Reducing the overhead of dynamic compilation

The execution model for mobile, dynamically‐linked, object‐oriented programs has evolved from fast interpretation to a mix of interpreted and dynamically compiled execution. The primary motivation for dynamic compilation is that compiled code executes significantly faster than interpreted code. However, dynamic compilation, which is performed while the application is running, introduces execution delay. In this paper we present two dynamic compilation techniques that enable high performance execution while reducing the effect of this compilation overhead. These techniques can be classified as (1) decreasing the amount of compilation performed, and (2) overlapping compilation with execution. We first present and evaluate lazy compilation, an approach used in most dynamic compilation systems in which individual methods are compiled on‐demand upon their first invocation. This is in contrast to eager compilation, in which all methods in a class are compiled when a new class is loaded. In this work, we describe our experience with eager compilation, as well as the implementation and transition to lazy compilation. We empirically detail the effectiveness of this decision. Our experimental results using the SpecJVM Java benchmarks and the Jalapeño JVM show that, compared to eager compilation, lazy compilation results in 57% fewer methods being compiled and reductions in total time of 14 to 26%. Total time in this context is compilation plus execution time. Next, we present profile‐driven, background compilation, a technique that augments lazy compilation by using idle cycles in multiprocessor systems to overlap compilation with application execution. With this approach, compilation occurs on a thread separate from that of application threads so as to reduce intermittent, and possibly substantial, delay in execution. Profile information is used to prioritize methods as candidates for background compilation. Methods are compiled according to this priority scheme so that performance‐critical methods are invoked using optimized code as soon as possible. Our results indicate that background compilation can achieve the performance of off‐line compiled applications and masks almost all compilation overhead. We show significant reductions in total time of 14 to 71% over lazy compilation. Copyright © 2001 John Wiley & Sons, Ltd.     

输电线脱冰过程中耐张串金具的受力仿真分析

针对脱冰过程中导线张力对输电线连接金具的破坏作用,运用ABAQUS软件分别建立特高压输电线路的导线模型和耐张串连接金具的整体模型.然后基于改变密度法模拟均匀覆冰的六分裂导线和覆冰脱落过程,研究不同步脱冰时各子导线的动态张力和导线束扭转角的变化规律,结果表明未脱冰子导线的动态最大张力大于导线覆冰静态张力,导线动张力与导线...     

Numerical analysis of dynamic problems with negative stiffness

In this paper, we present numerical studies for dynamic problems with negative stiffness. The solutions are obtained using a fourth order Runge-Kutta (R-K) method. We study the behaviour of this scheme on successively refined meshes with a view to examining the accuracy of the method.     

霜冰生长过程的数值模拟

开发了一个应用程序YICE2D用于霜冰的冰形预测,程序采用模块化方式设计.结冰过程的模拟采用时间步进的方法,水滴收集系数的计算使用欧拉法,水滴控制方程的求解和结冰量的计算通过FLUENT的自定义函数完成,网格更新在Gridgen中进行.YICE2D程序适时调用FLUENT和Gridgen以使整个计算过程全自动化.并计算了两组霜冰的测试算例,与LEWICE2.0的验证结果对比表明,该方法有效.     

三维地效翼展向效应数值模拟

为研究地效翼的展向效应,利用FLUENT软件求解定常不可压N-S方程和标准k-ε湍流模型,对在地面效应下三维地效翼的流场进行数值模拟.首先,对给定面积和离地高度下不同展弦比的地效翼进行数值模拟;然后,对给定弦长和相对飞行高度下不同展弦比和带端板的地效翼进行数值研究.计算结果给出不同展弦比和带端板地效翼的气动特性曲线,揭示展弦比和端板对翼尖涡涡核位置和下洗角的影响规律.在地面效应下,机翼的展向效应更为明显,端板将进一步提高机翼的空气动力性能;翼尖涡的位置受地面影响向外移动,下洗角相应减小.研究结果为地效飞行器的设计与优化提供理论依据.     

