临界区雷诺数下翼型流场电磁力控制的脱体涡模拟

利用Maxwell方程直接数值计算表面包覆电极与磁极翼型产生的三维Lorentz力分布,将其加入到流体控制方程的动量方程中,采用脱体涡模拟方法对临界区雷诺数下受Lorentz力作用下翼型绕流场进行数值模拟,研究分析了电磁力作用系数和攻角对翼型绕流场结构及其升阻力系数的影响机理和规律。结果表明,Lorentz力可以有效地改善翼型周围的流场结构,达到减阻、增升、消涡以及延缓和抑制其失速的目的,因此是翼型的一种有效流动控制手段。     

亚临界雷诺数下波浪型圆柱绕流的数值模拟及减阻研究

采用大涡模拟数值方法对亚临界雷诺数Re=3000下波浪型圆柱的绕流现象进行研究分析。研究结果表明,波浪型圆柱的三维尾迹涡结构能得到很好的控制,它在轴向方向呈现周期性正负涡的交替分布特性。随着幅值的增大,波浪型圆柱表面的自由剪切层得以延展,使得旋涡的脱落发生在波浪型圆柱下游较远处,从而达到减阻的目的。波浪型圆柱与普通直圆柱相比,其幅值对平均阻力系数及脉动升力系数的减少及尾迹控制有着更重要的影响,最大减阻可达16%。     

用SST-DES和SST-URANS方法数值模拟亚临界雷诺数下三维圆柱绕流问题

基于剪切应力运输(shear stress transport,SST)的分离涡模拟(detached-eddy simulation,DES)在近壁面的流动区域采用SST模型,而在其他区域采用亚格子模型求解流场。本研究中分别应用SST-DES和SST-URANS方法对亚临界雷诺数Re=3900下的三维圆柱绕流进行了数值模拟,比较分析了两种数值方法下圆柱表面的时均压力分布、圆柱后方的漩涡脱落特征、圆柱下游流场的时均速度剖面分布等,并将数值模拟结果同前人物理试验比较,结果表明SST-DES方法在亚临界雷诺数下的三维圆柱绕流数值模拟问题上比SST-RANS方法具有更好的效果。     

方柱绕流数值模拟

利用FLUENT中的RNG k-ε算法对长宽比H/B=1.5方柱绕流进行数值模拟,分别模拟了在雷诺数Re=800的亚临界条件下,4种不同情况下的流动。分别为:流体流过单个方柱,流体流过间距为1H,2H,3H的两并列方柱的流动。根据数值模拟结果,分别得出了4种流动状态下流场,壁面的升阻力系数。并对升力系数进行傅里叶变换,分析及归纳了方柱绕流旋涡脱落的规律。     

流向振荡圆柱绕流中的涡脱落模态分析

本文用有限体积法对平行于均匀来流方向受迫振荡的圆柱绕流问题进行了二维数值模拟。雷诺数选取Re=200、855等几种亚临界雷诺数情况。研究了不同振幅和振动频率下的流场结构、涡脱落模态和一些重要流动参数如升阻力系数、Strouhal数的变化关系。西文计算表明，流向振荡圆柱绕流中涡脱落方式可划分为对称与反对称两种形态，在反对称的涡脱落现象中又观察到了三种基本模态，相互竞争的各种模态间的共同作用对流动特征起决定性作用。同时计算结果还验证了在圆柱绕流实验中观察到的频率锁定现象。     

基于LES方法圆柱绕流三维数值模拟

该文采用计算流体软件CFX5中Large-eddy simulation(LES)模型计算了均匀流场中三维圆柱绕流的水动力特性。使用有限体积法对三维N-S方程进行求解。数值模拟着重研究了高雷诺数时展向各截面的压力、阻力、升力及涡管特性。数值计算结果表明:展向各截面柱体受力关于中截面对称且小于二维情况,柱体周围流场呈现明显的三维特性。     

