Effect of noncircular orifice plates on the near flow field of turbulent free jets

In this paper, we experimentally investigate the near-field flow characteristics of turbulent free jets respectively issued from circular, triangular, diamond, rectangular, and notched-rectangular orifice plates into air surroundings. All the orifice plates have identical opening areas or equivalent diameters（De） and their aspect ratios（AR） range from 1 to 6.5. Planar particle image velocimetry（PIV） is used to measure the velocity field at the same Reynolds number of Re = 5 × 10^4,where Re = Ue De/ν with Ue being the exit bulk velocity and ν the kinematic viscosity of fluid. The mean and turbulent velocity fields of all the five jets are compared in detail. Results show that the noncircular jets can enhance the entrainment rate, reflected by the higher acceleration rates of mean velocity decay and spread, shorten the length of the unmixed core,expedite the increase of turbulent intensity compared with the circular counterpart shortened unmixed core, and increase turbulent intensity comparing to the circular counterpart. Among the five jets, the rectangular jet（AR = 6.5） produces the greatest decay rate of the near-field mean velocity, postpones the position at which the 鈥榓xis-switching鈥檖henomenon occurs. This supports that axis switching phenomenon strongly depends on jet initial conditions. In addition, the hump in the centerline variation of the turbulence intensity is observed in the rectangular and triangular jets, but not in the circular jet, nor in diamond jet nor in notched-rectangular jet.     

Large eddy simulations of a circular orifice jet with and without a cross-sectional exit plate

The effect of a cross-sectional exit plane on the downstream mixing characteristics of a circular turbulent jet is in- vestigated using large eddy simulation （LES）. The turbulent jet is issued from an orifice-type nozzle at an exit Reynolds number of 5 ×104. Both instantaneous and statistical velocity fields of the jet are provided. Results show that the rates of the mean velocity decay and jet spread are both higher in the case with the exit plate than without it. The existence of the plate is found to increase the downstream entrainment rate by about 10% on average over the axial range of 8-30de （exit diameter）. Also, the presence of the plate enables the formation of vortex rings to occur further downstream by 0.5-1 .Ode. A physical insight into the near-field jet is provided to explain the importance of the boundary conditions in the evolution of a turbulent jet. In addition, a method of using the decay of the centreline velocity and the half-width of the jet to calculate the entrainment rate is proposed.     

薄膜自激拍打射流的流动特性实验

在射流喷嘴出口处安装一端固定的柔性薄膜,在流速足够大时,射流和柔性膜相互诱导产生自激拍打作用,针对这一现象,提出了一种新型的自激拍打射流混合技术。在直径D=40 mm的渐缩喷嘴上固定长度L=(0.5~2)D和厚度δ=50 μm的氟化乙烯丙烯共聚物(fluorinated ethylene propylene,FEP)薄膜,使用压差计测量了光滑渐缩喷嘴以及薄膜拍打运动所引起的压力损失。通过激光片光源和高速相机进行薄膜运动状态的显示和拍打幅度的测定,探究了拍打幅度受薄膜长度和Re的影响及其变化规律,利用测得的拍打幅度(A)和频率(f)作为Strouhal数(St=fA/U_o,U_o是射流的出口速度)的特征尺寸进行研究。在Re=3×104的条件下,使用热线风速仪测量了在不同薄膜长度下拍打射流轴向速度沿中心线的分布,并对湍流度、概率密度函数等特征进行分析。此外,经过数字迭代滤波后获得射流沿中心线的积分尺度、Taylor尺度和Kolmogorov尺度等统计量。实验结果显示,拍打射流的湍流度高于自由射流,意味着前者对周围流体具有更强的大尺度卷吸能力;但对不同薄膜长度的拍打射流而言,其湍流特性存在差异;在实验所用的薄膜长度范围内,1.25D膜长时射流混合效果最好。通过考察概率密度函数及其偏斜因子(Su)和平坦因子(Fu)发现,相较于自由射流,拍打射流速度更快地接近Gauss分布,这意味着拍打射流在增强大尺度卷吸的同时也促进了小尺度的掺混。      

圆柱尾流温度标量场的小波分析*

通过使用一排16根冷线探头排在多个空间点同时测量微加热圆柱的尾流温度场,用小波分析技术对瞬时温度场的时间序列信号进行多尺度分析,目的是研究不同尺度脉动温度对总体温度场的贡献.直径为d=12.7 mm的圆柱产生了被测的尾流,对应的雷诺数为5500,测量区域位于下游距离为2d和20d之间.基于小波多尺度分辨技术,尾流温度场被分解为不同温度脉动特征尺度的小波分量.通过分析这些小波分量的瞬时温度等值线图,能够直接观测到不同特征尺度的涡结构运动特征和湍流间歇过程.特别地,我们在近场区从原始信号分解获得的高频区域中发现了K-H涡的存在.不同尺度的温度方差沿流向的变化表明,在下游距离为x=3d和20d之间,中等尺度的结构比大尺度和小尺度结构对总的温度均方根的贡献更大.不同尺度的自相关函数表明,大尺度和中等尺度的结构显示出较大的相关性,而高频的小波分量则更快地失去了原有的拟序性.     

