共查询到20条相似文献,搜索用时 15 毫秒
1.
Bart Beulen Nathalie Bijnens Marcel Rutten Peter Brands Frans van de Vosse 《Experiments in fluids》2010,49(5):1177-1186
An ultrasound velocity assessment technique was validated, which allows the estimation of velocity components perpendicular
to the ultrasound beam, using a commercially available ultrasound scanner equipped with a linear array probe. This enables
the simultaneous measurement of axial blood velocity and vessel wall position, rendering a viable and accurate flow assessment.
The validation was performed by comparing axial velocity profiles as measured in an experimental setup to analytical and computational
fluid dynamics calculations. Physiologically relevant pulsating flows were considered, employing a blood analog fluid, which
mimics both the acoustic and rheological properties of blood. In the core region (|r|/a < 0.9), an accuracy of 3 cm s−1 was reached. For an accurate estimation of flow, no averaging in time was required, making a beat to beat analysis of pulsating
flows possible. 相似文献
2.
New insights into particle image velocimetry data using fuzzy-logic-based correlation/particle tracking processing 总被引:2,自引:0,他引:2
M. P. Wernet 《Experiments in fluids》2001,30(4):434-447
Digital particle image velocimetry (DPIV) data processing has been developed to the point where DPIV image data are processed
via auto- or cross-correlation techniques in near real time and the results are displayed on screen as they are processed.
Correlation techniques are highly desirable, since they provide velocity measurements on a regular grid, which are readily
comparable to CFD predictions of the flow field. In high-speed flows, particle lag effects are always of concern; however,
the correlation operation does not provide any means for minimization or elimination of systematic errors in the recorded
particle image data. In this paper, we present a combined correlation processing/particle tracking technique providing “super-resolution”
velocity measurements. Fuzzy-logic principles are employed to maximize the information recovery in the correlation operation
and to determine the correct particle pairings in the tracking operation. The combined correlation/particle tracking technique
is applied to DPIV data obtained in the diffuser region of a high-speed centrifugal compressor producing velocity vector maps
with an average density of 6 vectors/mm2. Inspection of the particle tracking results revealed large particles that were not following the flow. Using preknowledge
of the flow field, the biased velocity estimates arising from large particles in the flow were removed, thereby improving
the accuracy of the measurements.
Received: 21 October 1999/Accepted: 19 August 2000 相似文献
3.
The dynamics of particles in multi-phase jets has been widely studied due to its importance for a broad range of practical applications. The present work describes an experimental investigation on an initially non-dilute two-phase jet, aimed at improving the understanding in this field. A two-color PDPA has been employed to measure simultaneously the velocity and size of particles. The measurements are post-processed to check the reliability of the results and to derive information on particle volume flux as an indication of their concentration. Acoustic forcing is applied in order to control coherent structures, which are responsible for mixing and transport phenomena, and also to get phase-locked measurements. Phase-averaged statistics enabled to freeze the jet structure, not visible in the time-averaged data. The results along the jet centerline confirm that drag forces and the spread angle of the jet initially control particle dispersion, very near the nozzle exit (x/D < 4). However, as the vortical structures evolve forming tongue-shaped structures, the total particle volume flux is augmented when these structures connect with the main stream (x/D > 5). This is due to an increase of the number of smaller size particles, even when a decrease of the number of larger size particle is observed. Further analysis at five cross-stream sections across two consecutive vortices confirm that small particles are convected around the coherent structure and then incorporated to the main stream, increasing the particle concentration at the jet core. On the other hand, the number of larger particles (as well as their contribution to axial volume flux) starts to decay in regions of high azymuthal vorticity. This behaviour is partly ascribed to the transversal lift force, associated to the large spatial gradients observed in these regions. Saffman and Magnus forces have been estimated to be comparable or even greater than radial drag forces. The results suggest that the Saffman force might accelerate particles in radial direction, inducing a high radial volumetric flow rate from high to low axial velocity regions. 相似文献
4.
Statistical Properties of Turbulent Free Jets Issuing from Nine Differently-Shaped Nozzles 总被引:1,自引:0,他引:1
This paper reports the centerline evolutions of turbulent statistical properties in nine air jets issuing from differently-shaped
nozzles into still air surroundings. All nozzles of investigation have nominally identical opening areas or equal equivalent
diameters (D
e
) and their aspect ratio (AR) varies from AR = 1 (circle) to AR = 2.5 (isosceles triangle). Present measurements were made at the Reynolds number (based on D
e
) of approximately 15,000. Results show that the loss of jet-axisymmetry at the exit generally causes the mean velocity decaying
faster, and the fluctuating intensity growing, in the near field, thus indicating the increased overall entrainment rate.
