垂直管内油水两相流局部相分布特性实验研究

应用双头电导探针测量系统,对垂直上升管内油水两相分散流局部相分布特性进行了系统测量。得到了油水两相分散流的局部含油率分布类型图。研究结果表明低折算水速和低折算油速条件下,局部含油率在实验段截面上呈抛物线型局部分布特征,局部最大值出现在实验段中心区域。随折算油速增大,油滴受到横向力如升力的作用,逐渐向实验段壁面区域迁移,形成局部含油率的壁面峰值分布特性。当折算水速大于0．8 m／s时,局部含油率在实验段截面上呈均匀分布。     

Optimizing parameters of GTU cycle and design values of air-gas channel in a gas turbine with cooled nozzle and rotor blades

The authors have formulated the problem of joint optimization of pressure and temperature of combustion products before gas turbine, profiles of nozzle and rotor blades of gas turbine, and cooling air flow rates through nozzle and rotor blades. The article offers an original approach to optimization of profiles of gas turbine blades where the optimized profiles are presented as linear combinations of preliminarily formed basic profiles. The given examples relate to optimization of the gas turbine unit on the criterion of power efficiency at preliminary heat removal from air flows supplied for the air-gas channel cooling and without such removal.     

An analytical discrete-ordinates solution of the S-model kinetic equations for flow in a cylindrical tube

An integro-differential form of the linearized S-model kinetic equations for describing flow in a cylindrical tube is projected in such a way as to yield a pair of coupled transport equations that defines the desired velocity and heat-flow profiles. This system is then solved symbolically to yield a pair of coupled integral equations for the physical quantities required. At this point some transformations are carried out to yield a restatement of the original problem in terms of a “pseudo-problem” defined by plane-geometry variables. An analytical version of the discrete-ordinates method is then used to solve the pseudo-problem, and so, after both MATLAB and FORTRAN versions of the developed algorithm are implemented, results thought to be highly accurate are obtained for the case of diffuse reflection from the walls of a cylindrical tube. In addition to the velocity and heat-flow profiles, for the cases of Poiseuille flow and thermal-creep flow, the velocity slips, the heat-flow profiles evaluated at the wall, the particle-flow rates and the heat-flow rates for these two problems are reported for selected values of the tube radius.     

基于晶格-Boltzmann方法的纳米流体流动和传热模型

纳米流体是由流体与纳米粒子组成的悬浮体.悬浮在流体中的纳米粒子会受到运动阻力、布朗力、扩散力和重力等作用力的影响,因而其运动规律极其复杂.本文运用晶格-Boltzmann(LB)方法,建立纳米流体流动和传热模型模型,对纳米流体的流型结构和温度场进行了模拟和分析.     

Transport of self-propelling bacteria in micro-channel flow

Understanding the collective motion of self-propelling organisms in confined geometries, such as that of narrow channels, is of great theoretical and practical importance. By means of numerical simulations we study the motion of model bacteria in 2D channels under different flow conditions: fluid at rest, steady and unsteady flow. We find aggregation of bacteria near channel walls and, in the presence of external flow, also upstream swimming, which turns out to be a very robust result. Detailed analysis of bacterial velocity and orientation fields allows us to quantify the phenomenon by varying cell density, channel width and fluid velocity. The tumbling mechanism turns out to have strong influence on velocity profiles and particle flow, resulting in a net upstream flow in the case of non-tumbling organisms. Finally we demonstrate that upstream flow can be enhanced by a suitable choice of an unsteady flow pattern.     

Experimental study of the multiple scattering effect on the flow velocity profiles measured in Intralipid phantoms by DOCT

Time domain Doppler Optical Coherence Tomography (DOCT) technique was applied to measure flow velocity profiles in highly scattering media. We analyzed the distortions of the measured velocity profiles of the 1% Intralipid solution flow embedded into the scattering medium at different embedding depths. For this purpose a tissue phantom consisting of a plain glass capillary (inner diameter 0.3 mm) embedded into a slab of Intralipid solution mimicking human skin was designed. The measured flow velocity profiles and behavior of distortions caused by multiple scattering are shown.     

