|M.Sc Student||Cohen Asaf|
|Subject||Measurement of the Dynamics of Fibers Settling in Near|
Homogeneous, Isotropic Air Turbulence
|Department||Department of Mechanical Engineering||Supervisor||Dr. Rene Van Hout|
|Full Thesis text|
Fiber suspensions commonly occur in industrial processes such as paper and fiber glass insulation manufacturing. The orientation and position of the fibers affect the quality of the resulting products. Usually, the rotational motion of fibers in industrial and atmospheric processes is governed by the small scales of the turbulent flow. Up to date, mostly numerical studies have been performed while and experimental results are scarcely available. Here, a two-orthogonal view, high-speed (2 kHz) digital holography system was used to measure the translational and orientational dynamics of rigid, inertial nylon fibers suspended in near homogeneous isotropic air turbulence (HIT, Taylor scale Reynolds number 144, rms of velocity fluctuations of ~0.5 m/s). HIT was generated in the center of a 40?40?40 cm3 chamber by eight woofers mounted on each of its corners. The volume of interest was 17?17?17 mm3 positioned at the center of the chamber. Nylon fibers (rf = 1.15 gr/cm3) having a length of 0.5 mm and diameter of 13.7 mm were released from the top of the chamber. The measurement’s temporal resolution was about ~1/4.5 times the Kolmogorov time scale such that the measurements were time-resolved while the spatial resolution was 17 mm, enough to accurately determine the fiber’s length from the holographic reconstructions. Applying an orthogonal, two-view, high-speed digital holography system allowed the measurement of the 3D fiber trajectories as well as their 3D orientation dynamics. In total 644 fibers were tracked with an average track length of 8 ms (minimum and maximum lengths of 3.5 and 49.5 ms). Data processing algorithms were developed and will be discussed emphasizing on matching the two views, improving signal-to-noise ratio and determination of fiber translational and rotational characteristics. Fiber lengths extracted from the holographic reconstructions were validated against those measured under a microscope. Pdf’s of both measurements were similar with almost the same mean and standard deviation. Two case studies illustrating the data processing were presented indicating that individual fibers may be affected by different vortices. Pdf’s of fiber centroid translational velocities were similar as those of the air flow, however, effects of inertia were discernible especially in the direction of gravity. In addition, the fiber velocity and fiber orientation vector Lagrangian auto-correlations were calculated and compared to literature results of neutrally buoyant fibers.