过渡流下近壁插人圆柱瞬时流动传热特性研究

为了进一步探讨过渡流状态下,在近壁面区域插入圆柱对壁面传热强化影响的机理,本文利用基于复合网格系统的计算方法,对过渡流范围内Re=500,圆柱处于最佳插入位置,即C/D=0.6时近壁插入圆柱流场的瞬时传热特性进行数值研究。研究结果表明:在壁面附近插入圆柱能够导致圆柱下游壁面产生涡岛,而涡岛对壁面的洗刷效应正是促使圆柱下游壁面传热增强、摩擦降低的主要原因。     

过渡流下顺排双列圆柱流动、传热特性的数值研究

为了研究过渡流状态下管束传热强化机理,通过基于复合网格的专用程序,本文建立了顺排布置双列管束计算模型。计算结果表明:1)两列圆柱之间的流场结构分为稳定结构(对称死区)和非稳定结构(旋涡),非稳定结构的流场下的下列圆柱传热效率高于稳定结构流场的传热效率。2)由于旋涡的出现,在一定的横向间距内,传热效率升高速率要大于流动阻力升高的速率,大于这个横向间距后,情况相反,这一结果肯定了旋涡不稳定性对下游圆柱壁面传热强化的影响.3)随着纵向间距的缩小,涡脱频率升高,两列圆柱之间流体被加速,加速了旋涡的发展,这能在更小的横向间距内,率先使传热效率升高。     

开槽锥形孔对耦合传热特性的影响研究

本文通过数值模拟的方法对不同开槽孔结构对壁面的流场分布和冷却效率进行了研究,综合考虑固体内部导热和壁面表面对流换热的耦合传热特性,研究了开槽锥形孔和开槽圆柱孔对下游壁面冷却效果的影响。结果表明:在流速比V_r=0.5时,开槽锥形孔的冷却气能更好的被压覆在冷却壁面上,且有较大的横向扩展,对整个下游壁面有较好的冷却效果。     

圆柱近尾迹湍流涡结构的直接数值模拟

本文运用谱元方法对圆柱尾流进行直接数值模拟。通过对中等Re数（1000）时的圆柱近尾迹湍流涡结构的演变系统细致的研究,经过长时间流动计算,用速度矢量及相应涡结构示意图显示主涡,二次涡结构的相互作用,显示整个尾迹涡结构在一个周期内的演变过程,获得了在实验中难以清楚观察到的近壁流动特征。本文得到圆柱近壁处二次涡形成有两种不同的机理,并总结出涡的合并所需要的条件。     

低雷诺数下双向受迫振动圆柱尾涡结构分析

本文使用本团队自主开发的一款通用统一的流体力学计算软件UCFD,在Re=100和Re=500时对横、纵双向受迫振动圆柱绕流进行了数值模拟研究,研究发现了一些新的涡结构:在横向振动的基础上增加纵向振动后,在Re=100时,随着纵向振幅的增大,在某些工况下,圆柱的尾涡结构会在2P和P+S模式之间变化;当振动圆柱的纵向振幅增大至0.8D后,圆柱一周期脱落涡可达到5个;Re=500时,随着纵向振幅的增大,圆柱尾涡的2S与P+S结构分布逐渐减少,2P结构分布逐渐变多;当振动圆柱的纵向振幅增大至0.6D后,在某些工况下圆柱一周期脱落5个涡。     

过渡流下后向台阶流动与瞬态传热特性研究

为了研究过渡流下后向台阶的流动与瞬态传热特性,建立了二维不可压缩后向台阶流动的数值传热模型,采用非定常数值方式模拟了后向台阶的内部流场,比较了台阶下游底面瞬时努塞尔数Nu和时均努塞尔数Nu_m的分布,分析了台阶下游通道内温度分布规律。结果表明:过渡流下台阶底面Nu数沿着流动方向出现多个峰值,Nu数始终围绕Nu_m数发生波动。Re数为700时,主回流区未出现二次回流,Nu数在主回流区下游才发生波动;Re数为1200时,主回流区出现二次回流,Nu数在主回流区就发生了波动。台阶下游通道内温度沿法向呈阶梯式变化,Re数越大,近底面处的温度变化梯度也越大。     

