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1.
Flow characteristics in the interdisk midplane between two shrouded co-rotating disks were experimentally studied. A laser-assisted
particle-laden flow-visualization method was used to identify the qualitative flow behaviors. Particle image velocimetry was
employed to measure the instantaneous flow velocities. The flow visualization revealed rotating polygonal flow structures
(hexagon, pentagon, quadrangle, triangle, and oval) existing in the core region of the interdisk spacing. There existed a difference between the rotating frequencies of the
polygon and the disks. The rotating frequency ratio between the polygonal flow structure and the disks depended on the mode
shapes of the polygonal core flow structures—0.8 for pentagon, 0.75 for quadrangle, 0.69 for triangle, and 0.6 for oval. The
phase-resolved flow velocities relative to the bulk rotation speed of the polygonal core flow structure were calculated, and
the streamline patterns were delineated. It was found that outside the polygonal core flow structure, there existed a cluster
of vortex rings—each side of the polygon was associated with a vortex ring. The radial distributions of the time-averaged
and phase-resolved ensemble-averaged circumferential and radial velocities were presented. Five characteristic regions (solid-body
rotation region, hub-influenced region, buffer region, vortex region, and shroud-influenced region) were identified according
to the prominent physical features of the flow velocity distributions in the interdisk midplane. In the solid-body rotation
region, the fluid rotated at the angular velocity of the disks and hub. In the hub-influenced region, the circumferential
flow velocity departed slightly from the disks’ angular velocity. The circumferential velocities in the hub-influenced and
vortex regions varied linearly with variation of radial coordinates. The phase-resolved ensemble-averaged relative radial
velocity profiles in the interdisk midplane at various phase angles exhibited grouping behaviors in three ranges of polygon
phase angles (θ = 0 and α/2, 0 < θ < α/2, and α/2 < θ < α) because three-dimensional flow induced similar flow patterns to
appear in the same range of polygon phase angles. 相似文献
2.
Stereoscopic micro particle image velocimetry 总被引:1,自引:0,他引:1
A stereoscopic micro-PIV (stereo-μPIV) system for the simultaneous measurement of all three components of the velocity vector in a measurement plane (2D–3C) in a closed microchannel has been developed and first test measurements were performed on the 3D laminar flow in a T-shaped micromixer. Stereomicroscopy is used to capture PIV images of the flow in a microchannel from two different angles. Stereoscopic viewing is achieved by the use of a large diameter stereo objective lens with two off-axis beam paths. Additional floating lenses in the beam paths in the microscope body allow a magnification up to 23×. The stereo-PIV images are captured simultaneously by two CCD cameras. Due to the very small confinement, a standard calibration procedure for the stereoscopic imaging by means of a calibration target is not feasible, and therefore stereo-μPIV measurements in closed microchannels require a calibration based on the self-calibration of the tracer particle images. In order to include the effects of different refractive indices (of the fluid in the microchannel, the entrance window and the surrounding air) a three-media-model is included in the triangulation procedure of the self-calibration. Test measurement in both an aligned and a tilted channel serve as an accuracy assessment of the proposed method. This shows that the stereo-μPIV results have an RMS error of less than 10% of the expected value of the in-plane velocity component. First measurements in the mixing region of a T-shaped micromixer at Re = 120 show that 3D flow in a microchannel with dimensions of 800 × 200 μm2 can be measured with a spatial resolution of 44 × 44 × 15 μm3. The stationary flow in the 200 μm deep channel was scanned in multiple planes at 22 μm separation, providing a full 3D measurement of the averaged velocity distribution in the mixing region of the T-mixer. A limitation is that this approach requires a stereo-objective that typically has a low NA (0.14–0.28) and large depth-of-focus as opposed to high NA lenses (up to 0.95 without immersion) for standard μPIV. 相似文献
3.
Velocity distributions in a hydrocyclone separator 总被引:2,自引:0,他引:2
The internal three-dimensional flow field in a hydrocyclone was studied using laser velocimetry. Seven axial planes were
investigated for three different inlet flow rates and three independent and different rejects rates. Results at each measurement
plane showed that the measured tangential velocity profile behaves like a forced vortex at the region near the air core, and
like a free vortex in the outer portion of the flow. The peak nondimensional tangential velocity decreases as the distance
from the inlet region increases, however, the peak dimensional tangential velocity increases as the distance from the inlet
region increases. The nondimensional peak tangential velocities are approximately equal for all of the flow rates. The magnitude
of the tangential velocity increased in the inner forced vortex region as the rejects rate was increased. Backflows exist
in the axial velocity profile near the inlet region, but these reversed flows disappear in the exit region. The dimensional
vorticity is proportional to inlet flow rate and decreases with increasing rejects flow rates.
