PTV system과 CFD code를 이용한 Cylinder model 周圍의 3차원 流動解釋에 關한 比較硏究

Abstract

By introducing a refractor to the lens of a camera it is possible to take an accurate measurement of three dimensional position of any object by one camera. When the refractor is rotated around the optical axis at high speed, the tracer particles create annular streaks due to the effect caused by the refractor's circular shift. Since the radius of each annular streak, of a point being measured, is inversely proportional to its distance from the camera, it is therefore possible by analyzing these annular streaks, to determine the three dimensional positional information of the point. In this paper, three dimensional measurement by CCD camera using refractor is introduced and applied for the flow around Cylinder model. The use of 3-Dimensional information obtained by computer vision system is attractive in various fields and the automatic inference of 3-D information has been and continuous to be one of the primary aims of computer vision system. In order to obtain 3-Dimensional information, some methods have been developed up to now. One is based on stereovision with two or more TV cameras and another one is based on slit-ray projection method. In stereovision method, the depth of measuring points is measured by measuring the disparity of matching pairs betweenframes viewed from different angle. One problem related to the stereovision is finding matching pairs between frames causes problematic computer processing since there are several possibilities for the choice of matching points. In the slit ray projectionmethod, the measuring object is reconstructed by many images that include a structured light (slit ray) at the different position. The quantitative measurement is established by considering the geometric configuration between a TV camera and a structured light. In this method, the measuring object must be stationary while the structured light (slit ray) is scanned over the surface of the object. Therefore, moving object cannot be measured by the slit ray projection method. In this paper, a new technique tomeasure the instantaneous three-dimensional positions of multiple points by analyzing a single image is introduced. The main feature of our technique is to use a single TV camera with an image shifting apparatus[1]. By introducing a refractor on the TV camera lens, the image of the measuring point is displaced on the image plane with the corresponding displacements related to the distances between the TV camera and the measuring point[2][4]. The magnitude of displacement on the image plane is inversely proportional to the distance between the TV camera and measuring point. When the refractor is rotated physically at high speed during the exposure of the TV camera, annular streaks of measuring points appear on an image since the rotational shift is added to the image. Since the size of the annular streak is inversely proportional to the distance of the measuring point from the camera, each annular streak has three-dimensional information of a measuring point. The three-dimensional information can be calculated by analyzing annular streaks. That is, center position and the size of annular streak can be converted to three-dimensional world coordinate. Reliable information can be obtained since our system is free from difficult matching task of measuring points between different frames. This method has been applied to the measurement of tracer particles in flow and obtained the satisfactory results. In this paper, the surface measurement of moving surface such as swinging water is introduced for one of the applications of our system. This measurement can be realized by utilizing the peculiarity of our circular dynamic method and multi laser spots projector. These results proved a feasibility of our method. Circular dynamic stereo has special advantages as it enables a 3-D measurement using a single TV camera. Annular streaks are recorded using this system and the size of annular streaks directly concerns to the depth from the TV camera. That is, the size of annular streaks is inversely proportional to the depth fromtheTV camera and the depth can be measured by image processing technique. Since the three dimensional information can be extracted from a single image, moving points such as moving particles or moving surface can be measured. Experimental results show the feasibility of our system. The concluding part includes a basic comparison between the CFD codes and the PTV system by analyzing the data sheet from the experiment conducted. Though more research work is needed in this field, we can see from the conducted experiments that PTV system is at par with the CFD code analysis methods.