Sand Motion induced by oscillatory flows: sheet flow and vortex ripples
TL;DR: In this article, the wave-related sand transport is still very difficult to predict due to the complexity of its underlying processes, which mainly take place in a thin layer near the sea bed in the wave boundary layer.
Abstract: Shoaling short gravity waves at sea approaching the shore become asymmetric and are able to generate a net resulting sand transport in cross-shore direction (on-shore-offshore transport). The wave-related sand transport is still very difficult to predict due to the complexity of its underlying processes, which mainly take place in a thin layer near the sea bed in the wave boundary layer (thickness of order centimeters). The development of models for cross-shore sand transport heavily relies on experimental lab research, especially as taking place in large oscillating water tunnels (see, e.g., Nielsen, 1992). In oscillating water tunnels the near-bed horizontal orbital velocity, as induced by short gravity waves, can be simulated above fixed or mobile sandy beds (for a detailed description, see, e.g., Ribberink and Al-Salem, 1994). It should be realized that the vertical orbital flow and relatively small wave-induced residual flows as streaming and drift are not reproduced in flow tunnels. Research aimed at their contribution to the net sediment motion under surface waves is still ongoing (see Ribberink et al., 2000).
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509 citations
TL;DR: In this paper, U-tube measurements of instantaneous velocities, concentrations, and fluxes for a well-sorted, medium-sized sand in oscillatory sheet flow are analyzed.
Abstract: [1] U‐tube measurements of instantaneous velocities, concentrations, and fluxes for a well‐sorted, medium‐sized sand in oscillatory sheet flow are analyzed. The experiments involved two velocity‐asymmetric flows, the same two flows with an opposing current of 0.4 m/s, and a mixed skewed‐asymmetric flow, all with a velocity amplitude of 1.2 m/s and flow period of 7 s. We find that the net positive transport rate beneath velocity‐ asymmetric oscillatory flow results from large, but opposing sand fluxes during the positive and negative flow phase. With an increase in velocity asymmetry and, in particular, velocity skewness, the difference in the magnitude of the fluxes in the two half cycles increases, leading to larger net transport rates. This trend is consistent with the observed increase in skewness of the oscillatory bed shear stress. Phase‐lag effects, whereby sand stirred during the negative flow phase has not settled by the time of the negative‐to‐positive flow reversal and is subsequently transported during the positive flow phase, are notable but of minor importance to the net transport rate compared to earlier experiments with finer sands. In the vertical, the oscillatory flux is positive above the no‐ flow bed. Within the sheet flow pick‐up layer, the oscillatory flux is negative and similar in magnitude to the positive flux induced by the residual flow. The 0.4 m/s opposing current causes more sand to be picked up during the negative than during the positive flow phase. Above the no‐flow bed the resulting negative oscillatory flux is comparable in magnitude to the current‐related flux.
160 citations
Cites background or methods from "Sand Motion induced by oscillatory ..."
...This means that the experimental conditions fall well within the sheet flow regime [Ribberink et al., 2008]....
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...The positive streaming nearer to the bed (z < 6 mm) can be explained by the fact that, due to velocity skewness, the lowest levels in the sheet flow layer mobilized by the peak positive velocities are not mobilized during negative flow [Ribberink et al., 2008]....
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...The positive streaming nearer to the bed (z 6 mm) can be explained by the fact that, due to velocity skewness, the lowest levels in the sheet flow layer mobilized by the peak positive velocities are not mobilized during negative flow [Ribberink et al., 2008]....
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...The C1 ut(z) resembles the structure measured previously for velocity‐skewed oscillatory flow [Ribberink et al., 2008, Figure 7]....
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TL;DR: A review of the current status of computation of turbulent impinging jet heat transfer is presented in this paper, where the effects of different subgrid scale models, boundary conditions, numerical schemes, grid distribution, and size of the computational domain adopted in various large eddy simulations of this flow configuration are reviewed in detail.
Abstract: A review of the current status of computation of turbulent impinging jet heat transfer is presented. It starts with a brief introduction to flow and heat transfer characteristics of jet impinging flows considering the simplest jet impinging geometry: normal impingement of a single jet into a flat surface. Subsequently, a review of recent computational studies related to the same geometry is presented. The effects of different subgrid scale models, boundary conditions, numerical schemes, grid distribution, and size of the computational domain adopted in various large eddy simulations of this flow configuration are reviewed in detail. A review of direct numerical simulation of the same geometry is also presented. Further, some recent attempts in Reynolds-averaged Navier–Stokes modeling of impinging flows are also reviewed. A review of computation of other complex impinging flows is also presented. The review concludes with a listing of some important findings and future directions in the computation of impi...
130 citations
TL;DR: In this paper, a one-dimensional model solving the Reynolds-averaged Navier-Stokes and advection-diffusion equations in conjunction with a k − e model for turbulence closure is used to systematically explore net sediment transport rates under skewed asymmetric waves under sheet flow conditions.
