Molecular dynamics, smoothed-particle applied mechanics, and irreversibility

We explore the relationship of Monaghan's version of “smoothed-particle hydrodynamics,” here called “smoothed-particle applied mechanics,” to nonequilibrium molecular dynamics. We first use smoothed particles to model the simplest possible linear transport problems, as well as a liquid-drop problem. We then consider both gas-phase and dense-fluid versions of Rayleigh-Bénard convection, all in two space dimensions. We also discuss the possibility of combining the microscopic and macroscopic techniques in a hybrid scheme well-suited to the massively-parallel modelling of large-scale nonequilibrium flows.     

RoboNetSim: An integrated framework for multi-robot and network simulation

In networked multi-robot systems, communication plays a major role defining system’s dynamics and performance. Unfortunately, existing multi-robot simulators do not provide advanced communication models. Therefore, given the intrinsic unreliability of wireless communications, significant differences might be observed between simulation and real-world results.Addressing these issues, we present RoboNetSim, an integrated simulation framework for communication-realistic simulation of networked multi-robot systems. RoboNetSim integrates multi-robot simulators with network simulators. We present two model implementations based on ARGoS at the robotic side, and NS-2 and NS-3 as network simulators. We evaluate the framework in terms of accuracy and computational performance, showing that it can efficiently simulate systems consisting of hundreds of robots.Using the Stage simulator as an example, we also show the integration of a robotic simulator with RoboNetSim by only adapting robot controllers, without the need to adapt the general code of the simulator.Finally, we demonstrate the effects of communication on mobile multi-robot systems. We consider two different case studies: a distributed coordination and task assignment scenario, and a coordinated mobility scenario. We compare realistic network simulation with simplified communication models and algorithms, and we study the resulting behavior and performance of the multi-robot system and the impact of different parameters.     

Cache-based high-level simulation of microthreaded many-core architectures

The accuracy of simulated cycles in high-level simulators is generally less than the accuracy in detailed simulators for a single-core systems, because high-level simulators simulate the behaviour of components rather than the components themselves as in detailed simulators. The simulation problem becomes more challenging when simulating many-core systems, where many cores are executing instructions concurrently. In these systems data may be accessed from multiple caches and the abstraction of the instruction execution has to consider the dynamic resource sharing on the whole chip. The problem becomes even more challenging in microthreaded many-core systems, because there may exist concurrent hardware threads. Which means that the latency of long latency operations can be tolerated from many cycles to just few cycles. We have previously presented a simulation technique to improve the accuracy in high-level simulation of microthreaded many-core systems, known as Signature-based high- level simulator, which adapts the throughput of the program based on the type of instructions, number of instructions and number of active threads in the pipeline. However, it disregards the access to different levels of the caches on the many-core system. Accessing L1-cache has far less latency than accessing off-chip memory and if the core is not able to tolerate latency, different levels of caches can not be treated equally. The distributed cache network along with the synchronization-aware coherency protocol in the Microgrid is a complicated memory architecture and it is difficult to simulate its behaviour at a high-level. In this article we present a high-level cache model, which aims to improve the accuracy in high-level simulators for general-purpose many-core systems by adding little complexity to the simulator and without affecting the simulation speed.     

Mixing It Up

Micro's editor in chief introduces the topics covered by the four articles in this general-interest issue: an interconnection network using highly integrated photonic technology; the ManySim simulation framework for future large-scale chip-multiprocessors; the SimWattch simulation framework, which integrates the Simics functional simulator with the SimpleScalar/Wattch microarchitecture simulators; and self-configuring embedded systems.     