It is also shown that a change of shape of the nozzle exit does not affect the asymptotic decay rate of the centreline velocity
in the far field. The near-field structure of the isosceles-triangular jet is deduced to be most three-dimensional, compared
with the circular counterpart from smooth contraction being least. These discrepancies, however, weaken as the downstream
distance x is increased. Beyond x/D
e
= 20–30, the normalized velocity spectra for all jets of small AR collapse well, indicating similar statistical behaviors over a wide range of scales in the central region. Indeed, sufficiently
downstream from the exit, insignificant differences occur in jets’ velocity probability density function (PDF), the related
skewness and flatness factors, and also in their Taylor and Kolmogorov microscales. It is demonstrated that all the length
scales grow approximately linearly with x at x/D
e
≥ 20. 相似文献
5.
Vortex pairs are studied using a dye tracing visualisation technique and a particle tracking velocimetry system. The vortex
pairs are produced by gravity induced inlets of water issued through a uniform gap. The inlet Reynolds number is Re=Ud/ν≈875 in all tests (d being the gap width and U the cross sectional mean velocity), i.e. the flow is in the laminar regime. Initially, the dipolar vortex structure is two-dimensional,
but after travelling a distance of a few times its own width, the flow structure becomes unstable, breaks up and changes into
a three-dimensional flow structure. The breakup appears to be caused by an axial flow in the core centres of each vortex of
the dipolar structure. These axial flows are induced by boundary effects related to the von Karman viscous pump. After the
breakup, it is believed that a vortex ring is formed through reconnection of rudiments from the dipolar structure mediated
by the wall induced vorticity.
Received: 20 November 1995/Accepted: 14 November 1996 相似文献
6.
In relation to the development of the interfacial area transport equation, local flow measurements of vertical downward air–water flows in a pipe with an inner diameter of 50.8 mm were performed at three axial locations of z/D=6.50, 34.0, and 66.5 as well as ten radial locations from r/R=0 to r/R=0.9 using a multi-sensor probe. In the experiment, the superficial liquid velocity and the void fraction ranged from –0.620 m/s to –2.49 m/s and from 0.21% to 8.4%, respectively. The dependence of the interfacial area transport on the liquid velocity, void fraction, and bubble size is discussed in detail. 相似文献
7.
N. Andrés M. P. Arroyo H. Hinrichs K. Hinsch M. Quintanilla 《Experiments in fluids》2001,30(2):117-122
This paper investigates the feasibility of using holographic interferometry in wind tunnel flows for measuring velocity fields
rather than density or temperature fields. First results were obtained in a vortex street behind a cylinder at Re=190(U
∞=0.7 m/s). The light scattered from an illuminated fluid plane was holographically recorded twice with the same reference
beam. Using a time interval of 10 μs, local fluid displacements smaller than a few microns were recorded. The holographic
plate was placed in front and as close as possible to the fluid plane. The interferograms obtained from the hologram reconstruction
give information about one velocity component, at 45° with the illuminated plane. The alignment of the cylinder axis with
this 45° direction provided definite confirmation about the vortex street having a non-negligible axial velocity. The constant
velocity fluid region has proven to be very useful for quantifying the velocity information contained in the interferogram.
Received: 8 November 1999/Accepted: 14 March 2000 相似文献
8.
Particle tracer response across shocks measured by PIV 总被引:1,自引:0,他引:1
The experimental approach used for the evaluation of the particle response time across a stationary shock wave is assessed
by means of PIV measurements. The study focuses on the experimental requirements for a reliable and unbiased measurement of
the particle response time τ
p
and length ξ
p
based on a single-exponent decaying law. A numerical simulation of the particle response experiment returns the parameters
governing the measurement: namely the normalized spatial and temporal resolution, shock strength, and digital resolution.
Representing the velocity decay in logarithmic coordinates it is shown that measurements performed with laser pulse separation
time up to τ
p
and interrogation window up to ξ
p
still yield unbiased results for the particle response. A set of experiments on the particle response across a planar oblique
shock wave was conducted to verify the results from the numerical assessment. Liquid droplets of DEHS and solid tracer particles
of silicon and titanium dioxide with different primary crystal size are compared. The resulting temporal response ranges from
2 to 3 μs, corresponding to values commonly reported in literature, to almost 0.3 μs when particles are properly dehydrated
and a filter is applied before injection into the wind tunnel. It is the first experimental evidence of particle tracers with
a measured response time lower than 0.4 μs. The same procedure is applied to attempt the measurement of individual particle
tracers by particle tracking velocimetry to estimate the spread in the distribution of tracer time response. The latter analysis
is limited by the particle image tracking precision error, which biases the results introducing a wider broadening of the
particle velocity distribution. 相似文献
9.
