Modeling of Fluid Turbulence Modification Using Two-time-scale Dissipation Models and Accounting for the Particle Wake Effect

Presently developed two-phase turbulence models under-predict the gas turbulent fluctuation, because their turbulence modification models cannot fully reflect the effect of particles. In this paper, a two-time-scale dissipation model of turbulence modification, developed for the two-phase velocity correlation and for the dissipation rate of gas turbulent kinetic energy, is proposed and used to simulate sudden-expansion and swirling gas-particle flows. The proposed two-time scale model gives better results than the single-time scale model. Besides, a gas turbulence augmentation model accounting for the f＇mite-size particle wake effect in the gas Reynolds stress equation is proposed. The proposed turbulence modification models are used to simulate two-phase pipe flows. It can properly predict both turbulence reduction and turbulence enhancement for a certain size of particles observed in experiments.

Triatomic wake effect and the determination of the molecular structure of HD_2^+ from the Coulomb explosion

A new theoretical model of the triatomic molecular wake effect is proposed and applied to molecular ions D^＋3 and HD^＋2 while passing through a solid. The wake effects resulting from the reactions of the two similar ions with thin carbon foil are also investigated by using the Coulomb explosion technique. The experimental results are in good agreement with theoretical estimates and the molecular structure of HD^＋2 is determined by using the model.

Review on Research about Wake Effects of Offshore Wind Turbines

In recent years,the construction of offshore wind farms is developing rapidly.As the wake effect of the upstream wind turbines seriously affect the performance of the downstream wind turbines,the wake effect of offshore wind turbines has become one of the research hotspots.First,this article reviews the research methods of wake effects,including CFD numerical simulation method,wind turbine wake model based on roughness and engineering wake models.However,there is no general model that can be used directly.Then it puts forward some factors that affect the wake of offshore wind turbines.The turbulence intensity in offshore wind fields is lower than that in onshore wind fields.This makes the wake recovery length of offshore wind turbines longer than that of onshore wind turbines.Floating offshore wind turbines are simultaneously disturbed by wind loads and wave loads.Unsteady movement of the platform caused by wave loads.It affects the development and changes of the wake of wind turbines.In this regard,the focus of research on the wake effects of offshore wind farms will be the proposal of accurate prediction models for the wake effects of sea wind farms.

Modeling of Fluid Turbulence Modification Using Two-time-scale Dissipation Models and Accounting for the Particle Wake Effect

Presently developed two-phase turbulence models under-predict the gas turbulent fluctuation, because their turbulence modification models cannot fully reflect the effect of particles. In this paper, a two-time-scale dis- sipation model of turbulence modification, developed for the two-phase velocity correlation and for the dissipation rate of gas turbulent kinetic energy, is proposed and used to simulate sudden-expansion and swirling gas-particle flows. The proposed two-time scale model gives better results than the single-time scale model. Besides, a gas tur- bulence augmentation model accounting for the finite-size particle wake effect in the gas Reynolds stress equation is proposed. The proposed turbulence modification models are used to simulate two-phase pipe flows. It can prop- erly predict both turbulence reduction and turbulence enhancement for a certain size of particles observed in ex- periments.

Exergy Analysis of Single Array Wind Farm Using Wake Effects

The influence of wake parameters on the exergy analysis of single array wind farm is studied in this paper. Key parameters which influence wake effects in a wind farm are wind velocity, tip speed ratio, number of blades, rotor speed, rotor diameter and hub height. Three different models namely power, wake and exergy model were used in estimating the exergy efficiency of the single array wind farm. Even though it is ideal for wind farms to fix the wind turbines in rows and columns the conditions of the site may not always be condu- cive for it. Hence analysis has been done keeping the wind turbines at random in a row and the effect of positioning on the performance is analyzed. Energy and exergy efficiency calculations were made for different cases by varying the positions of wind turbines in the single array wind farm. Standard relations were used in estimating the energy deficit in the wind farm due to wake effects. The wake effects were found to have an aggregated influence on the energy production of the wind farm, which results from the changes in the key parameters mentioned above. Potential areas for reducing energy losses by proper location and selection of turbines based on rating are highlighted. The influence of individual parameters contributing to the wake ef-fect were analyzed and discussed in detail.

