Numerical analysis of flow and pollutant dispersion over 2-d bell shaped hills

The numerical simulations of flow and pollutant particle dispersion are described for twodimensional bell shaped hills with various aspect ratios. The Reynolds-averaged incompressible Navier-Stokes equations with low Reynolds numberk-ɛ turbulent model are used to simulate the flowfield. The gradient diffusion equation is used to solve the pollutant dispersion field. The code was validated by comparison of velocity, turbulent kinetic energy, Reynolds shear stress, speed-up ratio, and ground level concentration with experimental and numerical data. Good agreement has been achieved and it has been found that the pollutant dispersion pattern and ground level concentration have been strongly influenced by the hill shape and aspect ratio, as well as the location and height of the source.     

分布式能源用新型Senegal式垂直轴风力机尾流分析

针对现有的Jensen尾流模型分析水平轴风力机(HAWTs)的方法,对采用经典Jensen尾流模型时恒定的风速亏损和尾流衰减系数进行改进,提出基于改进型Jensen尾流模型对新型Senegal式垂直轴风力机(VAWTs)尾流分析的新方法。结合流场分析对该新型风力机进行尾流数值模拟,得出了基于改进的Jensen尾流模型的尾流参数二维分布的变化规律。由于尾流分布具有三维性,二维尾流分布不能完全反映尾流特性,因此还得到了尾流参数三维分布。全面地分析了单台VAWT的尾流特性,不仅为进一步的多台VAWTs尾流分析和风电场风力机排布研究打好基础,而且为该类Senegal式VAWT的尾流研究提供了借鉴和参考。     

Application of wind data from automated weather stations to wind resources estimation in Korea

The wind data measured from automated weather stations (AWS) at complex terrain in Korea was used to predict the wind speed at nearby sites that are several kilometers away. The 10-minute averaged wind data was measured at a height of 10 meters. A commercial CFD code, WindSIM, based on the Reynolds averaged Navier-Stokes equation was employed. The results were compared with the data measured using meteorological masts (MM) at heights of 30 to 50 meters installed for this study. The predictions using the AWS data and WindSIM showed good agreements with the measured data. The results are expected to be useful to find out a spot to install a meteorological mast for future wind farm development in complex terrain.     

风诱导塔振动对塔气动特性影响研究

塔设备风诱导振动对卡门涡脱落及气动特性的影响规律研究对于塔设备共振预测和设计计算十分重要,对塔设备振动下的气动特性问题进行数值模拟研究。采用数值求解N-S方程结合Transition SST湍流模型的方法模拟塔设备周围气流的非定常流动,获得塔外表面的风压系数分布,并结合试验数据验证了文中模拟方法的准确性;继而将塔设备振动简化为正弦运动,采用弹簧动网格技术实现网格动态生成。研究了塔设备振动频率以及振幅的变化对横向力系数、阻力系数和涡脱落频率等气动参数的影响规律,结合塔设备共振条件,指出了气动特性的改变对塔设备共振预测的影响规律。     

Supersonic moist air flow with condensation in a wavy wall channel

The characteristics of Prandtl-Meyer expansion of supersonic flow with condensation along a wavy wall in a channel are investigated by means of experiments and numerical analyses. Experiments are carried out for the case of moist air flow in an intermittent indraft supersonic wind tunnel. The flow fields are visualized by a Schlieren system and the distributions of static pressure along the upper wavy wall are measured by a scanning valve system with pressure transducers. In numerical analyses, the distributions of streamlines, Mach lines, iso-pressure lines, and iso-mass fractions of liquid are obtained by the two-dimensional direct marching method of characteristics. The effects of stagnation temperature, absolute humidity, and attack angle of the upper wavy wall on the generation and the locations of generation and reflection of an oblique shock wave are clarified. Futhermore, it is confirmed that the wavy wall plays an important role in the generation of an oblique shock wave and that the effect of condensation on the flow fields is apparent.     

基于模态分析和小波变换的声发射源定位新算法研究

针对传统声发射源定位中，声发射信号到达传感器的时间受设定门槛电压影响很大，导致声发射源定位效果较差，提出了一种声发射源定位新方法。根据模态声发射理论，携带声发射源信息的声发射信号在结构中传播过程中，具有频散现象和多模态特性。因此，声发射源定位应基于同一频率下、同一模态导波到达各个传感器的时间和传播速度。通过对声发射信号进行Gabor小波变换的方法，在时频空间内确定某一频率下某一模态导波到达传感器的时间；并通过数值计算得到该频率处模态导波的群速度，从而实现声发射源的准确定位。通过薄板中声发射线源定位试验，证明了该定位算法的有效性。     