This paper describes the tests of accuracy and the first application of a combined planar visualization technique. Its goal is two-phase flow discrimination, i.e. simultaneous measurements of velocity of droplets and ambient gas in the case of two-phase flow mixing, at the same location and with possible conditioning by “apparent diameter” (AD) of the droplets. It combines the mature techniques of particle image velocimetry (PIV), planar Mie scattering diffusion (PMSD), planar laser-induced fluorescence (PLIF), and it necessitates two synchronized cross-correlation systems, digital image treatment and analysis. This technique was developed with the objective of better describing the mixing between liquid and gaseous phases as in the case of high-pressure spray atomization in quiescent ambient gas. The basic principle of separation is to seed the ambient gas with micrometer particles and to tag the liquid with fluorescent dye. We use digital image treatment and analysis to discriminate between the phases. We use two cross-correlation PIV systems in order to obtain the velocity field of the droplets and gas simultaneously and separately at the same location. The digital image processing for separating the phases involves geometric measurement of droplet shapes. This leads to measurement of droplet parameters close to their real diameter, which could be used for analysis of actual mixing. A synchronized system composed of two CCD cameras is used for image recording, and two Nd:YAG lasers are used for generating pulsed light sheets at times t and t + δt. Tests were performed to check for different sources of errors. The combined technique was applied to measurements in high-pressure spray flow atomizing in a quiescent ambient gas, and first results are presented. 相似文献
10.
A specially constructed hot-wire probe was used to obtain very near-wall velocity measurements in both a fully developed
turbulent channel flow and flat plate boundary layer flow. The near-wall hot-wire probe, having been calibrated in a specially
constructed laminar flow calibration rig, was used to measure the mean streamwise velocity profile, distributions of streamwise
and spanwise intensities of turbulence and turbulence kinetic energy k in the viscous sublayer and beyond; these distributions compare very favorably with available DNS results obtained for channel
flow. While low Reynolds number effects were clearly evident for the channel flow, these effects are much less distinct for
the boundary layer flow. By assuming the dissipating range of eddy sizes to be statistically isotropic and the validity of
Taylor's hypothesis, the dissipation rate ɛ
iso in the very near-wall viscous sublayer region and beyond was determined for both the channel and boundary layer flows. It
was found that if the convective velocity U
c in Taylor's hypothesis was assumed to be equal to the mean velocity Uˉ at the point of measurement, the value of (ɛ+
iso)1 thus obtained agrees well with that of (ɛ
+)DNS for y
+ ≥ 80 for channel flow; this suggests the validity of assuming U
c=Uˉ and local isotropy for large values of y
+. However, if U
c was assumed to be 10.6u
τ
, the value of (ɛ+
iso)2 thus obtained was found to compare reasonably well with the distribution of (ɛ+
iso)DNS for y
+≤ 15.
Received: 31 May 1999/Accepted: 20 December 1999 相似文献
11.
A. A. Tawfek 《Heat and Mass Transfer》1999,35(4):327-333
An experimental investigations of heat transfer for a stationary isothermal circular cylinder exposed normal to an impinging
round air-jet has been reported. The circumferential heat transfer distributions as well as axial Nusselt number is measured.
The measurements are taken as a function of the Reynolds number ranging from 3.8 × 103 to 4 × 104, the cylinder separation distance to the nozzle diameter (z/d) varying from 7 to 30, and the nozzle to cylinder diameter ratio (d/D) changing from 0.06 to 0.14. The output results indicated that the axial and radial distributions of the local heat transfer
peaked at the impingement point. The heat transfer rate increases as the values of z decreases, for the same d and Re. The drop-off of the Nusselt number with increasing axial distance or radial angle from the impingement point was
more pronounced for smaller z and d. The peripheral and surface average Nusselt numbers were determined by integration. The experimental data was used to produce
correlations for both average and stagnation point heat transfer.
Received on 4 January 1999 相似文献
12.