螺旋侧板厚度对浮式风力机Spar平台涡激载荷的影响

Spar平台因其特殊的深吃水立柱式结构,在来流影响下极易发生涡激运动。为探究不同厚度螺旋侧板对浮式风力机Spar平台涡激载荷的影响,基于CFD方法,针对不同厚度螺旋侧板,对平台进行了数值模拟并详细分析平台的升阻力系数、压力场及涡量场等流场参数。结果表明:螺旋侧板可有效抑制涡激载荷,且随厚度增加,升力系数幅值先增加后减小,阻力系数逐步减小;在研究范围内,螺旋侧板厚度为0.10 D时,升力系数幅值最小,阻力系数与原型平台在同一范围内,对涡激载荷的抑制效果最好。     

绕轴对称体三维非定常空化流动的数值与实验研究

采用数值和实验相结合的方法研究了绕轴对称体三维非定常空化流动现象。实验中,采用高速摄像的技术和动态测力系统对绕半球型和平头型轴对称体的非定常空化流场及其动力特性进行了分析,观察了在不同空化数下,绕半球型和平头型轴对称体的空泡形态;测量了轴对称体受到的阻力,并对阻力信号进行了时频分析,得到了在非定常空化阶段,轴对称体动力特征频率。基于实验现象,发展了一种基于密度修正的分域空化流动的计算方法,并与实验结果进行了对比,结果表明:该方法可以较好的模拟绕轴对称体三维非定常空化流动现象。绕不同头型轴对称体的空化流场结构均存在明显的三维非定常流动特性,并且,空化流场形态与动力特征频率存在高度的相关性,不同头型轴对称体的非定常空化流场结构存在较大的差异,半球型轴对称体空泡流动的脉动主要是空泡尾部的高频小脱落引起的,而平头型轴对称体的空泡流脉动成分主要是大尺度的漩涡空泡团的周期性脱落。     

三维圆柱绕流数值模拟湍流方法的选择

研究湍流模拟方法对三维圆柱绕流数值模拟精度的影响,分别采用雷诺平均法（RANS）中的κ-ω模型、SST模型以及大涡模拟法（LES）对亚临界区内雷诺数Re=3 900时的三维圆柱绕流进行数值计算,分析了圆柱体表面的受力、圆柱后流场时均速度特性与瞬时涡量分布情况。结果表明,当流体流过圆柱表面时,圆柱表面出现的与流速方向相反的负压力差区域使流体从圆柱表面分离,引起了不稳定的周期性交替脱落的湍流涡泄,从而在圆柱表面产生周期性波动的升力,同时在圆柱后近流场区域形成回流区。研究还发现,LES法对圆柱体的受力以及流场时均速度特性的模拟效果要优于κ-ω模型与SST模型;相较于前人利用浸入边界法得到的模拟结果,LES法的模拟精度也有了较大提升;同样,通过对瞬时流场涡量等值线图的分析,并与已有的模拟结果进行对比,发现LES法不但可以从整体上表现出漩涡的周期性脱落,而且对流场中不同位置的、复杂的小尺度湍流涡泄也描述得非常细致,得到的自由分离剪切层长度与湍流涡泄的卷曲度更符合湍流涡泄的特征。所以,在亚临界区,LES法对湍流的模拟效果相对较好。     

基于LES的亚临界和超临界圆球绕流数值研究

三维圆球绕流是流体力学中的经典问题,国内外学者曾对此进行过许多研究,但大多局限于较低雷诺数。该文采用基于动力亚格子模型的大涡模拟对亚临界雷诺数(Re=10~4)和超临界雷诺数(Re=1.14×10~6)等高雷诺数下的圆球绕流进行了数值仿真计算。计算结果表明,得到的阻力时均值和经典Re-Cd曲线相吻合,相比于亚临界雷诺数,超临界雷诺数下近球面回流区剧烈收缩,阻力系数锐减;圆球尾流场明显偏离了中心轴(x轴),导致了侧向力的存在;球后有大尺度的规则涡从圆球表面脱落。     

用两相流模型模拟混流式水轮机内空化流动

本文采用两相流中两流体模型模拟混流式水轮机转轮内三维湍流空化流场, 基于两相流中两流体模型的理论和方程, 推导了水轮机转轮中空化两相流的两流体基本方程, 并利用k-ε-kp两相湍流模型计算水轮机转轮中三维空化两相湍流. 计算结果包括了液相和空泡相的主要流动特征, 模拟结果可初步预测流体机械空化性能, 缩短模型试验周期.     