Smart load control of the large-scale offshore wind turbine blades subject to wake effect

The smart fatigue load control of a large-scale wind turbine blade subject to wake effect was numerically investigated in this paper. The performances were evaluated and compared at selected typical wind speeds within the whole operational region under three turbine layout strategies, i.e., column, row and array arrangements, together with a single turbine case as reference, utilizing our newly developed aero-servo-elastic platform. It was observed that not only the blade fatigue loads but the stabilities of power and collective pitch angle were effectively controlled for all cases, especially at the highest studied hub velocity of20 m/s, leading to the averaged reduction percentages in the standard deviations of the flapwise root moment, the flapwise tip deflection and the root damage equivalent load, of about 30.0 %, 20.0 % and 20.0 %, respectively. Furthermore, the control effectiveness gradually lessened in the sequences of single, column, row and array cases, with successively increasing effective turbulence intensity,within regions II and III. The performances in region III,associated with the impaired flow separation on the blade by the effective pitching action, were much better than those in region II, related to enhanced flow detachment. In addition,at the rated wind velocity, the control for the array case was superior over other three cases, which was thought to be originated from the more pitch activities to impair the uncontrolled flow separation on the blade surface.

Power quality assessment in different wind power plant models considering wind turbine wake effects

The intense increase in the installed capacity of wind farms has required a computationally efficient dynamic equivalent model of wind farms.Various types of wind-farm modelling aim to identify the accuracy and simulation time in the presence of the power system.In this study,dynamic simulation of equivalent models of a sample wind farm,including single-turbine representation,multiple-turbine representation,quasi-multiple-turbine representation and full-turbine representation models,are performed using a doubly-fed induction generator wind turbine model developed in DIgSILENT software.The developed doubly-fed induction generator model in DIgSILENT is intended to simulate inflow wind turbulence for more accurate performance.The wake effects between wind turbines for the fullturbine representation and multiple-turbine representation models have been considered using the Jensen method.The developed model improves the extraction power of the turbine according to the layout of the wind farm.The accuracy of the mentioned methods is evaluated by calculating the output parameters of the wind farm,including active and reactive powers,voltage and instantaneous flicker intensity.The study was carried out on a sample wind farm,which included 39 wind turbines.The simulation results confirm that the computational loads of the single-turbine representation(STR),the multiple-turbine representation and the quasi-multiple-turbine representation are 1/39,1/8 and 1/8 times the full-turbine representation model,respectively.On the other hand,the error of active power(voltage)with respect to the full-turbine representation model is 74.59%(1.31%),43.29%(0.31%)and 7.19%(0.11%)for the STR,the multiple-turbine representation and the quasi-multiple representation,respectively.

Study on the Wake Shape behind a Wing in Ground Effect Using an Unsteady Discrete Vortex Panel Method

The unsteady evolution of trailing vortex sheets behind a wing in ground effect is simulated using an unsteady discrete vortex panel method. The ground effect is included by image method. The present method is validated by comparing the simulated wake roll-up shapes to published numerical results. When a wing is flying in a very close proximity to the ground, the optimal wing loading is parabolic rather than elliptic. Thus, a theoretical model of wing load distributions is suggested, and unsteady vortex evolutions behind lifting lines with both elliptic and parabolic load distributions are simulated for several ground heights. For a lifting line with elliptic and parabolic loading, the ground has the effect of moving the wingtip vortices laterally outward and suppressing the development of the vortex. When the wing is in a very close proximity to the ground, the types of wing load distributions does not affect much on the overall wake shapes, but parabolic load distributions make the wingtip vortices move more laterally outward than the elliptic load distributions.

The assessment of extactable tidal energy and the effect of tidal energy turbine deployment on the hydrodynamics in Zhoushan

In this study, we construct one 2-dimensional tidal simulation, using an unstructured Finite Volume Coastal Ocean Model （FVCOM）. In the 2-D model, we simulated the tidal turbines through adding additional bottom drag in the element where the tidal turbines reside. The additional bottom drag was calculated from the relationship of the bottom friction dissipation and the rated rotor efficiency of the tidal energy turbine. This study analyzed the effect of the tidal energy turbine to the hydrodynamic environment, and calculated the amount of the extractable tidal energy resource at the Guishan Hangmen Channel, considering the rotor wake effect.

