Digital Defocussing Particle Tracking Velocimetry in a Random Positioning Machine

MSc-project of Paul van Lunen

 

Summary:

The Random Positioning Machine(RPM) is a machine which is used to simulate some of the eff ects of micro-weight on a biological sample. This is accomplished by placing a biological sample inside the machine and varying the orientation of thus sample with respect to the gravity vector. The motion of the RPM is a bi-axial oscillating rotation, where the inner frame is placed perpendicular to the outer frame to accomplish a three dimensional motion. The side eff ect of this motion is that shear stresses can occur in the suspension fluid inside the container holding the specimen, which will exert an unwanted force onto the specimen. Previous research by Leguy et al. making use of a numerical model and a PIV measurement system, gave some description of the complex flow patterns inside this container, however, further research is necessary in order to get a full three dimensional description of the flow of the suspension fluid inside a container mounted onto an RPM.

The aim of this research is to develop a measurement system which is capable of giving a quantitative description of the flowprofi le of the suspension fluid which also is sufficiently small to be mounted inside the confined space available on the RPM. In 1992 Willert and Gharib presented a paper in which they described a measurement technique capable of tracking particles through a three dimensional measurement field with he use of only one camera, this technique is called Digital DefocussedParticle Tracking Velocimetry (DDPTV).
DDPTV makes use of an aperture plate which is placed between the lens and the CCD of the camera. This aperture plate contains some holes positioned at equidistant position away from the optical axis. By placing the focal plane outside the measurement domain, particles inside the measurement domain appear at diff erent positions on the CCD for each of the holes in this aperture plate. The distance between those particle images appearing on the CCD provides a measure for the amount the particle is out of focus and thereby for the position the particle has in the third dimension.
The measurement technique was adapted to fit in the available RPM facility and a validation setup was built to determine the resolution of the measurement system. From the validation it showed that the out-of-plane resolution is much lower than the in-plane resolution, in the order of a magnitude. This also became apparent in the results of measurements which were performed on the RPM, these result showed that the motion in the third direction is only slightly larger than the resolution.

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