共查询到20条相似文献,搜索用时 125 毫秒
1.
Karsten Roetmann Waldemar Schmunk Christoph S. Garbe Volker Beushausen 《Experiments in fluids》2008,44(3):419-430
The two dimensional molecular tagging velocimetry (2D-MTV) has been used to measure velocity fields of the flow in a micro
mixer. Instead of commonly used micro particles an optical tagging of the flow has been performed by using a caged dye. The
pattern generation is done by imaging a mask for the first time. This allows to generate nearly any imaginable pattern. The
flow induces a deformation of the optically written pattern that can be tracked by laser induced fluorescence. The series
of raw images acquired in this way were analyzed quantitatively with a novel optical flow based technique. The reference measurements
have been carried out allowing to draw conclusions about the accuracy of this procedure. A comparison to the standard technique
of μPIV has also been conducted. Apart from measuring flow velocities in microfluidic mixing processes, the spatial distribution
of concentration fields for different species has also been measured. To this end, a new technique has been developed that
allows spatial measurements from Planar Spontaneous Raman Scattering (PSRS). The Raman stray light of the relevant species
has been spectrally selected by a narrow bandpass filter and thus detected unaffectedly by the Raman stray light of other
species. The successful operation of this measurement procedure in micro flows will be demonstrated exemplary for a mixing
process of water and ethanol. 相似文献
2.
Evaluation of aero-optical distortion effects in PIV 总被引:1,自引:0,他引:1
Aero-optical distortion effects on the accuracy of particle image velocimetry (PIV) are investigated. When the illuminated particles are observed through a medium that is optically inhomogeneous due to flow compressibility, the resulting particle image pattern is subjected to deformation and blur. In relation to PIV two forms of error can be identified: position error and velocity error. In this paper a model is presented that describes these errors and particle image blur in relation to the refractive index field of the flow. In the case of 2D flows the model equations can be simplified and, furthermore, the background oriented schlieren technique (BOS) can be applied as a means to assess and correct for the optical error in PIV. The model describing the optical distortion is validated by both computer simulation and real experiments of 2D flows. Two flow features are considered: one with optical distortion normal to the velocity (shear layer) and one with optical distortion in the direction of the flow (expansion fan). Both simulation and experiments demonstrate that the major source for the velocity error is the second derivative of the refractive index in the direction of the velocity vector. The aero-optical distortion effect is less critical for shearing interfaces in comparison with compression/expansion fronts, the most critical case being represented by shock waves. Based on the results from the simulated experiments, it is concluded that for the 2D flow case the BOS method allows a measurement of the mean velocity error in PIV and can reduce it to a large extent. 相似文献
3.
Improved understanding of in-cylinder flows requires knowledge from well-resolved experimental velocimetry measurements and flow simulation modeling. Engine simulations using large eddy simulations (LES) are making large progress and the need for well documented velocimetry measurements for model validation is high. This work presents velocimetry measurements from PIV, high-speed PIV, stereoscopic PIV, and tomographic PIV to extensively describe the in-cylinder flow field in a motored optical engine operating at 800 RPM. These measurements also establish a comprehensive database designed for LES model development and validation. Details of the engine, engine accessory components, and well-controlled boundary conditions and engine operation are presented. The first two statistical moments of the flow field are computed and show excellent agreement among the PIV database. Analysis of statistical moments based on limited sample size is presented and is important for modeling validation purposes. High-speed PIV resolved the instantaneous flow field throughout entire engine cycles (i.e. 719 consecutive crank-angles), while tomographic PIV images are further used to investigate the 3D flow field and identify regions of strong vortical structures identified by the Q-criterion. Principle velocity gradient components are computed and emphasize the need to resolve similar spatial scales between experimental and modeling efforts for suitable model validation. 相似文献
4.
