Particle Image Velocimetry Velocity Field Research Articles

X-ray PIV 기법의 개발과 혈액 유동에의 적용연구

An x-ray PIV (Particle Image Velocimetry) technique was developed to measure quantitative information on flows inside opaque conduits and on opaque-fluid flows. At first, the developed x-ray PIV technique was applied to flow in an opaque Teflon tube. To acquire x-ray images suitable for PIV velocity field measurements, refraction-based edge enhancement mechanism was employed using detectable tracer particles. The optimal distance between with the sample and detector was experimentally determined. The resulting amassed velocity field data were in reasonable agreement with the theoretical prediction. The x-ray PIV technique was also applied to blood flow in a microchannel. The flow pattern of blood was visualized by enhancing the diffraction/interference-based characteristics of blood cells on synchrotron x-rays without any contrast agent or tracer particles. That is, the flow-pattern image of blood was achieved by optimizing the sample (blood) to detector distance and the sample thickness. Quantitative velocity field information was obtained by applying PIV algorithm to the enhanced x-ray flow images. The measured velocity field data show a typical flow structure of flow in a macro-scale channel.

Investigation of the near-field structure of an elliptic jet using stereoscopic particle imagevelocimetry

A stereoscopic particle image velocimetry velocity field measurement technique was used toinvestigate the flow structure and entrainment rate of a turbulent elliptic jet of aspect ratioAR = 2.A 3D calibration procedure was established to compensate for the distortion and refractioneffects embedded in the captured particle images; the results of this procedure were used tocalculate three orthogonal velocity components. The instantaneous velocity fields of threeorthogonal components in several cross sections perpendicular to the jet axis weremeasured and the spatial distributions of the turbulent statistics were obtainedby ensemble averaging 500 instantaneous velocity fields. The variation of theentrainment rate along the jet axis was evaluated from the mean velocity fieldsand compared with that of the round jet of the same equivalent diameter. Atx/De = 2,the jet widths along the major and minor axes are almost identical and axis-switchingoccurs. As the flow goes further downstream, the jet width grows much faster along theminor axis than along the major axis. After axis-switching, the flow structure of theelliptic jet shows two critical points on the minor axis, which are not observed inthe round jet. The total entrainment rate of the elliptic jet is about 1.5% largerthan that of the round jet. In addition, entrainment of the surrounding fluid ismore active on the minor axis as a result of the fast expansion of the jet alongthis axis due to the self-induction of the elliptic vortical structure in the nearfield.

PIV velocity field measurements of flow around a KRISO 3600TEU container ship model

The main purpose of this investigation was to demonstrate a useful application of the particle image velocimetry (PIV) method to analyze the complex flow characteristics around a ship. For a sample illustration, the KRISO 3600TEU container ship model was chosen. The flow structure in the stern and near-wake region of the model has been investigated experimentally in a circulating water channel. Instantaneous velocity fields measured by the PIV velocity field measurement technique have been ensemble-averaged to give details of flow structures such as the spatial distributions of the local mean velocity, vorticity, and turbulent kinetic energy. The free-stream velocity was fixed at Uo = 0.6 m/s, and the corresponding Reynolds number based on the length between perpendiculars was about 9.0 × 105. The container ship model shows a complicated three-dimensional flow structure in the stern and near-wake regions. The PIV results clearly revealed the formation of large-scale bilge vortices in the stern region and their effect on the flow in the near-wake. The results shown here provide valuable information for hull form design and the validation of viscous ship flow codes and of turbulence models.

X-ray particle image velocimetry for measuring quantitative flow information inside opaque objects

An x-ray particle image velocimetry (PIV) technique was developed to measure quantitative information on flows inside opaque objects. To acquire x-ray images suitable for PIV velocity field measurements, refraction-based edge enhancement was employed using detectable tracer particles with the object and detector separated by an experimentally determined optimal distance. The x-ray PIV method was applied to a flow in an opaque Teflon tube. The resulting amassed velocity field data were in reasonable agreement with theoretical predictions.

