不同湍流模型在螺旋桨水动力性能计算中的应用与比较

为了研究不同湍流模型在螺旋桨水动力性能预报中的适用性,运用计算流体力学软件对粘性流场中螺旋桨的敞水性能进行了计算研究.分别采用标准模型k-ε、RNGk-ε模型和雷诺应力方程模型(RSM)模拟了敞水螺旋桨在不同进速系数下的推力系数、转矩系数等.由3种湍流模型的预报结果与试验值的对比得出:标准模型对螺旋桨水动力性能的数值预报存在明显的缺陷;RNGk-ε模型相对于标准k-ε模型有所改进,但这种改进仍然没有抛弃基于涡粘性假设的基础,对预报精度的改进是有限的;而RSM模型完全抛弃了涡粘性假设,完全求解雷诺应力的微分输运方程,并且考虑了壁面对雷诺应力分布的影响,因此具有较其他2种模型更强的模拟能力.     

中性悬浮大颗粒对湍槽流影响的数值研究

采用虚拟区域方法对含有中性悬浮大颗粒的湍槽流进行双重直接数值模拟,研究当摩擦雷诺数为180、颗粒体积分数分别为0.79%和2.36%时颗粒对湍流场的影响.结果表明:颗粒的存在削弱了大尺度准流向涡结构,导致近壁处流向脉动速度强度的削弱,且诱导出较小尺度的涡结构,导致近壁处横向和展向脉动速度的增强.颗粒的存在对近壁区脉动速度概率密度分布的影响显著:降低了流向和横向脉动速度概率分布的倾斜度;减小了流向出现大速度脉动的概率,增加了横向和展向出现大速度脉动的概率.对于以方差归一化的脉动速度概率分布,颗粒的影响很小.     

燃气涡轮带肋冷却通道流动与换热的大涡模拟

为研究涡轮内部扰流肋结构的流动和传热机理,分别采用雷诺平均(RANS)方法和大涡模拟(LES)方法对肋通道周期性计算域和全尺寸计算域进行数值研究;结合实验数据,对比分析雷诺平均方法和大涡模拟在流动和换热方面的准确性.结果表明:大涡模拟捕捉到了肋顶扁平涡结构的产生与脱落过程,该旋涡在近似肋顶面的高度和下游空间内发展并掺混,强度逐渐衰减,该过程与湍流强度的分布规律一致;获得了肋前分离流动中向侧壁面偏转的横向二次流等更加详细的流场结构,更加真实地反映了流场的特性;在流动预测方面,大涡模拟更加准确地预测到了平均速度分布和脉动量湍动能的分布规律;在换热方面,大涡模拟方法对换热增强系数的预测误差比雷诺平均方法要小.     

纵向涡发生器对强化传热的影响

纵向涡发生器可以产生纵向涡,而纵向涡可以影响流体的结构,进而可以影响它们的传热过程.通过使用FLUENT计算软件和粒子图像测试仪(PIV),采用数值模拟和实验来研究布置一种新型纵向涡发生器的流场的非定常流动特性及它对槽道的传热的影响.新型涡流发生器为高宽比为0.5的斜截椭圆柱体.数值计算所用的湍流模型为大涡模拟.将数值模拟和PIV实验结果进行比较,两者的涡量场等极为相似,证明数值模拟的计算准确性.模拟和实验表明:高宽比为0.5的斜截椭圆柱体可以产生纵向涡,可以提高槽道的对流换热系数,起到强化传热的作用.     

五级旋风预热器冷模单体流场数值研究

采用雷诺应力(RSM)湍流模型对五级旋风预热器冷模单体流场进行了数值模拟，并同冷模测试结果进行了比较，取得了较好的吻合性。涡壳采用完全非结构网格，锥筒采用结构网格，有限体积法化湍流运动方程为差分方程．差分格式采用一阶迎风，压力速度耦合采用经典的SIMPLE算法数值求解湍流运动方程组，并在求解过程中采用网格自适应技术。通过对模拟结果分析，得到了旋风筒流场的基本特征。     

LDV用于搅拌槽流场测量的实验研究

对三维激光多普勒测速系统(3D-LDV)和测量原理进行了介绍,同时利用该设备对搅拌槽内的流场进行了湍流测量和流场分析,给出了搅拌槽内的时均速度场分布、脉动速度、湍流动能和雷诺应力等实验结果,为搅拌槽的设计和放大提供了很有价值的实验依据.     

