基于移频技术的短时傅里叶变换阶比分析

提出了一种基于移频技术的短时傅里叶变换阶比分析算法.该算法利用傅里叶变换在频域的卷积性质,对原始信号在时域乘以e-j2πf_it使f_i的频谱能量搬迁到零频处,按一定的频率间隔改变f_i就可以在零频处得到其他频率的频谱能量,以此来提高短时傅里叶变换在时频分析中的频率分辨率.然后在时频面上进行局部阈值降噪,同时跟踪转速的变化,最终应用到变速机械的阶比分析中.与短时傅里叶变换分析结果对比表明,本文方法可以更加准确地跟踪到实际的转速.实际降速过程中轴承信号利用本文方法进行阶比分析,成功提取到轴承的故障特征频率.      

考虑固壁作用力的微可压缩流体纳微米圆管流动分析

针对流体在纳微米尺度下的流体流动规律不符合泊肃叶规律的理论依据不足的难题,研究了纳微米圆管中流体的流动,将流体的微可压缩和固壁对流体的作用同时考虑进来,并将固壁对流体的作用采用固壁作用力的形式引入到流体力学方程,采用涡函数流函数将方程解耦,并用正则摄动法求得一阶精度的压力和速度的解析解.结果发现:固壁作用力导致零阶径向压力的出现,一阶压力的增强和一阶速度的降低;量纲一的体积流量偏离了不可压缩流体的体积流量,偏离效应受流体的微可压缩性和固壁作用力的共同影响.体积流量在同尺度下偏离泊肃叶流动的流量大小随着可压缩系数和流体中和壁面产生作用的离子浓度增大而增大,随着纳微米圆管管径减小而增大,纳微米圆管管径低于某一尺寸时,流体将不能流动.通过研究表明:纳微米尺度下产生微尺度效应的原因是流体的微可压缩性和壁面力的共同影响.      

基于分步傅里叶变换法对非线性薛定谔方程的数值仿真

介绍一种数值求解非线性偏微分方程的方法;分步傅里叶变换法.光脉冲信号在光纤中传输时,同时受到色散效应和非线性效应的影响.所以利用分步傅里叶变换法,考虑光信号在光纤中传输一段微小距离的情况下,先计算色散效应对光脉冲的影响,然后再计算非线性效应对光脉冲信号的影响,进而近似求出非线性薛定谔方程的数值解.最后,应用MATLAB软件来数值仿真这个数值解,仿真结果可以清晰看到色散效应对光脉冲的脉冲展宽,以及非线性效应对光脉冲的影响.     

分数阶对称平移不变过完备小波的构造及其在轴承故障诊断中的应用

提出了一种分数阶的对称性近似平移不变过完备小波的构造方法.首先,给出一种构造具有对称性且具有最小长度的低通滤波器方法.其次,通过拓普利兹矩阵分解法求出对应的具有近似平移不变性的高通滤波器,此方法比其他分解方法具有更低的计算复杂度.此外,利用此构造方法,也得到具有更高阶消失矩的分数阶过完备小波变换.最后,将构造出的分数阶对称平移不变过完备小波应用到轴承故障诊断中.实验结果表明,提出的小波变换能有效地提取出轴承的故障特征.      

轧辊偏心补偿控制系统研究

针对轧辊偏心问题,提出了采用相干时间平均算法进行信号的预处理,以减少或消除随机噪声,提高信噪比;采用快速傅里叶变换方法,对其进行进一步分析处理,得出偏心信号中所含各次正弦波的幅值、频率和相角,从而确定轧辊偏心信号的参数模型;以此为依据,控制轧机的压下系统,实现对轧辊偏心的有效补偿.     

