基于修正湍流模型的致动盘方法研究

针对现有致动盘模型高估尾流速度的问题,对动量源项进行修正,以正确反映风轮对来流的作用;对湍流模型进行修正,在标准k-ε方程中添加湍流动能源项和耗散率源项,使湍流动能的生成和耗散相互协调;修正耗散率源项系数,使其沿径向服从抛物线分布。选取风洞实验数据对模型进行验证,并对串列和错列排布下的尾流流场进行分析。结果表明:串列排布时尾流的速度损失和湍流强度均增加,并随间距的增大而减小;错列排布对尾流湍流强度的影响大于对速度的影响,下游尾流会出现湍流强度不对称现象,横向间距为1D时最为严重,横向间距为4D时仍可观察到不对称现象。     

基于熵产理论的三维涡激振动低速水流能俘能分析

以刚性圆柱作为俘能结构，通过强流固耦合数值方法模拟俘能结构在低速水流环境下的三维涡激振动，对共振约化速度(U_r=2～12)范围内进行俘能结构的俘能大小及俘能效率分析。为获得俘能结构三维尾流对俘能大小的影响，采用熵产理论并结合尾流特性以捕捉俘能结构尾涡的能量损失来源及分布，获得不同振动分支下俘能大小与尾流耗散间的关系。结果表明，基于熵产理论的能量损失分析可准确捕捉到俘能结构的能量耗散趋势。黏性熵产主要发生在圆柱表面，湍流熵发生在耗散率较大的尾流区。随着约化速度的增加，上端分支内，当U_r=6时俘能结构的俘能效率最大达到30.5%，俘能结构俘获的水动能增加，圆柱表面的黏性熵损耗和圆柱周围及尾流区湍流熵产损失也增加，俘能结构表面的黏性熵产损失和尾流区湍流熵产损失增大成为俘能效率下降的主要原因。     

山区河流阶梯—深潭结构水力特征数值模拟

为了探究阶梯—深潭结构在不同流量工况下的消能机理,采用紊流数值模拟方法分析研究了阶梯—深潭结构紊动能、紊动能耗散率、流场结构、潭底压力强度等主要水力参数。结果表明,随着流量的增大,平均紊动能耗散率不及平均紊动能增加速率快,验证了阶梯—深潭随着流量的增加消能率会有所下降;小流量工况下深潭内环流区明显,增大了水流滞留潭区时间,能量耗散效果较优;大流量工况下流量的增加促使结构发生自适应调整,水深加大,形成水垫及漩滚耗能,水流阻力增大且维持潭底压差分布趋于平稳,削弱了主流对河床的直接冲刷,有利于河床稳定。     

封闭梯形渠道水流特性的数值模拟

为深入了解封闭梯形渠道的水流特性,以3m长封闭梯形渠道为例,采用雷诺应力紊流模型对不同糙率顶板的封闭梯形渠道水流特性进行了数值模拟,分析了影响其流速场和二次流形态的因素。结果表明,断面流速呈非对称的弓形分布,随顶板糙率的加大,最大流速点向渠底移动;由于受边壁和顶板糙率的影响,过水断面的二次涡流主要集中在固壁拐角处;壁面附近的紊动强度和紊动能耗散量均比较大;粗糙顶板的壁面剪切应力大于光滑顶板的切应力。     

基于改进k-ω SST模型的风力机尾流数值模拟

基于双方程k-ω SST模型提出两种改进湍流模型,采用丹麦科技大学的自编求解器EllipSys3D结合致动盘方法,对单台风力机尾流流场进行数值模拟。该文将SST-sust模型引入到风力机尾流的数值研究中,为了更进一步增大预测到的湍流强度和提高尾流恢复速度,提出SST-Csust模型和SST-Dsust模型,它们是在SST-sust模型的基础上分别调整湍流模型的封闭常系数、修正ω方程的耗散项。通过与理论、实验以及LES模型进行对比分析,结果表明该文提出的改进模型在尾流区速度分布和湍流强度分布的预测方面可取得较好的应用效果。     

含双重植物明渠水流特性研究

以典型沉水植物狐尾藻和挺水植物菖蒲为研究对象,采用三维多普勒超声测速仪(ADV)对含双重植物明渠恒定水流的水力特性进行了试验研究,分析了含双重植物明渠水流的水力学特性及阻力特性并得出其曼宁糙率n的变化规律.结果表明,含双重植物的垂向流速分布基本呈"3"型分布,其紊流具有明显的各向异性特性,底层狐尾藻仅对其高度区域内的流速有较大影响,对高于其植物高度区域的断面流速影响不大.     