基于(火积)耗散极值原理的管外流场优化

本文基于(火积)耗散极值原理得到了对流传热的优化场协同方程,用Fluent数值模拟求解层流状态下的管外流动换热的优化流场。得到了管外流动的优化速度场为多纵向涡结构形态,对优化流场进行分析,发现优化流场的综合换热效果远远高于原型流场。通过在管外添加扰流片的方法,来产生带有纵向涡的流场,达到优化管外流动传热的目的。数值模拟结果表明,添加扰流片在一定程度上可以强化管外对流换热。     

锯齿型通道流动与换热的自维持振荡实验研究

本文对锯齿型通道内流动与换热的自维持振荡特性进行了实验研究。从可视化实验中观察到,Re300时,流场随时间几乎没有明显的变化;Re300,流动出现了明显的回流现象,且随着Re数增大,回流形成的涡增大,流动产生的涡的位置随时间呈有规律的周期性变化;在恒热流边界条件下,通道上、下壁面温度沿流动方向的变化与数值模拟所得到的Nu数、壁温的变化规律一致。     

圆柱和直桨叶突然启动瞬态流动的数值研究

采用基于动网格方法的有限体积法对圆柱和直桨叶突然启动引起的二维非定常不可压粘性流进行了数值模拟,给出了计算方法.通过计算得到了Re=5000和9500时圆柱突然启动后的流场随时间演化的非定常过程.将计算结果与实验结果进行了对比,两者吻合较好;采用大涡模拟对高雷诺数下的直桨叶突然旋转启动的流场进行了计算,得到了完成启动后叶片内部瞬态流场的分布结果.     

三维槽道两相流颗粒运动的大涡模拟

利用基于动态粘性亚网格模型的大涡模拟方法,分别模拟了Reτ=180的三维槽道湍流内的三种颗粒,以及Reτ分别为180和395时的塑料颗粒的运动状况。其中气相控制方程的求解采用分步投影方法,泊松方程采用基于FFT的快速求解,颗粒的拉氏控制方程采用二阶龙格-库塔积分．模拟统计得到了流体和颗粒的平均速度、脉动速度等,不同的颗粒在流场中(特别是近壁区)表现出来了不同的行为,并再现了颗粒在壁面附近局部富集的现象．随Reτ提高,颗粒各方向上的速度脉动都有所增强．     

圆柱振荡绕流中涡旋运动模式(pattern)的数值模拟

提出并发展的一种基于区域分解思想,综合了解N-S方程的有限差分法及涡法各自优点的新数值方法,计算了各种 Keulegan-Carpenter数下(Kc=2~24)振荡流绕圆柱的流动。系统地研究了振荡流中涡旋运动模式随Kc数变化的规律,模拟了不对称区、单对涡区(或模向区)、双对涡区(或对角区)和三对涡区四种不同的涡旋运动模式。将计算所得的阻力系数C_D、惯性系数C_M与国外近期发表的计算结果进行了比较。     

Viscous near-wall flow in a wake of circular cylinder at moderate Reynolds numbers

Here we present the results of experimental investigation of a cross flow around a circular cylinder mounted near the wall of a channel with rectangular cross section. The experiments were carried out in the range of Reynolds numbers corresponding to the transition to turbulence in a wake of the cylinder. Flow visualization and SIV-measurements of instantaneous velocity fields were carried out. Evolution of the flow pattern behind the cylinder and formation of the regular vortex structures were analyzed. It is shown that in the case of flow around the cylinder, there is no spiral motion of fluid from the side walls of the channel towards its symmetry plane, typical of the flow around a spanwise rib located on the channel wall. The laminar-turbulent transition in the wake of the cylinder is caused by the shear layer instability.     

