RM不稳定过程中预混火焰界面演化及混合区增长预测

预混火焰界面的RM (Richtmyer-Meshkov)不稳定导致的界面混合区增长过程在自然界和工程实践中十分常见,但化学反应对其增长的影响机理仍不明确,反应性界面混合区增长速率的预测也未见报道, 因此,开展RM不稳定过程中火焰界面演化和混合区预测的研究十分必要.本文采用带单步化学反应的Navier-Stokes方程和高精度数值格式,研究了正弦形预混火焰界面在平面入射激波及其反射激波作用下的RM不稳定过程.结果表明, 在入射激波作用后的阶段,除RM不稳定本身导致的界面演化为"钉-帽"和"泡"形结构外,化学反应一方面以预混火焰传播的方式促进了界面中"泡"结构的增长,另一方面通过与涡结构的复杂相互作用促进了"钉-帽"结构的增长.化学反应活性越强, 火焰界面的"泡" 结构和"钉-帽"结构的增长越快.在第一次反射激波作用后的阶段,化学反应以相同的火焰传播方式对"泡"和"钉-帽"结构产生影响, 两者效应相抵,因而导致反射激波作用后的阶段中界面混合区增长不受化学反应活性的影响.根据以上分析,分别针对入射激波和第一次反射激波作用后的火焰界面混合区增长速率提出了相应的预测模型,为探索反应性RM不稳定过程的理论预测方法提供了有益参考.      

Dynamic analysis of the double crank mechanism with a 3D translational clearance joint employing a variable stiffness contact force model

Nonlinear Dynamics - Oblique collisions are more likely to happen in the realistic translational joint with clearance, compared to the full front impacts. It can be a quite demanding task to...     

受限空间内甲醇喷雾液滴形成及其爆炸特性

为防控工业喷雾爆炸和完善喷雾爆炸测试方法,在20 L球形喷雾爆炸测试系统内，实验研究了不同环境压力、喷射压力及浓度下的甲醇喷雾液滴形成及爆炸特性规律。结果表明:增大喷射压力更易致使甲醇破碎成微小液滴，甲醇喷雾液滴爆炸极限范围变宽；环境压力的增大导致甲醇喷雾液滴粒径变大，喷雾液滴爆炸极限范围变窄，一定程度上可以有效抑制甲醇泄露可能导致的次生衍生事故发生。当爆炸容器内环境压力为0.1 MPa、喷射压力为2.1 MPa、甲醇喷雾浓度为356.4 g/m3、甲醇液滴索太尔平均直径为2.5 μm时，爆炸特性参数（最大爆炸压力、最大爆炸压力上升速率及层流燃烧速度）在上述拐点处取得最大值；小粒径（1～15 μm）的液滴在外界能量作用下，更易被点燃，且爆炸过程中瞬态物理化学反应更为迅速和剧烈；较大粒径(22 μm以上)的液滴会出现点火困难现象，然而点火成功后，爆炸特性参数均随甲醇喷雾浓度增加而增加，呈现近似线性规律，此时液滴粒径对上述爆炸特性参数的影响可以忽略。研究结果有助于理解喷雾液滴爆炸规律、完善相应测试方法和安全设计。     

Sampled-data output voltage regulation for a DC–DC buck converter nonlinear system with actuator and sensor failures

Nonlinear Dynamics - This paper considers the distributed adaptive neural consensus tracking control problem for a class of uncertain nonaffine nonlinear multi-agent systems. By making use of the...     

A new adaptive approach of the Metropolis-Hastings algorithm applied to structural damage identification using time domain data

In the present work, the formulation and solution of the inverse problem of structural damage identification is presented based on the Bayesian inference, a powerful approach that has been widely used for the formulation of inverse problems in a statistical framework. The structural damage is continuously described by a cohesion field, which is spatially discretized by the finite element method, and the solution of the inverse problem of damage identification, from the Bayesian point of view, is the posterior probability densities of the nodal cohesion parameters. In this approach, prior information about the parameters of interest and the quantification of the uncertainties related to the magnitudes measured can be used to estimate the sought parameters. Markov Chain Monte Carlo (MCMC) method, implemented via the Metropolis-Hastings (MH) algorithm, is commonly used to sample such densities. However, the conventional MH algorithm may present some difficulties, for instance, in high dimensional problems or when the parameters of interest are highly correlated or the posterior probability density is very peaked. In order to overcome these difficulties, a new adaptive MH algorithm (P-AMH) is proposed in the present work. Numerical results related to an inverse problem of damage identification in a simply supported Euler-Bernoulli beam are presented. Synthetic experimental time domain data, obtained with different damage scenarios, and noise levels, were addressed with the aim at assessing the proposed damage identification approach. An adaptive MH algorithm (H-AMH) and the conventional MH algorithm, already consolidated in the literature, were also considered for comparison purposes. The numerical results show that both adaptive algorithms outperformed the conventional MH. Besides, the P-AMH provided Markov chains with faster convergence and better mixing than the ones provided by the H-AMH.     

