Study of drag and orientation of regular particles using stereo vision, Schlieren photography and digital image processing

A new experimental, image-based methodology suitable to track the changes in orientation of non-spherical particles and their influence on the drag coefficient as they settle in fluids is presented. Given the fact that non spherical solids naturally develop variations in their angular orientation during the fall, none-intrusiveness of the technique of analysis is of paramount importance in order to preserve the particle/fluid interaction undisturbed. Three-dimensional quantitative data about the motion parameters is obtained through single-camera stereo vision whilst qualitative visualizations of the adjacent fluid patterns are achieved with Schlieren photography. The methodology was validated by comparing the magnitudes of the drag coefficient of a set of spherical particles at terminal velocity conditions against those estimated from drag correlations published in the literature. A noteworthy similarity was attained. During the fall of non-spherical solids, once the particle Reynolds number approximated 163 for disks, and 240 for cylinders, or exceeded those values, secondary motions composed by regular oscillations and tumbling were present They altered the angular orientation of the particles with respect to the main motion direction and caused complete turbulent patterns in the surrounding flow, therefore affecting the instantaneous projected area, drag force, and coefficient of resistance. The impact of the changes in angular orientation onto the drag coefficient was shown graphically as a means for reinforcing existing numerical approaches, however, an explicit relation between both variables could not be observed. (C) 2017 Elsevier B.V. All rights reserved.