We present detailed experimental results examining “negative wakes” behind spheres settling along the centerline of a tube
containing a viscoelastic aqueous polyacrylamide solution. Negative wakes are found for all Deborah numbers (2.43≤De(˙γ)≤8.75) and sphere-to-tube aspect ratios (0.060≤a/R≤0.396) examined. The wake structures are investigated using laser-Doppler velocimetry (LDV) to examine the centerline fluid
velocity around the sphere and digital particle image velocimetry (DPIV) for full-field velocity profiles. For a fixed aspect
ratio, the magnitude of the most negative velocity, U
min
, in the wake is seen to increase with increasing De. Additionally, as the Deborah number becomes larger, the location of this minimum velocity shifts farther downstream. When
normalized with the sphere radius and the steady state velocity of the sphere, the axial velocity profiles become self-similar
to the point of the minimum velocity. Beyond this point, the wake structure varies weakly with aspect ratio and De, and it extends more than 20 radii downstream. Inertial effects at high Reynolds numbers are observed to shift the entire
negative wake farther downstream. Using DPIV to investigate the transient kinematic response of the fluid to the initial acceleration
of the sphere from rest, it is seen that the wake develops from the nonlinear fluid response at large strains. Measurements
of the transient uniaxial extensional viscosity of this weakly strain-hardening fluid using a filament stretching rheometer
show that the existence of a negative wake is consistent with theoretical arguments based on the opposing roles of extensional
stresses and shearing stresses in the wake of the sphere.
Received: 10 November 1997 Accepted: 1 May 1998 相似文献
13.
The concentration fluctuation c of diluted fluorescein dye, a high-Schmidt-number passive scalar (Sc=ν/D ≈ 2000, ν and D are the fluid momentum and dye diffusivities, respectively), is measured in the wake of a circular cylinder using a single-point
laser-induced fluorescence (SPLIF) technique. The streamwise decay rate of the mean and rms values of c is slow in comparison to that of θ, the temperature fluctuation for which the molecular Prandtl number Pr=ν/κ is about 0.7 (κ is the thermal diffusivity). The comparison between mean and rms distributions of c and θ highlights the combined role the Reynolds and Schmidt numbers play in terms of dispersing the scalar. The streamwise
evolution of the probability density functions (pdfs) of c and θ suggest that while p(θ) is approximately Gaussian in the intermediate wake (x/d ≈ 80), p(c) is strongly non-Gaussian, and depends on both x/d and Re. The skewness of c is larger than that of θ along the wake centreline. Arguably, the asymmetry of p(c) reflects the relatively strong organisation of the large-scale motion in the far-wake.
Received: 27 July 2000/Accepted: 22 December 2000 相似文献
14.
Although equilibrium of spherical particles under radial migration has been extensively investigated, mostly in macroscale
flows with characteristic lengths on the order of centimeters, it is not fully characterized at relatively small Reynolds
numbers, 1 ≤ Re ≤ 100. This paper experimentally studies “inertial microfluidic” radial migration of spherical particles in circular Poiseuille
flow through a microcapillary. Microparticle tracking experiments are performed to obtain the spatial distribution of the
particles by adopting a depth-resolved measurement technique. Through the analysis of the radial distribution of particles,
inertial microfluidic circular Poiseuille flow is shown to induce a strong radial migration of particles at substantially
small Re, which is quite in contrast to the pipe flows at large Re previously reported. This particle migration phenomenon is so prominent that particle equilibrium positions are formed even
at small Re. However, it turns out that there exists a certain critical Re below which particle equilibrium position is almost fixed, but above which it seems to drift toward the channel wall. 相似文献
15.
Fluid flow through microtubes is of interest to many industries and there exists a need for detailed measurements of the
velocity field. Velocity profile data are critical for momentum, mass, and heat transport analysis, and thus the design of
devices utilizing microgeometries. This paper outlines a measurement technique that has led to time-resolved measurements
of velocity profiles in microtubes (less than 1,000 μm). The research program was experimental in nature and consisted of
an extension of molecular tagging velocimetry to the microscale. Average velocity and rms profile data in the fully developed
region, in addition to mass flow rate and pressure drop data, are presented for numerous Reynolds numbers ranging from 600
to 5,000 in a tube of diameter 705 μm.
Received: 20 December 1999 / Accepted: 20 March 2001 相似文献
16.
