Bottom Shear Stress Distribution Research Articles

Accuracy of bed-load transport models in eddy-resolving simulations

This work investigates the accuracy of commonly used bed-load transport models when applied in combination with high-resolution Navier-Stokes solvers. Empirical bed-load models predict the transport rate of sediments based on the average bottom shear-stress, while eddy-resolving approaches allow for a space- and time-dependent description of the bottom shear-stress distribution. We discuss the effect that a fine-graining of the stress distribution provided by the flow solver has on the transport model prediction, and we examine the space and time scales at which the averaged values of the transport rate, obtained using the local stress distribution, converge to the transport rate predicted using the average stress. To this aim, we performed Direct Numerical Simulation of a channel flow and used the resulting database to mimic Wall-Resolved and Wall-Modelled Large-Eddy Simulations. We compared the prediction of several bed-load transport models to experimental measurements in order to identify and highlight the limitations that stem from the coupling of these models with eddy-resolving techniques. We find that for small values of the Shields parameter (ratio of viscous and gravitational forces) the fine spatial and temporal resolution of wall-resolved simulations can yield overestimation of the bed-load transport rate; whereas more coarse-grained methods, such as wall-modelled Large Eddy Simulation, result in improved predictions. We also show that a short-time averaging of the force exerted by the fluid on the sediments, which we tested in three different configurations (channel flow with smooth and rough walls and flow over an idealized two-dimensional river dune), improves the accuracy of the bed-load transport predictions, thus providing indications about the flow scales that control the transport process.

Effect of Porous Media on Hydraulic Jump Characteristics by Using Smooth Particle Hydrodynamics Method

In order to understand the effect of porous media on hydraulic jumps, a smoothed particle hydrodynamics (SPH) model is applied to investigate the characteristics of hydraulic jumps interacting with porous media. Various porosities including cases without an obstacle or with a solid obstacle or porous media are considered. The opening of a gate was altered to adjust the hydraulic jump. The conjugate depth ratio, bottom shear stress distribution, and energy dissipation are reported. In the present study, validations are in a good agreement with previous studies. Overall, the result showed that the average error between numerical and experimental data was less than 7.2%. Energy dissipation is compared among cases with three porosities, with and without a solid obstacle. The porosity of 0.68 is found to dissipate more energy than do other porosities. Thus, porous media can be used to enhance energy dissipation of hydraulic jumps in an open channel. In conclusion, the proposed SPH model can simulate the effect of porous media on hydraulic jump characteristics.

Effect of Corrugated Bed on Hydraulic Jump Characteristic Using SPH Method

AbstractA hydraulic jump is a common phenomenon that can be observed in open channel flows such as rivers and spillways. It can cause damage of the downstream bed and bank of the channel due to the process of continuous erosion and degradation. In order to reduce the hydraulic jump destruction, the energy in the hydraulic jump must be dissipated as much as possible. One method to increase dissipation of energy is using a corrugated bed. In order to know the effect of a corrugated bed on the hydraulic jump, a smoothed particle hydrodynamics (SPH) model is applied to investigate the characteristics of hydraulic jumps in various corrugated beds. A variety of corrugated beds that are smooth, triangular, trapezoidal, and sinusoidal are considered. The opening of a gate is changed to adjust the hydraulic jump. The conjugate depth ratio, the jump length, the bottom shear stress distribution, and the energy dissipation are reported. The results of the present study are in a good agreement with previous studies. E...

Using Inclined Walls to Control the Bottom Shear Stress Distribution in an Annular Flume

This study proposes a method to alter secondary flow pattern in a rotating annulus flume and obtain a resulting uniform bottom shear stress distribution along the radial direction by changing the shape of the channel cross section. Two or three wall angles, except for the bottom wall angle, are simultaneously changed up to 25° for different cross-sectional shapes. The flow characteristics in the channel are analyzed using the computational fluid dynamics technique to find the most effective configuration of the walls, including the rotating top ring. The results show that the bottom shear stress decreases and becomes uniform as the outer wall and the top ring angles increase. In the case of the inner wall, the wall angle has little effect on the bottom shear stress. The stress distribution is most uniform when the top ring, the outer wall, and the inner wall angles are 25, 25, and 5°, respectively.

Wave- and current-induced bottom shear stress distribution in the Gulf of Lions

Simulations of both currents and waves were performed throughout the year 2001 to assess the relative contribution of each to their overall erosive potential on the Gulf of Lions shelf. Statistical analysis of bottom shear stress (BSS) was compared to sediment grain-size distribution on the bottom. The hydrodynamic features of the bottom layer coincide with the distribution of surficial sediments, and three areas with different hydro-sedimentary characteristics were revealed. (i) The sandy inner shelf (<30 m) area is a high-energy-wave dominated area but may be subjected to intense current-induced BSS during on-shore winds along the coast and during continental winds mainly in the up-welling cells. (ii) The middle shelf (30–100 m) is a low-energy environment characterised by deposition of cohesive sediments, where the wave effect decreases with depth and current-induced BSS cannot reach the critical value for erosion of fine-grained sediments. (iii) The outer shelf, which has a higher bottom sand fraction than the middle shelf, may be affected by strong south-westward currents generated by on-shore winds, which can have an erosive effect on the fine-grained sediments. Particular attention was paid to features of the current that were found to be predominant on the mid-outer shelf. These currents are strongly dependent on wind direction.