反旋向多漩流器阵列燃烧室流场的数值研究

采用雷诺应力模型(RSTM)对于反旋向多漩流器阵列贫油直喷(LDI)燃烧室的非反应冷态流场进行了数值模拟.研究中采用了一种3×3的漩流器阵列结构,相邻漩流器的旋向交替变化.计算结果表明,所研究的燃烧室流场结构十分复杂.由于相邻漩流器之间对于漩流的强化作用,因而各回流区维持了较长的长度.流场中存在的多个回流区及强湍流表明该燃烧室有潜力实现良好的燃烧性能.通过将雷诺应力模型的模拟结果与LDV实验数据进行比较发现,计算结果与实验值符合良好,说明雷诺应力模型较好地求解了回流区和高速度梯度问题.     

梢涡空化CFD数值方法

针对梢涡空化问题,应用雷诺平均的N-S方程并结合改进VOF(volume fraction)模型,对椭圆形水翼梢涡空化特性进行了研究,采用的湍流模型为代数雷诺应力模型.为减小网格离散误差,计算中对梢涡处的网格进行了加密.对未发生空化时梢涡内轴向速度和切向速度分布进行计算,为了准确模拟梢涡涡核内系统旋转和流线曲率影响,将...     

槽道内聚合物减阻流动对数层湍流结构的影响

为深入阐明聚合物减阻剂对壁湍流的影响,试验研究了聚合物槽道减阻流中近壁对数层的湍流特征结构。高分子聚合物溶液经过多孔壁面缓慢注入槽道,采用立体粒子图像测速技术测量对数层中流向与展向平面内3个方向的速度分量,采用二维粒子图像测速技术测量流向与垂直壁面方向平面的速度场。基于条件平均的方法,发现在对数层存在低动量流体的大尺度条带结构,在黏弹性聚合物溶液的影响下,条带在展向方向上扩展,在流向方向上拉伸;条带周围垂直壁面方向的脉动速度强度明显降低,类喷射运动被显著抑制。     

三维紊流特性测量与理论分析

为充分反映流场的三维紊动特性,基于PIV技术进行了明渠水槽实验.采集了不同工况下的瞬时流速数据,利用统计法分析了流场的变化.结果表明:水槽紊流场中心点A的脉动速度随时间的变化呈正弦(余弦)函数变化;随着点A的涡线上升,槽底附近的涡体逐渐由马蹄涡演变为发卡涡;横、纵向时均流速沿垂线分布均符合指数流速分布,且指数n=7时,实验误差值最小,水平面的相对误差为1.1%,纵剖面的相对误差为1.15%;在水深比为0.1附近,相对紊动强度在明槽底部变化剧烈,随着水深比的增加,强度逐步降低.结果进一步证实了前人对雷诺应力的研究结果.     

Heat transfer and flow characteristics in a channel with one corrugated wall

The present study numerically investigates the characteristics of three-dimensional turbulent flow and heat transfer in the channel with one corrugated wall heated with constant temperature by means of large eddy simulation.The corrugated wall is sinusoidal in the streamwise and spanwise directions.The Reynolds number in terms of bulk velocity and channel half-height is fixed at 2800 and the wave amplitude to wavelength ratio is varied in the rangeα/λ=0.01,0.02,0.04 in the streamwise direction andα/λ=0.01,0.02,0.04 in the spanwise direction.The results show that flow separation bubbles appear and near-wall streamwise vortices are generated with larger population in the upslope region of the bottom wall as wave amplitude increases.Compared with flat wall,the corrugated geometry increases the pressure coefficient and decreases the friction coefficient on the corrugated wall,and consequently increases the total drag coefficient owing to the increase of pressure coefficient,as expected,the heat transfer is higher.The waves in the spanwise direction converge the vortices into the trough along the streamwise direction and push them away from the bottom wall.Finally,thermal performance factor is defined and the effects of wave amplitude on the thermal performance are scrutinized.     

