激光多普勒测流系统中光学参数的确定

本文着重讨论了双光束条纹型多普勒测流系统中，光学分束系统参数与散射信号的关系，列出了有关公式。计算了圆形截面的散射颗粒直径与干涉条纹周期呈不同比值时对信号的影响，其结果比以前采用矩型截面进行计算的结果更为合理。结合激光海流计的设计，分析了光学系统中各项参数的选择对测量结果的影响。

激光Doppler测流系统中光学参数的确定

本文着重讨论了双光束条纹型激光多普勒测流系统中,光学分束系统参数与散射信号的关系,列出了有关的公式。计算了园形截面的散射颗粒直径与干涉条纹周期呈不同比值时对信号的影响,其结果与以前采用矩型截面进行计算的结果差异较大。以设计一台激光海流计为例,推荐一种以选择与散射粒径相匹配的条纹周期为出发点的光学参数选择方法。最后介绍作者提出的为实现这种选择的光学布置。

MEASUREMENT AND VISUALIZATION OF FLOW FIELD IN AXIAL FLOW FAN

An experimental investigation associated with the basic fluid mechanics in an axial flow fan is described in this paper. The flow field in the tip region has been studied by laser Doppler anemometer (LDA) and flow visualization technique. Some experimental data and images are interpreted to understand the complex interactions between the annulus wall boundary layer and the leakage flow. It shows that the vortex inside the blade passage is produced by the separation of annulus wall boundary layer rather than the rolling up of leakage flow.

Laser Doppler Measurements of Twin Impinging Jets Aligned with a Crossflow

This paper presents a detailed analysis of the complex flow beneath two impinging jets aligned with a low-velocity crossflow which is relevant for the future F-35 VSTOL configuration, and provides a quantitative picture of the main features of interest for impingement type of flows. The experiments were carried out for a Reynolds number based on the jet exit conditions of Rej = 4.3 × 10^4, an impingement height of 20.1 jet diameters and for a velocity ratio between the jet exit and the crossflow VR = V/Uo of 22.5. The rear jet is located at S = 6 D downstream of the first jet. The results show a large penetration of the first （upstream）jet that is deflected by the crossflow and impinges on the ground, giving rise to a ground vortex due to the collision of the radial wall and the crossflow that wraps around the impinging point like a scarf. The rear jet （located downstream） it is not so affected by the crossflow in terms of deflection, but due to the downstream wall jet that flows radially from the impinging point of the first jet it does not reach the ground. The results indicate a new flow pattern not yet reported so far, that for a VSTOL aircraft operating in ground vicinity with front wind or small forward movement may result in enhanced under pressures in the aft part of the aircraft causing a suction down force and a change of the pitching moment towards the ground.

Turbulent Energy Budgets of a Ground Vortex Flow

Turbulent kinetic energy budgets are presented for a highly curved flow generated by the collision of plane wall turbulent jet with a low-velocity boundary layer. The different terms are obtained in the vertical plane of symmetry by quadratic interpolation of the LDV （Laser Doppler Velocimetry） measurements, for a wall jet-to-boundary layer velocity ratio of 2. The results, which have relevance to flows encountered in powered-lift aircraft operating in ground effect, quantify the structure of the complex ground vortex flow. The analysis of turbulent energy equation terms using the measured data revealed that production by normal and shear stresses are both very important to the turbulent structure of the impact zone of the ground vortex. This is an indication that the modeling of turbulence of a ground vortex requires a good representation of the production by normal stresses which is most important in the collision zone.

Reconstruction of Turbulent Swirling Flow in a Dump Combustor

Experimental data of the continuous evolution of fluid flow characteristics in a dump combustor is very useful and essential for better and optimum designs of gas turbine combustors and ramjet engines. Unfortunately, experimental techniques such as 2D and/or 3D LDV （Laser Doppler Velocimetry） measurements provide only limited discrete information at given points; especially, for the cases of complex flows such as dump combustor swirling flows. For this type of flows, usual numerical interpolating schemes appear to be unsuitable. Recently, neural networks have emerged as viable means of expanding a finite data set of experimental measurements to enhance better understanding of a particular complex phenomenon. This study showed that generalized feed forward network is suitable for the prediction of turbulent swirling flow characteristics in a model dump combustor. These techniques are proposed for optimum designs of dump combustors and ramjet engines.

Large eddy simulation of a round jet into a counterflow

A round jet into a counterflow under different jet-to-current velocity ratios was investigated using large eddy simulation.The results agree well with experimental measurements from laser-Doppler anemometry and laser-induced fluorescence that include velocity and mean concentrations along the centerline and radial direction.Vortex rings appear in the region near the jet exit and large-scale vortex structures still occur near the stagnation point.The flow becomes more chaotic and three-dimensional with the presence of these structures.In particular,their presence near the stagnation point results in large velocity fluctuations that enhance the mixing process and dilution.These fluctuations are described by probability density functions that deviate from Gaussian distribution.The three-dimensional streamlines indicate that the jet not only oscillates in three directions but also rotates about the jet axis and around the vortex.The second and third moments of the velocity or scalar fluctuations identify that the mixing processes are greater in the region before the stagnation point.

A Laser Imaging-LDV Coupling Measurement of Single Bubble Forming and Rising in Shear-thinning Fluid

The shape evolution of bubble formed in carboxymethylcellulose(CMC) aqueous solution was real-time observed using laser image technique. The flow fields of liquid around growing and rising bubble were measured by laser Doppler velocimetry(LDV), and the liquid mean velocity and its contour curves were obtained. The results show that bubble grows as spherical shape because of the dominant role of surface tension in the early period, and then is stretched gradually as a teardrop shape due to the common effect of buoyancy and shear-thinning of fluid. The axial mean velocity of liquid phase takes on Gaussian distribution with the symmetrical axis passing through orifice center. However, the radial mean velocity increases first and then decreases with the increase of the distance from measured point to the symmetrical axis above. Further, the axial component along symmetrical axis decreases initially and increases with the rise of height, as well as its corresponding contour map diverging gradually. The radial component, yet, decreases steadily with the rise of height, and the maximum value deviates towards the two sides until disappear, as it contour shape of butterfly's "front wing".