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。      

层流与湍流等离子体冲击射流特性比较

本文采用数值模拟方法,对层流与湍流氩等离子体射流在空气环境中冲击平板时的流动与传热特性进行了对比研究.结果表明,在平板和射流进口间的距离较大时,平板的存在只对其附近的射流参数分布有较大影响,层流等离子体冲击射流的温度与轴向速度的轴向梯度明显小于湍流等离子体冲击射流情形;由于在平板表面形成的径向壁面射流对引射的附加贡献,层流和湍流等离子体冲击射流对环境空气的引射量明显增加.     

基于CFD的射流抛光喷射距离的分析和优化

分析了喷射距离对射流抛光效果的影响,基于计算流体动力学进行了喷射距离的分析和优化.通过构建射流抛光不同喷射距离的物理模型,采用能更好地处理流线弯曲程度较大的流动的RNG k-ε紊流模型应用于射流抛光的数学建模,使用SIMPLEC算法对射流模型进行数值计算,得到了不同模型的射流抛光冲击射流流场及工件壁面上的冲击压力、紊动强度、壁流速度分布.根据射流抛光对冲击射流特性的要求,比较和分析不同喷距模型的数值仿真结果,结果显示,射流抛光最优化喷距范围为喷嘴口径的10倍至12倍之间.     

稠密颗粒射流倾斜撞击颗粒膜特征

采用高速摄像仪对稠密颗粒射流倾斜撞击形成的类液体颗粒膜特征进行实验研究,考察了颗粒粒径、射流速度以及射流含固率等因素对颗粒膜形态及动态特征的影响.结果表明:随着颗粒粒径增大,稠密颗粒倾斜撞击流由颗粒膜向散射模式转变;随着射流速度增加,气固不稳定增强,射流流量出现脉动,正面与侧面分别表现为颗粒膜的非轴对称振荡和表面波纹结构;颗粒膜非轴对称振荡的振荡频率和振荡幅度随射流速度的增大而增大;表面波纹速度和波长沿传播方向增大,波纹间存在叠加现象.颗粒膜出现非轴对称振荡主要是因为喷嘴出口由气固不稳定性引起的射流流量脉动,射流流量脉动频率与撞击面振荡频率基本相当.     

A study in the developing region of square jet

The developing region of a turbulent square jet is investigated using high-resolution particle image velocimetry (PIV). The mean velocity and turbulence stresses are presented in various horizontal planes, along the jet centerline covering the initial region of the jet as well as the transition to the self-similar region. To study the flow structure away from the central plane, velocity measurements in two additional horizontal planes, one located halfway from the jet central plane toward the edge and the other at the edge of the square jet, are also examined. Analysis of the instantaneous velocity fields reveal the presence of an arrow-like feature in the square jet due to the higher instability generated in the jet shear layer compared with a round jet. To elucidate the imprints of the vortex structures present in the jets, a swirling strength-based vortex identification methodology is applied on a large ensemble of instantaneous velocity fields. Statistical analysis of the number of vortex cores, and their size and rotational strength in the measurement plane is undertaken. Vortex population at the edge was found to be very different compared with that in the central plane.     

基于MHD的磁射流抛光中流场的分析与模拟

从理论上分析了磁射流抛光中的磁场与流场的相互作用,构建了磁射流抛光的冲击射流模型,基于磁流体动力学对磁射流抛光过程的紊动冲击射流进行数值模拟,得到了磁射流抛光过程的连续流场和射流在工件壁面上的压力、速度、紊动强度分布。通过比较射流抛光和磁射流抛光的数值计算结果,分析了磁流变效应对射流稳定性的影响,从射流的流场、速度、紊动强度等方面分析射流在磁场中稳定的原因。     

出口雷诺数对平面射流自保持性的影响

通过实验研究出口雷诺数对平面湍流射流自保持性的影响. 测量的射流来自相同的喷嘴但不同的雷诺数Re（≡U_jh/ν,其中U_j是出口平均速度、h是窄缝出口的厚度和ν是黏性系数）,其变化范围是Re=4582—57735.所得的数据包括沿轴线的平均速度、湍流强度、积分尺度、高阶矩和能谱. 实验发现,随着Re的增大,平面射流发展减慢,平均速度和湍流强 关键词： 平面射流 雷诺数 自保持性     

紫外光诱导纳米颗粒胶体微射流碰撞特性（英文）北大核心CSCD

采用流体力学模拟方法,建立了垂直非淹没射流的计算流体动力学模型,研究了在紫外光诱导纳米颗粒胶体射流中用直径D为500μm的微孔光-液耦合喷嘴进行抛光加工的冲击动力学,分析了非淹没射流条件下光-液耦合喷嘴内、外的流场分布情况及其对工件表面的喷射冲击特征,对紫外光诱导纳米颗粒胶体射流冲击动力学过程进行了理论描述。计算结果表明,在1MPa入射压力时,微孔光-液耦合喷嘴口TiO2胶体的喷射速度约为30m/s,其集束匀速喷射距离约为5mm。在此喷射距离时进行垂直喷射,在胶束与工件表面的冲击射流作用区域,其射流静压最大值分布在射流冲击作用中心,但射流动压及射流合成速度在此区域的截面分布呈"W"形状,射流动压及速度最大值出现在胶体射流束的外环直径约2mm处。     