An Eulerian–Eulerian model for gravity currents driven by inertial particles

In this work we introduce an Eulerian–Eulerian formulation for gravity currents driven by inertial particles. The model is based on the equilibrium Eulerian approach and on an asymptotic expansion of the two-phase flow equations. The final model consists of conservation equations for the continuum phase (carrier fluid), an algebraic equation for the disperse phase (particles) velocity that accounts for settling and inertial effects, and a transport equation for the disperse phase volume fraction. We present highly resolved two-dimensional (2D) simulations of the flow for a Reynolds number of Re = 3450 (this particular choice corresponds to a value of Grashof number of Gr = Re 2 / 8 = 1.5 × 10 6 ) in order to address the effect of particle inertia on flow features. The simulations capture physical aspects of two-phase flows, such as particle preferential concentration and particle migration down turbulence gradients (turbophoresis), which modify substantially the structure and dynamics of the flow. We observe the migration of particles from the core of Kelvin–Helmholtz vortices shed from the front of the current as well as their accumulation in the current head. This redistribution of particles in the current affects the propagation speed of the front, bottom shear stress distribution, deposition rate and sedimentation. This knowledge is helpful for the interpretation of the geologic record.

Large-Eddy Simulation of Sediment Transport: Currents over Ripples

A large-eddy simulation has been performed to study the detailed sediment dynamics that occur in channel flow over ripples. The code solves the Navier-Stokes equations and an advection-diffusion equation for the sediment, and has been verified to produce quantitatively accurate hydrodynamic results for the flow domains used in this study. The main features of interest in this flow are seen because of the high resolution of the grid, especially near the ripple boundary. This yields a detailed bottom shear stress distribution, properly identified coherent structures, and resolved sharp gradients in the sediment concentration. Globally, the sediment is carried up into the flow from locations where the shear stress is high—on the upslopes of ripple crests—and is advected downstream by the current and upward by the vertical velocity component. Regions of positive vertical velocity are associated with Gortler vortices, which represent a significant sediment transport mechanism in this flow.

Wild-induced resuspension in a small shallow lake

Resuspension of inorganic sediments in a very shallow Swedish lake is studied using settling sediment traps and measurements of suspended matter. Theoretical aspects of resuspension dynamics is discussed emphasizing special shallow lake aspects. Bottom shear stress distribution is computed for different wind conditions.

Evolution of a classic sand ridge field: Maryland sector, North American inner shelf

ABSTRACTThe ridge and swale topography of the Middle Atlantic Bight is best developed on the Delaware‐Maryland inner shelf. Here sand ridges can be seen in all stages of formation. Several aspects of the ridge field are pertinent to the problem of ridge genesis. The first is ridge morphology. There is a systematic morphologic change from shoreface ridges through nearshore ridges to offshore ridges, which reflects the changing hydraulic regime. As successively more seaward ridges are examined, maximum side slope decreases, the ratio of maximum seaward slope to maximum landward slope decreases, and the cross‐sectional area increases. These changes in ridge morphology with depth and distance from shore appear to be equivalent to the morphologic changes experienced by a single ridge during the course of the Holocene transgression.A second aspect is the change in bottom sediment characteristics that accompanies these large‐scale morphologic changes. Megaripples, sand waves and mud lenses appear in the troughs between nearshore and offshore ridges. These changes indicate that the storm flows which maintain ridges are less frequently experienced in the deeper sector, and that the role of high‐frequency wave surge becomes less important relative to the role of the mean flow component in shaping the sea‐floor.A third aspect is the systematic relationship of grain size to topography. Grain size is 90° out of phase with topography, so that the coarsest sand lies between the axis of the landward trough and the ridge crest, while the finest sand lies between the ridge crest and the axis of the seaward trough. This relationship is characteristic of large‐scale bedforms.Finally, flow was measured and transport calculated on the same ridge during a one‐month period (November 1976). Threshold was exceeded only during storm events. Mean transport was southerly and a little seaward with respect to both the ridge crest and the shoreline. These flow measurements are in conformity with the pattern of smaller bedforms. A 43‐year time series of bathymetric change for this ridge reveals a systematic pattern of landward flank erosion, seaward flank deposition, and seaward crest migration.Sand ridges are considered the consequence of constructive feedback between an initial topography and the resulting distribution of bottom shear stress. The relationship between grain size and topography supports this model, but does not account directly for the oblique angle of the ridge with respect to the coastline. This feature may be due to a more rapid alongshore migration rate of the inshore edge of the ridge than the offshore edge, and the relationship between this migration rate, and the rate of shoreface retreat.

Relationship between sediment resuspension and the statistical frequency distribution of bottom shear stress

Detailed observations of near-bottom turbidity and currents are used to examine the relationship between bottom shear stress and sediment resuspension. Turbidity is related to the statistical frequency distribution of bottom shear stress in a way that implies that a few high-stress events may be responsible for resuspension. Such short-period processes will usually be masked by conventional methods of data handling.