高炉炉缸铁水流动的研究

使用水模型试验和数模研究了高炉炉缸中铁水的流动。建立水模型装置是为了通过测量铁趵时间及观察其流动形式来测定焦炭粒度，焦层结构，出铁速度和无焦空间对铁水流动性的影响。另外，利用有限元解出多孔介质的动量方程来计算流速，铁水滞留时间随焦炭粒度减小而增加，但是随出铁速度的增大而减少，如果死料柱中心有小粒焦，铁水环流加剧。流动路线由于存在焦炭无间隙区而改变。     

用首次积分降阶Lagrange方程的统一方法

通过定义对状态的变换,导出对Ｌａｇｒａｎｇｅ方程的变换．新方程以某个非循环坐标为自变量,时间变量成为“广义坐标”,并保持了Ｌａｇｒａｎｇｅ方程的形式．由此得到能同时利用原方程全部首次积分（循环积分和能量积分）降阶Ｌａｇｒａｎｇｅ方程的统一方法．示例说明了新方法的应用     

理想流体流动方程推导专题教学方法探讨

连续性方程和欧拉方程是动量传输的核心内容,其推导过程一直是教学中的重点和难点,以往教学中学生普遍反映难于理解。为了使学生更好的理解和掌握连续性方程及欧拉方程的推导过程,文章对此专题性教学方法进行探讨,采用易于学生理解和掌握的方法分别推导了流体流动的连续性方程及欧拉方程,并分享了连续性方程和欧拉方程推导专题教学中的一些认识与体会,为动量传输理论的教学提供一定的参考。     

伯努利方程推导分析

伯努利方程是流体力学描述理想流体定常流动的机械能守恒方程,可以从机械能守恒、动量守恒和能量守恒三个途径推导得到。从机械能守恒推导时,伯努利方程的前提是理想常密度流体沿流管流动的定常等温流动;从动量守恒推导时,前提是理想流体的定常流动,并可针对不可压缩、可压缩流体,有旋、无旋流动;从能量守恒推导时,前提是理想流体的等熵流动。这三种推导方程均可得到伯努利方程,但其推导前提和方程表达形式略有差别,在教学中将三种推导方法相结合,有助于全面准确理解伯努利方程。     

单轴加载条件下砂岩声发射特性分析

通过单轴加载条件下砂岩破裂过程的声发射试验,采集了砂岩变形各个阶段的声发射信号,得到了应力-时间-累计振铃计数、应力-时间-声发射事件率曲线.结合应力-应变曲线,对整个破坏过程中的声发射信号特征参数进行了分析,同时采用快速傅里叶变换,对其中按时间序列选取的110个声发射信号进行了频谱分析,得到了声发射信号对应的主频,对主频随应力的变化情况进行了分析.结果表明:在峰值强度50%之前,主频随压力随机变化,在峰值强度50%之后主频有降低的趋势,且主频主要集中在140~180 kHz之间.     

焙烧炉燃烧器射流发散的分析与消除

通过将燃烧器喷嘴锥阀模拟为弹簧——质量——阻尼二阶系统,并分析燃烧器脉冲阀关闭时喷嘴锥阀关闭响应,从流体介质重油的稳态和瞬态两方面加以计算求解,通过建立流体介质的动量方程,伯努力方程,运动微分方程,得出喷嘴锥阀关闭响应时间与脉冲时间的关系,从而找出焙烧炉燃烧器射流发散的原因以及相应的措施。     

附加外场力对锌锅表层锌液流动和锌渣分布的影响

实测了热镀锌锌锅表层锌液的流速,并分析了锌锅浮渣的分布特点,提出了一种在锌锅锌液上方主动加载外场力作用以驱动锌液流动进而改变锌渣分布的新方法,称为锌渣流动管理方法(DFMS-Dross Flowing Management System)。通过源力加载的方式修正了流体动量守恒方程,建立了锌液流场数值计算模型,模拟了不同外场力加载布局下锌锅表层锌液的流场。结果表明,附加外场力作用可有效促进锌锅表层锌液的切向流动,最大切向流速可达0.8 m/s;表层锌液的切向流动可拖动锌渣跟随流动,进而改变锌渣分布,促进锌锅排渣,证实了DFMS的有效性。模拟结果还表明,锌锅表层锌液的切向流速受外场力加载时间的影响较小,而随加载外场力的增大而增大,但加载外场力的大小对100~200 mm深锌液的流速影响较小。最后,通过定义锌锅表层锌液的排渣时间因子讨论了不同外场力加载方式的排渣效率。     