竖直圆管内超临界压力煤油传热特性实验研究

在压力2.5~4 MPa, 质量流量0.7~1.7 g/s, 热流密度0.06~1 MW/m2的实验条件下, 对煤油在内径1 mm, 长度300 mm竖直上升圆管内的流动与传热特性开展了实验研究, 并分析了传热系数随局部油温的变化及不同实验参数对传热的影响.结果表明, 超临界压力下煤油传热主要由自身物性和流动状态决定.超临界压力煤油传热过程大致可以分为3个区域:正常传热区传热强化区和传热恶化区.传热强化主要是湍流掺混增强和近壁面流体在拟临界温度附近物性剧烈变化的综合作用; 传热恶化则是因为壁温及近壁面流体温度远高于拟临界温度, 在近壁面发生了类似于亚临界状态下的“拟膜态沸腾”.      

Heat transfer measurement using thermochromatic liquid crystal

The present study describes the results of an experimental investigation of heat transfer rate on the wall surface downstream of vortex generators and includes the effect of different angles of attack of the vortex generators on heat transfer. Heat transfer measurements are made with thermochromatic liquid crystal (TLC) to provide the local distributions of heat transfer coefficients. The major conclusions are obtained from this study. The boundary layer is thinned in the regions where the secondary flow is directed to the wall and thickened where it is directed away from the wall. The peak augmentation of the local heat transfer occurs in the downwash region near the point of minimum boundary layer thickness, and the common-flow-down cases show better heat transfer enhancement than the common-flow-up cases.     

Experimental Investigation of Opposing Mixed Convection in a Channel with an open Cavity Below

Abstract

In this article, mixed convection in an open cavity with a heated wall bounded by a horizontal unheated plate is investigated experimentally. The heated wall is on the opposite side of the forced inflow. The results are reported in terms of wall temperature profiles of the heated wall and flow visualization. The range of pertinent parameters used in this experiment are Reynolds numbers (Re) from 100 to 2,000 and Richardson numbers (Ri) from 4.3 to 6,400. Also, the ratio between the length and the height of cavity (L/D) ranges from 0.5–2.0, and the ratio between the channel and cavity height (H/D) is equal to 1.0. The lack of experimental results on mixed convection in a channel with an open cavity below was an impetus for investigating this configuration when one cavity vertical wall is heated at uniform heat flux. The present results show that at the lowest investigated Reynolds number, the surface temperatures are lower than the corresponding surface temperatures for Re = 2,000 at the same ohmic heat flux. The flow visualization shows that for Re = 1,000, there are two nearly distinct fluid motions: a parallel forced flow in the channel and a recirculation flow inside the cavity. For Re = 100, the effect of a stronger buoyancy determines a penetration of thermal plumes from the heated plate wall into the upper channel. Moreover, the flow visualization shows that for lower Reynolds numbers, the forced motion penetrates inside the cavity, and a vortex structure is adjacent to the unheated vertical plate. At higher Reynolds numbers, the vortex structure has a larger extension while L/D is held constant.     

3D numerical analysis on mechanisms of flow and heat transfer in a square channel with right triangular wavy surfaces

Numerical investigations on thermo-hydraulic performance and mechanisms of flow and heat transfer in a square channel heat exchanger inserted with right triangular wavy surfaces are examined. The influence of the flow attack angles (30°, 45° and 60°) is investigated for laminar flow (Re = 100–2000). The configurations of the right triangular wavy surfaces are varied as inclined and V-shaped wavy surfaces (the pointing of V-tip with downstream and upstream called “V-downstream” and “V-upstream”, respectively). The insertions of the wavy surfaces in the channel heat exchanger are divided into two types: middle and diagonal insertions. The computational results reveal that the maximum thermal enhancement factor, TEF, is around 2.31 for the 30° V-downstream wavy surface with diagonal insertion at Re = 2000.     

