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  • 1
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    Hydrodynamic and sediment transport modeling of New River Inlet (NC) under the interaction of tides and waves (2015)
    Wiley
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    Publication Date: 2015-05-06
    Description: The interactions between waves, tidal currents, and bathymetry near New River Inlet, NC, USA are investigated to understand the effects on the resulting hydrodynamics and sediment transport. A quasi-3D nearshore community model, NearCoM-TVD, is used in this integrated observational and modeling study. The model is validated with observations of waves and currents at 30 locations, including in a recently dredged navigation channel and a shallower channel, and on the ebb tidal delta, for a range of flow and offshore wave conditions during May 2012. In the channels, model skills for flow velocity and wave height are high. Near the ebb tidal delta, the model reproduces the observed rapid onshore (offshore) decay of wave heights (current velocities). Model results reveal that this sharp transition coincides with the location of the breaker zone over the ebb tidal delta, which is modulated by semi-diurnal tides and by wave intensity. The modulation of wave heights is primarily owing to depth changes rather than direct wave-current interaction. The modeled tidally averaged residual flow patterns show that waves play an important role in generating vortices and landward-directed currents near the inlet entrance. Numerical experiments suggest that these flow patterns are associated with the channel-shoal bathymetry near the inlet, similar to the generation of rip currents. Consistent with other inlet studies, model results suggest that tidal currents drive sediment fluxes in the channels, but that sediment fluxes on the ebb tidal delta are driven primarily by waves. This article is protected by copyright. All rights reserved.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Convective instability in sedimentation: 3-D numerical study (2014)
    Wiley
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    Publication Date: 2014-11-09
    Description: To provide a probable explanation on the field observed rapid sedimentation process near river mouths, we investigate the convective sedimentation in stably stratified saltwater using 3D numerical simulations. Guided by the linear stability analysis (Yu et al. 2013, J. Geophys. Res. Oceans, 118, 256-272), this study focuses on the nonlinear interactions of several mechanisms, which lead to various sediment finger patterns, and the effective settling velocity for sediment ranging from clay (single-particle settling velocity V 0 = 0.0036 and 0.0144 mm/s, or particle diameter d = 2 and 4 μ m) to silt ( V 0 =0.36 mm/s, or d =20 μ m). For very fine sediment with V 0 =0.0036 mm/s, the convective instability is dominated by double diffusion, characterized by millimeter-scale fingers. Gravitational settling slightly increases the growth rate; however, it has notable effect on the downward development of vertical mixing shortly after the sediment interface migrates below the salt interface. For sediment with V 0 = 0.0144 mm/s, Rayleigh-Taylor instabilities become dominant before double-diffusive modes grow sufficiently large. Centimeter-scale and highly asymmetric sediment fingers are obtained due to nonlinear interactions between different modes. For sediment with V 0 = 0.36 mm/s, Rayleigh-Taylor mechanism dominates and the resulting centimeter-scale sediment fingers show a plume-like structure. The flow pattern is similar to that without ambient salt stratification. Rapid sedimentation with effective settling velocity on the order of 1 cm/s is likely driven by convective sedimentation for sediment with V 0 greater than 0.1 mm/s at concentration greater than 10 to 20 g/L.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 3
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    On the transport modes of fine sediment in the wave boundary layer due to resuspension/deposition—A turbulence-resolving numerical investigation (2015)
    Wiley
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    Publication Date: 2015-02-15
    Description: Previous field observations revealed that the wave boundary layer is one of the main conduits delivering fine sediments from the nearshore to continental shelves. Recently, a series of turbulence-resolving simulations further demonstrated the existence of a range of flow regimes due to different degrees of sediment-induced density stratification controlled by the sediment availability. In this study, we investigate the scenario in which sediment availability is governed by the resuspension/deposition from/to the bed. Specifically, we focus on how the critical shear stress of erosion and the settling velocity, can determine the modes of transport. Simulations reveal that at a given wave intensity, which is associated with more energetic muddy shelves and a settling velocity of about 0.5 mm/s, three transport modes, ranging from the well-mixed transport (mode I), two-layer like transport with the formation of lutocline (mode II) and laminarized transport (mode III) are obtained as the critical shear stress of erosion reduces. Moreover, reductions in the settling velocity also yield similar transitions of transport modes. We also demonstrate that the onset of laminarization can be well-explained by the reduction of wave-averaged bottom stress to about 0.39 Pa due to attenuated turbulence by sediments. A 2D parametric map is proposed to characterize the transition from one transport mode to another as a function of the critical shear stress and the settling velocity at a fixed wave intensity. This article is protected by copyright. All rights reserved.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 4
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    High Performance BaTiO3-Based BME-MLCC Nanopowder Prepared by Aqueous Chemical Coating Method (2012)
    Wiley
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    Publication Date: 2012-04-15
    Description: Dispersion plays an important role in aqueous chemical coating process for the preparation of BaTiO 3 -based powder for base-metal-electrode multilayer ceramic capacitors (BME-MLCCs) in our previous research. In this article, a powerful sand mill (SC-milling) machine is utilized instead of traditional ball milling to produce BaTiO 3 MLCC nanopowder, in which small ZrO 2 beads help to achieve high centrifugal accelerations and avoid BaTiO 3 agglomerates. As much as 1 kg BaTiO 3 nanopowder has been effectively dispersed very well in one time, then a uniform coating of additives on BT nanoparticles has been achieved by aqueous chemical coating method. High performance X7R BaTiO 3 nanoceramics with the average grain size of about 96 nm are obtained. The experimental result indicates that this technology can be feasible to the industry for manufacturing next generation BME-MLCCs.