Eulerian mean flow from an instability of convective plumes

The dynamical origin of large-scale flows in systems driven by concentrated Archimedean forces is considered. A two-dimensional model of plumes, such as those observed in thermal convection at large Rayleigh and Prandtl numbers, is introduced. From this model, we deduce the onset of mean flow as an instability of a convective state consisting of parallel vertical flow supported by buoyancy forces. The form of the linear equation governing the instability is derived and two modes of instability are discussed, one of which leads to the onset of steady Eulerian mean flow in the system. We are thus able to link the origin of mean flow precisely to the profiles of the unperturbed plumes. The form of the nonlinear partial differential equation governing the Eulerian mean flow, including nonlinear effects, is derived in one special case. The extension to three dimensions is outlined. (c) 2000 American Institute of Physics.     

Flow spectra from spectral power density calculations for pulsed Doppler

Range gated pulsed Doppler can be used to make localized velocity measurements within a blood vessel. Both the transducer and the sample volume are of finite size, and this prohibits the measurement of velocity at a point. A spectral flow profile can be created by stepping a sufficiently small sample volume across the lumen of a vessel. However no such set of spectra will correspond directly to the true velocity profile. In this study we developed a systematic theoretical treatment which allows Doppler spectral power density (SPD) functions to be calculated under a very wide range of conditions. Simulated flow spectra were created from sets of these spectra. The model is based on the beam intensity weighted volume method and incorporates, through the idea of a spread function, Guidi's individual flow line spectrum. Our method can be applied for different spread functions; with beam profiles which are uniform, Gaussian or arbitrarily narrow (needle beam); with range gated sample volumes which can be maximal (CW-type) or minimal (PW-type); and for beams which intersect the flow tube axis, or are off centre. Under all conditions we find the spread function parameter k, equal to the ratio of the central Doppler shift to half the bandwidth, plays a key role. After formulating the model analytically, we sought simplifications to allow results to be obtained from simple, practical formulae. Spread and unspread SPD functions are in most cases given as single integrals which contain measurable physical parameters and can be easily evaluated numerically. Model results are presented for flow spectra of parabolic flow, illustrating the interplay between different factors in determining the appearance of spectral flow profiles.     

Acoustic measurements of boundary layer flow and sediment flux

Results are reported on an assessment of the application of coherent Doppler and cross-correlation techniques to measure nearbed boundary layer flow. The approaches use acoustic backscattering from sediments entrained into the water column from the bed, to obtain high-resolution profiles of the nearbed hydrodynamics. Measurements are presented from a wave tunnel experiment in which sediment was entrained by unidirectional, oscillatory, and combined flows. The data collected have been used to evaluate the capability of the two flow techniques to measure boundary layer mean, turbulent, and intrawave velocity profiles. Further, the backscattered signal has been used to measure suspended sediment concentration profiles, which have been combined with the velocity profiles to obtain high-resolution measurements of boundary layer sediment flux.     

Simulations of the scattering of sound waves at a sudden area expansion

The scattering of acoustic plane waves at a sudden area expansion in a flow duct is simulated using the linearized Navier–Stokes equations. The aim is to validate the numerical methodology for the flow duct area expansion, and to investigate the influence of the downstream mean flow on the acoustic scattering properties. A comparison of results from numerical simulations, analytical theory and experiments is presented. It is shown that the results for the acoustic scattering obtained by the different methods gives excellent agreement. For the end correction, the numerical approach is found superior to the analytical model at frequencies where coupling of acoustic and hydrodynamic waves is significant. A study with two additional flow profiles, representing a non-expanding jet with an infinitely thin shear layer, and an immediate expansion, shows that a realistic jet is needed to accurately capture the acoustic–hydrodynamic interaction. A study with several different artificial jet expansions concluded that the acoustic scattering is not significantly dependent on the mean flow profile below the area expansion. The constructed flow profiles give reasonable results although the reflection and transmission coefficients are underestimated, and this deviation seems to be rather independent of frequency for the parameter regime studied. The prediction of the end correction for the constructed mean flow profiles deviates significantly from that for the realistic profile in a Strouhal number regime representing strong coupling between acoustic and hydrodynamic waves. It is concluded that the constructed flow profiles lack the ability to predict the loss of energy to hydrodynamic waves, and that this effect increases with increasing Mach number.     

