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1.
A micron-resolution particle image velocimetry (micro-PIV) system has been developed to measure instantaneous and ensemble-averaged
flow fields in micron-scale fluidic devices. The system utilizes an epifluorescent microscope, 100–300 nm diameter seed particles,
and an intensified CCD camera to record high-resolution particle-image fields. Velocity vector fields can be measured with
spatial resolutions down to 6.9×6.9×1.5 μm. The vector fields are analyzed using a double-frame cross-correlation algorithm.
In this technique, the spatial resolution and the accuracy of the velocity measurements is limited by the diffraction limit
of the recording optics, noise in the particle image field, and the interaction of the fluid with the finite-sized seed particles.
The stochastic influence of Brownian motion plays a significant role in the accuracy of instantaneous velocity measurements.
The micro-PIV technique is applied to measure velocities in a Hele–Shaw flow around a 30 μm (major diameter) elliptical cylinder,
with a bulk velocity of approximately 50 μm s -1.
Received: 26 November 1997/Accepted: 26 February 1998 相似文献
2.
Heat transfer properties vary locally and temporally in internal combustion engines due to variations in the boundary layer
flow. In order to characterize the dynamics in the boundary layer, crank-angle resolved high-speed micro particle image velocimetry
( μPIV) and particle tracking velocimetry (PTV) have been used for near-wall velocity measurements in a spark-ignition direct-injection
single cylinder engine. A 527-nm dual cavity green Nd:YLF laser was used for velocity measurements near the cylinder head
wall between the intake and exhaust valves in the tumble mean flow plane parallel to the cylinder axis. A long-distance microscope
was used to obtain a spatial resolution of 45 μm. Flow fields were determined from 180 to 490 CAD in the compression and expansion
strokes. The data show significant variation in the flow during the compression and expansion strokes and from cycle to cycle.
Flow deceleration was observed during the end of the compression that continued during the expansion stroke until 400 CAD
when the flow direction reverses. Sub-millimeter-sized vortical structures were observed within the boundary layer over extended
periods of time. 相似文献
3.
IntroductionThetwo_phaseflowwithcylinderparticlesisadifficultmatterintheoreticalresearchofmultiphaseflowandnon_Newtonianflow ,soitisacademicallyvaluabletostudyit.Meanwhile ,suchflowshaveabroadbackgroundinindustry .Inthematerialscience,themoldingandproces… 相似文献
4.
We address the problem of making quantitative measurements of local flow velocities in turbulent liquid helium, using tracer
particles. We survey and evaluate presently available particles and previous work to establish the need to develop new particles
for the purpose. We present the first practical solution for visualizing fluid motions using a suspension of solid hydrogen
particles with diameters of about 2 μm. The hydrogen particles can be used to study flows with Taylor-microscale Reynolds
numbers between 85 and 775. The particles can be used equally well with the PIV, LDV, or particle-tracking techniques. 相似文献
5.
We perform direct numerical simulation of three‐dimensional turbulent flows in a rectangular channel, with a lattice Boltzmann method, efficiently implemented on heavily parallel general purpose graphical processor units. After validating the method for a single fluid, for standard boundary layer problems, we study changes in mean and turbulent properties of particle‐laden flows, as a function of particle size and concentration. The problem of physical interest for this application is the effect of water droplets on the turbulent properties of a high‐speed air flow, near a solid surface. To do so, we use a Lagrangian tracking approach for a large number of rigid spherical point particles, whose motion is forced by drag forces caused by the fluid flow; particle effects on the latter are in turn represented by distributed volume forces in the lattice Boltzmann method. Results suggest that, while mean flow properties are only slightly affected, unless a very large concentration of particles is used, the turbulent vortices present near the boundary are significantly damped and broken down by the turbulent motion of the heavy particles, and both turbulent Reynolds stresses and the production of turbulent kinetic energy are decreased because of the particle effects. We also find that the streamwise component of turbulent velocity fluctuations is increased, while the spanwise and wall‐normal components are decreased, as compared with the single fluid channel case. Additionally, the streamwise velocity of the carrier (air) phase is slightly reduced in the logarithmic boundary layer near the solid walls. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
6.