Investigating the Performance of Twin Marine Propellers in Different Ship Wake Fields Using an Unsteady Viscous and Inviscid Solver

In this study,the performance of a twin-screw propeller under the influence of the wake field of a fully appended ship was investigated using a coupled Reynolds-averaged Navier–Stokes(RANS)/boundary element method(BEM)code.The unsteady BEM is an efficient approach to predicting propeller performance.By applying the time-stepping method in the BEM solver,the trailing vortex sheet pattern of the propeller can be accurately captured at each time step.This is the main innovation of the coupled strategy.Furthermore,to ascertain the effect of the wake field of the ship with acceptable accuracy,a RANS solver was developed.A finite volume method was used to discretize the Navier–Stokes equations on fully unstructured grids.To simulate ship motions,the volume of the fluid method was applied to the RANS solver.The validation of each solver(BEM/RANS)was separately performed,and the results were compared with experimental data.Ultimately,the BEM and RANS solvers were coupled to estimate the performance of a twin-screw propeller,which was affected by the wake field of the fully appended hull.The proposed model was applied to a twin-screw oceanography research vessel.The results demonstrated that the presented model can estimate the thrust coefficient of a propeller with good accuracy as compared to an experimental self-propulsion test.The wake sheet pattern of the propeller in open water(uniform flow)was also compared with the propeller in a real wake field.

Theoretical Investigation of the Vortex Shedding Noise from the Wake of Airfoil

The vortex shedding noise has been revealed as an important wing noise source on some modern commercial aircraft based on the fly-over measurements with a planar microphone array by Michel (1998). In this paper, an analytical model is presented for predicting this vortex shedding noise. The downstream wake of a 2-dimensional airfoil is assumed to be dominated by the von Karman vortex street, and the strength and the shedding frequency of the wake vortex are determined from the wake structure model. An aero-acoustic model is developed based on the Howe's unified theory of trailing edge noise and is incorporated with the wake model to predict the sound pressure level and directivity of vortex shedding noise. The predicted vortex shedding frequencies, sound pressure levels and directivities compare favorably with the measured results for 6 modern commercial aircraft.

基于动量理论的风力机三维工程全尾流模型研究

为综合研究风力机尾流特性,将尾流区域分为近尾流区域和远尾流区域,并考虑风切变效应引起的风速差对尾流速度的影响,提出一种基于动量理论的三维工程全尾流模型(3DFM)。与风场实测数据对比,3DFM模型的近尾流区预测精度高达95.82%,远尾流区预测精度高达95.00%,验证了模型的有效性和准确性。根据来流风不同大气稳定状态修正尾流膨胀系数和风切变系数,详细研究大气稳定状态对尾流速度分布和恢复的影响。研究结果表明大气层越稳定,风切变系数越高,下游相同位置处的尾流区域面积越大,尾流速度恢复得越慢。

中尺度模式下相邻海上风电场间尾流与功率干扰影响研究

为研究相邻海上风电场间的尾流与功率干扰特性,基于耦合风电场参数化的中尺度天气研究与预报模式(Weather Research and Forecasting,WRF),以北海海域某两相邻理想海上风电场为研究对象展开数值模拟,量化分析了在不同相邻场间距或下游风电场单机容量设置下的海上风电场尾流效应与功率输出特性。研究发现:当场间距从20D时增至40D时,上游场尾流引起的下游场最大风速亏损由3.5m/s降至2.5m/s,整场功率输出提升21.91%,最大单机功率亏损率由30.03%降至16.78%,场间距的增大有利于上游场尾流内的风速恢复、下游场速度亏损的减缓与整场功率提升。下游场单机容量从3.6MW增至4.5MW时,下游整场功率提升7.96%,最大风速亏损为2.2m/s,;下游场单机容量增至6.0MW,下游整场功率提升14.19%,最大风速亏损增至3.9m/s,相邻游风电场机型配置的阶梯式布局,有利于提升风电场发电量,提高风资源利用率。

火箭弹尾流冲击效应研究

以火箭弹尾流场及发射平台为研究对象,基于计算流体动力学方法,采用有限体积法进行数值离散,建立火箭弹尾焰燃气流场数学物理模型进行计算。分析火箭弹尾焰射流与发射平台的相互作用,探究燃气流场速度、温度和压力等参数分布特性,并研究发射过程中尾焰燃气流对发射平台的冲击效应,得到发射平台关注区域表面压力、温度变化规律。之后对不同材料表面进行数值模拟及实验模拟对比,研究结果表明由钢材料替换铝材料后,钢材料表面温度低于熔点,可减小温度对平台的影响,为火箭弹发射平台防护及优化提供了理论依据。