Velocity sensitivity of slice-selective excitation

The purpose of this study was to investigate how flow affects slice-selective excitation, particularly for radiofrequency (rf) pulses optimized for slice-selective excitation of stationary material. Simulation methods were used to calculate the slice profiles for material flowing at different velocities, using optimal flow compensation when appropriate. Four rf pulses of very different shapes were used in the simulation study: a 90° linear-phase Shinnar-LeRoux pulse; a 90° self-refocusing pulse; a minimum-phase Shinnar-LeRoux inversion pulse; and a SPINCALC inversion pulse. Slice profiles from simulations with a laminar flow model were compared with experimental studies for two different rf pulses using a clinical magnetic resonance imaging (MRI) system. We found that, for a given rf pulse, the effect of flow on slice-selective excitation depends on the product of the selection gradient amplitude, the component of velocity in the slice selection direction, and the square of the rf pulse duration. The shapes of the slice profiles from the Shinnar-LeRoux pulses were relatively insensitive to velocity. However, the slice profiles from the self-refocusing pulse and the SPINCALC pulse were significantly degraded by velocity. Experimental slice profiles showed excellent agreement with simulation. In conclusion, our study demonstrates that slice-selective excitation can be significantly degraded by flow depending on the velocity, the gradient amplitude, and characteristics of the rf excitation pulse used. The results can aid in the design of rf pulses for slice-selective excitation of flowing material.     

Shear zones and wall slip in the capillary flow of concentrated colloidal suspensions

We image the flow of a nearly random close packed, hard-sphere colloidal suspension (a "paste") in a square capillary using confocal microscopy. The flow consists of a "plug" in the center while shear occurs localized adjacent to the channel walls, reminiscent of yield-stress fluid behavior. However, the observed scaling of the velocity profiles with the flow rate strongly contrasts yield-stress fluid predictions. Instead, the velocity profiles can be captured by a theory of stress fluctuations originally developed for chute flow of dry granular media. We verified this both for smooth and rough walls.     

Spectral power density calculations for pulsed Doppler

Range-gated pulsed Doppler can be used to make localized velocity measurements within a blood vessel. A spectral flow profile can be created by stepping a sufficiently small sample volume across the lumen, but no set of spectra will correspond directly to the true velocity profile. Spectral flow profiles are affected by a complex interplay between different sources of spectral broadening. In this study we developed a systematic theoretical method which allows spectral power density functions to be calculated under a very wide range of conditions, and used it to obtain simulated flow spectra. The model was formulated analytically. It is based on the weighted-volume approach and incorporates, through the concept of a spread function, the intrinsic spectral broadening associated with a focused transducer. It can be applied for arbitrary values of the spread parameter; for non-uniform beam profiles; with maximal (continuous wave-type) or minimal (pulse wave-type) range-gated sample volumes; and for beams that intersect the flow tube axis, or are off centre. Results are presented for a Gaussian beam and parabolic flow. Simulated spectral flow profiles are given which illustrate how a profile's appearance can be altered by the different sources of spectral broadening.     

短周期涡轮试验台流量调节阀的设计

本文利用CFD对短周期涡轮试验台的流量调节阀进行了优化设计,改进了流量调节阀喉部的流动状况,提高了调节阀对流量的控制能力;对流量调节阀尾部锥体的三种设计方案进行了比较,得到了最佳方案。     

A study of the oscillating corner meniscus in a vertical constrained vapor bubble system

A vertical constrained vapor bubble, VCVB, made of fused silica was used to study the stability and oscillations of an evaporating wetting film of HFE- 7000^® in a corner. The film thickness profile was measured as a function of time and axial position using an advanced form of image analyzing interferometry. The curvature, apparent contact angle, and pressure profiles for the evaporating film were calculated from the measured film thickness profiles. Oscillation of the liquid film was observed and profiles for both the advancing and receding films were obtained. These are the first such detailed profiles obtained for an oscillating meniscus below a thickness of 0.1 μm.The film thickness profiles demonstrated the spreading of the meniscus during advance as well as the presence of a curvature gradient near the contact line region. The maximum curvature decreased for the advancing menisci and increased with time for the receding menisci. An increase in the adsorbed film thickness was associated with the advancing stage and a decrease with the receding stage. Pressure profiles were measured as a function of position indicating the potential for driving the flow of the fluid toward or away from the contact line. As the film advances or recedes, the pressure gradients change as a function of position fueling the next oscillation cycle.     