A GPU-based algorithm for efficient LES of high Reynolds number flows in heterogeneous CPU/GPU supercomputers

Αn optimized MPI+OpenACC implementation model that performs efficiently in CPU/GPU systems using large-eddy simulation is presented. The code was validated for the simulation of wave boundary-layer flows against numerical and experimental data in the literature. A direct Fast-Fourier-Transform-based solver was developed for the solution of the Poisson equation for pressure taking advantage of the periodic boundary conditions. This solver was optimized for parallel execution in CPUs and outperforms by 10 times in computational time a typical iterative preconditioned conjugate gradient solver in GPUs. In terms of parallel performance, an overlapping strategy was developed to reduce the overhead of performing MPI communications using GPUs. As a result, the weak scaling of the algorithm was improved up to 30%. Finally, a large-scale simulation (Re = 2 × 10⁵) using a grid of 4 × 10⁸ cells was executed, and the performance of the code was analyzed. The simulation was launched using up to 512 nodes (512 GPUs + 6144 CPU-cores) on one of the current top 10 supercomputers of the world (Piz Daint). A comparison of the overall computational time showed that the GPU version was 4.2 times faster than the CPU one. The parallel efficiency of this strategy (47%) is competitive compared with the state-of-the-art CPU implementations, and it has the potential to take advantage of modern supercomputing capabilities.     

Two-phase flow simulation for distinguishing deformable particles with a LiMCA system

In the metallurgical industry, Liquid Metal Cleanliness Analyser (LiMCA) commercial equipment cannot distinguish between hard particles (e.g., oxides, borides) and deformable particles (e.g., bubbles, molten salts). Therefore, hard particle concentrations can sometimes be grossly overestimated, which reduces the measurement accuracy. However, the method could potentially discriminate between deformable particles and hard particles by evaluating the particle's ability to deform. In this work, the coupled multiphysics problem of a particle deforming within current-carrying aluminium metal passing through the electric sensing zone (ESZ) is simulated using the conservative level-set (CLS) method. An emphasis is placed on understanding the transient deformation history, and the effect of the capillary number, Reynolds number, and confinement ratio on deformation are studied. Furthermore, a computational basis is given to estimate the influence of particle deformation on electrical resistance pulses (ERP). It is found that ERP features of deformation particles, including the peak magnitude and the pulse width, are different from those of hard particles. Based on the results, the effect of a particle's deformation and the feasibility to discriminate it from non-deformable particles in the LiMCA system is evaluated.     

离子浓度及表面结构对岩石孔隙内水流动特性的影响

酸性环境引发的岩石孔隙表面溶解增加了孔隙内水溶液的盐离子浓度,破坏了孔隙的表面结构.本文采用分子动力学模拟的方法研究了纳米级岩石孔隙内水溶液的流动特性,分析了盐离子浓度和孔隙表面结构对水流速度分布的影响及原因.研究结果表明:纳米级岩石孔隙内的水溶液流动符合泊肃叶流动特性,流速呈"抛物线"分布;随盐离子浓度增加,水溶液内部氢键网络变得更为致密,水黏度随其呈线性增长;水溶液中离子浓度越大,孔隙表面对水流动的阻力越大,最大流速越小,速度分布的"抛物线"曲率半径越大;岩石孔隙表面结构的破坏改变了流动表面的粗糙程度,增加了孔隙表面对H2O分子的吸引力.随表面结构破坏程度的增大,水溶液在近壁区域的密度增大,流速降低;当表面破坏程度达到50%时,水溶液在近壁区域出现了明显的负边界滑移现象.     

The symmetry and loading-independency of multiple inclusions enclosing uniform stresses in an infinite elastic plane

The identification of multiple interacting inclusions with uniform internal stresses in an infinite elastic matrix subjected to a uniform remote loading is of fundamental importance in the mechanics and design of particulate composite materials. In anti-plane shear and plane deformations, certain sufficient conditions have been established in the literature which guarantee uniform internal stresses inside multiple interacting inclusions displaying various symmetries when the matrix is subjected to specific uniform remote loading. Correspondingly, sufficient conditions which allow for the design of multiple interacting inclusions independent of any specific form of (uniform) remote loading have also been established. In this paper, we demonstrate rigorously that, in all cases, these sufficient conditions are also necessary conditions and indeed allow for the identification of all possible collections of such inclusions.     

Modelling and dynamic analysis of spline-connected multi-span rotor system

Meccanica - Self-excited oscillation induced by the internal frictions of spline joints is a major cause of rotor system instability. In this study, the dynamic equations of spline-connected...