Abstract: [1] A one-dimensional model solving the Reynolds-averaged Navier-Stokes and advection-diffusion equations in conjunction with a k − e model for turbulence closure is used to systematically explore net sediment transport rates beneath skewed asymmetric waves under sheet flow conditions The model, which reproduces net rates obtained in U tube experiments under purely skewed and asymmetric oscillatory flow well, is forced with an analytical expression of free stream oscillatory flow in which we varied the degree of skewness versus asymmetry, the magnitude of the total wave nonlinearity, the oscillatory flow amplitude, and the wave period We find that for a wide range of conditions sediment entrained into the flow during a particular wave half-cycle has not completely settled before flow reversal and tends to be transported during the next half-cycle Consistent with earlier work, these phase-lag effects reduce transport rates under oscillatory flow dominated by velocity skewness, while they enhance net transport rates under oscillatory flow dominated by velocity asymmetry Phase-lag effects are particularly important in fine-medium sands (<≈250 μm) For a given grain size phase-lag effects become more pronounced with an increase in wave nonlinearity and in velocity amplitude, and with a decrease (increase) in wave period under skewed (asymmetric) oscillatory flow When phase-lag effects are negligible, transport rates are largest under skewed oscillatory flow containing some asymmetry; otherwise, they are largest under asymmetric flow Our work implies that nearshore sediment transport equations based on the instantaneous bed shear stress may be restricted in applicability to situations when the grain size exceeds ≈250 μm
110 citations
Cites background or methods from "Sand Motion induced by oscillatory ..."
...7 m(2)/s(2)) may also increase the importance of phase-lag effects [e.g., Silva et al., 2006; Ribberink et al., 2008], even for relatively large...
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...…and wave-averaged sediment transport rates q [e.g., Davies and Li, 1997; Holmedal and Myrhaug, 2006; Ribberink et al., 2008] and to reproduce trends in q related to grain size, wave period, and orbital velocity magnitude [Ribberink et al., 2008], all under velocity-skewed U tube flow....
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...…shown to predict accurately depth-integrated and wave-averaged sediment transport rates q [e.g., Davies and Li, 1997; Holmedal and Myrhaug, 2006; Ribberink et al., 2008] and to reproduce trends in q related to grain size, wave period, and orbital velocity magnitude [Ribberink et al., 2008], all…...
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...[13] The model used here cannot represent the detailed flow and sediment dynamics in the sheet flow layer [Malarkey et al., 2003; Ribberink et al., 2008]....
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...velocity magnitude [Ribberink et al., 2008], all under velocity-skewed U tube flow....
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TL;DR: It is shown that the motion of a single fluid particle provides a clear manifestation of time irreversibility, and finds that the third moment of the power fluctuations along a trajectory, nondimensionalized by the energy flux, displays a remarkable power law as a function of the Reynolds number.
Abstract: The statistical properties of turbulence differ in an essential way from those of systems in or near thermal equilibrium because of the flux of energy between vastly different scales at which energy is supplied and at which it is dissipated. We elucidate this difference by studying experimentally and numerically the fluctuations of the energy of a small fluid particle moving in a turbulent fluid. We demonstrate how the fundamental property of detailed balance is broken, so that the probabilities of forward and backward transitions are not equal for turbulence. In physical terms, we found that in a large set of flow configurations, fluid elements decelerate faster than accelerate, a feature known all too well from driving in dense traffic. The statistical signature of rare “flight–crash” events, associated with fast particle deceleration, provides a way to quantify irreversibility in a turbulent flow. Namely, we find that the third moment of the power fluctuations along a trajectory, nondimensionalized by the energy flux, displays a remarkable power law as a function of the Reynolds number, both in two and in three spatial dimensions. This establishes a relation between the irreversibility of the system and the range of active scales. We speculate that the breakdown of the detailed balance characterized here is a general feature of other systems very far from equilibrium, displaying a wide range of spatial scales.
94 citations
References
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Book•
01 Jan 1983
TL;DR: In this article, the authors present selected theoretical topics on ocean wave dynamics, including basic principles and applications in coastal and offshore engineering, all from a deterministic point of view, and the bulk of the material deals with the linearized theory.
Abstract: The aim of this book is to present selected theoretical topics on ocean wave dynamics, including basic principles and applications in coastal and offshore engineering, all from the deterministic point of view. The bulk of the material deals with the linearized theory.
2,003 citations
"Sand Motion induced by oscillatory ..." refers background in this paper
...Considerable theoretical knowledge exists about wave propagation in shallow water, wave-induced orbital flows and mean currents, mass transport by waves and wave boundary layers [1, 6, 34]....
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Book•
01 Jan 1993
1,962 citations
"Sand Motion induced by oscillatory ..." refers background or methods in this paper
...The sediment (volume) concentration is calculated with a diffusion model, assuming εs = β · υt, with different models being proposed for the proportionality factor β [56]....
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...where θ cr represents the critical Shields parameter for initiation of sediment motion [56]....