Pipe Flow and Wall Turbulence Using a Modified Navier-Stokes Equation

We use a derived incompressible modified Navier-Stokes equation to model pipe flow and wall turbulence.We reproduce the observed flattened paraboloid velocity profiles of turbulence that cannot be obtained directly using standard incompressible Navier-Stokes equation.The solutions found are in harmony with multi-valued velocity fields as a definition of turbulence.Repeating the procedure for the flow of turbulent fluid between two parallel flat plates we find similar flattened velocity profiles.We extend the analysis to the turbulent flow along a single wall and compare the results with experimental data and the established controversial von Karman logarithmic law of the wall.     

非平衡气流与化学激光的增益饱和效应

提出气流与化学激光的“对流-非平衡交叉弛豫”的理论。对于气流介质的湍流和层流混合两类流动模型,分别导出了增益系数和饱和增益谱的表示式;得到了一些新的饱和规律;非饱和增益和饱和增益均与流向距离有关。具有Doppler加宽的饱和放大器的增益线形,呈现非均匀和均匀加宽并存的“反常”现象。本理论与实验结果相符合。 关键词：     

Doppler optical coherence tomography in cardiovascular applications

The study of flow dynamics in complex geometry vessels is highly important in various biomedical applications where the knowledge of the mechanic interactions between the moving fluid and the housing media plays a key role for the determination of the parameters of interest, including the effect of blood flow on the possible rupture of atherosclerotic plaques. Doppler Optical Coherence Tomography (DOCT), as a functional extension of Optical Coherence Tomography (OCT), is an optic, non-contact, noninvasive technique able to achieve detailed analysis of the flow/vessel interactions. It allows simultaneous high resolution imaging (∼10 μm typical) of the morphology and composition of the vessel and determination of the flow velocity distribution along the measured cross-section. We applied DOCT system to image high-resolution one-dimensional and multi-dimensional velocity distribution profiles of Newtonian and non-Newtonian fluids flowing in vessels with complex geometry, including Y-shaped and T-shaped vessels, vessels with aneurism, bifurcated vessels with deployed stent and scaffolds. The phantoms were built to mimic typical shapes of human blood vessels, enabling preliminary analysis of the interaction between flow dynamics and the (complex) geometry of the vessels and also to map the related velocity profiles at several inlet volume flow rates. Feasibility studies for quantitative observation of the turbulence of flows arising within the complex geometry vessels are discussed. In addition, DOCT technique was also applied for monitoring cerebral mouse blood flow in vivo. Two-dimensional DOCT images of complex flow velocity profiles in blood vessel phantoms and in vivo sub-cranial mouse blood flow velocities distributions are presented.     

应用于低密度等离子体产生的气体靶模拟

 利用流体力学计算软件对两种结构气体靶进行了数值模拟和分析。对于充气型毛细管气体靶,在充气达到稳定状态后,形成稳定的层流。气体密度的空间分布均匀,两进气口之间的密度不均匀性仅约1%。毛细管的结构参数如进气口的位置和宽度对气体密度分布的边缘有较大影响,但是对管内气体密度分布影响很小。采用锥形喷气靶可使气体密度分布的边缘更陡些,但是这种靶的超声流动可出现湍流,导致不稳定的气体流动以及更不均匀的气体密度分布。     

Melting heat transfer in boundary layer stagnation-point flow towards a stretching/shrinking sheet

An analysis is carried out to study the steady two-dimensional stagnation-point flow and heat transfer from a warm, laminar liquid flow to a melting stretching/shrinking sheet. The governing partial differential equations are converted into ordinary differential equations by similarity transformation, before being solved numerically using the Runge-Kutta-Fehlberg method. Results for the skin friction coefficient, local Nusselt number, velocity profiles as well as temperature profiles are presented for different values of the governing parameters. Effects of the melting parameter, stretching/shrinking parameter and Prandtl number on the flow and heat transfer characteristics are thoroughly examined. Different from a stretching sheet, it is found that the solutions for a shrinking sheet are non-unique.