Stereoscopic PTV 기법의 개발과 성능비교 연구

Image processing techniques for the measurement of two-phase bubbly pipe flows using particle image and tracking velocimetry (PIV/PTV)

Stereoscopic particle image velocimetry measurements of flow in a rod bundle with a spacer grid and mixing vanes at a low Reynolds number

Background Phase-contrast magnetic resonance imaging (PC-MRI) provides a powerful method for the quantification of blood velocity. Accuracy of flow measurement with PCMRI has been validated with several techniques such as Doppler ultrasound and electromagnetic flowmeters. However, these methods suffer from low accuracy, especially in pulsating flows where short response times are required. Methods Herein, a series of detailed experiments are reported for validation of MR measurements of steady and pulsatile flows with stereoscopic particle image velocimetry (SPIV) on three different stenotic models with 50%, 74%, and 87% area occlusions. Mean inlet Reynolds number was 190 for both steady and pulsatile cases, mimicking the flow of the human common iliac artery. Axial PC-MRI images were acquired at three sites: inlet (two diameters proximal to the stenosis), throat, and outlet (two diameters distal to the stenosis) using a 3T TX Achieva Philips MRI scanner with slice thickness = 4 mm, resolution = 1 × 1 mm, TE/TR = 3.0/4.0 ms, field of view =6 4 ×6 4 mm, and velocity encoding (Venc) = 30-200 cm/s depending on the imaging section. For SPIV purposes, a laser light sheet was passed perpendicular to the axis of the phantom to illuminate the flowing fluorescent particles (Fig 1). A set of image pairs were captured using two cameras looking at the phantom at different angles and the fluid velocity was extracted using a cross-correlation scheme, yielding a nominal spatial resolution of 0.2 mm for the velocity data. The temporal resolution of pulsatile flow measurements was 25 ms, corresponding to 40 measurements per second. Results Agreement between PC-MRI and SPIV was demonstrated for both steady and pulsatile flow measurements at the inlet by evaluating the linear regression between the two methods, which showed a correlation coefficient of >0.99 and >0.96 for steady and pulsatile flows, respectively. The difference between SPIV and PC-MRI measurements for steady and pulsatile mean flows was less than 5% for both inlet and throat and showed good agreement in all cases (Fig 2). The agreement, however, was weaker at the outlet especially for the 87% stenosis. The flow rate error distal to the stenosis was shown to be a function of narrowing severity. Conclusions Our experiments revealed that the most accurate measures of flow by PC-MRI are found at the throat of the stenosis. This study also illustrates that SPIV provides an excellent approach to in-vitro validation of new or existing PC-MRI flow measurement techniques. Funding

Influence of inhomogeneous refractive index fields on particle image velocimetry

Experiments using stereoscopic high-speed particle image velocimetry (PIV) to take measurements in a cross-stream (i.e. wall-normal/spanwise) plane in a turbulent boundary layer have been used to produce full 3D velocity fields. The 3D fields were constructed from planar 3C fields by using Taylor’s hypothesis to create a pseudo-spatial x-dimension from the temporally resolved measurements. This has produced a 3D viewof the elongated regions of high and low streamwise momentum found in the boundary layer, often referred to as ‘long structures’, and provided information on the arrangement, length and characteristic angle of these structures. Long structures are also seen to be associated with regions of high Reynolds stress. Vortical motions are visualised using swirling strength, and indicate that there is a prevalence for vortices to surround the low speed long structures. The vortices are characterised, and are found to resemble hairpin vortices in some respects.

