A high‐resolution scheme for low Mach number flows

A method for computing low Mach number flows using high‐resolution interpolation and difference formulas, within the framework of the Marker and Cell (MAC) scheme, is presented. This increases the range of wavenumbers that are properly resolved on a given grid so that a sufficiently accurate solution can be obtained without extensive grid refinement. Results using this scheme are presented for three problems. The first is the two‐dimensional Taylor–Green flow which has a closed form solution. The second is the evolution of perturbations to constant‐density, plane channel flow for which linear stability solutions are known. The third is the oscillatory instability of a variable density plane jet. In this case, unless the sharp density gradients are resolved, the calculations would breakdown. Under‐resolved calculations gave solutions containing vortices which grew in place rather than being convected out. With the present scheme, regular oscillations of this instability were obtained and vortices were convected out regularly. Stable computations were possible over a wider range of sensitive parameters such as density ratio and co‐flow velocity ratio. Copyright © 2004 John Wiley Sons, Ltd.     

Diffraction of normal shock wave by yawed wedges

The diffraction of normal shock with yawed wedges of small angles have been considered in this paper. Vorticity distribution of the fluid particle over the diffracted shock has been determined for several Mach numbers of the shock wave. The Mach reflection effects have also been investigated when the bend is concave to the flow.     

Low Mach number limit of inviscid Hookean elastodynamics

The low Mach number limit of inviscid Hookean elastodynamic equations is rigorously proved in bounded domain, whole space and periodic domain, respectively. The uniform existence of smooth solutions and convergence results as the Mach number tends to zero are obtained in three different domains.     

马赫杆加载下无氧铜的动态破碎

为了深入了解金属材料在复杂加载下的动态破碎行为,在有限元模拟的基础上,设计了两类马赫杆加载实验,用于研究无氧铜在复杂加载下的动态破碎行为。实验中,采用火炮加载马赫透镜和激光粒子速度干涉仪测量自由面速度,实现了峰值压力分别为95.75和32.38 GPa的动态加载。结果表明,实验中成功实现了稳定的马赫杆加载,并且观察到马赫杆加载下无氧铜的2种不同近表面破碎模式,即高压下产生微层裂、低压下产生三角波层裂,且层裂区呈凸形分布。

     

Refractometer based on a tapered Mach–Zehnder interferometer with Peanut-Shape structure

A novel refractometer based on tapered Mach–Zehnder modal interferometer (MZI) is proposed and experimentally demonstrated. This sensor is composed of a pair of Peanut-Shape structures and an embedded taper – the former excites high-order cladding modes, while the latter enhances the evanescent field. As the effective refractive index (RI) of cladding is based on the changes of surrounding RI, thus extinction ratio will change due to the alteration of the distribution of power in the fiber which is induced by various differences of core and cladding for RI. As a result, the maximum RI sensitivity of 240.78 extinction ratio/RIU (refractive index unit) is achieved within the range from 1.3334 to 1.4081.     

Anisotropic sound and shock waves in dipolar Bose-Einstein condensate

We study the propagation of anisotropic sound and shock waves in dipolar Bose-Einstein condensate in three dimensions (3D) as well as in quasi-two (2D, disk shape) and quasi-one (1D, cigar shape) dimensions using the mean-field approach. In 3D, the propagation of sound and shock waves are distinct in directions parallel and perpendicular to dipole axis with the appearance of instability above a critical value corresponding to attraction. Similar instability appears in 1D and not in 2D. The numerical anisotropic Mach angle agrees with theoretical prediction. The numerical sound velocity in all cases agrees with that calculated from Bogoliubov theory. A movie of the anisotropic wave propagation in a dipolar condensate is made available as supplementary material.     

Accuracy of high‐order density‐based compressible methods in low Mach vortical flows

At low Mach numbers, Godunov‐type approaches, based on the method of lines, suffer from an accuracy problem. This paper shows the importance of using the low Mach number correction in Godunov‐type methods for simulations involving low Mach numbers by utilising a new, well‐posed, two‐dimensional, two‐mode Kelvin–Helmholtz test case. Four independent codes have been used, enabling the examination of several numerical schemes. The second‐order and fifth‐order accurate Godunov‐type methods show that the vortex‐pairing process can be captured on a low resolution with the low Mach number correction applied down to 0.002. The results are compared without the low Mach number correction and also three other methods, a Lagrange‐remap method, a fifth‐order accurate in space and time finite difference type method based on the wave propagation algorithm, and fifth‐order spatial and third‐order temporal accurate finite volume Monotone Upwind Scheme for Conservation Laws (MUSCL) approach based on the Godunov method and Simple Low Dissipation Advection Upstream Splitting Method (SLAU) numerical flux with low Mach capture property. The ability of the compressible flow solver of the commercial software, ANSYS FLUENT , in solving low Mach flows is also demonstrated for the two time‐stepping methods provided in the compressible flow solver, implicit and explicit. Results demonstrate clearly that a low Mach correction is required for all algorithms except the Lagrange‐remap approach, where dissipation is independent of Mach number. © 2013 Crown copyright. International Journal for Numerical Methods in Fluids. © 2013 John Wiley & Sons, Ltd.     

广义相对论的几个问题

介绍了爱因斯坦如何建立广义相对论,以及人们应该如何理解这一关于时间、空间和引力的理论.     

基于马赫-曾德尔干涉仪的单个光子操控

分别利用空间和光纤马赫—曾德尔干涉仪对空间和光纤传输中的单个光子的干涉现象进行了研究,干涉对比度可达到90％以上,实现了空间和光纤中的单个光子的路由操控。实验采用脉冲调制加衰减的方法产生单个光子,获得了每个脉冲中只包含0．1个光子的准单光子源。通过改变压电换能器的电压控制马赫—曾德尔干涉仪的单个光子在两个输出端的选择,实现了光子在节点上的路由。采用同步符合检测技术,利用重复频率为1kHz,脉冲宽度为100ns的同步信号对输出信号进行符合,实现了量子效率高于70％,暗计数小于0．2s^-1的单光子高灵敏度检测,观测到了单个光子在相位操控下出射到两个输出端的有序分配现象。验证了基于马赫—曾德尔干涉仪的单个光子路由操控实现的可能性。