An integrated simulator for coupled domain problems in MEMS

An integrated circuit and microfluidic simulator is presented in this paper. It allows coupled simulation of flow, structure, thermal, and electrical domains. The overall architecture and various algorithms including the coupling of the circuit and microfluidic simulators are described. An application of the simulator is demonstrated for a controlled microliquid dosing system using detailed numerical models for the fluid field, a low-dimensional model for the flow sensor, and circuit elements for the electronic control. Unlike other simulators which employ mostly lumped models for the various components, we formulate here a methodology for distributed systems     

Dynamical properties of hybrid systems simulators

This paper analyzes the dynamical properties of a class of hybrid systems simulators. A hybrid system is a dynamical system with a state that can both flow and jump. Its simulator attempts to generate its solutions approximately. The paper presents mild regularity conditions on the hybrid system and its simulator to guarantee that simulated solutions are close to actual solutions on compact (hybrid) time intervals, and that asymptotically stable compact sets are preserved, in a semiglobal practical sense, under simulation. In fact, it is established that asymptotically stable compact sets are continuous in the integration step size parameter of the simulator; that is, as the step size of the simulator converges to zero, the asymptotically stable set observed in simulations approaches the asymptotically stable compact set of the true hybrid system. Examples are used to illustrate concepts and results.     

网络混合仿真及其在NS-2下的实现

将流仿真与数据包仿真结合的混合仿真可以同时利用两种仿真技术的优势。该文在Vishal Misra的流模型的基础上提出了混合仿真模型的实现框架。在仿真模型基础上，基于数据包仿真工具NS-2实现了混合仿真。在混合模型仿真实现中，对需要包级网络性能参数的子网采用包仿真技术，其余部分采用流仿真，在得到具体包级信息的同时，大大改善了仿真效率。混合模型实现的关键在于解决包仿真和流仿真间的接口问题，文中给出了解决接口问题的具体方法。该文设计了一组试验对该混合模型进行验证，仿真结果表明模型既可以准确捕获TCP流的动态特性，又可以提高整个网络的仿真效率。     

Area aggregation and time-scale modeling for sparse nonlinear networks

Model reduction and aggregation are of key importance for simulation and analysis of large-scale systems, such as molecular dynamics, large swarms of robotic vehicles, and animal aggregations. We study a nonlinear network which exhibits areas of internally dense and externally sparse interconnections. The densely connected nodes in these areas synchronize in the fast time-scale, and behave as aggregate nodes that dominate the slow dynamics of the network. We first derive a singular perturbation model which makes this time-scale separation explicit and, next, prove the validity of the reduced-model approximation on the infinite time interval.     

JiST: an efficient approach to simulation using virtual machines

Discrete event simulators are important scientific tools and their efficient design and execution is the subject of much research. In this paper, we propose a new approach for constructing simulators that leverages virtual machines and combines advantages from the traditional systems‐based and language‐based simulator designs. We introduce JiST, a Java‐based simulation system that executes discrete event simulations both efficiently and transparently by embedding simulation semantics directly into the Java execution model. The system provides standard benefits that the modern Java runtime affords. In addition, JiST is efficient, out‐performing existing highly optimized simulation runtimes. As a case study, we illustrate the practicality of the JiST framework by applying it to the construction of SWANS, a scalable wireless ad hoc network simulator. We simulate million node wireless networks, which represents two orders of magnitude increase in scale over what existing simulators can achieve on equivalent hardware and at the same level of detail. Copyright © 2005 John Wiley & Sons, Ltd.     

Swing-up and stabilization of a cart–pendulum system under restricted cart track length

This paper describes the swing-up and stabilization of a cart–pendulum system with a restricted cart track length and restricted control force using generalized energy control methods. Starting from a pendant position, the pendulum is swung up to the upright unstable equilibrium configuration using energy control principles. An “energy well” is built within the cart track to prevent the cart from going outside the limited length. When sufficient energy is acquired by the pendulum, it goes into a “cruise” mode when the acquired energy is maintained. Finally, when the pendulum is close to the upright configuration, a stabilizing controller is activated around a linear zone about the upright configuration. The proposed scheme has worked well both in simulation and a practical setup and the conditions for stability have been derived using the multiple Lyapunov functions approach.     