Velocity streak structure modifications induced by flow manipulation

The present research is based on the belief that the observation of the response of near wall turbulent organized motions to external perturbations is a powerful technique for investigating the physics of turbulence regeneration. The low speed streak behavior in the buffer layer of a flat plate turbulent boundary layer under the action of transversal wall oscillation has been statistically analyzed by observing particle image velocimetry velocity fields in a plane parallel to the wall at a distance of 20 wall units. It was found that the observed reduction in turbulence activity (velocity variance and Reynolds stress) is accompanied by an increase of the averaged velocity streaks width, spacing and waviness and by a reduction of their strength. The increase in streak width and spacing has been attributed mainly to coalescence processes between neighboring streaks. The occurrence of velocity streaks strength reduction, associated with the consequent decrease of the number of unstable low speed streaks, is believed to be the main reason leading to the reduction of turbulent activity. The interpretation of the present results supports the concept that the so-called streak regeneration cycle plays an important role in the near wall turbulence reproduction process.

Velocity field measurements of entilation flow in a vehicle interior

The ventilation flow in a 1/10-scale model of a vehicle interior was investigated experimentally. Five different ventilation modes with the same total flow rate were tested to study the effect of ventilation mode on the velocity field in the vehicle compartment. For each mode, more than 450 instantaneous velocity fields were measured using a PIV (particle image velocimetry) velocity field measurement technique. These fields were ensemble averaged to obtain the spatial distribution of the mean velocity and vorticity. The panel-vent ventilation mode was more effective than the foot-vent ventilation mode in terms of active momentum transfer and the uniformity of the speed distribution in the rear compartment. The foot-vent mode was characterised by a large-scale vortex structure in the rear compartment, which suppressed the heat and momentum transfer in this region. In contrast to the panel-vent and foot-vent modes, the hybrid-vent modes did not have a large-scale re-circulation flow in the rear passenger compartment; thus, the hybrid-vent modes are expected to efficiently reduce the thermal non-uniformity in the vehicle interior. The present work highlights the usefulness of the PIV velocity field measurement technique for the analysis of HVAC (hearting, ventilation and air conditioning) problems. These experimental results can be used not only to understand the ventilation flow in the passenger compartment in order to improve passenger comfort, but also to validate numerical predictions.

Velocity Field Measurements of Flow inside Snout of Continuous Hot-Dip Galvanizing Process Using a Single-frame PIV Technique.

In the continuous hot-dip galvanizing process of steel strips, the snout has been installed at the entering region of feeding strip into the molten zinc (Zn) pot. However, evaporated Zn particles in the snout cause ash imperfection on the galvanized steel strip surface. In order to resolve this problem, the flow field inside the snout, both on the deoxidization gas flow above the free surface and the molten Zn flow in the Zn pot. has been investigated experientially. For a 1/10 scale water model, flow visualization and PIV (Particle Image Velocimetry) velocity field measurements were carried out at the strip speed V s =1.5 m/s Aluminum flakes (1 μm) and atomized olive oil (3 μm) were used as seeding particles to simulate the molten Zn flow and the deoxidization gas flow, respectively. As a result, the liquid flow in the Zn pot is dominantly influenced by the up-rising flow in diagonal direction caused by the rotating sink roll. For gas flow in front of the strip inside the snout, the large-scale vortex formed by the downward moving strip is dominant. In the rear side of the strip, a counter-clockwise vortex is formed and some of the flow following by the moving strip impinges on the free surface of molten Zn. The liquid flow in front of the strip is governed by the up-rising flow entering the snout, caused by the rotating sink roll. The moving strip affects dominantly the liquid flow behind the strip inside the snout and large amounts of liquid are entrained and followed the moving strip toward the sink roll. A thin boundary layer is formed on the front side due to the up-rising flow, however, a relatively thick boundary layer is formed in the rear side of the strip. Inside the snout, the deoxidization gas flow above the free surface is much faster than the liquid flow in the Zn pot. More ash imperfections are anticipated on the rear surface of the strip where larger influx flow moves toward the strip in the region near the free surface.