明渠水流淹没植被阻力系数计算公式研究

阻力系数是评估明渠水流过流能力的一个重要参数,而对于含植被明渠水流运动,水流结构和流速分布特性较为复杂,传统的明渠水力学公式不再适用.采用经典Darcy-Weisbach方程,通过对含植被河道阻力系数理论分析,识别出影响阻力系数的关键因子为植被淹没度和无量纲化阻力长度,进而基于广泛搜集的水槽试验数据,利用最大差异性算法...     

潮流能水平轴水轮机流场分析

用OpenFOAM开源软件对水平轴水轮机进行数值模拟，以推力T作为体积力附加源项均匀加载到制动盘模型（AD），进行求解计算。在尖端速度比（TSR）为5.5的条件下，对自由区、单台水轮机、水轮机阵列3种情况下的流场进行分析。结果显示：在来流上游时，三者流速曲线吻合，流速增长趋势一致。水流在AD前端时，AD会对其有阻力作用，导致流速大幅下降。在尾流处，在水轮机阵列上游中，并列两侧水轮机尾流与下游水轮机流场发生动能交换，使其尾流恢复速度大于单台水轮机尾流速度。然而，无论是单台水轮机还是水轮机阵列，它们轴线方向上的中部尾流恢复严重不足，这是由于尾流的干涉使得下游水轮机的来流速度不断减小，其可吸收转化的能量也减少，速度随之减小，尾流也无法恢复。     

动能修正系数对城门洞形尾水隧洞水头损失及其系数的影响

基于Realizableκ-ε湍流模型及VOF算法,采用Fluent软件模拟计算某水电站尾水隧洞不同流速下的水流流态,分析了不同流速下动能修正系数的变化规律对水头损失及其水头损失系数的影响。结果表明,隧洞内的水流流速和流向对动能修正系数值均有影响,但边界条件是影响动能修正系数的决定性因素;流速越大,水头损失越大,而水头损失系数越小,且正反不同流向的水头损失及其系数亦不相等;在急变流的条件下,动能修正系数对水头损失及水头损失系数的计算有显著影响,且流速越大,影响越大;正向流速时,动能修正系数对水头损失及其水头损失系数的计算具有加大作用,而反向流速时则具有削减作用。结果可为工程应用提供参考。     

天然挺水植物明渠三维紊流特性研究

以天然挺水植物茭草、芦苇、水葱及模拟植物PVC管为试验材料并种植于大结构混凝土明渠中,研究其在流量及种植密度相同条件下明渠中水流的三维时均流速、脉动强度及雷诺切应力的分布特点。结果表明,明渠水流流速垂线更符合抛物线分布;天然挺水植物之间、天然挺水植物与PVC管间的时均流速分布相差较大,难以用一种分布模式描述;含挺水植物水流紊流特性表现出明显的各向异性;紊动强度与植物的生长形态及柔韧性密切相关;除水葱外,芦苇、茭草及PVC管的雷诺切应力-ρu′zu′x基本接近;相比天然挺水植物PVC管对水流切应力-ρu′yu′z影响最为显著;切应力-ρu′xu′y垂线分布于相对水深0.5处出现极值,总体呈ε形状。     

A wind‐tunnel study of the wake development behind wind turbines over sinusoidal hills

