|M.Sc Student||Rabencov Boris|
|Subject||Experimental Investigation of Bead Dispersal in a Turbulent|
Boundary Layer Using Time-Resolved Digital
|Department||Department of Mechanical Engineering||Supervisor||Dr. Rene Van Hout|
|Full Thesis text|
Single view, inline digital holographic cinematography (1 kHz) was used to measure bead dispersal of polystyrene beads (<dp> ≈ 583 ± 14 mm and <dp> ≈ 141 ± 2 mm) in a turbulent water channel flow. Algorithms fine-tuned to tracking “large” beads (i.e. having a significant perimeter) were developed. Overlapping beads were segmented using the distance and watershed transforms and subsequently tracked. In-plane tracking based on the nearest neighbor algorithm was performed prior to in-focus position detection. A comparison between different algorithms to determine the bead’s in-focus position indicated that the here proposed method based on intensity gradients outperformed the others. Thirteen data sets (26,144 holograms, 2616 “large” beads) were processed. The bead streamwise velocity profile was obtained and the friction velocity was estimated using the Clauser fit method. A case study of four beads near the bottom channel wall was used to highlight the data processing and tracking procedure. Bead diameters were accurately determined from the holographic reconstructions. Furthermore, Probability density functions (pdf’s) of transverse bead position indicated that on average preferential bead locations were absent. Pdf’s of the instantaneous bead velocity components were derived separately for upward and downward movements. The streamwise velocity values were comparable to the bulk flow velocity, while on average, downward moving beads corresponded to higher streamwise velocity than upward moving ones in correspondence with the well-accepted turbulent boundary layer structure of “sweeps” and “ejections”. Pdf’s of the wall-normal velocities indicated no dependence on the wall-normal position or movement direction. The bead spatial distribution was further investigated for the smaller beads (<dp> = 141 ± 2 mm) by Voronoï tesselation. The Voronoï analysis, which was applied to the collapsed 3D position on each one of two perpendicular planes, revealed preferential particle concentration (clustering) in the cross-sectional plane. The clusters were further characterized according to their area and perimeter and their coherence length in the streamwise direction was estimated to be 300 inner wall units, comparable to literature values for the extent of coherent hairpin packets that populate the boundary layer and that are instrumental in the transport of the beads.