(40f) Digital Particle Image Velocimetry in Flow Measurements

This paper presents experimental results in granular material flows carried out in a model built of Plexiglas with vertical walls. Eccentric filling and central and eccentric discharge were investigated. Flowing material was recorded by a high speed camera SensiCam through the transparent walls of the model. DPIV measurements allow to obtain plug flow zone evolution, velocity magnitude contours, velocity fields, velocity distributions, deformations in the flowing material or traces of individual particles. Discharge outlet was placed in three positions: 1 cm from the left edge, in the symmetry axis and 1 cm from the right edge of the model. Uniform and repeatable packing of the material with no particle segregation was obtained. Flax seed grains were used in the experiments. The grains have an oblong shape, exhibit the phenomenon of static electricity when flowing and sliding over Plexiglas. Quenot et.al presented a detailed description of the PIV technique and the reasons for introducing an alternative approach Digital Particle Image Velocimetry (DPIV). The experimental setups used for the flow analysis consists of a Plexiglas box, a set of illumination lamps, and a high resolution CCD camera (PCO SensiCam). The 12-bit pairs of images with resolution of 1280 pixels x 1024 pixels and maximum frequency of 3.75 Hz were acquired by Pentium 4 based personal computer. Long sequences of 100-400 images were taken at variable time intervals for subsequent evaluation of the velocity fields. The DPIV technique was already used by Sielamowicz et al. The Plexiglas flat bottomed silo model has a height of 80 cm, a depth of 10 cm, and a width of 26 cm. The model was placed on a stand and a granular material was supplied through a pipe. The width of the outlet was reduced to 1 cm. Figures1a,b,c present vertical velocity variations in time in the model for the three cases investigated here with eccentric filling located 1 cm from the left edge of the model and discharge a) 1 cm from the right, b) in the symmetry axis, c) 1 cm from the left. Figure1. Figure2 presents the velocity contours for the flowing seeds when the model is filled eccentrically from the left side and the outlet is located at three different positions. Figure2 Velocity contours for the eccentric filling in the left part of the model and a) discharge on the right, b) central discharge, c) discharge in the left part of the model The total time of flow was found as 123 s in the case a), 142 s in the case b) and 146 s in the case c).

Figure3 Velocity profiles for a) discharge from the right, b) central discharge and c) eccentric discharge, registered at the height h=10 cm above the outlet.

Figure4 presents a bunch of profiles for the first second of the flow. Figure4 A bunch of profiles obtained for the time t=1sec, for eccentric filling from the left and discharge from the left. In Figure5a,b the stagnant zone boundaries are almost similar and indicate eccentric flow mode. The flow channels forms in a regular shape. But the case c) shows that the bunch of profiles form in the shape almost as for central flow. After a very short time the flow forms in a symmetrical mode.

Figure5 Experimental measurements of the stagnant zone boundaries From measured velocity gradients one may calculate strains in the flowing material.

Figure6. Deformations in the flowing flax seed in the three cases of eccentric discharge. Figures 6a-c show the deformations of the flowing material after the first second of the flow. In Fig. 6b and c in the upper parts of the flowing material at the locations of 30 and 50cm above the outlet we can recognize the shear zones in the flowing material. Fig. 7. Volumetric discharge flow rate for flax seed calculated from (a) velocity profiles; (b) measurements of existing volume of the material in the model. In Figure7 the discharge flow rate calculated by two methods is presented. Each point corresponds to one rate value. The points were calculated from the velocity profiles and then smoothed to a line which represents the discharge volume rate. Acknowledgements: The authors express their gratitude to Professor Zenon Mroz of the Polish Academy of Sciences and Professor Andrew Drescher for discussions and consultations during the time of experiments. Many thanks to S. Blonski for his assistance in computer calculations and at experiments. QUENOT, G. M., PAKLEZA, J., KOWALEWSKI, T. A., Particle image velocimetry with optical flow, Experiments in Fluids, 25, 177-189, 1998. SIELAMOWICZ I., BLONSKI S. and KOWALEWSKI T.A., Optical technique DPIV in measurements of granular material flows, Part 1 of 3-plane hoppers, ChES, 60, 2, 589-598, 2005.