Date of Thesis

8-1-2013

Thesis Type

Masters Thesis (Bucknell Access Only)

Degree Type

Master of Science in Mechanical Engineering

First Advisor

M. Laura Beninati

Abstract

Experiments are presented to explore scour due to flows around support structures of model marine hydrokinetic (MHK) devices. Three related studies were performed to understand how submergence, scour condition, and the presence of an MHK device impact scour around the support structure (cylinder). The first study focused on clear-water scour conditions for a cylinder of varied submergence: surface-piercing and fully submerged. The second study centered on three separate scour conditions of different flow velocities (clear-water, transitional, and live-bed) around the fully submerged cylinder. Lastly, the third study examined the impact of an MHK turbine on scour around the support structure, in live-bed conditions. These three studies addressed the effect of structure blockage/drag, and the ambient scour conditions on scour around the support structure. The experiments were performed in the small-scale testing platform in the hydraulic flume facility (9.8 m long, 1.2 m wide and 0.4 m deep) at Bucknell University. The support structure diameter (D = 25.4 mm) was held constant for all tests. The submerged cylinder (l/D = 5) and sediment size (d50 = 790 microns) were held constant for all three studies. The MHK device (Dt = 101.6 mm) is a two-bladed horizontal axis turbine and the rotating shaft is friction-loaded using a metal brush motor. For each study, bed form topology was measured after a three-hour time interval using a two-dimensional traversing bed profiler. During the experiments, depth measurements of the scour-hole at the front face of the support structure (cylinder) were made at regular intervals to estimate scour rates. Measurements of the bed form were collected in half-diameter increments across the width of the test section. Results from these three studies show the scour-hole dimensions (length, width, depth) and scour rates are dependent on submergence, scour condition and presence of the MHK device. Increase in submergence in the first study showed increases in the scour-hole dimensions and scour rates. The second study indicated two distinct scour rate regimes in the clear-water condition, while four regimes were identified in the transitional and live-bed conditions. Scour-hole dimensions were found to be dependent on the change in average flow velocity between the three different scour conditions. Finally, the third study demonstrated that the addition of the MHK turbine increases the scour-hole dimensions in the live-bed condition. Scour rates increase with increasing average flow velocity, most notably in the latter two regimes.

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