Radiative Instability of an Unbounded Jet Flow

The problem of the linear dynamics of disturbances of an unbounded jet flow with a piecewise linear velocity profile is considered. The stable disturbances of a flow in an incompressible medium are so-called flow waves localized near a vorticity jump. In this work, it is shown that the amplitudes of these waves slowly increase in a compressible medium due to acoustic radiation; i.e., an instability appears. The asymptotic solution to the problem for small Mach numbers is represented in terms of Airy functions. An analytic expression for the growth increment of the disturbances is obtained.     

Bifurcation Instability of sheet metal during spring-back

In automotive and home appliance industries, there are many complex-shaped sheet metal components which need to be fabricated in multiple stamping operations. For example, the manufacturing of an outer case of washing machine consists of stamping followed by a bending operation. After the first stage of the stamping process, a large amount of spring-back takes place, and therefore, it is difficult to proceed to the next stage of the bending process. In the stamping process of that kind of sheet component with low geometric constraint, the forming area is large compared to the forming depth. Therefore, the formed part is in an unstable state and is less geometrically constrained, which causes a large amount of spring-back. To investigate this phenomenon, finite element analyses are carried out. During a spring-back analysis after forming, bifurcation takes place and the finite element solution procedure using the Newton–Raphson scheme becomes unstable. To get a stable post-bifurcation solution, a bifurcation algorithm is introduced at the bifurcation point. The deformed shapes obtained from finite element analyses are in good agreement with the experimental data. From this study, it is shown that the bifurcation behaviour enlarges the spring-back and the degree of dimensional error. To obtain additional possible post-bifurcation solutions, non-bifurcation analyses using initial guesses obtained in a modal analysis are carried. For the initial guesses, lowed four eigenmodes are utilized. Finally, the post-bifurcation behaviour and spring-back amount are investigated for various process parameters including the forming depth, punch width and corner radius.     

Adaptive Radiative Thermal Camouflage via Synchronous Heat Conduction

The advent of transformation thermotics has seen a boom in development of thermal metamaterials with a variety of thermal functionalities,including phenomena such as thermal cloaking and camouflage.However,most thermal metamaterials-based camouflage devices only tune in-plane heat conduction,which may fail to conceal a target from out-of-plane detection.We propose an adaptive radiative thermal camouflage via tuning out-ofplane transient heat conduction,and it is validated by both simulation and experiment.The physics underlying the performance of our adaptive thermal camouflage is based on real-time synchronous heat conduction through the camouflage device and the background plate,respectively.The proposed concept and device represent a promising new approach to fabrication of conductive thermal metamaterials,providing a feasible and effective way to achieve adaptive thermal camouflage.     

φ36的媒质效应和热不稳定性

在有限温度下运用硬热圈近似求得φ36理论单圈自能,进而求得有限质量和衰变率. 发现在临界温度Tc以上存在热不稳定性,而有效温度和衰变率则分别正比于gT和gT. 最后将所得结果与热QCD比较.     

热晕小尺度不稳定性的研究

报道了在实验条件下热晕小尺度不稳定性的研究结果，从实验及数值模拟两方面给出了光束质量Ｓｔｒｅｈｌ比，扰动积分能量的增长，各扰动谱项的能量竞争以及近场光斑半径随时间的变化。研究结果基本反映了热晕小尺度不稳定性的物理规律，实验测量与数值的结果具有较好的一致性。     

Thermal Instability of Non-Ideal Current-Carrying Plasmas of Metal Vapors

As a result of Ohmic heating the transition proceeds from the liquid metal state to the state of the gaseous plasma. The conductivity drops rapidly. This paper reports on the thermal instability of such a discharge. The instability results in the plasma stratification across the current direction. The discussion is given of the experiment by DIKHTER and ZEIGARNIK. The discharge is layered under the isobaric heating of cesium wire in the argon atmosphere.     

Dynamics of the Thermal Instability Evolution in Dielectric Breakdown

A mathematical model of the dynamics of temperature, electric field strength, and charge density in thermal dielectric breakdown is examined. The characteristics of the evolution of a thermal instability initiated by a local temperature disturbance are studied by numerical modeling. The conditions of initiation and growth of an electrothermal structure resulting in the formation of a highly conductive channel and shunting of the dielectrics current are identified.     

微光栅对SiC/光子晶体结构热辐射特性的调控

本文研究了一种SiC薄膜/光子晶体结构的热辐射特性,在SiC薄膜/光子晶体的界面处具有微光栅结构。利用RCWA方法数值模拟了该结构的热辐射特性,发现在SiC薄膜/光子晶体的界面处由于不同的电磁模态激发,导致其发射率在多个频段或方向具有很高的值,从而可通过利用不同电磁模态激发的条件及其相互作用实现对热辐射的调控。     

Transition to Longitudinal Instability of Detonation Waves is Generically Associated with Hopf Bifurcation to Time-Periodic Galloping Solutions

We show that transition to longitudinal instability of strong detonation solutions of reactive compressible Navier–Stokes equations is generically associated with Hopf bifurcation to nearby time-periodic “galloping”, or “pulsating”, solutions, in agreement with physical and numerical observation. In the process, we determine readily numerically verifiable stability and bifurcation conditions in terms of an associated Evans function, and obtain the first complete nonlinear stability result for strong detonations of the reacting Navier–Stokes equations, in the limit as amplitude (hence also heat release) goes to zero. The analysis is by pointwise semigroup techniques introduced by the authors and collaborators in previous works.     