Ghost Simulation Model for the Optimization of an Urban Subway System

The first part of the paper presents a model of a complex subway network that includes an operational cost and social costs measured in terms of passenger waiting times. We reformulate the model with a simple discrete event simulation model that considerably reduces the complexity of the simulation. The simplified model uses conditional expectations to filter out rapid dynamics, and it can be interpreted in terms of a subway network with “fluid” passenger levels. Because this network only sees train movements and no individual passengers are described, we call it the “ghost” model.In the second part of the paper, we explore the benefits of using stochastic approximations to adjust the service level (headway) of different subway lines as the network is operating, thus learning passenger traffic patterns and adaptively seeking the best service values. Our formulation of the ghost model is amenable for decentralized estimation of gradients of the cost function with respect to the control parameters (the line headways) and we use ersatz estimation methods to formulate a control scheme that uses minimal measurements and virtually no overhead.     

Trace生成对大规模并行性能模拟的影响及其改进策略

Trace生成是trace驱动体系结构模拟中不可或缺的步骤。Trace不仅需要占用大量存储空间,其生成过程还可能对目标应用程序的模拟执行产生一定程度的干扰,导致性能数据误差。Trace驱动并行性能模拟器由于其设计实现特点和所运行的宿主并行平台的多样性,使得trace生成的影响具有其独特性。本文选取典型并行模拟器BigSim和若干具有不同计算通信比的目标并行程序,在三个支持不同traceI/O方式的宿主机平台上设计实验评估了trace生成对并行性能模拟的影响,结果表明trace生成对模拟效率和精度均有较大的影响,并分析了这种影响与并行模拟器实现和宿主机平台I/O方式的关系,进而讨论了几种可行的改进方案,对trace驱动并行模拟器设计、实现和使用具有一定的指导意义。     

Speech enhancement based on undecimated wavelet packet-perceptual filterbanks and MMSE–STSA estimation in various noise environments

In this paper, we proposed a new speech enhancement system, which integrates a perceptual filterbank and minimum mean square error–short time spectral amplitude (MMSE–STSA) estimation, modified according to speech presence uncertainty. The perceptual filterbank was designed by adjusting undecimated wavelet packet decomposition (UWPD) tree, according to critical bands of psycho-acoustic model of human auditory system. The MMSE–STSA estimation (modified according to speech presence uncertainty) was used for estimation of speech in undecimated wavelet packet domain. The perceptual filterbank provides a good auditory representation (sufficient frequency resolution), good perceptual quality of speech and low computational load. The MMSE–STSA estimator is based on a priori SNR estimation. A priori SNR estimation, which is a key parameter in MMSE–STSA estimator, was performed by using “decision directed method.” The “decision directed method” provides a trade off between noise reduction and signal distortion when correctly tuned. The experiments were conducted for various noise types. The results of proposed method were compared with those of other popular methods, Wiener estimation and MMSE–log spectral amplitude (MMSE–LSA) estimation in frequency domain. To test the performance of the proposed speech enhancement system, three objective quality measurement tests (SNR, segSNR and Itakura–Saito distance (ISd)) were conducted for various noise types and SNRs. Experimental results and objective quality measurement test results proved the performance of proposed speech enhancement system. The proposed speech enhancement system provided sufficient noise reduction and good intelligibility and perceptual quality, without causing considerable signal distortion and musical background noise.     

A paris-model approach to modular simulation

The development of complex models can be greatly facilitated by the utilization of libraries of reusable model components. In this paper we describe an object-oriented module specification formalism (MSF) for implementing archivable modules in support of continuous spatial modeling. This declarative formalism provides the high level of abstraction necessary for maximum generality, provides enough detail to allow a dynamic simulation to be generated automatically, and avoids the “hard-coded” implementation of space-time dynamics that makes procedural specifications of limited usefulness for specifying archivable modules. A set of these MSF modules can be hierarchically linked to create a parsimonious model specification, or “parsi-model”. The parsi-model exists within the context of a modeling environment (an integrated set of software tools which provide the computer services necessary for simulation development and execution), which can offer simulation services that are not possible in a loosely-coupled “federated” environment, such as graphical module development and configuration, automatic differentiation of model equations, run-time visualization of the data and dynamics of any variable in the simulation, transparent distributed computing within each module, and fully configurable space-time representations. We believe this approach has great potential for bringing the power of modular model development into the collaborative simulation arena.     