In the present work, the wake development behind small‐scale wind turbines is studied when introducing local topography variations consisting of a series of sinusoidal hills. Additionally, wind‐tunnel tests with homogeneous and sheared turbulent inflows were performed to understand how shear and ambient turbulence influence the results. The scale of the wind‐turbine models was about 1000 times smaller than full‐size turbines, suggesting that the present results should only be qualitatively extrapolated to real‐field scenarios. Wind‐tunnel measurements were made by means of stereoscopic particle image velocimetry to characterize the flow velocity in planes perpendicular to the flow direction. Over flat terrain, the wind‐turbine wake was seen to slowly approach the ground while it propagated downstream. When introducing hilly terrain, the downward wake deflection was enhanced in response to flow variations induced by the hills, and the turbulent kinetic energy content in the wake increased because of the speed‐up seen over the hills. The combined wake observed behind 2 streamwise aligned turbines was more diffused and when introducing hills, it was more prone to deflect towards the ground compared to the wake behind an isolated turbine. Since wake interactions are common at sites with multiple turbines, this suggested that it is important to consider the local hill‐induced velocity variations when onshore wind farms are analysed. Differences in the flow fields were seen when introducing either homogeneous or sheared turbulent inflow conditions, emphasizing the importance of accounting for the prevailing turbulence conditions at a given wind‐farm site to accurately capture the downstream wake development.     

侧风条件下带消能导流装置的直接空冷岛冬季冻结风险模拟研究

哈密某电厂在其空冷岛中使用了一种新型消能导流装置来抵抗侧风影响、稳定机组背压。然而,该电厂冬季空冷单元散热管束冻结的情况依然存在。为了探究该装置对空冷岛冬季防冻的影响,利用Fluent软件对该电厂冬季大风时空冷岛的流动传热特性及各空冷单元的冻结风险进行了模拟研究。研究表明：空冷岛“消能导流装置”整体上对空冷单元的防冻起负面作用;在冬季大风条件下该装置迎风侧空冷单元的散热量平均超出警戒值27%以上,最高达到50%;消能导流装置主要通过提升轴流风机空气流速来增加对应空冷单元的换热量,该装置迎风侧空冷单元轴流风机的轴向空气流速甚至能达到与环境侧风相同的水平,这导致对应空冷单元换热量激增,更容易出现冻结事故;大风条件下该装置在空冷岛下方形成的高压区域分布并不均匀,临近主厂房与相邻空冷岛一侧的高压区域压力更高、面积更大,这些区域空冷单元的冻结风险更高。     

Turbulent natural convection in a vertical parallel-plate channel with asymmetric heating

Turbulent natural convection in a vertical parallel plate channel has been investigated both experimentally and numerically. The experimental channel is formed of a uniform temperature heater wall and an opposing glass wall. A fibre flow laser doppler anemometer (LDA) is used to measure velocity profiles along the channel. Simultaneous velocity and temperature profile measurements are made at the channel outlet. A commercial computational fluid dynamics (CFD) code is used to simulate heat transfer and fluid flow in the channel numerically. The code is customised building in some low Reynolds number (LRN) k–ε turbulence models. The numerical method used in this study is found to predict heat transfer and flow rate fairly accurately. It is also capable of capturing velocity and temperature profiles with some accuracy. Experimental and numerical data are presented comparatively in the form of velocity, temperature, and turbulent kinetic energy profiles along the channel for a case. Correlating equations are obtained from the numerical results for heat transfer and induced flow rate and, are presented graphically comparing with other studies available in the literature.     

Self‐similarity and turbulence characteristics of wind turbine wakes via large‐eddy simulation

Mean and turbulent properties of the wake generated by a single wind turbine are studied in this paper with a new large eddy simulation (LES) code, the wind turbine and turbulence simulator (WiTTS hereafter). WiTTS uses a scale‐dependent Lagrangian dynamical model of the sub‐grid shear stress and actuator lines to simulate the effects of the rotating blades. WiTTS is first tested by simulating neutral boundary layers without and with a wind turbine and then used to study the common assumptions of self‐similarity and axisymmetry of the wake under neutral conditions for a variety of wind speeds and turbine properties. We find that the wind velocity deficit generally remains self similarity to a Gaussian distribution in the horizontal. In the vertical, the Gaussian self‐similarity is still valid in the upper part of the wake, but it breaks down in the region of the wake close to the ground. The horizontal expansion of the wake is always faster and greater than the vertical expansion under neutral stability due to wind shear and impact with the ground. Two modifications to existing equations for the mean velocity deficit and the maximum added turbulence intensity are proposed and successfully tested. The anisotropic wake expansion is taken into account in the modified model of the mean velocity deficit. Turbulent kinetic energy (TKE) budgets show that production and advection exceed dissipation and turbulent transport. The nacelle causes significant increase of every term in the TKE budget in the near wake. In conclusion, WiTTS performs satisfactorily in the rotor region of wind turbine wakes under neutral stability. Copyright © 2014 John Wiley & Sons, Ltd.     