Numerical methods for analysis of structure and ground vibration from moving loads

An overview of the main theoretical aspects of finite-element and boundary-element modelling of the response to moving loads is given. The moving loads represent sources of noise and vibration generated by moving vehicles, and the analysis describes the propagation of the disturbances generated in soil. A finite-element time-domain analysis in convected coordinates with a simple upwind scheme is presented, including a special set of boundary conditions permitting the passage of outgoing waves in the convected coordinate system. The modification of frequency-dependent damping to convected coordinates is described, and the convected formulation of boundary elements is presented and used for illustrating the effect of ‘high-speed’ motion. Finally, a procedure for the coupling of a local finite-element model with a boundary-element model of an exterior, or open, domain is described. The paper uses recent results from the Danish research programme ‘Damping Mechanisms in Dynamics of Structures and Materials’ as a basis for a general discussion and review of the recent literature on the subject.     

基于DSP和ARM的电气化铁路接触线覆冰监测系统设计

为预防接触线上的冰害事故以及为除冰融冰提供支持,设计了集成视频传输、覆冰厚度测量、导线温度测量、现场微气象采集的接触线覆冰监测系统。介绍了由控制单元和测量传感器组成的监测系统结构,简略阐述了微气象传感器的选择、无线导线测温传感器的设计和摄像机电磁屏蔽与防冻设计。重点描述了基于图像处理的控制单元的软硬件设计,研究应用DSP与ARM的HPI接口使DSP兼具视频传输与图像处理的功能,ARM通过EPON传输控制信号与测量数据。使用多阈值下的边缘融合算法优化了基于模极大值的边缘检测算法,并提出覆冰厚度和冰凌长度的计算方法。     

浮筒转塔式系泊系统中的动态管缆数值分析

借助有限元法,结合非线性时域分析,对浮筒转塔式系泊(Buoyant Turret Mooring,BTM)系统与FPSO脱离前后分别采取不同的分析策略,解决其柔性立管、动态脐带缆和动态电缆(合称动态管缆)的设计分析工作量大以及所面临的高度非线性和强耦合等难题.以我国南海某BTM系统的动态管缆的分析为例,借助专业立管分析软件Orcaflex展示该类系统的数值建模和分析技术,并通过极值和疲劳的分析与评价阐述此类工程设计的技术特点.     

随机非线性动态系统数值模拟与性态分析

针对具有随机参数非线性系统和随机非线性系统的数值模拟与性态分析问题,提出将蒙特卡罗模拟办法与数值积分法相结合,建立了迭代积分算法求取随机非线性系统在不副随机序列作用下的数值解集,分析了计算误差．其次,利用数值解集的样本值进行解过程性态分析,建立了系统状态的概率分布、均方和矩函数的数值汁箅公式,实现了该类系统数值模拟分析的完整算法和程序流程设计．最后,通过两类典型示例系统的数值仿真结粜验证本文算法与流程的正确性和有效性．     

架空跨海电缆净空高度动态监测系统设计

根据架空跨海电缆的特点,设计了一种以单片机为中心的净空高度动态监测系统。文中给出了一些主要硬件模块的设计方法和GPS数据的解析程序。经实验测试,该系统能有效地保证电缆的安全运行。     

Numerical simulation of ice accretion prediction on multiple element airfoil

For studying ice accretion on aircraft and helicopter airfoils,a modified model of the mass and heat transfer on icing surface was first proposed based on the classical Messinger model.Then an approach for predicting ice accretion on multi-element airfoils was set up through introducing the interpolation calculation of airflow field around the multi-element airfoils.Consid-ering the equivalent thermal power from anti-ice system,a method of the prediction of ice accretion under anti-ice situation was propose...     

Numerical prediction of unsteady vortex shedding for large leading-edge roughness

A full two-dimensional Navier-Stokes algorithm is used to investigate unsteady, incompressible viscous flow past an airfoil leading edge with surface roughness that is characteristic of ice accretion. The roughness is added to the surface through the use of a Prandtl transposition and can generate both small-scale and large-scale roughness. The focus of the study is a detailed flow analysis of the unsteady velocity fluctuations and vortex shedding induced by the surface roughness. The results of this study are compared to experimental data on roughness-induced transition for the same roughness geometry. A comparison is made between “fluctuation intensity” values from the current algorithm to experimentally determined turbulence intensity values. The effects of the roughness Reynolds number, Re_k, are investigated and compared to experimental values of the critical roughness Reynolds number. The authors speculate that there may be a possible correlation between unsteady roughness-induced vortex shedding and the onset of experimentally measured transitional flow downstream of large-scale roughness.     