Models for the center frequency of cavitating-vortex induced pressure-fluctuations, in a flow around propellers, require knowledge of the vortex strength and vapor cavity size. For this purpose, stereoscopic particle image velocimetry (PIV) measurements were taken downstream of a fixed half-wing model. A high spatial resolution is required and was obtained via correlation averaging. This reduces the interrogation area size by a factor of 2–8, with respect to conventional PIV measurements. Vortex wandering was accounted for by selecting PIV images for a given vortex position, yielding sufficient data to obtain statistically converged and accurate results, both with and without a vapor-filled vortex core. Based on these results, the low-order Proctor model was applied to describe the tip vortex velocity outside the viscous core, and the cavity size as a function of cavitation number. The flow field around the vortex cavity shows, in comparison with a flow field without cavitation, a region of retarded flow. This layer around the cavity interface is similar to the viscous core of a vortex without cavitation.

The development of optical experimental techniques has contributed to obtaining explanations of the behaviour of blood flowing in micro-channels. Although past results have been valuable, detailed studies on the flow properties of in vitro blood in micro-channels have been limited by several technical factors such as poor spatial resolution and difficulty in obtaining quantitative detailed measurements at such small scales. In recent years, due to advances in computers, optics, and digital image processing techniques, it has become possible to combine both particle image velocimetry (PIV) and particle tracking velocimetry (PTV) methods with confocal microscopes. As a result, this combination has greatly increased the resolution of conventional micro-PIV/PTV systems and consequently provided additional detailed description on the motion of blood cells not obtainable by traditional methods. In this chapter the most relevant theoretical and technical issues related to both conventional and confocal micro-PIV/PTV methods are discussed. Additionally, the most recent studies on the blood flow behaviour in micro-channels obtained by our confocal micro-PIV/PTV system are also reviewed.

The current work presents simultaneous high-resolution temperature and three-component velocity measurements taken in a series of turbulent (Re = 10,000, 20,000, and 30,000) piloted, non-premixed jet flames using filtered Rayleigh scattering (FRS) thermometry and stereoscopic particle image velocimetry (sPIV). This manuscript details the experimental approach with a key focus on the experimental protocol and specific challenges unique to simultaneous single-shot FRS/sPIV measurements in turbulent non-premixed flames. Specific areas of discussion include the experimental particle scattering rejection for FRS measurements, elimination of signal “crosstalk” between FRS and PIV channels, tracer particle selection, data processing for noise removal, and spatial resolution for both the temperature and velocity measurements. Sample joint temperature/velocity fields are presented highlighting interactions between the flow turbulence and the temperature fields, and a detailed statistical assessment of the temperature and velocity measurements demonstrates the quantitative nature of the results. Dissipation spectra from the measured temperature and velocity fluctuations are used to assess the true spatial resolution of the measurements. Results indicate that the sPIV measurements are resolved well into the dissipative range and the highest spatial frequencies (smallest dissipative scales) are resolved for the temperature measurements under all flame cases and measurement locations. The current joint FRS/sPIV approach enables the first simultaneous single-shot temperature/velocity imaging in turbulent non-premixed flames.

Experiments were performed within the Rutgers University Mach 3.4 supersonic wind tunnel to quantify the turbulent boundary layer in the test section, using stereoscopic particle image velocimetry (SPIV). Seeding of the tunnel was accomplished using atomized mineral oil, resulting in a Stokes number of 0.09. The Reynolds number based on momentum thickness and the friction-velocity Reynolds number were 64,000 and 4320, respectively, marking this as a high Reynolds number boundary layer. SPIV measurements were capable of resolving the mean velocity profile, along with turbulence quantities, permitting comparison against the known literature. Furthermore, the convection of vorticity concentrations representing hairpin vortices was observed within instantaneous vector fields. The resulting Reynolds stresses demonstrate that the boundary layer had typical values of anisotropy, while comparing well with data obtained within a slightly adverse pressure gradient. This was particularly noticeable from the trends in the Reynolds stresses, which exhibited higher-than-expected peak levels. These data provide a unique measurement of a supersonic turbulent boundary layer at Reynolds numbers not often encountered within existing data sets.