Methods for parallel computation of complex flow problems

This paper is an overview of some of the methods developed by the Team for Advanced Flow Simulation and Modeling (TAFSM) [http://www.mems.rice.edu/TAFSM/] to support flow simulation and modeling in a number of “Targeted Challenges”. The “Targeted Challenges” include unsteady flows with interfaces, fluid–object and fluid–structure interactions, airdrop systems, and air circulation and contaminant dispersion. The methods developed include special numerical stabilization methods for compressible and incompressible flows, methods for moving boundaries and interfaces, advanced mesh management methods, and multi-domain computational methods. We include in this paper a number of numerical examples from the simulation of complex flow problems.     

Distributed state space minimization

We present a new algorithm, and its distributed implementation, for reducing labeled transition systems modulo strong bisimulation. The base of this algorithm is the Kanellakis–Smolka “naive method”, which has a high theoretical complexity but is successful in practice and well suited to parallelization. This basic approach is combined with optimizations inspired by the Kanellakis–Smolka algorithm for the case of bounded fanout, which has the best known time complexity. The distributed implementation is improved with respect to previous attempts by a better overlap between communication and computation, which results in an efficient usage of both memory and processing power. We also discuss the time complexity of this algorithm and show experimental results with sequential and distributed prototype tools.     

A model of kinesthetically and visually controlled arm movement

This paper describes a preliminary simulation of kinesthetic control systems that operate a humanoid arm having three degrees of freedom. The design is in part a literal interpretation of the stretch and tendon reflexes considered as control systems. A second level of control converts independent control of three joint angles into a trio of systems controlling the tip of the arm in pitch, yaw and distance coordinates centered on the shoulder. The basic properties of muscles are included, and the arm movements are calculated using equations describing the physical dynamics of the arm. A “visual servo” level of control is included in preliminary form. The model exhibits realistic behavior, producing stable and fast control without computing either inverse kinematics or inverse dynamics.     

Overload effects and their prevention

The so-called “thrashing effect”, well known from virtual storage, but also reported from data-base systems and packet switching networks, has turned out to be a common phenomenon of large systems with concurrent processing. It simply means that beyond a saturation point an increase of the load (e.g. number of jobs) leads to a (sometimes sudden) decrease in performance (e.g. throughput). With growing size and complexity of computer systems and the general trend towards distribution, overload phenomena of different origin can interfere and superimpose mutually, resulting in a composite overload effect that can hardly be broken down into its constituents. Because the complexity of such systems defies detailed modeling, it is more appropriate to look at those systems in a more macroscopic, behavioral way, considering only the two externally measurable variables “load” and “throughput”. The resulting abstraction from internal details can smooth the way to a more general treatment and application. The article deals with such overload phenomena and their prevention in a general way using a control-theoretic approach. Special emphasis is placed on dynamic behavior, where load characteristics are changing with time, making feedback mechanisms necessary. The problem is approached as a dynamic optimum search problem for which different algorithms are presented and compared by simulation.     

Emulation of complex open quantum systems using superconducting qubits

With quantum computers being out of reach for now, quantum simulators are alternative devices for efficient and accurate simulation of problems that are challenging to tackle using conventional computers. Quantum simulators are classified into analog and digital, with the possibility of constructing “hybrid” simulators by combining both techniques. Here we focus on analog quantum simulators of open quantum systems and address the limit that they can beat classical computers. In particular, as an example, we discuss simulation of the chlorosome light-harvesting antenna from green sulfur bacteria with over 250 phonon modes coupled to each electronic state. Furthermore, we propose physical setups that can be used to reproduce the quantum dynamics of a standard and multiple-mode Holstein model. The proposed scheme is based on currently available technology of superconducting circuits consist of flux qubits and quantum oscillators.     

蓄电池模拟装置主电路的拓扑及控制研究

蓄电池模拟装置在用电设备进行设计或检测时可以反复地、长时间地进行检测作业,可以提高效率并节约成本[1]。蓄电池模拟装置对于其功率电路的功能要求有：网侧可回馈电能,具有一定的动态跟随特性。本文设计了一种单相供电的网侧可回馈拓扑结构,一定程度上满足了蓄电池模拟装置对功率电路的功能要求。