减阻沟槽表面对湍流边界层高阶矩特性的影响

以光滑平板及间隔V型减阻沟槽面为研究对象,利用二维LDV系统获得光滑平板及沟槽板上部区域湍流边界层流场的流动参数。通过象限分析法深入研究减阻沟槽表面湍流边界层中雷诺切应力的变化情况,着重考察沟槽面对湍流边界层高阶统计参数的影响,并将高阶统计参量同湍流边界层相干运动规律结合起来,由此来探讨沟槽面与湍流边界层之间的相互作用规律。结果表明,沟槽的存在抑制了整个近壁区低速条带的喷射运动,降低了y+<15区域内高速流体的下扫强度;雷诺切应力以及湍动能的法向输运主要是由喷射和下扫这两种运动来完成的,沟槽的存在削弱了雷诺切应力及湍动能的法向输运水平;沟槽的存在减小了y+<13区域内的流向速度脉动的偏斜因子与y+<3区域内流向速度脉动的平坦因子。     

壁湍流周期性吹吸减阻的实验研究

为了研究局部周期性吹吸小扰动对湍流边界层的影响和湍流相干结构减阻控制机理,用实验方法研究了局部周期性吹吸小扰动与湍流边界层的相互作用.采用热线测速技术精细测量了施加吹扰动前后平板湍流边界层不同法向位置的流向、法向和展向速度分量的时间序列信号;通过测量扰动源下游近壁区域平均速度剖面的变化,发现在扰动下游一定空间范围内,粘性底层增厚和壁面减阻;通过对湍流度变化的分析,发现扰动影响猝发的发生;此外通过对展向平均涡量变化的分析发现,在扰动下游形成了2个定常的、强度相似的、大尺度的展向涡,它们起到流体滚动轴承的作用,减小了壁面摩擦阻力.     

Analysis of solid particle clusters in coherent structures of turbulent channel flow

A particle-laden turbulent channel flow is investigated to study particle clusters in large-scale turbulent coherent structures.The fluid phase is calculated by large eddy simulation and particles are tracked using Lagrangian trajectory method.The flow Reynolds number is 180 based on the friction velocity and half-width of the channel.The particle is lycopodium with St=0.93which may well follow the fluid phase.The mean and fluctuating velocities of both two phases are obtained,which are in good agreement with previous data.The strongest accumulations of particles in low-speed streak structures are observed at y+=11.3.Moreover,once particles are captured in low-speed streaks,most of them will reside there for a long period.Particles clustered in low-speed streaks obtain smaller instantaneous wall-normal and spanwise velocities than those out of there,which induce a larger particle flux into low-speed streaks than that out of there.The study is important for understanding particle dispersion mechanisms in gas-particle turbulent channel flows.     

Influence of rheological parameters in all drag reduction regimes of turbulent channel flow with polymer additives

The influence of rheological parameters on vortex dynamics and the extent of drag reduction(DR) were deciphered via extensively analyzing the hi-fidelity direct numerical simulation results of the turbulent channel flow with polymer solutions.It has been observed that in all drag reduction regimes from the onset of DR to maximum drag reduction(MDR) limit,the Deborah number is defined as the product of an effective Weissenberg number,and the root mean square streamwise vorticity fluctuation remains O(1) in the near wall region.The ratio of the average lifetime of axial vortices to the vortex rotating duration decreases with increasing DR,and MDR is achieved when these time scales become nearly equal.Based on these observations a simple framework is proposed adequately to describe the influence of polymer additives on the extent of DR from onset to MDR as well as the universality of the MDR in flow systems with polymer additives.     