Michael J. Patrick Chia-Yuan Chen David H. Frakes Onur Dur Kerem Pekkan 《Experiments in fluids》2011,50(4):887-904
In hemodynamics, the inherent intermittency of two-phase cellular-level flow has received little attention. Unsteadiness is
reported and quantified for the first time in the literature using a combination of fluorescent dye labeling, time-resolved
scanning confocal microscopy, and micro-particle image velocimetry (μPIV). The near-wall red blood cell (RBC) motion of physiologic
high-hematocrit blood in a rectangular microchannel was investigated under pressure-driven flow. Intermittent flow was associated
with (1) the stretching of RBCs as they passed through RBC clusters with twisting motions; (2) external flow through local
obstacles; and (3) transitionary rouleaux formations. Velocity profiles are presented for these cases. Unsteady flow clustered
in local regions. Extra-cellular fluid flow generated by individual RBCs was examined using submicron fluorescent microspheres.
The capabilities of confocal μPIV post-processing were verified using synthetic raw PIV data for validation. Cellular interactions
and oscillating velocity profiles are presented, and 3D data are made available for computational model validation. 相似文献
5.
An optical flow MTV based technique for measuring microfluidic flow in the presence of diffusion and Taylor dispersion 总被引:1,自引:0,他引:1
Christoph S. Garbe Karsten Roetmann Volker Beushausen Bernd Jähne 《Experiments in fluids》2008,44(3):439-450
A novel technique is presented for accurately measuring flow fields in microfluidic flows from molecular tagging velocimetry
(MTV). Limited optical access is frequently encountered in microfluidic systems. Therefore, in this contribution we analyze
the special case of tagging a line across the thin dimension of a microchannel and subsequent imaging along this line. This
represents a set-up that is applicable to a wide range of microfluidic applications. A volume illumination has to be used,
resulting in an integration of the visualized dye across the flow profile. This leads to the well-known effect of Taylor dispersion.
Our novel technique consists of measuring motion from digital image sequences in a gradient-based approach. A motion model
is developed which explicitly deals with brightness changes due to Taylor dispersion and additional molecular diffusion of
dyes. The presented approach is specific to the case of a parabolic velocity profile. In the presence of these effects, an
accurate computation of motion is only possible by applying this novel motion model. Our technique is tested on simulated
sequences corrupted with varying levels of noise and on actual measurements. Measurements were conducted in a microfluidic
mixer of precisely known flow properties. In the same mixer, a comparative study of our MTV technique to μPIV was performed.
Also, the results were compared to bulk measurements of the fluid flow velocity. The novel algorithm compared favorably and
also, measurements were conducted on inhomogeneous flow configurations. 相似文献
6.
In vivo whole-field blood velocity measurement techniques 总被引:3,自引:2,他引:3
In this article a number of whole-field blood velocity measurement techniques are concisely reviewed. We primarily focus on
optical measurement techniques for in vivo applications, such as laser Doppler velocimetry (including time varying speckle),
laser speckle contrast imaging and particle image velocimetry (including particle tracking). We also briefly describe nuclear
magnetic resonance and ultrasound particle image velocimetry, two techniques that do not rely on optical access, but that
are of importance to in vivo whole-field blood velocity measurement. Typical applications for whole-field methods are perfusion
monitoring, the investigation of instantaneous blood flow patterns, the derivation of endothelial shear stress distributions
from velocity fields, and the measurement of blood volume flow rates. These applications require individual treatment in terms
of spatial and temporal resolution and number of measured velocity components. The requirements further differ for the investigation
of macro-, meso-, and microscale blood flows. In this review we describe and classify those requirements and present techniques
that satisfy them. 相似文献
7.
Markus Raffel Ricardo Hernandez-Rivera Benjamin Heine Andreas Schröder Karen Mulleners 《Experiments in fluids》2011,51(2):573-578
Density tagging velocimetry, a novel optical technique for point-wise measurement of flow velocity is proposed here. This
new method is based on the detection and subsequent tracking of a local density variation deliberately inserted in the flow.