Mechanics of the turbulent-nonturbulent interface of a jet

We report the results of an experimental investigation of the mechanics and transport processes at the bounding interface between the turbulent and nonturbulent regions of flow in a turbulent jet, which shows the existence of a finite jump in the tangential velocity at the interface. This is associated with small-scale eddying motion at the outward propagating interface (nibbling) by which irrotational fluid becomes turbulent, and this implies that large-scale engulfment is not the dominant entrainment process. Interpretation of the jump as a singular structure yields an essential and significant contribution to the mean shear in the jet mixing region. Finally, our observations provide a justification for Prandtl's original hypothesis of a constant eddy viscosity in the nonturbulent outer jet region.     

Use of a plane jet for flow-induced noise reduction of tandem rods

Unsteady wake from upstream components of landing gear impinging on downstream components could be a strong noise source.The use of a plane jet is proposed to reduce this flow-induced noise.Tandem rods with different gap widths were utilized as the test body.Both acoustic and aerodynamic tests were conducted in order to validate this technique.Acoustic test results proved that overall noise emission from tandem rods could be lowered and tonal noise could be removed with use of the plane jet.However,when the plane jet was turned on,in some frequency range it could be the subsequent main contributor instead of tandem rods to total noise emission whilst in some frequency range rods could still be the main contributor.Moreover,aerodynamic tests fundamentally studied explanations for the noise reduction.Specifically,not only impinging speed to rods but speed and turbulence level to the top edge of the rear rod could be diminished by the upstream plane jet.Consequently,the vortex shedding induced by the rear rod was reduced,which was confirmed by the speed,Reynolds stress as well as the velocity fluctuation spectral measured in its wake.This study confirmed the potential use of a plane jet towards landing gear noise reduction.     

PIV measurements of the flow and turbulent characteristics of a round jet in crossflow

The instantaneous and ensemble averaged flow characteristics of a round jet issuing normally into a crossflow was studied using a flow visualization technique and Particle Image Velocimetry measurements. Experiments were performed at a jet-to-crossflow velocity ratio, 3.3 and two Reynolds numbers, 1,050 and 2,100, based on crossflow velocity and jet diameter. Instantaneous laser tomographic images of the vertical center plane of the crossflow jet show that there exists very different natures in the flow structures of the near field jet due to Reynolds number effect even though the velocity ratio is same. It is found that the shear layer becomes much thicker when the Reynolds number is 2,100 because of the strong entrainment of the inviscid fluid by turbulent interaction between the jet and crossflow. The mean and second order statistics are calculated by ensemble averaging over 1,000 realizations of instantaneous velocity fields. The detail characteristics of mean flow field, streamwise and vertical rms velocity fluctuations, and Reynolds shear stress distributions are presented. The new PIV results are compared with those from previous experimental and LES studies.     

Research on the jet characteristics of the deflector–jet mechanism of the servo valve

In view of the complicated structure of the deflector-jet mechanism,a mathematical model based on the turbulent jet flow theory in the deflector-jet amplifier is proposed.Considering the energy transformation and momentum variation,an equation of the flow velocity distribution at the key fluid region is established to describe the morphological changes of the fluid when it passes through the deflector and jets into the receiver.Moreover,the process is segmented into four stages.According to the research results,the oil enters the deflector and impinges with the side wall.Then one part of the oil's flow velocity decreases and a high pressure zone is formed by the oil accumulation,the other part of the oil reverses out of the deflector along the side wall.Prior to entering the receiver,the flow is a kind of plane impinging jet.Virtually,the working pressure of the receiver is generated by the impact force,while the high speed fluid flows out of the receiver and forms a violent vortex,which generates negative pressure and causes the oil to be gasified.Compared with the numerical simulation results,the turbulent jet model that can effectively describe the characteristics of the deflector-jet mechanism is accurate.In addition,the calculation results of the prestage pressure characteristic have been verified by experiments.     

Equations for describing a family of self-similar solutions of the far field of a round submerged jet

The flow of a turbulent round submerged jet in far field has been considered. The modified Navier?Stokes equations have been derived with the aid of known self-similar properties of the time-mean flow as well as the spatial and temporal characteristics of the entire spectrum of the scales of turbulent fluctuations. A technique has been proposed for the numerical computation of these equations to describe a family of self-similar solutions in a cylindrical region with periodic boundary conditions in the longitudinal direction, and the velocity of the jet expansion enters the obtained equations explicitly as a parameter.     