    Print ISSN: 0002-7820
    Electronic ISSN: 1551-2916
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 5
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    The effects of flocculation and bed erodibility on modeling cohesive sediment resuspension (2011)
    Wiley
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    Publication Date: 2011-03-23
    Description: Flocculation and bed erodibility are two main processes causing the transport of cohesive sediments to be more complicated than typical noncohesive sediments. Earlier flocculation models assume a constant fractal dimension and/or a constant floc yield strength. However, recent studies have shown that considering both the fractal dimension and the floc yield strength to be variable is critical to the prediction of temporal evolution of floc size. Due to consolidation, it is also well established that critical bed shear stress of a mud bed cannot be parameterized as a constant. This study further investigates how flocculation models with different degrees of complexity and bed erodibility can affect the resulting cohesive sediment resuspension driven by tidal flows. A one-dimensional vertical numerical model for sediment transport is revised to incorporate modules for flocculation and bed erodibility. Model results are compared with data measured in the Ems/Dollard estuary. Model study suggests that it is important to incorporate variable critical shear stress in order to properly model the supply of sediment from the bed. When flocculation is neglected or incorporated incompletely, numerical model predicts nearly zero sediment concentration during slack water and very steep concentration gradient, which are inconsistent with the observed data. When the fractal dimension and the floc yield strength are both considered to be variable, the numerical model predicts much smaller settling velocity and hence captures the more well-mixed condition consistent with field observations.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 6
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    A numerical investigation of lutocline dynamics and saturation of fine sediment in the oscillatory boundary layer (2011)
    Wiley
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    Publication Date: 2011-09-14
    Description: Understanding the state of the muddy seabed is critical to sediment transport, hydrodynamic dissipation and seabed properties. However, this endeavor is challenging because the availability and settling velocity of sediment in muddy environments are highly variable. For a given Reynolds number, typical of fine sediment settling in a moderately energetic muddy shelf, recent 3D numerical simulations have revealed four distinct regimes of wave-induced fine sediment transport. These regimes depend on the availability (or concentration) of sediment and range from well-mixed condition to the formation of lutocline, and eventually a complete flow laminarization. By keeping the sediment availability unchanged, this study further demonstrates the existence of these flow regimes for a range of sediment settling velocities. Simulation results suggest that when settling velocity is larger, the location of the lutocline becomes lower (closer to the bed) and the flow eventually laminarizes when there is further increase in the settling velocity. Hence, the dynamics of lutocline is clearly related to the transition between these flow regimes. The vertical flow structure in the presence of lutocline is revealed through the budgets of sediment flux and turbulent kinetic energy. The suppressed mixing in the lutocline layer is further illustrated from a new perspective, i.e., the budgets of turbulent suspension and concentration fluctuation variance. The concept of saturation, commonly used for tidal boundary layer, is extended here for wave boundary layer.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 7
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    A numerical investigation of convective sedimentation (2011)
    Wiley
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    Publication Date: 2011-09-21
    Description: Understanding the fate of riverine sediment in the coastal environment is critical to the health of the coastal ecosystem and the changing morphology. One of the least understood mechanisms of initial deposition is the convective sedimentation of hypopycnal plumes. This study aims at investigating convective sedimentation by means of a numerical model for fine sediment transport solving the non-hydrostatic Reynolds-averaged Navier-Stokes equations for stratified turbulent flow. Model validation is sought by comparison to laboratory results for turbidity and saline currents over a changing slope. The model is shown to be capable of predicting both the upstream supercritical and the downstream subcritical flows. The numerical model is then utilized to study convective sedimentation and its depositional and mixing characteristics. By analyzing model results of more than 40 runs for different inlet sediment concentration (density ratio γ), settling velocity (particle Reynolds number Rep), and inlet velocity/height (inlet Reynolds number Re), four distinct flow regimes are revealed. For large γ, we observe divergent plumes with significant deposits near the inlet. For intermediate γ and large Rep, intense convective fingers are predicted which are only marginally affected by ambient shear flow. Further reducing the density ratio γ or Rep gives weak convective fingers that are significantly affected by the ambient shear flow. Eventually, no convective fingers are observed during the computation for very small γ or Rep. Sediment deposits in the divergent plume and intense convective finger regimes are relatively insensitive to Re. Deposit increases with Re in the weak convective finger regime.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 8