A numerical study of transient flow around a cylinder and aerodynamic sound radiation

The fully 3D turbulent incompressible flow around a cylinder and in its wake at a Reynolds number Re = = 9×10⁴ based on the cylinder diameter and Mach number M = 0.1 is calculated using Large Eddy Simulations (LES). Encouraging results are found in comparison to experimental data for the fluctuating lift and drag forces. The acoustic pressure in far-field is commutated through the surface integral formulation of the Ffowcs Williams and Hawkings (FWH) equation in acoustic analogy. Five different sound sources, the cylinder wall and four permeable surfaces in the flow fields, are employed. The spectra of the sound pressure are generally in quantitative agreement with the measured one though the acoustic sources are pseudo-sound regarding the incompressible flow simulation. The acoustic component at the Strouhal number related to vortex shedding has been predicted accurately. For the broad band sound, the permeable surfaces in the near wake region give qualitative enough accuracy level of predictions, while the cylinder wall surface shows a noticeable under-prediction. The sound radiation of the volumetric sources based on Lighthill tensors at vortex shedding is also studied. Its far-field directivity is of lateral quadrupoles with the weak radiations in the flow and cross-flow directions.     

Numerical Simulation of Two-Dimensional Flow over Three Cylinders by LatticeBoltzmann Method

The numerical simulation using the multiple relaxation time lattice Boltzmann method (MRT-LBM) is carried out for the purpose of investigating the two-dimensional flow around three circular cylinders. Among these three circular cylinders, one of the three cylinders on which a forced in-line vibrating is used to do this research and attempt to find out the effects of the movingcylinder and the other two rigid cylinders on the wake characteristics and vortex formation. As a benchmark problem to discuss the problem of lift coefficient r.m.s for these cylinders with spacing ratios T/D between other rigid side-by-side cylinders, and the calculation is carried out with two compared cases at Reynolds number of 100, two of the cylinders are rigid and the other one is an in-line vibrated cylinder lying downstream, in addition, forced vibrating amplitude and frequency are A/D= 0.5 and f_ν=0.4 (where A is the forced amplitude, D is the cylinder diameter, and f_ν stands for the vibrating frequency, respectively). The calculated results not onlyindicate that the spacing ratios T/D (T is the center-to-center spacing between the two upstream cylinders) have influence on the wake patterns and the formation of vortex shedding, but also analyze the lift coefficient r.m.s for the three cylinders with the spacing ratios S/D (where S is the center-to-center spacing between the center of upstream two side-by-side cylinders and downstream cylinder).     

电磁力控制湍流边界层分离圆柱绕流场特性数值分析

在亚临界区高雷诺数Re=1.4×105下,采用脱体涡模拟结合湍流分离的方法对弱电解质中电磁力作用下湍流边界层分离圆柱绕流场及其升(阻)力特性进行了数值模拟和分析.结果表明,电磁力可以提高圆柱体湍流边界层内的流体动能,延缓圆柱体湍流边界层的流动分离,减弱圆柱体湍流绕流场中在流向和展向上大尺度漩涡的强度,减小圆柱体阻力时均值及其升力脉动幅值.当电磁力作用参数大于某个临界值后,湍流边界层流动分离消失,在圆柱体尾部产生射流现象,从而电磁力对圆柱体产生净推力作用,出现负阻力现象,而且升力脉动幅值接近于零,出现圆柱体升力消失现象.     

纳米尺度绕流现象中流体分子运动行为研究

采用分子动力学模拟方法,研究纳米尺度低Re数(Re=20)条件下,氩流体流过铂(FCC(100))金属圆柱的绕流现象.从流线和速度矢量分布两方面刻画涡的周期性产生、发展和脱落过程.结果显示,一个涡脱落周期大约为0.3 ns,斯特劳哈尔数St约为0.2;时均对称涡的长度是圆柱直径的1.4倍,涡径与圆柱直径相当.从速度场和密度场的角度刻画回流区,并进一步研究典型区域(圆柱中心线附近区域及下游邻近区域)的速度分布特性.