Received: 27 February 2001/Accepted: 19 June 2001 相似文献
4.
The detailed flow structure behind an impulsively started circular cylinder has been investigated experimentally. The Reynolds
number based on the steady state velocity and the diameter of the cylinder was 500 to 3,000. This work is unique in that unsteady
spatial velocities were measured simultaneously by a quantitative visualization technique — Laser Induced Photochemical Anemometry
(LIPA). The surface vorticity at g/q = π/2 and vorticity distribution behind the cylinder in the Lagrangian coordinates (i.e. coordinates fixed on the cylinder)
were calculated from the measured velocities. The surface vorticity shows in the early stage of flow development a close agreement
with the previous results obtained by analytical and numerical approaches. The large-field velocity and vorticity information
provides an insight into the formation process of the vortices downstream of the cylinder. In addition to the quantitative
information, the results of visualized flow pattern obtained by LIPA technique are also presented.
A preliminary version of this paper was presented at the Twelfth Symposium on Turbulence, University of Missouri-Rolla, Sept.
24–26, 1990 相似文献
5.
H. Rubinsztein-Dunlop B. Littleton P. Barker P. Ljungberg Y. Malmsten 《Experiments in fluids》2001,30(1):36-42
A flow tagging technique based upon ionic fluorescence in strontium is investigated for applications to velocity measurements
in gas flows. The method is based upon a combination of two laser based spectroscopic techniques, i.e. resonantly-enhanced
ionisation and laser-induced ionic fluorescence. Strontium is first ionised and then planar laser-induced fluorescence is
utilised to give 2D `bright images' of the ionised region of the flow at a given time delay. The results show that this method
can be used for velocity measurements. The velocities were measured in two types of air–acetylene flames – a slot burner and
a circular burner yielding velocities of 5.1 ± 0.1 m/s and 9.3 ± 0.2 m/s, respectively. The feasibility of the method for
the determination of velocities in faster flows than those investigated here is discussed.
Received: 5 November 1998/Accepted: 19 January 2000 相似文献
6.
The acoustic Doppler velocimeter (ADV) is widely used for the characterization of fluid flow. Secondary flows (“acoustic streaming”)
generated by the ADV’s acoustic pulses may affect the accuracy of measurements in experiments with small velocities. We assessed
the impact of acoustic streaming on flow measurement using particle image velocimetry. The probes of two different ADVs were
successively mounted in a tank of quiescent water. The probes’ ultrasound emitters were aligned with a laser light sheet.
Observed flow was primarily in the axial direction, accelerating from the ultrasound emitter and peaking within centimeters
of the velocimeter sampling volume before dropping off. We measured the dependence of acoustic streaming velocity on ADV configuration,
finding that different settings induce streaming ranging from negligible to more than 2.0 cm s−1. From these results, we describe cases where acoustic streaming affects velocity measurements and also cases where ADVs accurately
measure their own acoustic streaming. 相似文献
7.
The two refractive indices in the flow of a colloidal birefringent liquid are measured separately by means of a Mach-Zehnder
interferometer. For a quantitative evaluation of the resulting interferograms it is not necessary to linearize the respective
equations relating the refractive index distribution to the deformation velocity in the flow. Therefore it becomes possible
to perform velocity measurements in the non-Newtonian flow range. An additional measurement of the mean flow rate enables
one to determine the velocity field without the need of a calibration of the observed interference fringes. 相似文献
8.
Solid transport measurements through image processing 总被引:1,自引:0,他引:1
The paper describes a novel technique which enables evaluation of sediment fluxes on the upper layer of a granular bed by means of separate measurements of concentration and velocity of the moving particles. Specific elaboration techniques based on digital image processing were applied to films of the solid fluxes, allowing for automatic measurement. Sediment concentration was measured via a technique based on image subtraction and following proper filtering procedures, while grain velocity was measured by Particle Image Velocimetry. The method was applied in laboratory experiments of one dimensional bed load; the solid discharges measured by the proposed image processing technique were compared to those obtained by manual count of the grains passing over a fixed plate used as a sight. After calibration of the automatic technique, dimensionless solid discharges ranging from 1.0 × 10−3 to 1.2 × 10−1 were measured with a maximum error as large as 25%. The technique proposed also enables measurement of the time variation of the quantities and the two dimensional direction of sediment motion, for a complete characterization of grain kinematics in solid transport processes. 相似文献
9.
