Sheet flow and suspension of sand in oscillatory boundary layers
TL;DR: In this article, time-dependent measurements of flow velocities and sediment concentrations were conducted in a large oscillating water tunnel, aimed at the flow and sediment dynamics in and above an oscillatory boundary layer in plane bed and sheet-flow conditions.
About: This article is published in Coastal Engineering.The article was published on 1995-07-01. It has received 257 citations till now. The article focuses on the topics: Boundary layer & Suspension (vehicle).
Citations
More filters
TL;DR: In this paper, the validity of a bed-load transport formula based on the bed-shear concept of Meyer-Peter and Mueller was investigated for steady unidirectional flows, oscillatory flows and oscillatory flow with superimposed net currents.
362 citations
TL;DR: In this paper, a model for concentrated sediment transport that is driven by strong, fully developed turbulent shear flows over a mobile bed is presented, where balance equations for the average mass, momentum and energy for the two phases are phrased in terms of concentration-weighted (Favre averaged) velocities.
Abstract: A model is presented for concentrated sediment transport that is driven by strong, fully developed turbulent shear flows over a mobile bed. Balance equations for the average mass, momentum and energy for the two phases are phrased in terms of concentration–weighted (Favre averaged) velocities. Closures for the correlations between fluctuations in concentration and particle velocities are based on those for collisional grain flow. This is appropriate for particles that are so massive that their fall velocity exceeds the friction velocity of the turbulent fluid flow. Particular attention is given to the slow flow in the region of high concentration above the stationary bed. A failure criterion is introduced to determine the location of the stationary bed. The proposed model is solved numerically with a finite–difference algorithm in both steady and unsteady conditions. The predictions of sediment concentration and velocity are tested against experimental measurements that involve massive particles. The model is further employed to study several global features of sheet flow such as the total sediment transport rate in steady and unsteady conditions.
223 citations
TL;DR: In this paper, the authors consider a sheet flow in which heavy grains near a packed bed interact with a unidirectional turbulent shear flow of a fluid and employ a relatively simple model of the turbulent shearing of the fluid and use kinetic theory for the collisional grain flow to predict profiles of the mean fluid velocity, the mean particle velocity, and the particle concentration.
Abstract: We consider a sheet flow in which heavy grains near a packed bed interact with a unidirectional turbulent shear flow of a fluid. We focus on sheet flows in which the particles are supported by their collisional interactions rather than by the velocity fluctuations of the turbulent fluid and introduce what we believe to be the simplest theory for the collisional regime that captures its essential features. We employ a relatively simple model of the turbulent shearing of the fluid and use kinetic theory for the collisional grain flow to predict profiles of the mean fluid velocity, the mean particle velocity, the particle concentration, and the strength of the particle velocity fluctuations within the sheet. These profiles are obtained as solutions to the equations of balance of fluid and particle momentum and particle fluctuation energy over a range of Shields parameters between 0.5 and 2.5. We compare the predicted thickness of the concentrated region and the predicted features of the profile of the mean fluid velocity with those measured by Sumer et al. (1996). In addition, we calculate the volume flux of particles in the sheet as a function of Shields parameter. Finally, we apply the theory to sand grains in air for the conditions of a sandstorm and calculate profiles of particle concentration, velocity, and local volume flux.
180 citations
Cites methods from "Sheet flow and suspension of sand i..."
...Sheet flows have been studied in laboratory experiments in flumes or water tunnels by Wilson (1966, 1989), Sawamoto & Yamashita (1987), Asano (1992), Nnadi & Wilson (1992), Ribberink & Al-Salem (1995), and Sumer et al. (1996)....
[...]
TL;DR: In this article, a new empirical equation is presented which characterises the time-dependent concentration profile in the sheet flow layer, and the dependence of both parameters on flow and bed conditions is analysed.
172 citations
TL;DR: In this paper, the authors measured flow velocity and total sediment load in the swash zone on a steep beach using ducted impeller flow meters and capacitance water level probes and found that the measured immersed weight total load transport rate displayed a strong relationship with the time-averaged velocity cubed, which was consistent with equations for both bedload transport and total load transfer under sheet flow conditions.
162 citations
References
More filters
TL;DR: In this paper, Bagnold's energy-based total load sediment transport model is used as a basis for the development of a total load model of time varying sediment transport over a plane sloping bed.
