Variational integrators for electric circuits

Variational integrators are symplectic-momentum preserving integrators that are based on a discrete variational formulation of the underlying system. So far, variational integrators have been mainly developed and used for a wide variety of mechanical systems. In this work, we develop a variational integrator for the simulation of electric circuits. An appropriate variational formulation is presented to model the circuit from which the equations of motion are derived. Finally, a corresponding time-discrete variational formulation provides an iteration scheme for the simulation of the electric circuit. In this way, a variational integrator is constructed that gains several advantages. A comparison to standard integration techniques shows that even for simple LCR circuits a better long-time energy behavior and frequency preservation can be obtained. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

用冲击波应力历史曲线确定材料物态方程参数

提出一种确定材料物态方程参数的新方法，即从一次平面碰撞实验中单个拉格朗日量计记录的应力历史曲线出发，建立非线性优化模型，进而确定材料的物态方程参数。计算结果表明了这种方法的有效性。还考察了权重的选取对于最后优化结果的影响。     

Approximation of the effective conductivity of ergodic media by periodization

 This paper is concerned with the approximation of the effective conductivity σ(A, μ) associated to an elliptic operator ∇ _xA (x,η)∇ _x where for xℝ ^d , d≥1, A(x,η) is a bounded elliptic random symmetric d×d matrix and η takes value in an ergodic probability space (X, μ). Writing A ^N (x, η) the periodization of A(x, η) on the torus T ^d _N of dimension d and side N we prove that for μ-almost all η

A subharmonic dynamical bifurcation during in vitro epileptiform activity

Epileptic seizures are considered to result from a sudden change in the synchronization of firing neurons in brain neural networks. We have used an in vitro model of status epilepticus (SE) to characterize dynamical regimes underlying the observed seizure-like activity. Time intervals between spikes or bursts were used as the variable to construct first-return interpeak or interburst interval plots, for studying neuronal population activity during the transition to seizure, as well as within seizures. Return maps constructed for a brief epoch before seizures were used for approximating the local system dynamics during that time window. Analysis of the first-return maps suggests that intermittency is a dynamical regime underlying the observed epileptic activity. This type of analysis may be useful for understanding the collective dynamics of neuronal populations in the normal and pathological brain.     

A New Algorithm for Digital Image Encryption Based on Chaos Theory

In recent decades, image encryption, as one of the significant information security fields, has attracted many researchers and scientists. However, several studies have been performed with different methods, and novel and useful algorithms have been suggested to improve secure image encryption schemes. Nowadays, chaotic methods have been found in diverse fields, such as the design of cryptosystems and image encryption. Chaotic methods-based digital image encryptions are a novel image encryption method. This technique uses random chaos sequences for encrypting images, and it is a highly-secured and fast method for image encryption. Limited accuracy is one of the disadvantages of this technique. This paper researches the chaos sequence and wavelet transform value to find gaps. Thus, a novel technique was proposed for digital image encryption and improved previous algorithms. The technique is run in MATLAB, and a comparison is made in terms of various performance metrics such as the Number of Pixels Change Rate (NPCR), Peak Signal to Noise Ratio (PSNR), Correlation coefficient, and Unified Average Changing Intensity (UACI). The simulation and theoretical analysis indicate the proposed scheme’s effectiveness and show that this technique is a suitable choice for actual image encryption.     

Computational simulation of variable magnetic force on heat characteristics of backward-facing step flow

Journal of Thermal Analysis and Calorimetry - The application of the nanofluid is highly increased due to its high heat rate in the attendance of the magnetic field. In current investigation, CFD...     

Expanding scissor-based UGV for large obstacles climbing

This research proposes a new extendable platform for an unmanned ground vehicle to overcome the obstacle climbing issue. The new platform is basically established on scissor mechanism principles which have been innovated to achieve long and rigid displacement. A couple of scissor mechanisms are embedded in the rover platform adjusting the mass center of rover respect to the rear and front wheels. Accordingly, it yields geometric control of the contact forces, which can simultaneously reduce the slip of the wheels and increase the performance of the obstacle climbing up. To demonstrate the performance of the proposed platform, the 3D kinematics is derived. Subsequently, the stick-slip Euler-Lagrange dynamics is derived and a three-level controller including the torque optimization is implemented to simulate the rover facing obstacles. Finally, without any hardware prototyping, the extendable rover is simulated and compared with a typical fixed-geometry rover to show the enhancement of the climbing ability by using the proposed concept. Moreover, controlling the normal contact forces of the wheels yields the slip reduction, which subsequently, increases the traction force.     

不同气流环境下氟化氘激光对45#钢靶的辐照效应

通过表面形貌观察、温度场分析,研究了切向空气气流、切向氮气气流、自然对流3种环境下氟化氘（DF）激光对45#钢靶的辐照效应,结果表明：切向空气气流环境下,钢靶烧蚀效果最显著,靶板后表面中心温升最高;切向氮气气流环境下,钢靶有一定的烧蚀,但温升最低;自然对流环境下,烧蚀效果最差。实验结果表明：切向气流可移除部分熔化物,特别在切向空气气流环境下剧烈的氧化反应可促进钢靶温度升高,显著增强激光对钢靶的烧蚀,停止激光辐照后切向气流的冷却效应起主要作用。根据实际物理问题建立了相应的数值计算模型,模拟了不同气流环境下激光对钢靶的辐照效应,其中,利用生死单元的方法,模拟了切向空气气流环境下激光对钢靶的烧蚀,并考虑了氧化放热的影响。模拟结果与实验结果基本相符,解释了气流在激光辐照效应中的作用。     

Dynamic half-Fourier single-shot turbo spin echo for assessment of deep venous thrombosis: initial observations

Objective

The objective of this study was to retrospectively analyze the value of dynamic half-Fourier single-shot turbo spin echo (HASTE) imaging in patients with suspected deep venous thrombosis (DVT).

Materials and Methods

Fifty-five veins in 24 patients were interrogated using a HASTE sequence with the patients relaxed and in various degrees of Valsalva. Veins were analyzed for changes in caliber (+CAL) and signal intensity (+SI) or in their absence (−CAL and −SI, respectively) and compared with the presence of thrombus on gadolinium-enhanced magnetic resonance imaging.

Results

There was no thrombus in veins with the +CAL, +SI pattern (n=40) (P<.01). Five of seven veins (71.4%) with the −CAL, −SI pattern had thrombus (P<.01). A qualitative change in CAL had a sensitivity of 100% and a specificity of 91% for the presence of thrombus. An increase of 1.5 mm in CAL had a sensitivity of 100% and a specificity of 93% for this diagnosis.

Conclusion

Dynamic HASTE imaging offers a physiological method to evaluate veins for deep venous thrombosis.     

A cutoff phenomenon in accelerated stochastic simulations of chemical kinetics via flow averaging (FLAVOR-SSA)

We present a simple algorithm for the simulation of stiff, discrete-space, continuous-time Markov processes. The algorithm is based on the concept of flow averaging for the integration of stiff ordinary and stochastic differential equations and ultimately leads to a straightforward variation of the the well-known stochastic simulation algorithm (SSA). The speedup that can be achieved by the present algorithm [flow averaging integrator SSA (FLAVOR-SSA)] over the classical SSA comes naturally at the expense of its accuracy. The error of the proposed method exhibits a cutoff phenomenon as a function of its speed-up, allowing for optimal tuning. Two numerical examples from chemical kinetics are provided to illustrate the efficiency of the method.