Study On The Three-Dimensional Particle Tracking Velocimetry Based On The Defocusing Microscopic Image

In order to carry out the research of three-dimensional microscopic particle tracking velocimetry,a set of velocity measuring system with high speed microscope defocusing particle image was established in this pap er.The research for defocused image acquisition method and the calibration of particles spatial coordinates were conducted,and the algorithm about particle's identification and trajectory tracking were coded.The experimental studies for three different micro-channels whose structures are square long straight,inverted step and arrayed cube were completed,respectively.The main studies include as follows:Firstly,according to the principle of defocused imaging,the light path system was upgraded on the basis of the original two-dimensional micro-PTV/ PIV system.A three-pinhole mask fabricated three holes with linear distribution of thickness between each hole was embedded in the microscope objective lens' back-end and the hole edge thickness is merely 0.1mm.The characteristics of similar triangle was used in the process of their defocusing images recognition,and the centroid of particle defocusing image spot was calculated by its grayscale satisfied two-dimensional Gaussian distribution.In this study,29 sample particles were selected in the process of measurement algorithm analysis about particle's three-dimensional position,and hence 29 different depth data were utilized for calibrating in the particles' depth direction with the method of linear fitt ing.The drift compensation functions about the in-plane position of particle were acquired by quadratic polynomial fitting with 870 groups of data.Secondly,the experiments of defocusing microscopic particle image velocimetry were implemented.A microsco pe objective lens of 20?0.4 was used to observe the flow field of 205?m ? 205?m,and a high speed CMOS camera was employed to trace the fluorescent particles of 2 ?m at a rate of 1000 frames per second.The configuration parameters of defocusing particle image,namely its diameter and centroid,were obtained by the optical and image analysis.The actual depth of the particle position was calculated by the calibration function of depth,and the particle position horizontally could be modified by its compensation function.Eventually the three-dimensional trajectory and velocity vector of the tracer particle were obtained.In order to verify the reliability of defocusing microscopic particle image velocimetry system and measurement accuracy,the measurement s of flow fields in a rectangular long straight micro-channel were completed first,whose Reynolds number are 0.05 and 0.1,respectively.The results indicated that the measured velocity profiles were in good agreement with the numerical simulation results.Next,the measurements of flow around the inverted step structure in a micro-channel were implemented,and three-dimensional velocity distribution and traced trajectory of the particle were obtained.The trajectories of particles were consistent with the flow pattern obtained through the numerical simulation.The measurement of three-dimensional flow field about flow around the arrayed cube structure fabricated in the microchannel was completed on the basis of the above validation experiments.The results showed that the particle trajectory tracking within three-dimensional complex structure could be realized by the measuring system.The flow field had a periodic structure as a whole,but the periodicity was broken at some local positions.The reason may be due to that the flow instability happens as the complexity of the flow past three-dimensional obstacle.