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Book•
21 Jul 1992
TL;DR: In this article, a review of bottom boundary layer flows including the boundary layer interaction between waves and steady currents is presented, and the concept of eddy viscosity for these flows is discussed in depth because of its relation to sediment diffusivity.
Abstract: This book is intended as a useful handbook for professionals and researchers in the areas of Physical Oceanography, Marine Geology, Coastal Geomorphology and Coastal Engineering and as a text for graduate students in these fields. With its emphasis on boundary layer flow and basic sediment transport modelling, it is meant to help fill the gap between general hydrodynamic texts and descriptive texts on marine and coastal sedimentary processes. The book commences with a review of coastal bottom boundary layer flows including the boundary layer interaction between waves and steady currents. The concept of eddy viscosity for these flows is discussed in depth because of its relation to sediment diffusivity. The quasi-steady processes of sediment transport over flat beds are discussed. Small scale coastal bedforms and the corresponding hydraulic roughness are described. The motion of suspended sand particles is studied in detail with emphasis on the possible suspension maintaining mechanisms in coastal flows. Sediment pickup functions are provided for unsteady flows. A new combined convection-diffusion model is provided for suspended sediment distributions. Different methods of sediment transport model building are presented together with some classical models.
1,311 citations
"Sand Motion induced by oscillatory ..." refers background or methods or result in this paper
...In general, due to the complexity of the flow dynamics near the sea bed, model development strongly relies on experimental laboratory research, especially in large oscillating water tunnels and large wave channels [6]....
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...The ripple height can be estimated using a ripple-height predictor [6, 41]....
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...They used an analytical solution of the advection–diffusion equation (Equation (12)) with constant diffusion coefficient, as found earlier by Nielsen (see [6]), to obtain the exponential time-averaged concentration profile (Equation (10)) with concentration decay length Rc = εs/Ws (stirring height of sediment)....
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...At high mobility number the ripple dimensions reduce and a transition takes place to the flat bed sheet-flow regime [6, 40]....
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...It has therefore been argued [6] that both convective and diffusive mechanisms are involved in the sand entrainment process and that a combined description of these processes is required....
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TL;DR: In this article, a general theory of mass transport is developed, which takes account of the viscosity, and leads to results in agreement with observation, and is shown that the nature of the motion in the interior depends upon the ratio of the wave amplitude a to the thickness δ of the boundary layer.
Abstract: It was shown by Stokes that in a water wave the particles of fluid possess, apart from their orbital motion, a steady second-order drift velocity (usually called the mass-transport velocity). Recent experiments, however, have indicated that the mass-transport velocity can be very different from that predicted by Stokes on the assumption of a perfect, non-viscous fluid. In this paper a general theory of mass transport is developed, which takes account of the viscosity, and leads to results in agreement with observation. Part I deals especially with the interior of the fluid. It is shown that the nature of the motion in the interior depends upon the ratio of the wave amplitude a to the thickness $\delta $ of the boundary layer: when a$^{2}$/$\delta ^{2}$ is small the diffusion of vorticity takes place by viscous 'conduction'; when a$^{2}$/$\delta ^{2}$ is large, by convection with the mass-transport velocity. Appropriate field equations for the stream function of the mass transport are derived. The boundary layers, however, require separate consideration. In part II special attention is given to the boundary layers, and a general theory is developed for two types of oscillating boundary: when the velocities are prescribed at the boundary, and when the stresses are prescribed. Whenever the motion is simple-harmonic the equations of motion can be integrated exactly. A general method is described for determining the mass transport throughout the fluid in the presence of an oscillating body, or with an oscillating stress at the boundary. In part III, the general method of solution described in parts I and II is applied to the cases of a progressive and a standing wave in water of uniform depth. The solutions are markedly different from the perfect-fluid solutions with irrotational motion. The chief characteristic of the progressive-wave solution is a strong forward velocity near the bottom. The predicted maximum velocity near the bottom agrees well with that observed by Bagnold.
1,186 citations
Book•
01 Nov 1992
TL;DR: The main objective of as mentioned in this paper is to describe from a deterministic point of view the sediment transport in the general wave-current situation, which is useful for students with a background in basic hydrodynamics.
Abstract: The main objective of the book is to describe from a deterministic point of view the sediment transport in the general wave-current situation. For this purpose, the book is divided into two major parts. The first part of the book is related to flow and turbulence in combined wave-current. This part covers the turbulent wave boundary layer, bed friction in combined wave-current motion, turbulence in the surf zone, and wave-driven currents in the long- and cross-shore direction. The second part treats the sediment transport as a result of the wave-current action. This part includes an introduction to basic sediment transport concepts, distribution of suspended sediment in the sheet flow regime, description of bedforms formed by current and waves, and their influence on sediment transport pattern. Finally, the modelling of cross- and long-shore sediment transport is described. This book is useful for students with a background in basic hydrodynamics.
926 citations
"Sand Motion induced by oscillatory ..." refers background in this paper
...For fully turbulent flow over a rough wall the maximum phase lead reduces in the Reynolds number range 105< Re < 106 from 45 degrees to a minimum of 10 degrees [30]....
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