Abstract: Bagnold's energetics-based total load sediment transport model for streams is used as a basis for the development of a total load model of time varying sediment transport over a plane sloping bed. In both the bedload and suspended load, the transport rate vectors are found to be composed of a velocity-induced component directed parallel to the instantaneous velocity vector and a gravity-induced component directed down slope. The model is applied to idealized surfzone conditions, leading to estimates of the local longshore and onshore-offshore sediment transport rates as well as the equilibrium beach slope as a function of the local wave and current conditions. The model is combined with a nonlinear longshore current model and spatially integrated to obtain predictions of the total longshore transport rate as a function of the incident wave conditions. The results support the general form of the wave power equation except that the wave power coefficient is no longer constant but is instead a complex function of the incident wave and beach characteristics.
743 citations
TL;DR: In this paper, random waves normally incident on a dissipative beach induce a variety of cross-shore flows, such as asymmetric oscillatory flow, wave grouping-induced long-wave flow, breaking-induced turbulent flow, and momentum decay-induced undertow.
Abstract: Random waves normally incident on a dissipative beach induce a variety of cross-shore flows, such as asymmetric oscillatory flow, wave grouping-induced long-wave flow, breaking-induced turbulent flow, and momentum decay-induced undertow. These flows are identified, analyzed and hindcasted in a set of laboratory experiments with the aim of revealing the role of each of the flow mechanisms in the two-dimensional case of bar generation on a beach.
384 citations
TL;DR: In this paper, a simple model for the near-bottom combined wave and current flow over a moveable sediment bed is presented, which is an extension of the Grant and Madsen (1979) combined wave-and current model, with moveable bed effects on the physical bottom roughness included as by Grant and MADsen (1982).
Abstract: A simple model for the near-bottom combined wave and current flow over a moveable sediment bed is presented. The model is an extension of the Grant and Madsen (1979) combined wave and current model, with moveable bed effects on the physical bottom roughness included as by Grant and Madsen (1982). The unsteady conservation of fluid momentum and sediment mass equations, which are coupled through an eddy diffusivity closure scheme, are solved for the wave and current velocity profiles, along with the suspended sediment concentration and transport profiles. Sample runs are presented to illustrate the effect of varying wave conditions on the current, the mean sediment concentration, and the mean sediment transport. Results show that for fine to medium sands, self-stratification of the flow can be important during storms owing to the large amount of sediment suspended by the waves and the enhanced turbulent mixing associated with the wave-current interaction. The theory easily can be generalized for inclusion in a full Ekman layer model.
291 citations
Abstract: An experimental study was focused on the process of sediment transport in unsteady flow conditions due to wave action. Wave-induced oscillatory flow conditions near the seabed were simulated at full scale (1:1) in a new large oscillating water tunnel. Two sets of experiments (series A and B) were carried out. During series A, bed forms and wave-cycle averaged suspended sediment concentrations were measured under sinusoidal waves. Series B focused on measurement of the wave-cycle averaged sediment transport rates under regular and random asymmetric oscillatory flows (upper shoreface conditions) and was aimed at the verification of quasi-steady formulas for the description of cross-shore sediment transport.
Bedform dimensions appeared to decrease considerably under the influence of wave asymmetry and wave randomness. Only the measured ripple dimensions under asymmetric waves (series B) showed good agreement with Nielsen's (1979) relations. For most of the series B experiments the bed was plane (sheet flow), the net sediment transport was directed “onshore” and the measured transport rates showed a strong correlation with the velocity moment . An empirical quasi-steady transport model is proposed which is based on the new tunnel data and other existing data sets. The limitations of the quasi-steady model approach became clear in the rippled-bed regime and through the presence of a consistent influence of the wave period in plane-bed conditions. In rippled-bed conditions the suspended concentration profiles followed a negative exponential distribution. For most of the experiments the measured concentration decay length showed a linear relation with the ripple height. In plane-bed/sheet flow conditions the measured suspended concentration profiles followed a negative power function. The power or concentration decay parameter was constant (α ≃ 2.1) for a wide range of velocity conditions. It is suggested that the mobile bed (sheet flow layer) has a strong damping effect on the mixing of suspended sediments (turbulence damping).
262 citations