Application of the ‘natural flow paradigm’ in a New Zealand context

The natural flow paradigm (NFP) emphasizes the need to partially or fully maintain or restore the range of natural intra‐ and interannual variation of hydrologic regimes to protect native biodiversity and the evolutionary potential of aquatic, riparian and wetland ecosystems. Based on our studies of natural and managed flow regimes in New Zealand, we do not believe that all components of the natural flow regime are necessary to achieve the objectives of the NFP, either partially or fully, because many aquatic species have very flexible niches and life‐history requirements (i.e. there is ‘ecological redundancy’). Obviously, maintaining the natural flow regime will maintain the hydrologic and hydraulic conditions necessary for sustaining natural ecosystems. However, if there is adequate knowledge of what ‘values’ need to be maintained in a waterway, and the aspects of the flow regime that are required to maintain those values are also known, then regimes can be designed that target these requirements and thus optimize conditions for the ‘values’. We believe that an assessment of ecosystem requirements using information on river processes together with habitat requirements and life‐history strategies of biota can achieve the best balance between resource use and sustaining ecosystem function and value, and show examples where changes to natural flow regimes have maintained, or even improved, instream values in some New Zealand rivers. We caution that simple flow‐based rules, such as those that might be developed under the NFP, could be unnecessarily restrictive on multiple use of water in New Zealand while, at the same time, preclude the opportunity for enhancement of key ecosystem values in many waterways. Copyright © 2008 John Wiley & Sons, Ltd.     

弯曲管段内流动的大涡模拟

本文应用湍流大涡模拟方法对三维空间的弯管流进行了数值模拟。计算中使用了隐含流线迎风耗散作用具有三阶部分展开的Taylor-Galerkin离散格式，导出了经Gauss滤波流大涡模拟方法适用于边界形状复杂和存在各向异性大尺度涡的流动仿真。     

中间渠道内非恒定流数值模拟

利用平面二维非恒定流数值模型，来研究中间渠道内非恒定流问题，该模型基于任意三角形计算网络，采用隐式求解自由表面的剖开算子法，离散求解控制方程，有效地处理了非线性混合算子和计算自由表面的困难，避免人工滤波措施，提高了计算精度和计算稳定性，并在三峡通航工程方案比较中得到应用。计算结果表明，该模型能较好地反映中间渠道内水流特征，与分析模拟船闸不同运行方式下渠道内水流波动特性，达到优化运行方案，改善渠道内航行条件的目的。     

Instream flow methods: a comparison of approaches

Minimum flows in rivers and streams aim to provide a certain level of protection for the aquatic environment. The level of protection is described by a measure such as a prescribed proportion of historic flows, wetted perimeter or suitable habitat. Conflicting minimum flow assessments from different instream flow methods are arguably the result of different environmental goals and levels of protection. The goals, the way in which levels of protection are specified, and the relationship between levels of protection and the aquatic environment are examined for three major categories of flow assessment methods: historic flow, hydraulic geometry and habitat. Basic conceptual differences are identified. Flow assessments by historic flow and hydraulic methods are related to river size and tend to retain the ‘character’ of a river. Habitat-based methods make no a priori assumptions about the natural state of the river and flow assessments are based primarily on water depth and velocity requirements. Flow and hydraulic methods assume that lower than natural flows will degrade the stream ecosystem, whereas habitat methods accept the possibility that aspects of the natural ecosystem can be enhanced by other than naturally occurring flows. Application of hydraulic and habitat methods suggests that the environmental response to flow is not linear; the relative change in width and habitat with flow is greater for small rivers than for large. Small rivers are more ‘at risk’ than large rivers and require a higher proportion of the average flow to maintain similar levels of environmental protection. Habitat methods are focused on target species or specific instream uses, and are useful where there are clear management objectives and an understanding of ecosystem requirements. Flow and hydraulic methods are useful in cases where there is a poor understanding of the ecosystem or where a high level of protection for an existing ecosystem is required. © 1997 John Wiley & Sons, Ltd.     