With the prevalence of particle image velocimetry (PIV) as a quantitative tool for fluid mechanics diagnostics, its application
for analyzing complicated multiphase flows has been steadily increasing over the last several decades. While the primary issue
in using PIV for multiphase flows is in separating the information of the phases for independent analysis with a minimum of
spurious “cross-talk,” an equally crucial but often overlooked point is in the accurate quantitative measurement of the dispersed
phase concentration. Accurate concentration measurement is important due to the fact that the dispersed phase is often heterogeneously
distributed in both space and time, either due to a non-uniformity of the source of particulates (such as a spray nozzle or
sediment boundary) or due to inertial migration of the particles even from originally homogeneous spatial distributions. In
the current work, we examine the effects of light sheet profile distortion and attenuation by tracer seeding particles, as
well as reflected light from local wall boundaries on the effective light sheet thickness. The effective thickness is critical
for concentration measurements, as it dictates the dispersed phase detection volume. A direct calibration method is demonstrated
to measure the effective light sheet thickness in a water/glass bead system, which shows that systematic bias errors on the
order of 30% can result if the reflective bed condition is not accounted for, and the errors can be as high as 50% or more
if a single-point measure of the sheet width is used. 相似文献
10.
The accuracy of tomographic particle image velocimetry for measurements of a turbulent boundary layer 总被引:1,自引:0,他引:1
Callum Atkinson Sebastien Coudert Jean-Marc Foucaut Michel Stanislas Julio Soria 《Experiments in fluids》2011,50(4):1031-1056
To investigate the accuracy of tomographic particle image velocimetry (Tomo-PIV) for turbulent boundary layer measurements,
a series of synthetic image-based simulations and practical experiments are performed on a high Reynolds number turbulent
boundary layer at Reθ = 7,800. Two different approaches to Tomo-PIV are examined using a full-volume slab measurement and a thin-volume “fat” light
sheet approach. Tomographic reconstruction is performed using both the standard MART technique and the more efficient MLOS-SMART
approach, showing a 10-time increase in processing speed. Random and bias errors are quantified under the influence of the
near-wall velocity gradient, reconstruction method, ghost particles, seeding density and volume thickness, using synthetic
images. Experimental Tomo-PIV results are compared with hot-wire measurements and errors are examined in terms of the measured
mean and fluctuating profiles, probability density functions of the fluctuations, distributions of fluctuating divergence
through the volume and velocity power spectra. Velocity gradients have a large effect on errors near the wall and also increase
the errors associated with ghost particles, which convect at mean velocities through the volume thickness. Tomo-PIV provides
accurate experimental measurements at low wave numbers; however, reconstruction introduces high noise levels that reduces
the effective spatial resolution. A thinner volume is shown to provide a higher measurement accuracy at the expense of the
measurement domain, albeit still at a lower effective spatial resolution than planar and Stereo-PIV. 相似文献
11.
Stereoscopic particle image velocimetry (SPIV) is applied to measure the instantaneous three component velocity field of pipe
flow over the full circular cross-section of the pipe. The light sheet is oriented perpendicular to the main flow direction,
and therefore the flow structures are advected through the measurement plane by the mean flow. Applying Taylor’s hypothesis,
the 3D flow field is reconstructed from the sequence of recorded vector fields. The large out-of-plane motion in this configuration
puts a strong constraint on the recorded particle displacements, which limits the measurement accuracy. The light sheet thickness
becomes an important parameter that determines the balance between the spatial resolution and signal to noise ratio. It is
further demonstrated that so-called registration errors, which result from a small misalignment between the laser light sheet
and the calibration target, easily become the predominant error in SPIV measurements. Measurements in laminar and turbulent
pipe flow are compared to well established direct numerical simulations, and the accuracy of the instantaneous velocity vectors
is found to be better than 1% of the mean axial velocity. This is sufficient to resolve the secondary flow patterns in transitional
pipe flow, which are an order of magnitude smaller than the mean flow. 相似文献
12.
We investigate the utility of particle imaging velocimetry (PIV) for performing kinematic measurements in wet aqueous foam
with a liquid film beneath it. The flow velocities are measured near the walls of a square cross-section horizontal duct.
The flow velocities are useful for validating the rheological models. We show that there is a discrepancy between the velocity
profiles in the wet foam and the Bingham plastic model of flow. The velocity measurements reveal a more complex flow pattern,
which may be analysed following three different regimes: a plug flow, a shear flow in a vertical plane and a three-dimensional
shear flow. The transition between the plug flow and the shear flows may be explained by a shear-induced migration of bubbles.
