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Donya P. Frank-Gilchrist, Allison Penko, and Joseph Calantoni

Abstract

Accurately assessing the response of sediments to oscillatory flows requires high-resolution fluid velocity and sediment transport measurements at the fluid–sediment interface. Fluid and sediment grain velocities were measured simultaneously with combined particle image and tracking velocimetry under oscillatory flows over movable sand ripples. Three high-speed cameras equipped with varying optical filters were used to distinguish between fluorescent fluid tracers and the grains, from which the fluid and grain velocities were determined, respectively. Individual grains were tracked during transport to determine velocities and trajectories. Sediment grains were first mobilized by a vortex impacting the bed during flow reversal and suspended into the water column just prior to vortex ejection from the ripple crest, similar to previous observations. During phases of maximum flow velocity, additional grains were mobilized by the shear stress and were subsequently suspended. The flow reversed and similar observations were made in the opposite direction. Consequently, four peaks in suspended sediment concentration were observed throughout the flow cycle, consistent with previous observations. However, some previous researchers attributed peaks in suspended sediment concentration occurring during phases of maximum flow velocity to sediment-laden vortices that were shed from adjacent ripples. The measured sediment grain velocities were of similar magnitude and phase to the near-bed fluid velocities when the grains were being advected with the flow. Measurements of suspended sediment concentration agreed well with semiempirical formulations having an average root-mean-square deviation of approximately 4 × 10−5 m3 m−3. Predictions of settling velocity also compared well with the laboratory estimates, agreeing to within 90%.

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Donya Frank, Diane Foster, Pai Chou, Yu-Min Kao, In Mei Sou, and Joseph Calantoni
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Donya Frank, Diane Foster, Pai Chou, Yu-Min Kao, In Mei Sou, and Joseph Calantoni

Abstract

Measurements within the mobile bed layer have been limited by previous Eulerian-based technologies. A microelectromechanical system device, called a smart sediment grain (SSG), that can measure and record Lagrangian observations of coastal sediments at incipient motion has been developed. These sensors have the potential to resolve fundamental hypotheses regarding the incipient motion of coastal sediments. Angle of repose experiments verified that the sensor enclosure has mobility characteristics similar to coarse gravel. Experiments conducted in a small oscillating flow tunnel verified that the sensors detect incipient motion under various hydrodynamic conditions. Evidence suggests the influence of pressure-gradient-induced sediment motion, contrary to the more commonly assumed bed shear stress criterion. Lagrangian measurements of rotation measured with the newly developed SSG agreed to within 5% of the rotation estimates made simultaneously with high-speed video cameras.

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Mark D. Orzech, Fengyan Shi, Jayaram Veeramony, Samuel Bateman, Joseph Calantoni, and James T. Kirby

Abstract

A coupled model system has been developed to investigate the physics of wave attenuation and ice edge retreat in the marginal ice zone (MIZ) at small scales [O(m)]. A phase-dependent finite-volume/finite-difference fluid dynamics model is used to simulate waves and currents, and a discrete element software package is employed to represent ice floes as bonded collections of individually tracked smaller particles. We first review the development of the coupled system, with an emphasis on the coupling software and the representation of wave–ice shear stress. Then we describe a series of simulations that were conducted to evaluate and qualitatively validate the performance of the coupled models. The system produced reasonable results for cases of a vertically oscillating ice block and a free-floating ice floe in monochromatic waves. In larger-scale simulations involving multiple ice floes and pancake ice, estimated transmission and reflection coefficients were similar to those obtained from alternate models and/or data, although numerical dissipation may have reduced estimates of transmitted wave energy in longer wave flumes. Challenges and limitations involving relative length scales in the coupled wave and ice domains are explained and discussed.

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