Thomas Schweizer 《Rheologica Acta》2007,46(5):629-637
Creep experiments with a solution of polystyrene (M
w = 2.6 MDa, 16 vol.%, 25 °C) in diethyl phthalate are reported for stresses between 100 and 2,500 Pa (≈ 3G
N
0/4). The aim was to look for a flow transition as reported for strongly entangled poly(isobutylene) solutions. The experiments
with the polystyrene solution were repeated for cone angles of 2, 4, and 6° (radius 15 mm) and showed no dependence on cone
angle. The Cox–Merz rule was not fulfilled for stresses beyond about 800 Pa. The tangential observation with a CCD camera
showed that the edge took a concave shape because of the second normal stress difference. Beyond 1,000 Pa, the concave edge
develops into a crevice, thus substantially reducing the effective cross-section. This leads to runaway in a constant torque
experiment. At p
21 = 800 Pa, head-on particle tracking confirms that the originally linear velocity profile takes a gooseneck shape, thus revealing
shear banding. When the creep stress is stepped down to 100 Pa, this velocity profile evolves back to a linear one. The conclusion
from this work is that even if nonlinear creep experiments are reproducible and a steady state is reached, this does not mean
that the flow field is homogeneous.
This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27–29, 2006. 相似文献
17.
Wind-blown sand is a typical example of two-phase particle-laden flows. Owing to lack of simultaneous measured data of the
wind and wind-blown sand, interactions between them have not yet been fully understood. In this study, natural sand of 100–125 μm
taken from Taklimakan Desert was tested at the freestream wind speed of 8.3 m/s in an atmospheric boundary layer wind tunnel.
The captured flow images containing both saltating sand and small wind tracer particles, were separated by using a digital
phase mask technique. The 2-D PIV (particle imaging velocimetry) and PTV (particle tracking velocimetry) techniques were employed
to extract simultaneously the wind velocity field and the velocity field of dispersed sand particles, respectively. Comparison
of the mean streamwise wind velocity profile and the turbulence statistics with and without sand transportation reveal a significant
influence of sand movement on the wind field, especially in the dense saltating sand layer (y/δ < 0.1). The ensemble-averaged streamwise velocity profile of sand particles was also evaluated to investigate the velocity
lag between the sand and the wind. This study would be helpful in improving the understanding of interactions between the
wind and the wind-blown sand. 相似文献
18.
The dye visualization experiments show that a dual leading-edge vortex (LEV) structure exists on the suction side of a simplified
butterfly model of Papilio ulysses at α = 8°−12°. Furthermore, the results of particle image velocimetry (PIV) measurement indicate that the axial velocity of the
primary (outer) vortex core reaches the lower extreme value while a transition from a “wake-like” to a “jet-like” axial velocity
profile occurs. The work reveals for the first time the existence of dual LEV structure on the butterfly-like forward-sweep
wing configuration. 相似文献
19.
Masaru Sumida 《Experiments in fluids》2007,43(6):949-958
This paper presents the results of an experimental study on the developing pulsatile flow in curved pipes with a long, straight
pipe upstream. In order to examine the dependence of flow-field development on the governing parameters, LDV measurements
were conducted systematically for six cases of flow, where the Womersley number α was varied from 5.5 to 18, the mean Dean number D
m
was 200 and 300, the flow rate ratio η was 0.5 and 1, and the curvature radius ratio Rc was 10 and 30. Peculiar flow phenomena, such as flow reversal for all values of α and a depression in the axial velocity profile for α = 10, were analyzed by decomposing the axial velocity into a time-mean and a varying component, as well as by obtaining the
bias of their profiles. The velocity distributions abruptly change with the phase at turn angles Ω of 15–30°, corresponding to the nondimensional axial length z′ ≅ 1–2 from the bend entrance, and their development along the pipe axis is the most complicated for the flow at a moderate
α of 10 and large η of 1. The entrance length in the case of pulsatile flow is shorter than that for steady flow with the same flow rate as the
maximum pulsatile flow rate. 相似文献
20.
In this paper the present authors measured the gas-particle two-phase velocity correlation in sudden expansion gas-particle flows with a phase Doppler particle anemometer(PDPA) and simulated the system behavior by using both a Reynolds-averaged Navier-Stokes(RANS) model and a large-eddy simulation(LES).The results of the measurements yield the axial and radial time-averaged velocities as well as the fluctuation velocities of gas and three particle-size groups(30 μ m,50 μ m,and 95 μ m) and the gas-particle velocity correlation for 30 μ m and 50 μ m particles.From the measurements,theoretical analysis,and simulation,it is found that the two-phase velocity correlation of sudden-expansion flows,like that of jet flows,is less than the gas and particle Reynolds stresses.What distinguishes the two-phase velocity correlations of sudden-expansion flow from those of jet and channel flows is the absence of a clear relationship between the two-phase velocity correlation and particle size in sudden-expansion flows.The measurements,theoretical analysis,and numerical simulation all lead to the above-stated conclusions.Quantitatively,the results of the LES are better than those of the RANS model. 相似文献