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    High Performance BaTiO3-Based BME-MLCC Nanopowder Prepared by Aqueous Chemical Coating Method (2012)
    Wiley
    Details
    Publication Date: 2012-01-12
    Description: Dispersion plays an important role in aqueous chemical coating process for the preparation of BaTiO 3 -based powder for base-metal-electrode multilayer ceramic capacitors (BME-MLCCs) in our previous research. In this article, a powerful sand mill (SC-milling) machine is utilized instead of traditional ball milling to produce BaTiO 3 MLCC nanopowder, in which small ZrO 2 beads help to achieve high centrifugal accelerations and avoid BaTiO 3 agglomerates. As much as 1 kg BaTiO 3 nanopowder has been effectively dispersed very well in one time, then a uniform coating of additives on BT nanoparticles has been achieved by aqueous chemical coating method. High performance X7R BaTiO 3 nanoceramics with the average grain size of about 96 nm are obtained. The experimental result indicates that this technology can be feasible to the industry for manufacturing next generation BME-MLCCs.
    Print ISSN: 0002-7820
    Electronic ISSN: 1551-2916
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 9
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    A numerical investigation of wave-breaking-induced turbulent coherent structure under a solitary wave (2014)
    Wiley
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    Publication Date: 2014-09-23
    Description: To better understand the effect of wave-breaking-induced turbulence on the bed, we report a 3D Large-Eddy Simulation (LES) study of a breaking solitary wave in spilling condition. Using a turbulence-resolving approach, we study the generation and the fate of wave-breaking-induced turbulent coherent structures, commonly known as obliquely descending eddies (ODEs). Specifically, we focus on how these eddies may impinge onto bed. The numerical model is implemented using an open-source CFD library of solvers, called OpenFOAM, where the incompressible 3D filtered Navier-Stokes equations for the water and the air phases are solved with a finite volume scheme. The evolution of the water-air interfaces are approximated with a volume of fluid method. Using the dynamic Smagorinsky closure, the numerical model has been validated with wave flume experiments of solitary wave breaking over a 1/50 sloping beach. Simulation results show that during the initial overturning of the breaking wave, 2D horizontal rollers are generated, accelerated and further evolve into a couple of 3D hairpin vortices. Some of these vortices are sufficiently intense to impinge onto the bed. These hairpin vortices possess counter-rotating and downburst features, which are key characteristics of ODEs observed by earlier laboratory studies using Particle Image Velocimetry. Model results also suggest that those ODEs that impinge onto bed can induce strong near-bed turbulence and bottom stress. The intensity and locations of these near-bed turbulent events could not be parameterized by near-surface (or depth integrated) turbulence unless in very shallow depth.
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    Topics: Geosciences , Physics
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  • 10
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    On the mechanisms of low-frequency wave attenuation by muddy seabeds (2014)
    Wiley
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    Publication Date: 2014-04-12
    Description: Low-frequency (lf-) wave energy increases as the waves shoal into shallow waters. However, recent field observations reported an unexpected nearshore lf-wave energy dissipation on muddy seabeds, which cannot be explained by the classic two-layer formulation. Therefore, this phenomenon has been ascribed to either direct dissipation or non-linear energy transfer. We investigate, by means of a two-layer nonlinear model, the role of the wave nonlinearity and mud viscosity in controlling these two competing mechanisms of mud-induced lf-wave attenuation. Bispectral analysis of the simulated cases reveals the existence of three distinct lf-wave attenuation regimes, which determine if the lf-wave energy losses are owing to either non-linear energy transfer or direct dissipation. These regimes can be predicted based on the Ursell number, whereas the mud viscosity controls the amount of energy transfer. The present findings clarify apparent inconsistencies in the literature regarding the mechanisms of lf-wave attenuation by mud.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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