A new acoustic emission source location technique based on wavelet transform and mode analysis

For wave propagation in dispersive media, the arrival time of the acoustic emission signal to the sensor is dependent on the setting of the threshold voltage, which results in the inaccuracy of the acoustic emission location. Based on the wavelet transform and the theory of modal acoustic emission, a new method is proposed to improve the accuracy of acoustic emission source location. It is believed that the acoustic emission signal propagation in the structure has the characteristics of multi-mode and dispersion, and the acoustic emission source location should use the arrival time to sensors obtained from the output signals not only at the same mode but also at the same frequency. The wavelet transform is used to resolve the problem. By utilizing the time-frequency data of the wavelet, the frequency-dependent arrival time traveling is easily obtained; by numerical computation of the wave’s propagation in structure, the group velocity of the guided mode is also obtained, therefore the accuracy source location is realized. The acoustic emission source location experiments were conducted in a thin steel plate and results show that the technique is an effective tool for acoustic emission source location. __________ Translated from Chinese Journal of Scientific Instrument, 2005, 26(3) (in Chinese)     

Numerical analysis of injected current effects on thermal characteristics of vertical-cavity surface-emitting laser

The present study aims to numerically investigate thermal characteristics of the Vertical-cavity surface-emitting lasers (VCSELs) considering current flows, non-radiative recombination, spontaneous emission transfer, and heat generation. The finite-volume method is used for discretizing the governing equations, and the comparison between prediction and measurement is made to evaluate the simulation code developed in this study. From literature, the numerical models are established for resistive heating inside Bragg reflector and contacts, non-radiative recombination between electrons and holes in the active region, and absorptive heating of created photons, and spontaneous emission. It is found that the numerical prediction shows good agreement with experimental data of temperature rise, and local heating exists mainly near the active layer of VCSEL during operation. Near the active region, thermal sources and temperature increase with injected current, whereas the electrical potential is mainly distributed in the active and p-mirror regions. Also, the maximum temperature rise appears in the active region owing to non-radiative recombination and reabsorption of spontaneous light emission. Even though the heat source significantly increases at the edge of the active region and high resistive regions, the temperature does not change much because of the small size of the active region. Moreover, the resistive and active heating, and total heating dissipation increase with injected currents. The resistive heating dissipation is larger than the active heat dissipation because of high resistivity materials.

     

Numerical simulation of buoyant plume dispersion in a stratifice atmosphere using a lagrangian stochastic model

In the present paper, numerical simulations of buoyant plume dispersion in a neutral and stable atmospheric boundary layer have been carried out. A Lagrangian Stochastic Model (LSM) with a Non-Linear Eddy Viscosity Model (NLEVM) for turbulence is used to generate a Reynolds stress field as an input condition of dispersion simulation. A modified plume-rise equation is included in dispersion simulation in order to consider momentum effect in an initial stage of plume rise resulting in an improved prediction by comparing with the experimental data. The LSM is validated by comparing with the prediction of an Eulerian Dispersion Model (EDM) and by the measured results of vertical profiles of mean concentration in the downstream of an elevated source in an atmospheric boundary layer. The LSM predicts accurate results especially in the vicinity of the source where the EDM underestimates the peak concentration by 40% due to inherent limitations of gradient diffusion theory. As a verification study, the LSM simulation of buoyant plume dispersions under a neutral and stable atmospheric condition is compared with a wind-tunnel experiment, which shows good qualitative agreements.     

基于物联网的风电自动预测系统

因为受到全球模式提供的侧边界条件的限制,采用区域数值天气预报的风电预测,难以有效提高预测水平。利用我国的季风特性的空间相关性预测,在季风期间可以更有效地改善数值天气预报提供的预测结果。采用物联网,可以把风场观测、专用观测塔等得到的观测数据,通过无线和有线方式的广域网进行传送,实现和全国风电预测中心的交互。预期该网可以可靠提高风电预测的效果。     

Numerical investigation of plume dispersion characteristics for various ambient flow velocities

Floating LNG bunkering offshore terminal (FLBT) has been adopted as a floating unit by the marine industry equipped with the natural gas liquefaction plant on the ship’s deck with the practical interest in controlling the plume emission discharged from the ship. The downwash of emitted plumes has adverse consequences on the ship’s engine intake and ventilation system followed by the interference of the smoke with helicopter operations. Owing to this fact, understanding of the plume behavior is considered to be so significant in the aspect of ship design and thereby, flow visualization techniques assist to study the plume path ensuring the safety of the personnel and functioning system. The unignited flare gas emitted from the tower invites heat transfer due to the temperature difference between the atmospheric wind and the ship’s exhaust, which follows the examination of mixing enhancement of fluid mediums as well as the mitigation of plume concentration depending upon wind speeds. The parametric study is intended to investigate the plume dispersion pattern around a ship based on the flare motion and bending angle for light and strong wind speeds. It is observed that the plume gas rises higher and disperses over the larger area with wider streamline separation due to the effect of buoyancy for light wind speeds. On the other hand, the motion of flare gas is found to be narrow for strong winds restricting vertical movements due to dominant inertial forces than gravity pull. Thus, the current numerical investigation facilitates in understanding the configuration and plume distribution by the variation in streamline behavior with contours plots. In this work, the calculated results are analyzed systematically in pretty realistic conditions and simple measures are obtained, which will be applied to the preliminary design of plume stack depending on the ship’s deck arrangement.

