which the prey species exhibits herd behavior and the predator species with quadratic mortality, has been studied. The stability of positive constant equilibrium, Hopf bifurcations, and diffusion‐driven Turing instability are investigated under the Neumann boundary condition. The explicit condition for the occurrence of the diffusion‐driven Turing instability is derived, which is determined by the relationship of the diffusion rates of two species. The formulas determining the direction and the stability of Hopf bifurcations depending on the parameters of the system are derived. Finally, numerical simulations are carried out to verify and extend the theoretical results and show the existence of spatially homogeneous periodic solutions and nonconstant steady states. Copyright © 2014 John Wiley & Sons, Ltd.     

Modeling and analysis of a marine plankton system with nutrient recycling and diffusion

This article describes a nutrient‐phytoplankton‐zooplankton system with nutrient recycling in the presence of toxicity. We have studied the dynamical behavior of the system with delayed nutrient recycling in the first part of the article. Uniform persistent of the system is examined. In the second part of the article, we have incorporated diffusion of the plankton population to the system and dynamical behavior of the system is analyzed with instantaneous nutrient recycling. The condition of the diffusion driven instability is obtained. The conditions for the occurrence of Hopf and Turing bifurcation critical line in a spatial domain are derived. Variation of the system with small periodicity of diffusive coefficient has been studied. Stability condition of the plankton system subject to the periodic diffusion coefficient of the zooplankton is derived. It is observed that nutrient‐phytoplankton‐zooplankton interactions are very complex and situation specific. Moreover, we have obtained different exciting results, ranging from stable situation to cyclic oscillatory behavior may occur under different favorable conditions, which may give some insights for predictive management. © 2014 Wiley Periodicals, Inc. Complexity 21: 229–241, 2015     

Numerical Experiments and Structural Transformation of NonLinear Evolution in Illposed Systems

Numerical experiments on nonlinear equations of the 1st and 3rdorder derivatives have been carried out through structural analyses in the phase space according to the numerical instability of illposed systems, with changes of initial values and parameters, etc.. The results show that the quantitative instability in an illposed system may reveal reversed transformation in system evolution by structural representation, and confirm A·Dauglas’ theorem that “a nonlinear equation does not satisfy the existence of the initial value in a linear wellposed system”.     

J-TEXT托卡马克高速单色成像系统研制

在J-TEXT托卡马克上研制了一套高速单色成像系统用于研究等离子体杂质行为与磁流体力学(MHD)不稳定性之间的关系.用STRAH代码模拟估算了碳杂质(CV227.09nm,CIII464.7nm)辐射强度.采用光纤耦合方法设计了系统光路结构,光路覆盖高场侧区域0.3a～0.95a(a为小半径),其空间分辨率为1.3cm...