The experimental implementation comprising tagging, detection, and velocity evaluation reverts to and combines principles
of well-known optical measurement techniques. Density tagging velocimetry has the potential for in-flight application and
is particularly suited for measuring flow velocities in regions where the use of tracer particles is difficult or undesired.
The applicability of this new technique is illustrated by a jet flow measurement. 相似文献
8.
Experiments are conducted on the influence of turbulence and longitudinal vortices on subcooled flow boiling in a vertical, rectangular channel. Different flow inserts are used to create turbulence and vortices in the channel. Studied boiling regimes range from the onset of nucleate boiling over the critical heat flux up to fully developed film boiling. A wide range of measuring techniques is applied: time averaged particle image velocimetry (PIV) is used in cold flows for the evaluation of the effects the inserts have on the flow, high speed PIV and photography are used to determine the effects on the fluid and vapor movement in boiling experiments. Digital Holographic Interferometry is used for the evaluation of temperature distributions in the boiling flow. Furthermore, optical microprobes are used to obtain pointwise measurements in areas inaccessible to the imaging techniques. The experiments show that the flow inserts can have considerable impact on the heat fluxes and the distribution of vapor and temperature along the channel. All used inserts lead to an increase in critical heat flux, which is more pronounced for stronger turbulence and higher flow rates and fluid subcoolings. The measuring techniques reveal both a better transport of vapor from the heater surface as well as an increase in mixing in the liquid phase with flow inserts. 相似文献
9.
The development and applications of a new class of water-soluble compounds suitable for molecular tagging diagnostics are
described. These molecular complexes are formed by mixing a lumophore, an appropriate alcohol, and cyclodextrin. Using 1-BrNp
as the lumophore, cyclohexanol is determined to be the most effective overall among the alcohols for which data are currently
available. Information is provided for the design of experiments based on these complexes along with a less complex method
for generating the grid patterns typically used for velocimetry. Implementation of a two-detector system is described which,
in combination with a spatial correlation technique for determining velocities, relaxes the requirement that the initial tagging
pattern be known a priori, eliminates errors in velocity estimates caused by variations in the grid pattern during an experiment, and makes it possible
to study flows with non-uniform mixtures. This detection and analysis combination also solves one of the problems associated
with using caged fluorescein to study high-speed flows. In addition to the traditional implementation for velocimetry, novel
applications for studying the Lagrangian evolution of both reacting and non-reacting interfaces and obtaining combined passive
scalar/velocity measurements are demonstrated.
Received: 26 August 1996/Accepted: 13 March 1997 相似文献
10.
Because of the inherent small size of optical fiberscopes, they provide access and relative handling ease in given closed vessels, which are hardly equipped with extra windows for conventional flow visualization. The use of an optical fiberscope in conjunction with a conventional particle image velocimetry/particle tracking velocimetry (PIV/PTV) system without optimization can lead to degraded transmission of images. The present study proposes a processing technique to filter background noise contained within the coarse bundle image by subtracting the original image of the bundle as reference image. Additionally, efforts were made to increase the reliability of vector processing using particle streak images via judicious pulse interval and duration adjustments. As an applications test we measured classic jet flow using the developed system and using established conventional measurement techniques. Our tests confirmed that our fiberscope PTV system provides vector fields with sufficient accuracy. 相似文献
11.
Shahram Pouya Alexander Van Rhijn Marcos Dantus Manoochehr Koochesfahani 《Experiments in fluids》2014,55(8):1-5
We present results for first molecular tagging velocimetry (MTV) measurements in water under resonant femtosecond excitation/emission process of a phosphorescent supramolecule. Both two-photon and three-photon absorption processes are examined, and the feasibility of measurements is demonstrated by single component velocimetry in a simple jet flow. The new capabilities enabled by FemtoMTV include elimination of the need for short wavelength UV excitation source and UV optical access in flow facilities, and potential for high rep-rate flow imaging. 相似文献
12.