Large eddy simulation of unsteady transitional flow on the low-pressure turbine blade

The aim of this paper is to predict the phenomenon of laminar separation,transition and reattachment in a low-pressure turbine(LPT).Self-developed large eddy simulation program of compressible N-S equations was used to describe the flow structures of T106A LPT blade profile at Reynolds number of 1.1X105 based on the exit isentropic velocity and chord length.The computational results show the distributions of time-averaged wall-static pressure coefficient and mean skin-friction coefficient on the blade surface.The locations of laminar separation and reattachment points occur around 87%and 98%axial chord,which agree well with experiment data.The two-dimensional shear layer is gradually unstable along the downstream half of the suction side as a result of the spanwise fluctuation and the roll up of shear layer via Kelvin-Helmholtz(KH)instability.Three-dimensional motions appear near 84%axial chord which later triggers spanwise vortexes and streamwise vortexes,leading to transition to turbulence in the separation bubble.Through introducing the concept of dissipation function,the high loss mainly comes from the places where strong shear layer and intense fluctuation exist.Furthermore,the separation region is only an accumulation center of the low-energy fluid rather than an area of loss source.     

小尺度沟槽面边界层湍流参数的测量

利用二维激光多普勒测速仪对零压力梯度下光滑面和小尺度沟槽面的沟槽及峰上部区域中湍流边界层流场进行了对比测量。着重考察了在相同流动条件下两位置的平均速度、流向速度脉动强度、高阶矩以及雷诺剪应力的分布特性。实验中发现 :沟槽能够增加粘性底层的厚度 ,减小壁面剪切力 ,有减阻效应。而此处的流向速度脉动的强度、高阶矩以及雷诺剪应力在距离壁面的某一区域中有减小或降低现象 ,说明沟槽具有削弱湍流湍动的功能 ;而峰上部的湍流统计平均量则表现出与槽相反的结果或趋势。同时还讨论了该沟槽面减阻的原因     

岸坡植被对复式河道水动力特性影响的数值模拟

广泛分布于天然河道岸坡的植被能够改变河道水流特性，进而影响坡脚淘刷、河岸稳定及河道形态演变趋势。为精细揭示岸坡植被对复式河道水动力特性的影响，提出植被固壁边界处理方法，利用雷诺应力模型封闭紊流控制方程构建了复式河道三维水动力数学模型，并对复式河道水流特性进行了模型验证，模拟结果与实测结果吻合较好。将该模型应用于岸坡有植被的复式河道水流特性研究，对比非淹没刚性植被、淹没刚性植被、淹没与非淹没刚性植被交替和无植被情况下流速、流量、河床剪应力、雷诺应力以及紊动能分布的差异。结果表明：岸坡植被加剧了滩槽间横向动量交换，引起二次流涡旋数量、强度和范围的增加，同时使得横断面流速、流量分配、河床剪应力、雷诺应力及紊动能在滩槽交界区显著减小，紊动交换在临近植被的周边区域明显增加。其中岸坡种植非淹没刚性植被时效果最为显著，相比无植被情况，二次流涡旋数量增加了一对，岸坡植被区流速、流量分配、河床剪应力、雷诺应力及紊动能大幅减小，甚至趋近于零，而临近植被的周围区域紊动交换剧烈，无量纲化的紊动能值可达到20。     

Effects of surface shapes on properties of turbulent/non-turbulent interface in turbulent boundary layers

The effects of directional riblets surfaces on the turbulent/non-turbulent(T/NT) interface in turbulent boundary layers are experimentally investigated using two-dimensional time-resolved particle image velocimetry(PIV). The velocity field of streamwise—wall-normal plane for the smooth surface, converging and diverging riblets surfaces are measured. The interface is detected using the criterion of local kinetic energy. The statistical properties of interface height and conditional averaged velocity for different surfaces are analyzed. It is shown that, the converging and diverging riblets surfaces have little effect on the fractal dimension of the T/NT interface, but they cause the intermittency profile deviate from error function and the probability distribution of interface height deviate from Gaussian function. To be specific, the distribution of interface height for the converging riblets surface shows a positive skewness while it shows a negative skewness for the diverging riblets surface.Moreover, the conditional averaged streamwise velocity and spanwise vorticity across the interface are analyzed, and it is found that their self-similarities are preserved for different surfaces when normalized with respective friction velocity. The correlation analysis reveals that near-wall streamwise velocity fluctuation and interface height show a negative correlation.