Numerical simulation of an electrical explosion of thin aluminum wires

Numerical simulation of an electrical explosion of a thin aluminum wire is carried out in the investigation of the dynamics of forming the core-corona structure and redistribution of current over a discharge channel. The parameters of the aluminum wire (l = 12 mm in length and d = 25 μm in diameter) and of the current pulse (I = 10 kA is the peak value and dI/dt ∼ 50 A/ns is the current rise rate) correspond to the data of the experiment, conducted in Lebedev Physics Institute of the Russian Academy of Sciences (LPI RAS). Several series of the calculations in the framework of radiative magnetic hydrodynamics (RMHD), both with regard to radiation and not in regard to it, as well as the calculations from the “cold start,” with regard to the phase transitions and the possibility of implementation of metastable states, are carried out. The obtained results are compared with the experimental data.     

An analysis of the average message overhead in replica controlprotocols

Management of replicated data has received considerable attention in the last few years. Several replica control schemes have been proposed which work in the presence of both node and communication link failures. However, this resiliency to failure inflicts a performance penalty in terms of the communication overhead incurred. Though the issue of performance of these schemes from the standpoint of availability of the system has been well addressed, the issue of message overhead has been limited to the analysis of worst case and best case message bounds. In this paper we derive expressions for computing the average message overhead of several well known replica control protocols and provide a comparative study of the different protocols with respect to both average message overhead and system availabilities     

Inertial effects from single body segments in dynamic analysis of lifting

Biomechanical calculations of loads on the locomotor system in lifting tasks are often based on static models. Several investigators have shown, though, that in a dynamic act like lifting, inertial effects are not generally negligible. A complete dynamic analysis includes the calculation of linear and angular accelerations of body segments and is rather complicated to carry out. So it was of interest to find out whether the contribution of single body segments to the dynamic effects of the whole body might be negligible. Ten male subjects lifted a 12.8 kg box using two different techniques and two speeds. Ground reactions and body segment movements were recorded and reaction forces and net moments at different joints were calculated with static, dynamic and 'semidynamic' methods. The latter method incorporated the ground reaction forces as measured from a force plate but was otherwise equal to the static method. The results from semidynamic analyses agreed well with those from complete dynamic analyses for the ankle, knee, hip, and L5/S1 joints. For example, the semidynamically calculated peak moments at L5/S1 differed on average less than 3% from the dynamically calculated values. The contributions of the lower extremities and the pelvis to the dynamic effects of the whole body seemed to be quite small. The less complicated semidynamic method offers a good alternative to a fully dynamic analysis for estimations of peak moments in the lower back, at least for the studied lifting techniques.     

Tutorial on dynamic analysis of the Costas loop

Costas loop is a classical phase-locked loop (PLL) based circuit for carrier recovery and signal demodulation. The PLL is an automatic control system that adjusts the phase of a local signal to match the phase of the input reference signal. This tutorial is devoted to the dynamic analysis of the Costas loop. In particular the acquisition process is analyzed. Acquisition is most conveniently described by a number of frequency and time parameters such as lock-in range, lock-in time, pull-in range, pull-in time, and hold-in range. While for the classical PLL equations all these parameters have been derived (many of them are approximations, some even crude approximations), this has not yet been carried out for the Costas loop. It is the aim of this analysis to close this gap. The paper starts with an overview on mathematical and physical models (exact and simplified) of the different variants of the Costas loop. Then equations for the above mentioned key parameters are derived. Finally, the lock-in range of the Costas loop for the case where a lead-lag filter is used for the loop filter is analyzed.     