A rule of minimal energy dissipation in modeling of channel flow turbulence

ABSTRACT

A model of turbulence is proposed to solve Reynolds equations for fully developed flow in a wall-bounded straight channel. We show that for the channel flow the Reynolds number can be defined as a ratio of flow kinetic energy to the work of friction/dissipation forces. Then, we introduce a turbulent Reynolds number as a balance between energy losses due to the momentum exchange by turbulent vortices traveling from lowto high-velocity areas and wall friction. The main idea of the model is expressed in the following phenomenological law: The minimal energy dissipation rule requires that a local deformation of the axial velocity profile can and, in the presence of finite-size instabilities, should generate turbulence with such intensity that it keeps the local turbulent Reynolds number below the critical value. Thus, the only empirical parameter in the model is the critical Reynolds number.

The model is applied to several basic channel flows such as the fully developed flow in a circular tube, in an infinite plane channel, and in an annulus. The application of the minimal energy dissipation rule requires an additional integral equation, and this can be considered as an integral-equation algebraic model of turbulence.     

Four-equation turbulence model for prediction of the turbulent boundary layer affected by buoyancy force over a flat plate

Turbulent convective heat transfer with appreciable buoyancy effect over a heated or cooled horizontal flat plate is numerically analyzed by solving four equations for mean square temperature variance , its rate of destruction _θ, turbulent kinetic energy κ and the rate of kinetic energy dissipation . Turbulent time-scale ratio R of temperature fluctuations relative to velocity fluctuations defined by is found to vary widely across the boundary layer. For both highly stable and highly unstable conditions, the ‘four-equation’ model yields better results for mean temperature profile and surface heat flux than the two-equation model. It is also found that the magnitude of thermal von Karman constant κ_θ is not a universal constant but it depends on the thermal stratification of the boundary layer.     

Field synergy equation for turbulent heat transfer and its application

A field synergy equation with a set of specified constraints for turbulent heat transfer developed based on the extremum entransy dissipation principle can be used to increase the field synergy between the time-averaged velocity and time-averaged temperature gradient fields over the entire fluid flow domain to optimize the heat transfer in turbulent flow. The solution of the field synergy equation gives the optimal flow field having the best field synergy for a given decrement of the mean kinetic energy, which maximizes the heat transfer. As an example, the field synergy analysis for turbulent heat transfer between parallel plates is presented. The analysis shows that a velocity field with small eddies near the boundary effectively enhances the heat transfer in turbulent flow especially when the eddy height which are perpendicular to the primary flow direction, are about half of the turbulent flow transition layer thickness. With the guide of this optimal velocity field, appropriate internal fins can be attached to the parallel plates to produce a velocity field close to the optimal one, so as to increase the field synergy and optimize the turbulent heat transfer.     

大河枢纽溢洪道水力试验研究

为寻找高效率消能工型式，促进从泄槽来的高速水流由动能转化为位能，也减轻了对睛游的冲刷，节省工程投资，所确定的推荐方案消能效率经设计方案有所提高。     

可变滚流GDI发动机缸内气体流动特性模拟研究

在一台可变滚流比直喷汽油机(GDI)上对不同滚流强度下缸内冷态湍流流场进行了数值模拟研究,并通过PIV结果进行了实验验证。研究结果表明:进气翻板关闭将显著提高缸内滚流强度并产生较强的湍流,尤其在气门升程最大时刻,其滚流比约为翻板开启时的5倍,湍动能为后者的4倍左右;缸内流场在高滚流比工况时较早地形成单一大尺度涡,同时涡心更明显,流场更加规则,流速相对较高,在进气下止点时平均流速为20m/s;在压缩过程中,高滚流比工况湍流的黏性耗散较大,湍动能衰减较快;但在压缩末期缸内湍动能较低滚流比工况高,同时分布更加均匀。