ASHRAE 4674-2004 An Algorithm of Stereoscopic Particle Image Velocimetry for Full-Scale Room Airflow Studies《全尺寸室内气流研究用 立体粒子图像测速仪的运算法则》.pdf
《ASHRAE 4674-2004 An Algorithm of Stereoscopic Particle Image Velocimetry for Full-Scale Room Airflow Studies《全尺寸室内气流研究用 立体粒子图像测速仪的运算法则》.pdf》由会员分享,可在线阅读,更多相关《ASHRAE 4674-2004 An Algorithm of Stereoscopic Particle Image Velocimetry for Full-Scale Room Airflow Studies《全尺寸室内气流研究用 立体粒子图像测速仪的运算法则》.pdf(6页珍藏版)》请在麦多课文档分享上搜索。
1、4674 An Algorithm of Stereoscopic Particle Image Velocimetry for Full-Scale Room Airflow Studies Yigang Sun, Ph.D. Lingying Zhao, Ph.D. Associate Member ASHRAE Yuanhui Zhang, Ph.D., P.E. Member ASHRAE Xinlei Wang, Ph.D. Member ASHRAE ABSTRACT One challenge in indoor air qualitystudies is the measure
2、- ment of three-dimensional air velocity profiles in an airspace so that the nature of airflow can be better understood and appropriate ventilation systems can be designed. There is much dispute over a variety of computationalfluid dynamics (CFD) models, primarily due to a lack of credible data to v
3、ali- date those models. This study aimed to develop a stereoscopic particle imaging velocimetry (SPIV) system suitable for measurement of full-scale room three-dimensional airflow. The SPIV method is based on the principle of parallax to extract a third (z-direction) velocity component using two cam
4、eras. We have developed a new three-dimensional algo- rithm in the particle streak mode (PSM) that only requires two cameras to acquire three velocity components (x, y, z) andflow directions in a three-dimensional volume, rather than in a thin layer of a plane. The two-camera approach is expected to
5、 greatly simplifi the image acquisition and data processing and improve the accuracy by eliminating the error caused by the third camera image or some special techniques employed to resolve directional ambiguity. The three-dimensional image volume can contain a full range of three-dimensional veloci
6、- ties. In this new three-dimensional SPIVsetup, two cameras are placed at diferent angles to view the illuminated$eld and to capture particle displacement images that contain the influ- ence of the third velocity component. Theparallax efect allows us to obtain different two-velocity component vect
7、or maps from each camera. The diferences between the two images arise from the third velocity component and the geometrical config- uration of the two cameras. Afer image calibration, this third velocity component can be calculated. The two cameras will be set at diferent exposure times, thus acquir
8、ing diflerent lengths of streaks for the same particle path. The differences of the exposure then can be used to distinguish theflow direc- tions. INTRODUCTION Stereoscopic particle image velocimetry (SPIV), which can provide instantaneous velocity distributions over an area or space in a flow field
9、, has been studied by a number of researchers (Arroyo and Greated 199 1 ; Coupland and Halli- well 1992; Sinha and Kuhlman 1992; Malik et al. 1993; Robinson and Rockwell 1993; Prasad and Adrian 1993; Grant et al. 1995; Hinsch 1995; Scholzen and Moser 1996; Willert 1997; Grant et al. 1998; Lawson and
10、 Wu 1999; Bearman et al. 1999; Palero et al. 2000). Existing stereoscopic configuration may be classified as translational method or angular displace- ment method. These currently available stereoscopic systems, however, are very difficult to precisely position. The transla- tional systems (Arroyo a
11、nd Greated 199 1 ; Prasad and Adrian 1993) require that the object plane, lens plane, and image plane all be parallel to each other, and the lens of all the cameras must be in the same plane. The errors in the system setups can be significant when the concerned area or space is large, e.g., in full-
12、scale rooms. The common area viewed by all cameras is usually small unless some special techniques are adopted, which increases the difficulties in the system setups and produces additional image recording errors. Angular displacement methods (Prasad and Jensen 1995; Scholzen and Moser 1996; Lawson
13、and Wu 1997; Willert 1997; Lawson and Wu 1999) have fewer requirements for the alignment of the system setups than translational systems. But these methods still require that the optical axes of all cameras remain in the same plane and intersect the object plane at the system axis. Yigang Sun is a v
14、isiting scholar, Yuanhui Zhang is a professor, and Xinlei Wang is an assistant professor in the Department of Agricultural Engineering, University of Illinois at Urbana-Champaign. Lingying Zhao is an assistant professor in the Department of Food, Agricultural and Biological Engineering, Ohio State U
15、niversity, Columbus. 02004 ASHRAE. 75 The errors in the system arrangement could again be signifi- cant when the concerned area or space is large. The measure- ment volume of both configurations is a mere sheet of very limited depth. To resolve directional ambiguity, often more than two cameras are
16、installed or special techniques, such as image shifting, are employed, which multiplies the work for system setup and/or image processing and adds an error source. An algorithm for stereoscopic PIV in the particle streak mode is presented in this paper. The SPIV technique based on the algorithm requ
17、ires no special conditions except for the two cameras having a common viewing area of the object as large as possible without blocking each other. Through calibration, the orientations, locations, and imaging parameters of two cameras can be determined with satisfactory precision. Direc- tional ambi
18、guity is eliminated by making use of the difference between the exposure times of the two cameras. The setup of the system is substantially simplified compared with the exist- ing technology, and the measurement volume is much larger than other systems available now. HARDWARE OF A THREE-DIMENSIONAL
19、PIV SYSTEM IN PSM The hardware of a three-dimensional PIV system in the particle streak mode consists of a particle seeding system, an illumination system, an image recording system, and an image and data processing system. Particle Seeding System To visualize a flow for PIV, the flow has to be seed
20、ed with small, neutrally buoyant tracer particles. The seeding particles should have the physical density of the host flow and be as small as possible to guarantee good follower behavior but large enough to reflect sufficient light in suitable illumination and to be captured by the image recording s
21、ystem. In the parti- cle streak mode, the concentration of seeding particles should be low enough that individual particle streak images can be identified. illuminating System Illumination is a key component in a traditional PIV measurement system. Unlike an illumination system for a traditional PIV
22、, the light for this algorithm is not required to be transferred into a thin sheet because it is used for volume measurement. The light source should also not be pulsed for this operating mode. The light intensity, however, should be high enough and its distribution be uniform enough in the whole ob
23、servation volume to ensure clear images of streaks in the area of concern. Image Recording System Two high resolution cameras are required for the system. The shutters of the two cameras can be synchronously opened with a controller, and their exposure times can be set at high accuracy. If the airfl
24、ow in a room of 18 feet (5.5 m) width is (x, y, z) QiParticlc Figure 1 Schematic of stereoscopic PIK measured using the system, and the diameters of the seeding particles are about 2 mm, a resolution of about 3000 pixels horizontally is needed for the cameras. Intensified cameras or some digital cam
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