Classification of freshwater ice conditions on the Alaskan Arctic Coastal Plain using ground penetrating radar and TerraSAR-X satellite data

Arctic freshwater ecosystems have responded rapidly to climatic changes over the last half century. Lakes and rivers are experiencing a thinning of the seasonal ice cover, which may increase potential over-wintering freshwater habitat, winter water supply for industrial withdrawal, and permafrost degradation. Here, we combined the use of ground penetrating radar (GPR) and high-resolution (HR) spotlight TerraSAR-X (TSX) satellite data (1.25 m resolution) to identify and characterize floating ice and grounded ice conditions in lakes, ponds, beaded stream pools, and an alluvial river channel. Classified ice conditions from the GPR and the TSX data showed excellent agreement: 90.6% for a predominantly floating ice lake, 99.7% for a grounded ice lake, 79.0% for a beaded stream course, and 92.1% for the alluvial river channel. A GIS-based analysis of 890 surface water features larger than 0.01 ha showed that 42% of the total surface water area potentially provided over-wintering habitat during the 2012/2013 winter. Lakes accounted for 89% of this area, whereas the alluvial river channel accounted for 10% and ponds and beaded stream pools each accounted for <1%. Identification of smaller landscape features such as beaded stream pools may be important because of their distribution and role in connecting other water bodies on the landscape. These findings advance techniques for detecting and knowledge associated with potential winter habitat distribution for fish and invertebrates at the local scale in a region of the Arctic with increasing stressors related to climate and land use change.     

End-wall effects on vortex shedding in planar shear flow over a circular cylinder

A prevailing controversy regarding the suppression of periodic vortex shedding from a circular cylinder embedded in a planar shear flow has been addressed. Three-dimensional computer simulations utilizing the advanced MGLET software [11] demonstrated the importance of the end-wall conditions. Earlier results from two-dimensional simulations at Re = 100 were reproduced only with free-slip conditions. With no-slip conditions imposed at one or both end-walls, the vortex shedding was suppressed near the no-slip boundary and the shedding pattern was substantially affected even at mid-span. The Strouhal number decreased when the shear-rate parameter was increased from 0.1 to 0.2, irrespective of the choice of boundary conditions.     

An analysis on the airship dynamic model linearization

The existing results on airship dynamic model linearization are usually achieved under the assumption that the airship trim state has a linear velocity with nonzero axial and normal components. This means that the direction of airship velocity is not coincident with the axis in the trim state and the deflection angles of the air rudders are different from their attack angles. However, these differences are not explicitly distinguished when calculating the air rudders’ aerodynamic force. In this paper, to build a more accurate airship dynamic model, aerodynamic forces and moments produced by the air rudders are deduced for the case that the direction of airship velocity is not coincident with the trim axis. A special trim state that all linear and angular velocities are zeros except axial and normal linear velocity, adopted by most research on airship dynamic model linearization, is selected to linearize the six degree airship dynamic model for simplicity and for comparison. Both the control forces of the propelling system and the air rudders are linearized to achieve more control inputs for the linear model. The extra control inputs provided by the propelling system can be used to stabilize the airship when the air rudders are invalid due to low speed of the airship or the constraint of air rudder mechanical strength. The nuance between the new linearized model and those deduced in the literature is discussed.     

On hierarchical modelling of motion for workflow analysis from overhead view

Understanding human behaviour is a high level perceptual problem, one which is often dominated by the contextual knowledge of the environment, and where concerns such as occlusion, scene clutter and high within-class variations are commonplace. Nonetheless, such understanding is highly desirable for automated visual surveillance. We consider this problem in a context of a workflow analysis within an industrial environment. The hierarchical nature of the workflow is exploited to split the problem into ‘activity’ and ‘task’ recognition. In this, sequences of low level activities are examined for instances of a task while the remainder are labelled as background. An initial prediction of activity is obtained using shape and motion based features of the moving blob of interest. A sequence of these activities is further adjusted by a probabilistic analysis of transitions between activities using hidden Markov models (HMMs). In task detection, HMMs are arranged to handle the activities within each task. Two separate HMMs for task and background compete for an incoming sequence of activities. Imagery derived from a camera mounted overhead the target scene has been chosen over the more conventional oblique views (from the side) as this view does not suffer from as much occlusion, and it poses a manageable detection and tracking problem while still retaining powerful cues as to the workflow patterns. We evaluate our approach both in activity and task detection on a challenging dataset of surveillance of human operators in a car manufacturing plant. The experimental results show that our hierarchical approach can automatically segment the timeline and spatially localize a series of predefined tasks that are performed to complete a workflow.