Large-eddy simulations (LES) of a vertical turbulent channel flow laden with a very large number of solid particles are performed.
The motivation for this research is to get insight into fundamental aspects of co-current turbulent gas-particle flows, as
encountered in riser reactors. The particle volume fraction equals about 1.3%, which is relatively high in the context of
modern LES of two-phase flows. The channel flow simulations are based on large-eddy approximations of the compressible Navier–Stokes
equations in a porous medium. The Euler–Lagrangian method is adopted, which means that for each individual particle an equation
of motion is solved. The method incorporates four-way coupling, i.e., both the particle-fluid and particle–particle interactions
are taken into account. The results are compared to single-phase channel flow in order to investigate the effect of the particles
on turbulent statistics. The present results show that due to particle–fluid interactions the mean fluid profile is flattened
and the boundary layer is thinner. Compared to single-phase turbulent flow, the streamwise turbulence intensity of the gas
phase is increased, while the normal and spanwise turbulence intensities are reduced. This finding is generally consistent
with existing experimental data. The four-way coupled simulations are also compared with two-way coupled simulations, in which
the inelastic collisions between particles are neglected. The latter comparison clearly demonstrates that the collisions have
a large influence on the main statistics of both phases. In addition, the four-way coupled simulations contain stronger coherent
particle structures. It is thus essential to include the particle–particle interactions in numerical simulations of two-phase
flow with volume fractions around one percent. 相似文献
7.
A biological seeding particle approach for μ-PIV measurements of a fluid flow provoked by microorganisms is presented. Results
obtained with yeast cells and milk as tracer particles are compared. The use of milk enables more detailed visualisation and
analysis of the fluid flow at higher magnifications. 相似文献
8.
Particle-laden flows in a horizontal channel were investigated by means of a two-phase particle image velocimetry (PIV) technique.
Experiments were performed at a Reynolds number of 6 826 and the flow is seeded with polythene beads of two sizes, 60 μm and 110 μm. One was slightly smaller than and the other was larger than the Kolmogorov length scale. The particle loadings were relatively
low, with mass loading ratio ranging from 5×10 −4 to 4×10 −2 and volume fractions from 6×10 −7 to 4.8×10 −5, respectively. The results show that the presence of particles can dramatically modify the turbulence even under the lowest
mass loading ratio of 5×10 −4. The mean flow is attenuated and decreased with increasing particle size and mass loading. The turbulence intensities are
enhanced in all the cases concerned. With the increase of the mass loading, the intensities vary in a complicated manner in
the case of small particles, indicating complicated particle-turbulence interactions; whereas they increase monotonously in
the case of large particles. The particle velocities and concentrations are also given. The particles lag behind the fluid
in the center region but lead in the wall region, and this trend is more prominent for the large particles. The streamwise
particle fluctuations are larger than the gas fluctuations for both sizes of particles, however their varying trend with the
mass loadings is not so clear. The wall-normal fluctuations increase with increasing mass loadings. They are smaller in the
60 μm particle case but larger in the 110 μm particle case than those of the gas phase. It seems that the small particles follow the fluid motion to certain extent while
the larger particles are more likely dominated by their own inertia. Finally, remarkable non-uniform distributions of particle
concentration are observed, especially for the large particles. The inertia of particles is proved to be very important for
the turbulence modification and particles behaviors and thus should be considered in horizontal channels.
The project supported by the National Natural Science Foundation of China (50276021), and Program for New Century Excellent
Talents in University, Ministry of Education (NCET-04-0708) The English text was polished by Yunming Chen. 相似文献
9.