Received: 25 April 2000 / Accepted: 26 February 2001 相似文献
13.
To develop a quantitative understanding of unsteady and interacting turbomachine flow fields, it is necessary to quantify
the instantaneous efficiency of high speed turbomachines. This requires the measurement of both the unsteady velocity and
total temperature variation in the exit flow of a high speed rotor. In this paper, techniques to utilize a single slant-film
anemometer to measure unsteady total temperature are developed and evaluated. Then a series of preliminary experiments are
performed in a high speed axial fan facility to quantify the instantaneous rotor efficiency. This is accomplished by utilizing
these single slant-film methods to measure the total temperature in the rotor wakes. Results show that measurements at multiple
overheats and several probe orientations are required. The simplest method proves to be useful for determining parameters
used in other methods. An analysis based on King’s law gives good results even when measurements are outside the calibration
range. Within the calibration range, a polynomial representation of the wire response to mass flux and total temperature yields
good total temperature fluctuation results. A model analysis technique is also assessed.
Received: 13 November 1997/Accepted: 16 February 1998 相似文献
14.
Nonintrusive measurements in a sediment-laden flow using two laser-based techniques, Discriminator Laser-Doppler Velocimetry
(DLDV) and Particle Tracking Velocimetry (PTV), are compared. DLDV was previously developed at the Iowa Institute of Hydraulic
Research, while PTV was specially configured for this application. Mean and fluctuating velocity components for both flow
fractions were simultaneously measured in a laboratory-scale, submerged water jet loaded with alluvial sand. This information
cannot be obtained using existing measurement techniques. The jet Reynolds number was 6120, and the sediment sieve diameter
ranged from 0.5 to 0.6 mm. Small mass loadings of sand with inertial time constant τ
p
of 0.6 ms were examined.
The configuration, operation, and results obtained using the DLDV and PTV are presented. For each technique, means to precisely
distinguish between the light scattered by suspended sand and that originating from seed-particles following the water were
implemented. The agreement in measurement for the two methods validates one another since they are based on completely different
principles of operation. The capabilities of DLDV and PTV to reliably measure sand and water velocities in sediment-laden
flows are further indicated by the agreement of the present findings with those obtained previously in similar studies. The
comparison suggests that PTV, due to its whole field nature, could become a powerful tool for flow and particle-related diagnostics,
yielding fundamental information in an area with a long history of conflicting experimental evidence.
Received: 27 August 1996 / Accepted: 25 June 1997 相似文献
15.
Sebastian Pfadler Frank Beyrau Friedrich Dinkelacker Alfred Leipertz 《Experiments in fluids》2010,49(4):839-851
In this study, we report on the direct measurement of the density-weighted subgrid scale (SGS) stress tensor in turbulent
premixed flames. In large-eddy simulations (LES), this unresolved tensor is typically modelled using eddy viscosity approaches.
Additionally to the direct measurement, we provide a pure experimentally based a-priori test of the commonly used eddy viscosity
model suggested by Smagorinsky. For two turbulent premixed V-shaped methane–air flames, a statistical analysis is presented
where the correlation between the directly measured SGS stress tensor and the eddy viscosity model following Smagorinsky is
tested. The measurement strategy is based on the application of a dual-plane stereo-PIV technique which enables the measurement
of the 3D flow field in two parallel planes. This allows the determination of velocities as well as velocity gradients in
all three directions. Here, a vector resolution of 118 μm was achieved. For a priori testing, the data are subjected to a
spatial filtering procedure that reproduces the application of the filter function in LES. The calculation of velocity gradients
is performed after the application of this spatial averaging. Additionally to the velocity field, the flame front position
is deduced from the clearly observable step in the tracer particle number density between burnt and unburnt regions of the
flame. This facilitates the direct single-shot-based evaluation of all components of the density-weighted SGS stress tensor.
Additionally, the model expressions related to these terms can be determined, which is done in this first study for the static
Smagorinsky model. With that, the instantaneous local comparison between directly measured stress terms and modelled terms
is possible, based on the instantaneous local evaluation procedure. The measurement procedure is described, and first results
are presented and discussed. They show a rather poor performance of the static form of the Smagorinsky model (with fixed Smagorinsky
constant). Our future aims are to use the directly measured SGS data for the a-priori comparison with more advanced models. 相似文献
16.
