Study Of Digital Particle Image Velocimetry Based On Multispectral Imaging

Digital Particle Image Velocimetry (DPIV) plays a very important role in modern flow research. It becomes one of the most important tendencies in modem fluid velocity. With main characteristics of non-intrusive, whole field and dynamic measurement, DPIV has widely used in varied researches as hydrodynamics, combustion, aerodynamics and hydromechanism. With rapid progresses in optics research, imaging technology and computer image processing methods in a recent decade, DPIV has been a mature technology. The study on DPIV has important science values and great economic values.This thesis is mainly concerned with the technology of DPIV and the relative techniques. The researches on the theory and realization methods of DPIV are fully carried out. Especially, emphasis is put on two very important topics in DPIV: technology to improve superior limit of measurable velocimetry of DPIV system and measurement of complex structure flow.In a DPIV system, two consecutive particle images are taken with a known time interval. The velocity in the flow field can be analyzed by finding different position of particles in two images. The superior limit of measurable velocimetry of DPIV system depends on the time interval between two particle images. To measure a high velocity, the time interval should be as short as possible. Frame straddling technology is widely used in DPIV system which highly depends on professional high speed camera to reduce the time interval between two particle images. We describe in this paper a DPIV system based on multi-wavelength pulse laser beam illumination and multispectral imaging instead of Frame Straddling. The time interval between two particle images can be reduced to hundreds of nanoseconds.Complex structure flow is a kind of flow in which very high speed flow and low speed flow both exist. To measure a flow with very high speed, the time interval between two particle images should be short enough to make sure that particles won't escape from image acquisition area. To measure a flow with low speed, the time interval should be long enough to make sure that displacement of particles is large enough to be recognized. Complex structure flow can't be measured by normal DPIV systems for the reason of that normal DPIV are double-pulsed systems with only one time interval. In this paper, we describe a double-time-interval system based on multi-wavelength pulse laser beam illumination and multispectral imaging technology which can be used to measure complex structure flow.Particle image matching is crucial for measuring two-dimensional velocity field. Therefore this thesis focuses on the matching methods. By comparing large amount of experimental results, the cross-correlation coeficient based on FFT is selected as the cross-correlationm easure, which can greatly decrease the computational time. We also describe a new method for efficient detection of spurious vectors in complex structure flow measurement. The DPIV system based on multispectral imaging is firstly calibrated and then tested by measuring several different flows.