The effect of particle size on two-phase turbulent jet flow structure is studied in the present experimental investigation. Polystyrene solid particles of 210, 460, and 780 μm were used. The particles' mass loading ratios ranged from 0 to 3.6. The flow Reynolds number was 2 ‘ 10 4, which was based on the pipe nozzle diameter and the fluid-phase centerline velocity at the nozzle exit. A two-color laser-Doppler anemometer (LDA), combined with the amplitude discrimination method and the velocity filter method, was employed for measurement. The measurement range of the jet flow was from the initial pipe exit to 90D downstream. Results are presented for the mean velocities of particle and fluid phases, the flow's turbulent intensities and the flow's Reynolds stresses. The energy spectra and the correlation functions of the two-phase jet flow were also obtained by using another one-component He-Ne LDA system. 相似文献
10.
Frequency spectra of air turbulence of particle-laden flows were investigated by use of a laser-Doppler velocimeter to discover
the eddy-length scales that are influenced by the transported particles. The influence of glass and steel particles of 100–1,000 μm
diameter was measured in a horizontal channel and a horizontal pipe for the streamwise and transverse components of the velocity
vector. Particles that were small compared with the integral length scale of the particle-laden flow decrease the turbulent
power density of the greatest eddies in varying degrees, depending on mass loading and distance from the wall. All fractions
create turbulence in their wakes, the size of which depends on loading and slip velocity. These results support the hypothesis
that the particles consume energy by following the large eddies that are much greater than the particle diameters, and in
so doing, turbulence is created by this energy.
Received: 28 September 2000/Accepted: 9 April 2001 相似文献
11.
To investigate the behaviour of inter-particle collision and its effects on particle dispersion, direct numerical simulation of a three-dimensional two-phase turbulent jet was conducted. The finite volume method and the fractional-step projection algorithm were used to solve the governing equations of the gas phase fluid and the Lagrangian method was applied to trace the particles. The deterministic hard-sphere model was used to describe the inter-particle collision. In order to allow an analysis of inter-particle collisions independent of the effect of particles on the flow, two-way coupling was neglected. The inter-particle collision occurs frequently in the local regions with higher particle concentration of the flow field. Under the influence of the local accumulation and the turbulent transport effects, the variation of the average inter-particle collision number with the Stokes number takes on a complex non-linear relationship. The particle distribution is more uniform as a result of inter-particle collisions, and the lateral and the spanwise dispersion of the particles considering inter-particle collision also increase. Furthermore, for the case of particles with the Rosin–Rammler distribution (the medial particle size is set d50 = 36.7 μm), the collision number is significantly larger than that of the particles at the Stokes number of 10, and their effects on calculated results are also more significant. 相似文献
12.
The dynamics of particles of the disperse phase in a turbulent gas flow in planar shock waves sliding along a solid surface
with a trapezoid cavity is examined numerically. Lifting of particles from the cavity walls is calculated in the approximation
of a rarefied gas suspension. It is shown that the intensity of the transient shock wave and the initial positions of particles
have a significant effect on the particle-lifting properties. The height of particle lifting is found to nonmonotonically
depend on the initial streamwise coordinate and shock-wave Mach number. It is shown that zones of aggregation and subtraction
of particles may be formed at the cavity bottom.
__________
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 1, pp. 24–34, January–February, 2007. 相似文献
13.
This paper reports the results from a study of the effects of particles on the boundary layer characteristics for cross flow over a single circular cylinder in the near separation region and on a cylinder located in the second row of a small mock-up heat exchanger array. Two size ranges of particles (45–85 m and 100–200 m) with two concentrations were used for the single cylinder tests. For the array, the larger particles were used at a single concentration. For the single cylinder, the results with the larger particles at mass loading ratio of 1.0 show an obvious effect on the velocity profile shape and on the rms levels. For reduced concentrations, and for the smaller particles, the effects are less noticeable. For the array, similar trends are observed and a delay in the separation point is reported. 相似文献
14.
