열·유체 시스템 해석용 고해상 다차원 이미징 측정법의 개발

Abstract

The multi-dimensional imaging techniques such as, 4D PTV, SPIV, SPTV, FSIMS, Panoramic PIV and 2D PIV have been developed or constructed for the measurements of the flows not only to provide databases for fundamental researches in engineering and industrial applications, but also to provide better design guides for thermal-fluid flow systems. The purpose of the study is to develop multi-dimensional imaging techniques that can provide quantitative three-dimensional vector fields over the whole measurement volumes in thermal flows.

A new 4-dimensional particle tracking velocimetry (4D PTV) has been developed using three high-resolution-high-speed cameras(1k x 1k, 2000fps), and it was used to measure spatial vector fields of a sphere (d=30mm) wake. The Reynolds number is 1,113. Measurements with a Stereoscopic PIV(SPIV) and a Stereoscopic PTV(SPTV) were carried out under the same experimental conditions in order to compare those results obtained by the three measurement techniques. Further, measurement uncertainty analyses were carried out for the 4D PTV system, and it was confirmed that the uncertainties of the constructed 4D PTV system for the three components(U, V, W)of the velocity vector were ？}1.2mm, ？}1.6mm and ？}1.6mm, respectively.

In 4D PTV measurement, more than 5,000 instantaneous correct vectors were able to be measured by the system and they were interpolated onto the three-dimensional cell grids to obtain physical properties of the wake. A experimental database on turbulent properties such as turbulent kinetic energy and Reynolds stress over the whole measurement volume have been constructed and some features of the wake have been evaluated. The Strouhal number (St) was obtained at x/D=1.53, y/D=0.53 and z/D=0.03 through FFT analysis using the instantaneous velocity vectors and its value was 0.19, which showed good agreements with those of the previous researches. An eigenvalue analyses was carried out to get temporal evolution of the wake structures using the grids vectors. Separation could be seen on the circumference of the sphere and the separated flow was recirculated near x/D=2.5 toward the sphere. It was verified from the 4D PTV, SPIV and SPTV measurements that the sphere wake has a two-shells-structure that consists of the inner shell (vortex core part) and the outer shell (vortex loop part). The outer shell, vortex loop, was convected downstream rotating clockwise and counterclockwise with some irregular time interval. The inner shell looked like a hair pin shape. The axis of this hair pin appeared vertically and horizontally alternatively with some regular time intervals. It is thought that the velocity profiles of the sphere wake has strong relations with a spiral motion of the vortices that were shedded from the surface of the sphere.

The constructed 4D PTV system was also applied to the measurements of an impinged jet flow. The distance between the nozzle exit and the rigid flat plate, x/D=7.0 for the nozzle diameter D=20mm. The Reynolds number is 40,000. More than 7,000 instantaneous three-dimensional velocity vectors were measured by the system. A ring vortex was clearly reconstructed by the obtained velocity vectors. The location of the ring vortex, 0.9D from the plate, was at reasonable positions. The thickness of the ring vortex was about 0.8D. The results showed that the sweeping velocity of the ring vortex over the surface of the impinged plate was about 20msec. It seems that the ring vortex appears periodically and shows a squeezing motion in horizontal and in vertical with some intervals. Some prerequisite features that the hardware of 4D PTV system should have were investigated regarding to Kolmogorov's time and length scales.

Due to the wide range of applicabilities of the impinged jet that has a phenomenon of fluid-to-flexible-structure interactions, A measurement system that can capture simultaneously the motions of an elastic flat plate and the motions of the flows impinged onto that flexible plate has been constructed. The flow-structure interaction measurement system (FSIMS) consists of four cameras, two(0.5k x 0.5k) are for the measurements of the flow fields and two(1.0k x 1.0k) are for the measurements of the motions of the plate.

For the measurements of the flow fields, the GA-3D PTV algorithm has been used. For the measurements of the motions of the flexible plate, a new measurement algorithm called 'bidirectional tracking algorithm' has been developed. The performances of this algorithm have been tested using a set of artificial images of which data had been obtained by a theoretically generated plate's virtual motion and by an actual experiment. Further, its performances were also compared with that of a commercial measurement system through an actual test on the motions of a 6-degree-of-freedom platform. The measurement uncertainty of the motion tracking system were ？}1.2mm, ？}1.6mm and ？}1.6mm for x, y and z, respectively, which implies 1？`2 % relative error for the whole measurement volume size.

The jet flow coming from the nozzle diameter (D=15mm) was impinged onto the flexible plate. The injection of the jet was made upward from the bottom of a water tank by opening the valve installed under the nozzle. The Reynolds number with the nozzle diameter is 20,000. The flexible plate (t=0.5mm, d=400mm) is made of silicon and the distance between the nozzle and the flexible plate is x/D=4.0. To make clear visibility of the targets which were attached onto the surface of the silicon plate and were used for the motion tracking, a violet light was used.

It could be seen from the measurement results and from experimental experiences that the direction of the flexible plate's motion was just the opposite of the jet direction at the first stage of the jet's spouting. The vortices came from the stagnation point at the flexible plate had a tendency of separating relatively far away from the separation location of the case of rigid plate (r/d=1.0？`1.5).

In order to make maximum use of the benefits of the above multi-dimensional imaging techniques onto engineering and industrial applications, a Panoramic PIV which can be regarded as a revision of the 4D PTV system has been developed and applied to the measurements of the air flows of the air-conditioning system of a building model. In this experiment, it has been verified that the similarity between models and the practical one are not dependant on Reynolds numbers but on mass flow rates, especially over Re=1,000. Further, 2D PIV measurement on the freezing room of a refrigerator was carried out for finding the best conditions of the room not to be frosted. It revealed that the freezing of water droplets decreased largely when the chance of meeting of the cooled air vented from the cooling coil was made rare with the air in circulation regions like the room corners.

The experimental data on the sphere obtained by 4D PTV, SPIV and SPTV can be used for the studies from fundamental to applications in engineering fields. The measurement data on the impinged jet obtained by 4D PTV can also be used for the theoretical studies of the physics of the vortex separations from the impinged plate, which will help develope a better physical model for CFD research area. The database on the flow-elastic-structure interaction that has been obtained by the FSIMS will open a new era in the research activities on the FSI problems appearing in various engineering fields. The experimental results obtained by a Panoramic PIV and a 2D PIV on the practical engineering applications, on a building air-conditioning ventilating system and on a refrigerator, can be a good guide for best applications of the imaging techniques into thermal-flow systems.