In this paper, a micro-stereoscopic PTV system is developed using single camera system. Two viewing holes are installed just behind the object lens of the microscopic system to construct a stereoscopic viewing. The system consists of one high-definition camera (1028 x 1024 pixel, 500fps), a plate of two-viewing holes and the host computer. Since two image pair for one tracer particle exists on the same image, it is not so easy to find the same particle pairs among many particles in the images.
Two calculation methods have been introduced to calculate three-dimensional positions of the particles. One is to introduce a hybrid genetic algorithm (GA) to calculate three-dimensional vector fields. The other one is to utilize the distances between the two image centers of the particles made by the two viewing holes. As for the first calculation, the hybrid genetic algorithm (GA) has been used to find the same particles' image pairs seen on the experimental images. An epipolar line was used to reduce the number of candidates for one particle. In the genetic algorithm, crossover operation, mutation operation and reproduction operation are introduced. Percentile values of these three values are changed to find the most optimal pairring conditions.
In order to make a performance test for the constructed algorithm, the camera parameters obtained in the process of camera calibrations have been used for generating a set of virtual image. Using the constructed virtual images the optimal calculation conditions have been found. That is, it has been empirically verified that optimal percentile values for the GA algorithm are crossover 100%, mutation 20%, Reproduction 50%, respectively. Further, the optimal diameter of the two holes were 3.5mm and the distance between the two holes were 5mm. The measurement errors for X, Y and Z coordinates could be estimated using the results of camera calibration as 0.083μm, 0.045μm and 0.083μm, respectively. And the deviations of the measurement error could be estimated as 0.725μm, 0.452μm, 4.108μm.
As for the second calculation method for the calculation of three-dimensional vector fields, the distance between the photographic centers of the two images that have been constructed by the two holes. A relation between Z-positions and the distances are calculated before the main experiment so that the Z positions of any unknown particles can be predicted. The center points of the distance have been regarded as the photographical coordinates on X and Y axises.
The constructed micro-stereoscopic PTV(a 3D-PTV) has been used to measure the velocity fields of a micro-back-step channel flow. The height of the back-step is 36 μm and the height of the channel is 60 μm with a width of 3mm. The Reynolds number is 0.017. The data obtained by the constructed system have been compared with those of CFD. It has experimentally verified that the velocity profiles shows a good agreement quantitatively and qualitatively. The constructed micro measurement system is planed to be used for the measurements of the wall boundary layers in turbulent flows.