Based on the AfKdV equation of Xu et al.[1], a theory on the velocities of the precursor soliton generation in two-layer flow over topography is presented in the present
paper. Moving velocities of precursor solitons, of the first zero-crossing of the tailing wavetrain and of the flow behind
the topography are found theoretically. It is shown that for a given topography, when its moving velocities are at the resonant
points, we have the following rules: the ratio of the moving velocity of the precursor solitons to that of the first zero-crossing
of the tailing wavetrain equals −4/3. At the same time, the ratio of the width of generating region of the precursor solitons
to that of the depressed water region equals also −4/3. The theoretical results are examined by means of numerical calculation.
The comparison between the theoretical and numerical results are found in good agreement. For different stratified parameters
of two-layer flow, the velocities of the precursor soliton generation are also predicted in terms of the present theoretical
results.
The project supported by the National Natural Science Foundation of China under the Grant No. 49776284 相似文献
17.
An accurate non-intrusive method of measurement of liquid and sediment velocities, called Discriminator Laser Doppler Velocimetry (DLDV) is described. The DLDV arrangement consists of a LDV, and a discriminator system that utilizes near on-axis diffraction from sediment particles passing through or grazing the LDV measurement volume to result in strong voltage signals. For liquid velocity statistics, velocity measurements associated with a discriminator voltage above a threshold are discarded; the discriminator signal is used to validate that only particle velocities are recorded during particle velocity measurement. Possible error sources in the use of DLDV are discussed. Measurements using DLDV in an open-channel alluvial sand-laden flow indicate differences between liquid and particle velocities even for dilute sand concentrations.The initial financial support of the project was provided by NSF, under grant CTS-9021149. Financial support from the Iowa Institute of Hydraulic Research in gratefully acknowledged. 相似文献
18.
In wall-bounded turbulent flows, determination of wall shear stress is an important task. The main objective of the present
work is to develop a sensor which is capable of measuring surface shear stress over an extended region applicable to wall-bounded
turbulent flows. This sensor, as a direct method for measuring wall shear stress, consists of mounting a thin flexible film
on the solid surface. The sensor is made of a homogeneous, isotropic, and incompressible material. The geometry and mechanical
properties of the film are measured, and particles with the nominal size of 11 μm in diameter are embedded on the film’s surface
to act as markers. An optical technique is used to measure the film deformation caused by the flow. The film has typically
deflection of less than 2% of the material thickness under maximum loading. The sensor sensitivity can be adjusted by changing
the thickness of the layer or the shear modulus of the film’s material. The paper reports the sensor fabrication, static and
dynamic calibration procedure, and its application to a fully developed turbulent channel flow at Reynolds numbers in the
range of 90,000–130,000 based on the bulk velocity and channel full height. The results are compared to alternative wall shear
stress measurement methods. 相似文献
19.
Transient dynamics of two injection flows, upstream and downstream a swirl injector, are investigated. Capillary n-heptane
pipe flow is measured using laser Doppler anemometer to obtain instantaneous time series of centerline velocity and to reconstruct
series of instantaneous and integrated flow rates and pressure gradient. A collimated laser sheet and a high-speed video camera
visualize injected spray flow. Finally, the phase Doppler anemometer measurements are introduced to analyze instantaneous
patterns of droplets velocity-size and number density into fuel spray. All measurements are employed at similar temporal resolution
close to 30 μs. Results indicate that both flows are strongly time-dependent and well correlated in time-phases. Initial transitions
are completed by 100 μs. Opening or closing of the injector valve affects both flows as strong delta oscillation causes spray
penetration dynamics and a post injection effect. A combination of intrusive laser-based techniques allows indication of the
basic injection and spraying characteristics need to optimize high-pressure fuel injectors and combustion late injection mode
at a high speed.
Received: 19 December 1998/Accepted: 13 August 1998 相似文献
20.
A novel carbon nanotube (CNT) sensor is being developed to measure the mean and fluctuating wall shear stress (WSS) in a turbulent
boundary layer. The CNT WSS sensor is based on the thermal principle and featured by high spatial and temporal resolutions
(in the order of nm and kHz, respectively), low power consumption (in the order of μW), and a compact fabrication process
compared with traditional WSS measurement sensors. The CNT WSS-sensing element was characterized in detail before its calibration.
The CNT sensor was operated under a constant temperature (CT) operation mode and an overheat ratio range of −0.15 to −0.19
and calibrated in a fully developed turbulent channel flow. It has been observed for the first time in a macroscopic flow
that the sensor output power is approximately proportional to the 1/3 powered WSS, as expected for a thermal-principle-based
WSS sensor, and the wall shear stress measurement is demonstrated for a low Reynolds number flow. 相似文献