     

Combustion and emission characteristics of low rank-coals utilized in Korea

The use of low-rank coal (physical and combustion characteristics are different from those of high-rank coal) is rapidly expanding both in Korea and globally owing to economic constraints; fully understanding the impact of low-rank coal on combustion and emission processes is thus imperative. Observations from several studies on low-rank coal are reviewed in this paper; these studies were conducted at Pusan clean coal center (PC³) with entrained flow reactors (laminar flow reactor and drop tube furnace). This study provides several contributions to the analysis of the combustion and emission characteristics of low-rank coal. Such contributions include the establishment of a simplified prediction model of burning coal and analysis of the effect of particle size and environmental conditions. A numerical model that incorporates the chemical percolation devolatilization sub-model is also developed, and the effect of volatile matter and in-furnace blending method is investigated. Results obtained from analyzing low rank-coal and the developed models provide insights into the application of low-rank coal in power plants.     

A numerical study on electrohydrodynamic induction pumps using CFD modeling

The flow characteristics of EHD induction pumps have been investigated by a numerical method which adds an electric modeling to the conventional CFD. The governing equation for the electric modeling is derived and discretized using the electronic potential as the only dependent variable so that the boundary conditions are satisfied independent of convergence. The numerical results make a good prediction about the frequency-dependent characteristics, which are consistent with both the experiment and the theory. An improvement in understanding the cause of instabilities is achieved to show that the decrement of volumetric force in the local region induces backflow resulting in the instability. The vulnerability was discovered under the conditions of low frequency and large channel depth. The micropump has a maximum flow rate at an optimum channel depth due to the flow instability at larger depths and the flow resistance at smaller one have an undesirable influence upon the one-directionality of flows.     

Large scale PIV-measurements at the surface of shallow water flows

To measure the flow dynamics at the surface of shallow water flows over a large measuring field, a simple and reliable method has been developed using the advantages of Particle Image Velocimetry (PIV). Besides the determination of mean flow conditions and turbulent flow characteristics, this method makes it possible to track two-dimensional large coherent structures, which are the dominating flow phenomena in many shallow flow applications. As basic equipment, a commercial PIV software system has been used. The measurements are carried out at the water surface, which means that no laser light sheet is needed. Depending on the time scales of the flow and camera characteristics, it is even possible to work with a constant light source. A particle dispenser to provide a homogeneous distribution of particles on the water surface is also presented. Because floating particles have a strong tendency of sticking together, different types of particles and special coatings have been tested to reduce this problem. A laboratory application of this method is presented to analyze the effects of shallow dead-water zones on exchange processes in rivers where large coherent two-dimensional flow structures in the mixing layer dominate the flow characteristics.     

基于组合神经网络的风机轮毂处短期风速预测

综合考虑风电机组轮毂处历史风速规律和近期数值气象预报风速变化,对时间尺度为6 h以内的风电机组轮毂处风速进行预测。为提高风速骤变情况下短期风速预测精度,提出一种新的组合神经网络。针对信号分解后的不同频率子序列特点,采用深度卷积神经网络和门控循环递归单元对趋势项子序列进行预测,用建立的Elman循环递归神经网络对细节项子序列进行预测。利用降尺度后的数值气象预报风速来判断风速骤变拐点,采用广义自回归条件异方差模型对非拐点风速预测值进行修正。最后,利用新疆某风电场实际风速数据进行实验,以平均绝对误差、均方误差及平均绝对百分比误差计算确定性预测精度,以覆盖率和区间宽度计算不确定性预测精度,结果验证了所提算法的有效性。     

Unsteady viscous flow over elliptic cylinders at various thickness with different reynolds numbers

