Turbulent Boundary Layers and Sediment Resuspension in the Absence of Mean Shear PDF Download

Author: Blair Anne Johnson
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Languages : en
Pages : 128

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Book Description
Sediment resuspension is initiated through a variety of mechanisms across the wave breaking, run-up, and run-down stages that occur in the wave breaking, surf, and swash zones. When a wave breaks offshore, the plunging jet rarely reaches the bed directly; however, this injection of momentum generates turbulence that propagates through the water column, creating local stresses as turbulent eddies interact with the seafloor. When these stresses are sufficient to resuspend sediment from the bed, the sediment can become entrained in the flow, and thus both turbulence and sediment are advected shoreward during onshore directed phases of the wave cycle. It is at this time that the fluid is highly turbid, filled with sediment and debris. As the flow enters the swash zone, it runs upshore and slows, and the sediment settles out of entrainment and is deposited onshore. Then, as this relatively clear fluid flow retreats and regains speed in flowing down the beach face, sediment is again resuspended into the flow. But upon careful observation, the turbid region is constrained to the base of the water column due to shear-dominated resuspenions as a boundary layer grows. The resuspension during the latter event can be attributed to an increased bed shear stress, due to the uniform mean flow along the sediment bed. This mechanism for resuspension is well-understood, as thorough experimental research has led to well-characterized parameters such as the Shields Curve that identify requisite shear stresses for sediment resuspension. However, the former mechanism, in which sediment is resuspended due to the interaction of large-scale turbulent motions, has seen far less research, because of the difficulty in isolating this particular flow in the laboratory and identifying meaningful parameters of the flow necessary for incipient particle motion from the bed. We have chosen to isolate the phenomenon of sediment resuspension by turbulence absent mean shear in the laboratory to better understand this fundamental process. By adapting a turbulence chamber in the DeFrees Hydraulics Lab developed by Variano & Cowen (2008), in which a Randomly Actuated Synthetic Jet Array (RASJA; Variano and Cowen, 2008) is suspended above a tank of water, with jets directed downward, we have a nearly ideal facility for examining decaying horizontally homogeneous isotropic turbulence above a bed. The jets fire according to a specified random algorithm to generate homogeneous isotropic turbulence without inducing mean flows. With a solid glass bottom boundary in place, data were collected with Acoustic Doppler Velocimetry (ADV) and Particle Image Velocimetry (PIV) measurements to characterize profiles of the mean flow, turbulence intensity, and kinetic energy. Statistics such as spatial and temporal spectra and parameters of mean flow strength were computed to understand the nature of the turbulence in the facility, revealing a well developed inertial subrange and very weak mean flows. The glass was then replaced with a sediment bed, and the tests were repeated to make a direct comparison between the solid and sediment beds. During the sediment tests, resuspension was observed intermittently. We were surprised to find that a rippled pattern quickly evolved in the sand bed, even with relatively few visible resuspension events. Preliminary tests have been performed to examine the relationships between the forced turbulence levels and the mechanisms of resuspension, and we have also performed qualitative studies to investigate the influence of the presence of solid boundaries and turbulence on bed morphology. In order to quantitatively analyze the nec- essary stresses and fluid structures present, a conditional quadrant analysis is performed on the Reynolds stresses. A small project was included to test the performance of a Nortek Aquadopp High Resolution Profiler in the facility. Though the profiler is designed to capture large-scale environmental flows that are uniform across its beams at a given elevation from its transducer face, this investigation was performed to test the instrument's capability to capture high levels of turbulence in a small facility where this assumption breaks down. Direct comparisons were made to simultaneous PIV measurements, and measurements from the Profiler were included in the overall tank characterization.