雷诺数对圆形渐缩喷嘴湍流射流的影响

本文研究雷诺数(Re)对圆形渐缩喷嘴湍流射流的影响.实验在射流出口雷诺数为 Re = 4050—20100 的范围内进行,分别测量了射流出口、中心线的平均及湍流流场以及部分径向剖面速度分布.所有测量均采用单热线恒温热线风速仪进行高频采样,所测流场范围在轴向上为 0—30d(这里d为射流出口直径).虽然出口速度分布均为"平顶帽"形,但测量结果依然反映出Re对射流出口以及下游流场有强烈的影响.当Re小于临界值(～10000)时 关键词： 雷诺数 圆形射流 热线风速仪     

圆柱尾流温度标量场的小波分析$lt;font color="#ff0000"$gt;$lt;font color="#ff0000"$gt;（已撤稿）$lt;/font$gt;$lt;/font$gt;

通过使用一排16根冷线探头排在多个空间点同时测量微加热圆柱的尾流温度场, 用小波分析技术对瞬时温度场的时间序列信号进行多尺度分析, 目的是研究不同尺度脉动温度对总体温度场的贡献.直径为d = 12.7 mm 的圆柱产生了被测的尾流, 对应的雷诺数为5500, 测量区域位于下游距离为2d 和 20d 之间. 基于小波多尺度分辨技术, 尾流温度场被分解为不同温度脉动特征尺度的小波分量. 通过分析这些小波分量的瞬时温度等值线图, 能够直接观测到不同特征尺度的涡结构运动特征和湍流间歇过程. 特别地, 我们在近场区从原始信号分解获得的高频区域中发现了K-H涡的存在. 不同尺度的温度方差沿流向的变化表明, 在下游距离为x=3d和 20d之间, 中等尺度的结构比大尺度和小尺度结构对总的温度均方根的贡献更大. 不同尺度的自相关函数表明, 大尺度和中等尺度的结构显示出较大的相关性, 而高频的小波分量则更快地失去了原有的拟序性. 关键词： 湍流尾流 被动标量 小波分析     

Mean and fluctuating velocity fields of a diamond turbulent jet

The present paper reports the first investigation on a turbulent jet issuing from a diamond orifice(hereafter termed a "diamond jet") with an aspect ratio of 1.7.Velocity measurements were conducted in the transitional region,and the exit Reynolds number of the jet was 50000.For comparison,a round jet with identical normalized boundary conditions was also measured.It is shown that the diamond jet decays and spreads faster than the round jet does over the measured flow region.The axis-switching phenomenon is observed in the diamond jet.Although both jets display primary coherent structures in the near field,these structures are found to break down more rapidly in the diamond jet,due to the higher three-dimensionality of the flow.Moreover,the streamwise components of the Reynolds normal stress and all the shear stresses reach their maxima around the location of the maximal mean shear while the maxima of the lateral components of the Reynolds normal stresses occur around the centreline of the jet.     

等腰三角形湍流射流的流动特性实验研究

对喷嘴等效直径相同出口Reynolds数均为15000的3种等腰三角形（顶角分别为30,60和90°）以及圆形孔口射流进行了流场显示与速度场测量.结果表明:相比于圆形射流,等腰三角形射流的出口中心线速度衰减更快,湍流度更高,三角形射流卷吸周围流体能力显著增强.随着三角形顶角减小,近场区涡结构三维性更强,卷吸效果更明显.此外,对不同射流的中心线湍动能谱概率密度函数Taylor尺度和Kolmogorov尺度进行了分析讨论,发现出口形状对湍流小尺度运动的影响较小.      

圆形射流中心线上小尺度湍流的统计特性及其受高频噪声的影响

本文采用热线风速仪测量了出口雷诺数为Re (≡ U_jd/ν) = 20100的圆形射流的中心线轴向速度,其中U_j为动量平均出口速度,d为喷嘴出口直径,ν为运动黏性系数.在有效去除热线测量数据中的高频噪声后,作者对射流中心线上小尺度湍流统计量的变化规律进行了系统的分析.研究发现,射流在经过一定距离的发展后,其小尺度统计量逐渐进入自相似状态,湍动能平均耗散率ε随下游距离的增加以指数形 关键词： 恒温热线 圆形湍射流 耗散率 小尺度