Two-dimensional incompressible Navier-Stokes equations are solved using SIMPLER method in the intrinsic curvilinear coordinates system to study the unsteady viscous flow physics over two-dimensional ellipses Unsteady viscous flows over various thickness-to-chord ratios of 06, 08, 10, and 1 2 elliptic cylinders are simulated at different Reynolds numbers of 200, 400, and 1,000 This study is focused on the understanding the effects of Reynolds number and elliptic cylinder thickness on the drag and lift forces The present numerical solutions are compared with available experimental and numerical results and show a good agicement Through this study, it is observed that the Reynolds number and the cylinder thickness affect significantly the frequencies of the force oscillations as well as the mean values and the amplitudes of the drag and lift forces     

Bipolar corona assisted jet flow for fluidic application

In this paper, we present a study on a jet flow, assisted by low net charge ion wind from bipolar corona discharge setup. The ion wind is simultaneously generated from both positive and negative electrodes placed in parallel, adding momentum to the bulk flow directed alongside the electrodes and focused in the middle of interelectrode space. The electrodes are connected to a single battery-operated power source in a symmetrical arrangement, where the electrode creating charged ions of one polarity also serves as the reference electrode to establish the electric field required for ion creation by the opposite electrode, and vice versa. Multiphysics numerical simulation is carried out with programmable open source OpenFOAM, where the measured current-voltage is applied as a boundary condition to simulate the electrohydrodynamics flow. The jet flow inside the device is verified by hotwire anemometry using hotwires embedded within the device, with the measured values in good agreement with simulation. The corona discharge helped to focus the jet and increased the flow peak velocity from 1.41 m/s to 2.42 m/s with only 27.1 mW of consumed discharge power. The device is robust, ready-to-use and minimal in cost. In addition, as the oppositely charged corona flows are self-neutralized, the generated air flow remains neutral and therefore does not attach to a particular target, which expands the application range. These are important features, which can contribute to the development of multi-axis fluidic inertial sensors, fluidic amplifiers, micro blowers, gas mixing, coupling and analysis with space constraints and/or where neutralized discharge process is required, such as circulatory flow heat transfer or the formation of low charged aerosol for inhalation and charged particle deposition.     

Flow analysis of engine cooling system for a passenger vehicle

A numerical simulation is carried out to analyze the flow field of cooling air through the radiator and engine compartment. In order to consider the strong effect of the suction-type flow by the cooling fan at engine idling condition, a potential flow analysis is attempted by the assumption of a line sink located at the position of the cooling fan. The governing equations for steady two-dimensional, incompressible, turbulent flow are solved with the two-equationk-ε model for turbulence. The velocity profiles in the underhood engine compartment and around the front-end of a real vehicle are measured to compare with the numerical results. The agreement between the numerical and experimental results is fairly good. It is concluded that a two-dimensional computation is a fast and efficient tool for predicting the effect of front-end design on the cooling air flow through the radiator.     

基于钝尾缘翼型叶片的三维风力机性能分析

随着风力机向大型化发展,为有效提升风力机叶片的性能以及结构强度,将钝尾缘翼型应用于风力机叶片设计。以NACA639XX系列翼型为基准翼型,通过Hicks-Henne型函数和钝尾缘函数对翼型进行参数化拟合,使用多岛遗传算法优化得到层流钝尾缘翼型族(USST-XXX)。将此翼型族中相对厚度为21%的USST-211翼型与NACA63921层流翼型替换NREL PhaseVI叶片截面的S809翼型,建模得到两种三维风力机叶片,采用数值模拟的方法,对这两种叶片不同风速下的流场进行分析,并与NREL Phase VI风力机叶片的气动性能进行对比。数值模拟结果表明,在额定风速附近,采用层流钝尾缘翼型所构造的新叶片风力机的风能利用系数高于其他两种叶片。研究结果表明优化得到的层流钝尾缘翼型族可以有效提升风力机气动性能,在大型水平轴风力机叶片设计方面具有良好的应用前景。     

炉底漏风对炉内燃烧及NOx生成排放特性的影响

干式排渣系统的冷却风会影响炉内燃烧的组织,现有研究多采用现场试验的方法,研究对象多为排烟温度或锅炉效率。以一台湿渣机改干渣机的660 MW超临界机组为研究对象,采用数值模拟的方法,研究了干渣机漏风对炉内燃烧和NO_x生成排放特性的影响,对比分析了不同炉底漏风来源时,炉膛火焰中心、飞灰含碳量、NO_x排放浓度等关键参数的变化。研究结果表明,炉底漏风量为燃尽风减量的工况对于炉内燃烧影响不明显;炉底漏风量全部或部分为燃烧器二次风减量时,随着漏风量占二次风比例的增大,主燃区氧量水平、出口飞灰含碳量、NO_x排放浓度均随之增高。从冷却风量对炉内燃烧及NO_x排放特性的角度分析了干渣机不同冷却风来源对机组运行的影响,为干式排渣机设计、改造及运行提供理论指导。