Measurements of particle mean and r.m.s. velocity were obtained by laser-Doppler anemometry in a descending solid-liquid turbulent flow in a vertical pipe with volumetric concentrations of suspended spherical particles of 270 μm mean diameter in the range 0.1–14%. Similar measurements were obtained in the flow downstream of an axisymmetric baffle of 50% area blockage placed in the pipe with volumetric concentrations of 310 μm particles up to 8% and of 665 μm particles up to 2%. In order to enable measurements in high particle concentrations without blockage of the laser beams the refractive index of the particles was matched to that of the carrier fluid. The results show that the particle mean velocity profiles become more uniform and the particle r.m.s. velocity decreases with increasing concentration in both flow cases. The particle mean velocity in the pipe flow also decreases with concentration and the relative velocity, the difference between the particle velocity and the fluid velocity in single-phase flow, decreases with increasing Reynolds number. The length of the recirculation region downstream of the baffle was shorter than in single-phase flow by 11 and 24% for particle concentrations of 4 and 8%, respectively. The particle mean velocities were hardly affected by size for concentrations up fo 2%, but the r.m.s. velocities were lower with the larger particles. 相似文献
15.
The motion is considered of a Stokes-like spherical particle in a turbulent nonisothermic gas flow whose viscosity depends
on the temperature. The field of the turbulent velocity is assumed to be homogeneous, isotropic, and steady. It is shown that
if there is a mean temperature gradient in the gas, and, consequently, a heat flow due to turbulent pulsations, then there
may be turbulent migration of particles in a direction collinear with the gradient of the mean temperature. The migration
is due to statistical correlation of turbulent pulsations of velocity and temperature, and is not connected with the phenomenon
of ordinary thermophoresis. Upon the introduction of a number of simplifying assumptions, the rate of migration is calculated
in dependence on the characteristics of the particle and the flow.
Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 53–58, November–December, 1986.
The author is grateful to V. S. Galkin, V. A. Zharov, M. N. Kogan, and V. A. Sabel'nikov for discussions of the study. 相似文献
16.
The flow and distribution of Newtonian, polymeric and colloid suspension fluids at low Reynolds numbers in bifurcations has
importance in a wide range of disciplines, including microvascular physiology and microfluidic devices. A bifurcation consisting
of circular capillaries laser etched into a hard polymer with inlet diameter 2.50 ± 0.01 mm, bifurcating to a small diameter
outlet of 0.76 ± 0.01 mm and a large diameter outlet of 1.25 ± 0.01 mm is examined. Four distinct fluids (water, 0.25%wt xanthan
gum, 8 and 22%vol hard-sphere colloidal suspensions) are flowed at flow rates from 10 to 30 ml/h corresponding to Reynolds
numbers based on the entry flow from 0.001 to 8. PGSE NMR techniques are applied to obtain dynamic images of the fluids inside
the bifurcation with spatial resolution of 59 × 59 μm/pixel in plane over a 200-μm-thick slice. Velocity in all three spatial
directions is examined to determine the impact of secondary flows and characterize the transport in the bifurcation. The velocity
data provide direct measurement of the volumetric distribution of the flow between the two channels as a function of flow
rate. Water and the 8% colloidal suspension show a constant distribution with increasing flow rate, the xanthan gum shows
an increase in fluid going into the larger outlet with higher flow rate, and the 22% colloidal suspension shows a decrease
in fluid entering the larger channel with higher flow rate. For the colloidal particle flow, the distribution of colloid particles
down the capillary is determined by examining the spectrally resolved propagator for the oil inside the core–shell particles
in a direction perpendicular to the axial flow. Using dynamic magnetic resonance microscopy, the potential for using magnetic
resonance for “particle counting” in a microscale bifurcation is thus demonstrated. 相似文献
17.
The diffusion equations and boundary condition for particle deposition from a turbulent flow are obtained on the basis of
the kinetic equation for the probability density of the particle distribution. This approach makes it possible to calculate
the deposition fairly simply without introducing additional empirical information relating to the particles (empirical constants
are needed only for calculating the characteristics of the turbulent carrier flow).