Gravitational Instability of Fluid of Finite Electrical and Thermal Conductivity Flowing through Porous Medium

In view of the importance of porosity in astrophysical context the problem of gravitational instability of fluid of finite electrical and thermal conductivity flowing through porous medium is studied. Equations of the problem are stated and from them linearized perturbation equations are derived. Dispersion relations are obtained and Jeans' criterion of instability is discussed taking properties of the medium in different combinations for parallel and perpendicular directions to the magnetic field. Thermal conductivity modifies the Jeans' expression. Magnetic field and porosity of the medium also modify the Jeans' expression in case of wave propagation in perpendicular direction to the magnetic field but finite electrical conductivity neutralizes the effect of porosity on Jeans' criterion of instability.     

Electroosmosis-Optimized Thermal Model for Peristaltic Transportation of Thermally Radiative Magnetized Liquid with Nonlinear Convection

The present study addresses the heat transfer efficiency and entropy production of electrically conducting kerosene-based liquid led by the combined impact of electroosmosis and peristalsis mechanisms. Effects of nonlinear mixed convection heat transfer, temperature-dependent viscosity, radiative heat flux, electric and magnetic fields, porous medium, heat sink/source, viscous dissipation, and Joule heating are presented. The Debye–Huckel linearization approximation is employed in the electrohydrodynamic problem. Mathematical modeling is conducted within the limitations of δ << 1 and Re → 0. Coupled differential equations after implementing a lubrication approach are numerically solved. The essential characteristics of the production of entropy, the factors influencing it, and the characteristics of heat and fluid in relation to various physical parameters are graphically evaluated by assigning them a growing list of numeric values. This analysis reveals that heat transfer enhances by enhancing nonlinear convection and Joule heating parameters. The irreversibility analysis ensures that the minimization of entropy generation is observed when the parameters of viscosity and radiation are held under control. Fluid velocity can be regulated by adjusting the Helmholtz–Smoluchowski velocity and magnetic field strength.     

Spectral and Non Radiative Decay Studies of Lead Di Bromide Single Crystals by Mode Matched Thermal Lens Technique

In the present paper, the investigations on the non radiative decay mechanism, optical band gap determination from absorption spectroscopic studies and fluorescence emission by photo luminescence techniques using different excitation wavelengths on gel derived lead di bromide single crystals are reported. Non radiative decay of the sample is studied using high sensitive dual beam mode matched thermal lens technique. For the thermal lensing experiment the crystal in solution phase is incorporated with rhodamine 6G dye for enhancing the absorption of the crystal sample. The thermal diffusivity of lead di bromide is determined using the probe beam intensity v/s time measurements.     

PIC Simulation of Current‐Driven Buneman Instability in Presence of Collisional and Thermal Effects

Collisional and thermal effects on the nonlinear evolution of Buneman instability in an unmagnetized current‐driven plasma are investigated by particle in cell simulations. These simulations show that, as the time passes, the electron distribution function profiles deviate from initial shifted Maxwellian distribution and the electron phase‐space holes can be formed. The electron distribution function profiles also indicate the counter‐streaming and plateau formation. Moreover, the contour of the electric field and profiles of the growth rate display the resonance condition for this instability. Finally, the time evolution of the electric field energy density and ion kinetic energy in the presence of collisional and thermal effects are presented. These evolutions illustrate that in the presence of collisions, the growth rate of the Buneman instability is smaller than the collisionless case. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

第一性原理计算高压下金红石TiO2的结构不稳定性及热学性质

采用第一性原理计算研究了金红石TiO₂结构在高压下的不稳定性及热力学性质. 计算的高压下结构参数和零压的声子色散曲线与实验数据十分吻合. 进一步模拟了在不同压力下的声子曲线,在压力下,声子曲线不断软化,?点附近的振动频率不断减小直至虚频,意味着结构的不稳定,根据计算的不同压力下的弹性常数获得了其力学不稳定性,结果表明金红石结构TiO₂在压力高于17.7 GPa时变得不稳定. 根据准谐近似,获得了金红石TiO₂结构的热力学性质,计算结果与现有     

Effects of Perpendicular Thermal Velocities on the Transverse Instability in Electron Phase Space Holes

A multi-dimensional electron phase-space hole （electron hole） is considered to be unstable to the transverse instability. We perform two-dimensional （2D） particle-in-cell （PIC） simulations to study the evolutions of electron holes in weakly magnetized plasma （Ωe〈 Wpe, where Ωe and Wpe are the electron gyrofrequency and plasma frequency, respectively）, and the effects of perpendicular thermal velocities on the transverse instability are investigated. The transverse instability can cause decay of the electron holes. We find that with the increasing perpendicular thermal velocity tending to stabilize the transverse instability, the corresponding wave numbers decrease.     

Radiative reaction and quantum mechanics

According to the principles of quantum mechanics, the classical Lorentz-Dirac equations of the electron should follow from quantum electrodynamics in the classical limit. We show this is indeed true for the special case in which the charge does not radiate, provided the momentum operators in the Dirac theory are identified, in the classical limit, with the effective momenta of the Lorentz-Dirac equations.     

Radiative pion-proton scattering

We present the results of a non-relativistic calculation of the π^± proton bremsstrahlung cross section at the pion kinetic energy of 298 MeV for backward photon angles. The pion-nucleon interaction is given by models that are based on the p-wave Chew-Low theory. An interaction current is included in an attempt to make the overall bremsstrahlung amplitude gauge-invariant. The predicted cross sections show little of the expected resonance, and are in fair agreement with the data. We have also calculated the cross sections at other kinetic energies, and we have studied effects of the off-mass-shell electromagnetic vertex.