A variety of techniques have been proposed in the literature for sprinkler drop characterization. An optical particle tracking velocimetry (PTV) technique is proposed in this paper to determine drop velocity, diameter and angle. The technique has been applied to the drops emitted by an isolated impact sprinkler equipped with two nozzles (diameters 3.20 and 4.37 mm) operating at a pressure of 175 kPa. PTV has been previously used to determine the velocity vector of different types of particles. In this research, PTV was used to photograph sprinkler drops over a region illuminated with laser light. Photographs were taken at four horizontal distances from the sprinkler, which was located at an elevation of 1.65 m over the soil surface. Drop angle and velocity were derived from the displacement of the drop centroid in two images separated by a short time step. Centrality and dispersion parameters were obtained for each drop variable and observation point. Results derive from the analysis of 2,360 images. Only 37.5 % of them (884 images) contained drops which could be processed by the PTV algorithm, resulting in a total of 3,782 drops. A filtering algorithm just validated 1,893 valid drops, which were successfully analyzed. The proposed technique uses expensive equipment requiring continued protection against irrigation water. This methodology has proven valuable to characterize irrigation water drops. Despite its robust measurement procedure, further comparison with other techniques seems necessary before this optical technique can be recommended for practical use in sprinkler drop characterization.

Measurements of the laminar burning velocity of hydrogen–air premixed flames

An improved correlation based Particle Tracking Velocimetry (PTV) algorithm was proposed in the present paper. The path tracking of the tracer particles was achieved through a correlation operation of the small interrogation window around the studied tracer particles at two-time steps. The central positions of the tracer particles were determined by the correlation operation of the tracer particle image with a Gaussian particle mask in order to improve the accuracy to identify the central positions of particles up to sub-pixel level. The performance of the present improved correlation based Particle Tracking Velocimetry (PTV) algorithm was evaluated by using both synthetic VSJ standard PIV images and actual PIV images of a self-induced sloshing. Compared with other conventional PTV methods, the present improved correlation based PTV algorithm was found to be able to provide better solution and more robust for suppression the effect of background noise in the PIV images.

Integrated monitoring method of flood free surface and surface velocity in a laboratory compound channel

A comparison of gas velocity measurements in a full-scale enclosure fire

An experimental study investigates the natural convection flow through a horizontal ceiling vent of a fire compartment. This flow is governed by the buoyancy forces and the friction forces, and is bidirectional. The orifice used is of circular cross section and is characterized by the aspect ratio L/D of its thickness L over its diameter D. The reduced scale apparatus consist of two superimposed compartments of $$1.5 \times 1 \times 1 \, \mathrm{m}^3$$ for the lower and $$1 \times 1 \times 1 \, \mathrm{m}^3$$ for the upper. These two rooms are connected by a 0.038 m length circular orifice with different diameters available ( $$ D \in [63.5, \, 76.2, \, 127, \, 152.4, \, 190.5, \, 260$$ mm]). For the study of natural convection, only the lower compartment is used; the upper compartment is left at atmospheric conditions by removing all its walls. The temperature gradient ( $$\Delta T \approx 50^\circ $$ C between the bottom and the top of the orifice) is generated by a 2 kW electrical resistor located in the lower compartment. This electrical resistor makes it possible to generate, without mass inlet, a plume with no soot and to reach a thermal steady state in the “fire room”. The optical stereoscopic particle image velocimetry measurement technique provides the velocity field through the opening for different diameters. This technique, based on the finding of the displacement of a set of particles between two consecutive snapshots of a seeded fluid, makes it possible to obtain all three components of the velocity vector field by non-intrusive measurement. The use of this technique is a novelty for such vent flows and allows accessing information not only on the variation of the flow rate but also on the distribution of the fluids moving upward and downward through the orifice. Despite an unsteady dynamic regime, the analysis of averaged fields shows a geometrical organization of the bidirectional flow through the orifice, with a central area occupied by the hot fluid flowing upward and a peripheral zone where the cold fluid flows downward. The flow section occupancy ratio reverses with the extrema of flow velocities, for $$L/D \approx 0.41$$ .