Investigation of piezoelectrically generated synthetic jet flow

The purpose of this paper is to investigate the compressible turbulent synthetic jet flow characteristics of a dual diaphragm piezoelectric actuator. Experimentally, a flow visualization system was established to obtain the particle streak images scattered from 10-μm red fluorescent spheres for observing the synthetic jet flowfield produced by a dual diaphragm piezo actuator. The centerline velocity of the synthetic jet was also measured by using a hot-wire anemometry system. In the analysis, the computational approach adopted the transient three-dimensional conservation equations of mass and momentum with the moving boundary specified to represent the piezo diaphragm motion. The standard k-∈ two-equation turbulent model was employed for turbulence closure. For the actuator operating at the frequency of 648 Hz, the particle streakline images in the cross-sectional plane visualized a turbulent jet flow pattern in the far-field area. The hot-wire anemometry data indicated that the measured centerline velocity of synthetic jets reached 3.8 m/s at y/d= 50. The predictions were compared with the visualized particle streak images and centerline velocity of the synthetic jet in order to validate the computer code. The numerical simulation revealed the time-periodic formation and advection of discrete vortex pairs. Caused by the outward movement of the piezo diaphragms, air near the orifice was entrained into the actuator cavity when the vortex pairs were sufficiently distant from the orifice.     

Influence of dispersed phase on the turbulent structure of submerged axisymmetric impact jet

The PIV/LIF method was used to experimentally examine turbulent characteristics of a submerged gassaturated axisymmetric impact jet. A novel method to analyze the dynamics of vortex formations is proposed, making it possible to obtain spectral characteristics of turbulent motion at low sampling frequency. A comparison of data obtained with theoretical models is reported.     

Incomplete similarity of a plane turbulent wall jet on smooth and transitionally rough surfaces

This study assesses the hypothesis of incomplete similarity for a plane turbulent wall jet on smooth and transitionally rough surfaces. Typically, a wall jet is considered to consist of two regions: an inner layer and an outer layer. The degree to which these two regions reach equilibrium with each other and interact to produce the property of self-similarity remains an open question. In this study, the analysis of the outer and inner regions indicates that each region is characterised by a half-width which exhibits its own distinct dependence on the streamwise distance x from the slot, and a single self-similar structure for both regions does not exist. More specifically, the inner and outer layers of the wall jet exhibit different scaling laws, which results in two self-similar mean velocity profiles, both of which retain a dependence on the slot height H. As such, incomplete similarity of the wall jet on smooth and transitionally rough surfaces is confirmed by this study. In addition, comparison of the experimental results for the transitionally rough surface with the smooth wall case indicates that the surface roughness modifies the development of the mean velocity profile in both the inner and outer regions, although the effect on the outer region is relatively small and close to the experimental uncertainty.     

Features of diffusion of an incident particle

Mechanisms of turbulent diffusion of particles moving in a turbulent medium under the action of gravity and Stokes forces are studied. It is observed that the velocity variance of particles decreases upon an increase in the inertia of the motion. At the same time, the diffusion coefficients of a particle are independent of its inertia. It is shown that, as the velocity of incidence increases, diffusion in the vertical plane becomes stronger than in the horizontal plane, and the ratio of the horizontal and vertical diffusion coefficients decreases to 1: 2.     

Flow mapping of a jet in crossflow with Stereoscopic PIV

Stereoscopic Particle Image Velocimetry (PIV) has been used to make a three-dimensional flow mapping of a jet in crossflow. The Reynolds number based on the free stream velocity and the jet diameter was nominally 2400. A jet-to-crossflow velocity ratio of 3.3 was used. Details of the formation of the counter rotating vortex pair found behind the jet are shown. The vortex pair results in two regions with strong reversed velocities behind the jet trajectory. Regions of high turbulent kinetic energy are identified. The signature of the unsteady shear layer vortices is found in the mean vorticity field.     

Investigation of a turbulent wall jet in forced convection issuing into a directed coflow stream

In the present paper, we have studied numerically the directed coflow stream effects on mean and turbulent flow properties of a turbulent plane wall jet in forced convection emerging into a directed coflow stream. The system of equations governing the studied configuration is solved with a finite difference scheme using a staggered grid for numerical stability, not uniform in the two directions of the flow. The modified version of the first-order low Reynolds number k–? turbulence model is used and compared to existing experimental findings. It is found that predicted results are in satisfactory agreement with the experimental data and that the wall jet fluid decays faster in presence of a directed coflow stream. Results show also that the increase of coflow deviation angles causes an increase of the growth rates of the dynamic and thermal half-width of the jet and enhances the turbulent mixing. It is found that the longitudinal development of normalised forms of the jet characteristics parameters at different directed coflow velocity ratios can be reasonably well collapsed onto universal trends through the use of momentum length scale.     

稠密颗粒射流撞击壁面颗粒膜表面波纹特征

采用高速摄像仪对稠密颗粒射流撞击有限尺寸壁面的流动过程进行了实验研究,重点研究了颗粒膜及其表面波纹特征,考察了颗粒粒径、射流速度和固含率等因素对颗粒膜形态和表面波纹的影响.研究结果表明,随着颗粒粒径增大,稠密颗粒撞壁流由颗粒膜向散射模式转变.与液体射流撞壁液膜相比,颗粒膜扩展角较大,射流速度对其影响不显著.稠密颗粒射流撞壁颗粒膜表面波纹存在明显的叠加现象,颗粒膜表面波纹频率比液膜大约低一个数量级.颗粒膜表面波纹主要由射流脉动引起,表面波纹频率与射流脉动频率具有相同的数量级.