A model of blood interaction with optical-fluid guide for laser angioplasty

A mathematical model is developed to describe the flow and mixing of blood and optical fluid used in liquid-guided light for laser angioplasty. The model is based on a two-fluid formulation in which separate transport equations are solved for the blood and the optical fluid. Empirical relations, established in prior work, are used to represent interfluid transport of momentum. Both steady and phasic inflow conditions are considered. Parametric calculations are performed showing effect of relative flow rates of blood and optical fluid, on the mixing phenomena. The relative velocity considered (based on average blood velocity) ranged from 0.08 to 0.28 m/sec. No allowance has been made for ablation of the plaque. The predicted results include spatial distribution of the velocity field and the existence probabilities (volume fractions) that provide a measure of the extent of mixing between the fluids. It is found that the degree of mixing is adversely affected by the relative inflow velocity between blood and optical fluid and the pulsatility of blood inflow. Deep penetration of the optical fluid is predicted at high relative velocity and at the end of diastolic and early systolic stages of the cardiac cycle.     

Comparison of Zero-Inertia and Volume Balance Advance-Infiltration Models

Results of the advance-infiltration phase from a zero-inertia model and a volume balance model in level basins are analyzed and compared. Level basin irrigation systems are traditionally more efficient than other surface irrigation systems (e.g., furrows, borders). An important factor in the design and operation of level basins is the time of advance, which is primarily a function of the inflow rate, soil infiltration parameters, and roughness coefficient. The advance-infiltration phase is determined by two well-known and also very distinct mathematical approaches. The first approach is known as the zero-inertia model, which is categorized as “simplified hydrodynamics.” The simplification consists of ignoring some of the terms in the momentum equation. The second approach is based on the volume balance model and is considered to be “kinematics” because the momentum equation is ignored. The volume balance model, despite being less complex and less mathematically demanding than the zero-inertia model, provides satisfactory predictions of the advance-infiltration phase. The Lewis-Milne equation defines, in this case, the advance-infiltration phase with the modified Kostiakov infiltration function describing the infiltration process. The obtained solution is relatively simple to program. In spite of great advances in computers that facilitate the solution of complex mathematical schemes, hydrodynamic models, even simplified versions, are infrequently used in the daily practice of engineering because of their complexity. Time steps to be used in these simulations are extremely small to guarantee good accuracy and to avoid instability in the numerical scheme. Errors in the estimation of the time of advance between the zero-inertia and volume balance models range from 3.87 to 8.44% from unit inflows ranging from 2.0 to 7.0 L∕s∕m. The time of advance tL from the zero-inertia model is larger than that from the volume balance model, meaning that the zero-inertia model yields smaller average flow velocity for the entire basin than for the volume balance model.     

Double-Averaging Concept for Rough-Bed Open-Channel and Overland Flows: Theoretical Background

The goal of this paper is to discuss the spatial averaging concept in environmental hydraulics and develop it further by considering transport equations for fluid momentum, passive substances, and suspended sediments. The averaging theorems, the double-averaged (in time and in space) fluid momentum equation, and advection-diffusion equations for a passive substance and suspended sediments are introduced and their limitations and applications for modeling rough-bed flows, experimental design, and data interpretation are discussed. The suggested equations differ from those considered in terrestrial canopy aerodynamics and porous media hydrodynamics by accounting for roughness mobility, change in roughness density in space and time, and particle settling effects for the case of suspended sediments. We show that the form of the double-averaged equations may depend on the type of decomposition of flow variables and that this difference may have important implications for modeling. We also show that the suggested methodology offers better definitions for hydraulic characteristics, variables, and parameters such as flow uniformity, flow two dimensionality, and bed shear stress.     

Hydraulics of Stacked Drop Manholes

This paper presents the results of an experimental investigation on flows inside stacked drop manholes (SDM). An SDM consists of two identical rectangular or square manhole chambers stacked together at an elevation difference. SDMs for different conditions were assessed on their ability to dissipate the energy of the approaching flow and their suitability to perform adequately under different flow conditions. Flow regimes were classified based on the inflow conditions and geometry of the structure in the first chamber and downstream outflows in the second chamber. An analysis based on the integral momentum equation was developed to estimate pool depths and energy losses under critical flow conditions. A fully surcharged stage with inflow and outflow pipes running full was also tested and velocity profiles were measured at a horizontal center plane to the opening connecting both shafts. Additionally, air flow rates were measured to assess the air demand into a large-height SDM.     

