A Boroscopic Quantitative Imaging Technique For Sheet Flow Measurements
Dudley, Russell David
A boroscopic imaging system is developed to measure sediment velocities within a highly concentrated sheet layer in open channel flows. A detailed literature review is presented to investigate previous attempts at sheet layer measurements, followed by a set of experiments to determine clean water flow characteristics in the open channel flume located in the DeFrees Hydraulics Laboratory. Experiments are carried out in the same flume under sediment-laden, sheet flow conditions to determine the steadiness of the flow as well as to verify velocities in the suspended sediment transport region with existing theories. Sediment velocities are captured in the highly concentrated sediment sheet for the same flow conditions using the new boroscopic imaging system. The boroscope is a minimally intrusive lens with a diameter that is only an order of magnitude larger than the natural sand being studied. Minimum Quadratic Difference techniques, along with several filtering techniques, are utilized to determine particle displacements. A rigorous investigation into calibrations is included in order to convert pixel displacements into physical velocities. Three separate metrics are investigated, all involving light intensity values. These metrics are examined from calibrations performed under fixed, moving and in-situ experimental conditions. The limitation of each calibration as well as a discussion of the effectiveness of the calibration techniques is included. Physical velocities within the sheet layer are determined using the in-situ variance of intensity calibration. A complete streamwise velocity profile spanning from the non-moving bed to the free stream is included which combines velocity data collected from both the boroscopic imaging technique as well as an acoustic Doppler velocimeter. The joint profile seems fairly continuous but more attention needs to be focused on the velocities located along the interface of the sheet flow and suspended regions as well as possible reasons for differences between these experimental results and results published previously. Finally, future development is discussed including the further investigation into calibration techniques, the possibility of measuring sediment concentrations as well as water velocities and other possible uses for the boroscopic quantitative imaging technique.
sheet flow; sediment transport; particle image velocimetry; open channel flow
dissertation or thesis