计及尾流的风电场智能等效建模及偏航优化控制

大型风电场中存在的尾流干扰会降低风电场整体输出功率,因此需要对风电场建立等效模型,并通过优化尾流分布来提升风电场整体功率输出能力。在所提出的新型风电场偏航优化框架中,提出了一种利用参数监督学习和正则化技术改进的径向基函数神经网络算法,并建立了面向多自由度偏航控制的风电场功率转换智能等效模型。在该模型的基础上,定义了以风电机组偏航角为决策变量的场级最大输出功率优化问题,提出了一种改进的精英多种群遗传算法并进行优化求解,得到各机组偏航角优化值以减少机组间尾流干扰。利用实际风电场布局和风况数据进行了仿真测试。结果表明:所建立的风电场功率转换智能等效模型与其实际特性吻合良好,在特定风况下,偏航优化控制使得风电场总功率提升718.79 kW;连续风况下,所提优化控制方法在不同风况下均有明显的优化效果,风电场总功率平均提升1208 kW,验证了所提出的新型风电场偏航优化框架在提升风电场整体输出功率上的优越性。

灌门水道水轮机阵列布置对潮流运动特性的影响

潮流能水轮机机组的运行会改变潮流运动结构,引起局部区域的水动力特性发生变化,进而影响机组的获能效率。对此,以舟山群岛灌门水道为例,基于Delft 3D软件建立三维水动力学模型,研究机组阵列交错布置对运动潮流的影响。通过网格嵌套分别模拟舟山群岛和灌门水道的潮流运动特征,采用多孔盘法模拟水轮机阵列对潮流空间结构的作用。研究表明:多排机组阵列运行之后,水轮机及下游潮流流速会显著下降,并引起垂向上转轮上、下边缘区流速增加;水轮机迎流方向水位上升,涨、落急时刻最大壅水差值达10cm;水轮机底部及机组之间的床面切应力增大,尾流影响区切应力减小;尾流叠加,床面切应力进一步减小,落急时刻尾流区的床面切应力减小量可达54.03%,阵列两侧床面切应力增大量可达177.93%。在未来的潮流能开发选址中,应全面评估布置水轮机后可能引起的潮流运动特性变化,优化阵列布置,降低对环境的影响。

风电机组双高斯分布致动盘模型研究

准确高效的风电机组尾流模拟对风电场开发设计和优化运行至关重要,CFD方法与简化风轮的致动盘模型相结合是目前工程中常用的尾流数值模拟方法。针对传统的非均匀分布致动盘模型在近尾流区模拟精度不足的问题,提出双高斯分布致动盘模型。该模型考虑了叶片受力特性和尾流分布特征,使体积力在致动盘面上沿径向呈双高斯曲线分布。以风洞实验和风电场实测数据为例进行流场数值模拟。结果表明:双高斯分布致动盘模型适用于推力系数较高的大型风电机组,整体尾流速度模拟精度较高,可准确模拟出尾流半径沿流向的变化,在近尾流区显著改善了传统非均匀致动盘模型低估速度损失的问题。

基于三维卷吸尾流模型的风电场功率计算

风电场中风力机尾流产生叠加效应。为更加准确地计算风电场的尾流风速的分布,该文基于作者前期建立的单台风力机的三维余弦形状的卷吸尾流模型(3DCLE模型),考虑尾流叠加效应,建立了风电场尾流模型(3DCLEWFM)。通过Horns Rev海上风电场及Nørrekær Enge陆上风电场的实测数据来进行验证。结果表明,3DCLEWFM能够精准预测不同风向下风电场中风力机的输出功率,两种案例下,精度较Jensen风电场尾流模型(Jensen wind farm model,JWFM)提高65%和86%,较BPA风电场尾流模型(BPAWFM)提高48%和66%。

考虑大气稳定度的风电场等效粗糙度模型研究

目前模型未充分考虑大气稳定度的影响,针对该问题,在不同大气稳定度下,通过建立风电场边界层的动量平衡关系和揭示风电场的流动不均匀性,提出一种适用于不同大气稳定度的风电场等效粗糙度模型。采用大涡模拟方法对所提模型进行验证,结果显示该方法能有效评估大气稳定度对风电场流动不均匀性以及等效粗糙度的影响,所得等效粗糙度平均误差约10%。

考虑尾流效应的风电场分群综合惯性控制研究

为解决尾流效应导致风电场风速分布不均匀、风电场单机等值模型不能进行精准综合惯性控制的问题,提出一种风电场分群综合惯性控制策略。该策略以具备相近调频能力为机组分群原则,定义调频能力因子作为分群指标,通过聚类算法对机组进行动态分群;综合惯性控制环建立以系统频率偏差量、系统频率变化率为输入,惯性控制比例系数、惯性控制微分系数为输出的模糊自适应综合惯性控制器,进而实现风电场分群综合惯性控制。仿真结果表明,所提出的分群指标是有效的,建立的风电场多机等值模型较准确地体现了各机组群的调频能力,且建立的风电场分群综合惯性控制策略可以提供更为可靠的惯性支撑。