入口均匀性对直方管内湍流影响的大涡模拟研究

采用大涡模拟方法对直方管内的湍流流动进行了研究，对比计算了进口处为均匀来流和非均匀来流三种工况。从瞬时速度、脉动速度和平均速度分布等方面对湍流流动进行了分析。计算结果表明：入口处均匀来流和非均匀来流在横截面都会产生二次流运动；四个角域强化了壁面流体和主流流体的动量交换；在平均雷诺数相同情况下．非均匀来流时湍流程度大，各方向湍流脉动加强，横截面二次流加强，出现多个复杂的涡流区．瞬时流场结构、涡结构要复杂得多。     

The effect of particle size and morphology on the in-flight behavior of particles during high-velocity oxyfuel thermal spraying

A one-way coupled mathematical model is formulated to simulate the effects of particle size and morphology on the momentum and thermal energy transfer of particles during high-velocity oxyfuel (HVOF) thermal spraying. First, computational fluid dynamic techniques are implemented to solve the Favre-averaged mass, momentum, and energy conservation equations in the gas phase. The gas dynamic data are then used to model the behavior of particles in the gas field. The concept of sphericity is used to incorporate the effect of particle morphology into the model. The calculated results show that the particle velocity and temperature, before impinging onto the substrate, are strongly affected by particle size, morphology, and spray distance. Smaller particles are accelerated to a higher velocity but slowed down rapidly due to their smaller momentum inertia, while the larger particles are accelerated with some difficulty. The same tendency is observed regarding the effect of particle size on its thermal history.     

A comparative experimental and numerical study to investigate the relative merits of convectors and “C” inserts in cooling cold-rolled coils

The coil cooling and storage unit (CCSU) is used to cool cold-rolled coils to the temper rolling temperature after the annealing cycle is over at the batch annealing furnace (BAF) in a cold rolling mill (CRM). In the CCSU, the coils are kept on the cooling bases for any fixed time irrespective of the grade and tonnage. Therefore, the need for a mathematical model to accurately predict the cooling time of the coils was felt. The current study involves experimental and numerical analysis of a stack of coils with respect to heat transfer and fluid flow. A comparative study was carried out to ascertain the relative merits of convectors and “C” inserts (CIs) in the cooling the coils. The air flow distribution for the case of different convectors and CIs was measured by means of a full scale physical model. Two different mathematical models were applied to model the fluid flow and flow distribution through the stack of coils. The first flow model uses the hydraulic resistance concept for estimating the air flow rate distribution, whereas the second flow model uses commercial computational fluid dynamics (CFD) software and predicts the velocity distribution in the flow path between two coils in a stack. The predictions from these two models compare well with the experimental data. The flow models were used to calculate the average heat-transfer coefficient in different flow passages in a stack. The heat-transfer coefficients thus obtained were used to tune and validate a two-dimensional transient heat-transfer model of coils. The heat-transfer model predicts the cooling time of coils accurately and also suggests a possible reduction of cooling time if CIs are used in place of convectors.     

Turbulence Characteristics and Interaction between Particles and Fluid in Particle-Laden Open Channel Flows

Mechanism of sediment transport is composed of complicated interactions between turbulent flow, particle motion, and bed configurations. Of particular significance is the interaction between turbulence and particle motion, although turbulence measurements of particle-laden two phase flow have been a problem for a long time, especially in the near-wall region. In this study, simultaneous measurements of both the particles and fluid (water) were conducted in particle-laden two phase open channel flows by means of a discriminator particle-tracking velocimetry. The mean velocity and turbulence characteristics for fluid and particles each were examined in comparison with those in clear-water (particle-free) flow, together with previous existing data measured by laser Doppler anemometer and phase Doppler anemometer. The relative velocity and the turbulence modulation, which are the most important topics in two phase-flow approach, were revealed by varying the particle diameter and specific density. The fluid-sweeps are more contributory to the motion of particles than the fluid ejections in the near-wall region. In turn, the particle-sweeps transport the high momentum to the carrier fluid and enhance the turbulence intensities of fluid.