感潮河段弯道水流的水力特性

网走湖水质主要为生物化学污染。论文利用声学多普勒流速剖面仪(ADCP)和咸水温度及深度测量仪(STD),于1996年的4月和8月在该湖出口下游河道的一个180°弯道处分别对三维流速场和咸淡异重流进行了原型观测。结果表明:在4月的融雪季节,从湖里流出的淡水主要为由上游流向下游的单向水流流动,且其流速u为矩形分布,量值与潮位有关;环流流速v与u值密切相关,下层流向指向凹岸。在8月的春潮季节,主要为上淡下咸且主流流向不同的成层异重流流动,且u的分布及流速梯度随着流向的不同而不同。经过观测数据与计算值的比较表明:融雪季节观测的主流和环流流速分布与理论计算值十分接近;春潮季节观测的环流流速表现为:(1)上、下层中的环流方向相反;(2)除过渡流外,环流强度与理论计算值较为接近。含盐度分布特征也因水流条件的不同而不同。     

圆柱尾迹流场中横向振荡翼型绕流的数值模拟

该文数值模拟了静止圆柱尾迹中横向振荡翼型绕流的涡结构、力及频谱特性,揭示前方尾迹流场对振荡翼绕流特性的影响,以了解鱼类游动时自主利用前方流场有利干扰的流体力学机理。研究表明圆柱尾迹的影响可导致翼型前缘涡脱落模态发生变化;翼型尾迹的频谱特性更加复杂;在一定的参数下翼型阻力系数可以整体下降。     

匀加速流中圆柱起动流动的研究

本文用氢泡流动显示和数值计算计算求解Ｎ－Ｓ方程的方法说细研究了匀加速来流中圆柱起动流动。匀加速起动的圆柱绕流和突然起动的圆柱绕流的流动结构相似，中在低Ｒｅａ数时出现“ａ－现象”；在高Ｒｅａ时出现“β－现象”，但只有一个二次涡，起动涡形成初期，内部基本上是层流；起动涡形成后期，内部流动变为湍流，用多种布置铂丝的方法，测量了流场的速度分布，计算结果和实验结果进行了比较，二者基本一致。     

应用PIV进行二维后向台阶流流动特性的研究(2)二维后向台阶流起动流的研究

利用PIV技术，对二维后向台阶起动流的瞬态速度场进行了观测。得到层流至湍流情况下起动流的一些运动规律。本文分析了流动中起动涡等旋涡的运动和发展特征。分析了起动流过程中再附着点的位置、起动涡等旋涡涡心的位置等的变化规律。     

Performance of the bio-inspired leading edge protuberances on a static wing and a pitching wing

It is shown that the leading edge protuberances on the flippers of a humpback whale can significantly improve the hydrodynamic performance. The present study numerically investigates the flow control mechanisms of the leading edge protuberances on a static wing and a pitching wing. For static wings, the performance in both laminar flow and turbulent flow are studied in the context of the flow control mechanisms. It is shown that the protuberances have slight effects on the performance of static wings in laminar flow. Also, it could be deduced that non-uniform downwash does not delay the stall occurrence in either laminar flow or turbulent flow. In turbulent flow, the leading edge protuberances act in a manner similar to vortex generators, enhancing the momentum exchange within the boundary layer. Streamwise vortices do contribute to the delay of the stall occurrence. The normal vorticity component also plays an important role in delaying the stall occurrence. However, for the pitching wing, the effect of leading edge protuberances is negligible in turbulent flow. Detailed analysis of the flow field indicates that for the wing with the leading edge protuberances, the leading edge vortices become more complex, while the thrust jet and the vortices in the wake are not changed significantly by the leading edge protuberances.