Particle imaging techniques for microfabricated fluidic systems 总被引:7,自引:0,他引:7
This paper presents the design and implementation of velocimetry techniques applicable to the analysis of microfluidic systems. The application of both micron-resolution particle image velocimetry (micro-PIV) and particle tracking velocimetry (PTV) to the measurement of velocity fields within micromachined fluidic channels is presented. The particle tracking system uses epifluorescent microscopy, CCD imaging, and specialized image interrogation algorithms to provide microscale velocity measurement resolution. The flow field in a straight channel section is measured using cross-correlation micro-PIV and compared to the analytical solution for a measured mass flow rate. Velocity field measurements of the flow at the intersection of a cross-channel are also presented and compared with simulations from a commercially available flow solver, CFD-ACE+. Discussions regarding flow seeding, imaging optics, and the flow setup for measuring flows in microfabricated fluidic devices are presented. A simple process for estimating measurement uncertainty of the in-plane velocity measurements caused by three-dimensional Brownian motion is described. A definition for the measurement depth for PTV measurements is proposed. The agreement between measured and predicted values lends further support to the argument that liquid microflows with characteristic dimensions of order 50-μm dimension channels follow macroscale flow theory. 相似文献
13.
Thermal convection in a small vertical gap is studied experimentally applying digital particle image velocimetry/thermometry.
This optical method enables the simultaneous measurement of two-dimensional flow and temperature fields in a liquid. The principle
is based on seeding the liquid flow medium with thermochromic liquid crystal particles. The temperature is measured by the
crystal particles which change their reflected colour as function of temperature. The flow velocity is measured by using the
same particles as flow tracers. The investigation shall contribute to the understanding of the fluid mechanical behaviour
of biological liquids within micro reactor systems. However, the problem is also of fundamental interest as far as heat and
mass transfer is concerned. Measured temperature and flow velocity fields are presented and discussed.
Presented in part at the 4th Chemnitz/Hamburger Colloquium (CHC) on Microflows, Hamburg, Germany, November 2004. 相似文献
14.
The flow around a neutrally buoyant circular cylinder in simple shear was studied using particle-image velocimetry. Steady
experimental results are presented for both torque-free and fixed cylinders at shear-based Reynolds numbers between 8 and
25 at two different flow confinement values. These data, which are to our knowledge the first experimental results for this
Reynolds number range, show that the normalized rotation rate decays much more slowly with Reynolds number than previous theoretical
predictions for unconfined flow, with the rate of decay decreasing as confinement increases. A new simple model is proposed
to explain these effects. Given physical and technological limitations upon steady and unconfined flow experiments and simulations,
the effect of confinement on a cylinder in simple shear merits further study.
Received: 17 February 2000/Accepted: 22 June 2000 相似文献
15.
S. Dubsky R. A. Jamison S. P. A. Higgins K. K. W. Siu K. Hourigan A. Fouras 《Experiments in fluids》2012,52(3):543-554
Computed tomographic X-ray velocimetry has been developed for simultaneous three-dimensional measurement of flow and vessel
geometry. The technique uses cross-correlation functions calculated from X-ray projection image pairs acquired at multiple
viewing angles to tomographically reconstruct the flow through opaque objects with high resolution. The reconstruction is
performed using an iterative, least squares approach. The simultaneous measurement of the object’s structure is performed
with a limited projection tomography method. An extensive parametric study using Monte Carlo simulation reveals accurate measurements
with as few as 3 projection angles, and a minimum required scan angle of only 30°. When using a single/source detector system,
the technique is limited to measurement of periodic or steady flow fields; however, with the use of a multiple source/detector
system, instantaneous measurement will be possible. Synchrotron experiments are conducted to demonstrate the simultaneous
measurement of structure and flow in a complex geometry with strong three-dimensionality. The technique will find applications
in biological flow measurement, and also in engineering applications where optical access is limited, such as in mineral processing. 相似文献
16.