Moscow. Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 96–104, September–October, 1988. 相似文献
18.
In this paper a typical fluid-structure interaction scenario is investigated for a turbulent flow past a circular cylinder
at a relatively low subcritical Reynolds number. Numerous experimental and numerical studies have been undertaken for a baseline
Reynolds number of 4,000 involving a stationary cylinder to study in detail the near wake mean flow and turbulence characteristics.
These studies conclusively show that the turbulent wake displays significant coherent periodic structures of large eddies
that could be adequately and profitably resolved by “low order modelling” of turbulence. In this study, an unsteady numerical
framework is employed for the simulations, incorporating an Arbitrary Lagrangian–Eulerian (ALE) method for the associated
grid deformation to simulate the coupled motion of the circular cylinder with a single degree of freedom in the initial zone
in a typical cylinder-flow response map or what is called “initial regime”. Particular attention is paid towards resolving
the large scales of the fluid motion and the inherent coupling of the cylinder’s motion towards the associated evolution of
the time averaged flow field. The flow-induced vibration effects regarding the kinetic energy exchange between the mean flow
and the coherent periodic scales are investigated further. The predictions discussed and analyzed in detail in the paper display
reasonable agreement with the chosen benchmark tests of the stationary cylinder and suggest that the conclusions outlined
regarding the coupled flow-cylinder system potentially provides a valuable contribution to the state of the art. 相似文献
19.
μPIV is a widely accepted tool for making accurate measurements in microscale flows. The particles that are used to seed the
flow, due to their small size, undergo Brownian motion which adds a random noise component to the measurements. Brownian motion
introduces an undesirable error in the velocity measurements, but also contains valuable temperature information. A PIV algorithm
which detects both the location and broadening of the correlation peak can measure velocity as well as temperature simultaneously
using the same set of images. The approach presented in this work eliminates the use of the calibration constant used in the
literature (Hohreiter et al. in Meas Sci Technol 13(7):1072–1078, 2002), making the method system-independent, and reducing the uncertainty involved in the technique. The temperature in a stationary
fluid was experimentally measured using this technique and compared to that obtained using the particle tracking thermometry
method and a novel method, low image density PIV. The method of cross-correlation PIV was modified to measure the temperature
of a moving fluid. A standard epi-fluorescence μPIV system was used for all the measurements. The experiments were conducted
using spherical fluorescent polystyrene-latex particles suspended in water. Temperatures ranging from 20 to 80°C were measured.
This method allows simultaneous non-intrusive temperature and velocity measurements in integrated cooling systems and lab-on-a-chip
devices. 相似文献
20.
A fibre Bragg grating (FBG) sensor was proposed as an alternative to strain gauges to measure the strain ɛ of a vibrating
cylinder in a uniform cross flow. In order to validate the measurements of the FBG sensor, the transverse fluctuating bending
displacement Y of the cylinder was also measured using a laser vibrometer. The two measurements were found to be consistent in terms of
the natural frequency of the fluid–structure system and the vortex shedding frequency. The spectral coherence between ɛ and
Y at the same point of the cylinder attains 1 at these frequencies, thus indicating a near perfect correlation between the
two quantities. When the transverse bending displacement is small, the measured ɛ and Y are linearly related. Therefore, the results indicate that the FBG sensor can be used with confidence to measure the fluctuating
strain arising from the vortex-induced forces on a structure in a uniform cross flow. As such, it can be used in conjunction
with a laser Doppler anemometer to study fluid–structure interactions in flow-induced vibration problems. Furthermore, it
is expected that the FBG sensor, because of its physical uniqueness, will have an important role to play in the study of fluid–structure
interaction problems with multiple structures arranged in an array.
Received: 17 August 1998 / Accepted 27 January 1999 相似文献
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