Imaging laser Doppler velocimetry (ILDV) is a novel flow measurement technique, which enables the measurement of the velocity
in an imaging plane. It is an evolution of heterodyne Doppler global velocimetry (HDGV) and may be regarded as the planar
extension of the classical dual-beam laser Doppler velocimetry (LDV) by crossing light sheets in the flow instead of focused
laser beams. Seeding particles within the flow are illuminated from two different directions, and the light scattered from
the moving particles exhibits a frequency shift due to the Doppler effect. The frequency shift depends on the direction of
the illumination and the velocity of the particle. The superposition of the two different frequency-shifted signals on the
detector creates interference and leads to an amplitude modulated signal wherein the modulation frequency depends on the velocity
of the particle. This signal is detected using either a high-speed camera or alternatively a smart pixel imaging array. This
detector array performs a quadrature detection on each pixel with a maximum demodulation frequency of 250 kHz. To demonstrate
the feasibility of the technique, two experiments are presented: The first experiment compares the measured velocity distribution
of a free jet using ILDV performed with the smart pixel detector array and a high-speed camera with a reference measurement
using PIV. The second experiment shows an advanced setup using two smart pixel detector arrays to measure the velocity distribution
on a rotating disk, demonstrating the potential of the technique for high-velocity flow measurements. 相似文献
17.
Particle image velocimetry is applied to measure the vertical (r–z) plane flow structures in a light-duty direct-injection diesel engine with a realistic piston geometry. The measurements are corrected for optical distortions due to the curved piston bowl walls and the cylindrical liner. Mean flow fields are presented and contrasted for operation both with and without fuel injection and combustion. For operation with combustion, the two-dimensional divergence of the measured mean velocity fields is employed as a qualitative indicator of the locations of mean heat release. In agreement with numerical simulations, dual-vortex, vertical plane mean flow structures that may enhance mixing rates are formed approximately mid-way through the combustion event. Late in the cycle a toroidal vortex forms outside the bowl mouth. Imaging studies suggest that soot and partially oxidized fuel trapped within this vortex are slow to mix with surrounding fluid; moreover, the vortex impedes mixing of fluid exiting the bowl with air within the squish volume. 相似文献
18.
19.
Stratified flows with small density difference commonly exist in geophysical and engineering applications, which often involve interaction of turbulence and buoyancy effect. A combined particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) system is developed to measure the velocity and density fields in a dense jet discharged horizontally into a tank filled with light fluid. The illumination of PIV particles and excitation of PLIF dye are achieved by a dual-head pulsed Nd:YAG laser and two CCD cameras with a set of optical filters. The procedure for matching refractive indexes of two fluids and calibration of the combined system are presented, as well as a quantitative analysis of the measurement uncertainties. The flow structures and mixing dynamics within the central vertical plane are studied by examining the averaged parameters, turbulent kinetic energy budget, and modeling of momentum flux and buoyancy flux. At downstream, profiles of velocity and density display strong asymmetry with respect to its center. This is attributed to the fact that stable stratification reduces mixing and unstable stratification enhances mixing. In stable stratification region, most of turbulence production is consumed by mean-flow convection, whereas in unstable stratification region, turbulence production is nearly balanced by viscous dissipation. Experimental data also indicate that at downstream locations, mixing length model performs better in mixing zone of stable stratification regions, whereas in other regions, eddy viscosity/diffusivity models with static model coefficients represent effectively momentum and buoyancy flux terms. The measured turbulent Prandtl number displays strong spatial variation in the stratified jet. 相似文献
20.
Endoscopic 2D particle image velocimetry (PIV) flow field measurements in IC engines 总被引:1,自引:0,他引:1
The paper describes camera and laser endoscopes designed for particle image velocimetry (PIV) applications like measurements in IC engines or turbomachinery. Endoscopic PIV measurements through 8-mm optical access on an IC engine are presented and compared with the measurements using standard optical access through a window. 相似文献