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Comparison between the Q criterion and Rortex in the application of an in-stream structure

The scouring process near spur dikes could jeopardize the stability of riverbanks. Thus, accurate estimation of the maximum scour depth near spur dikes is crucial in river engineering. Howe...

Prediction of Maximum Scour Depth near Spur Dikes in Uniform Bed Sediment Using Stacked Generalization Ensemble Tree-Based Frameworks

Computational modeling of open-channel flows is usually carried out using the rigid-lid (RL) assumption to prescribe the water surface. Such an approach deems more appropriate for sub-critical flow regimes with Froude number ≪ 1. When the Froude number is locally high, e.g. flows around hydraulic structures, the free-surface fluctuations affect the induced hydrodynamics. Indeed, prior investigations on the effect of the RL assumption on large-eddy simulation (LES) of open-channel flows around bridge abutments revealed that such an assumption may influence both the first and second order turbulence statistics. In this work, we investigate the effect of the RL assumption on bed-morphodynamics calculations. The fully coupled flow and bed morphodynamics virtual flow simulator (VFS-Geophysics) model is herein employed to numerically investigate the sediment transport around a laboratory-scale model of abutment. Numerical simulations using a mass-conserving level-set (LS) method and a RL assumption are performed to evaluate the influence of free-surface elevation on the computed bed-morphodynamics at a Reynolds number of 7.9 × 104. We also conducted an experimental study to observe the scour development around the abutment model. The instantaneous scour patterns and bed profiles computed with the LS and RL are compared with the measurements. The mean absolute errors of bed-profile predictions at or near quasi-equilibrium of bed evolution, for the LS and RL computed results, are about 4.1% and 4.7%, respectively. Despite the differences in flow field computations of the two methods, the computed bed morphology appeared relatively insensitive to the two numerical approaches. However, the computational cost of the coupled LES-LS-morphodynamics is at least one order of magnitude greater than that of the LES-RL-morphodynamics.

https://par.nsf.gov/servlets/purl/10158223

A comparative study of rigid-lid and level-set methods for LES of open-channel flows: morphodynamics

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2019WR026720

Impacts of Bridge Piers on Scour at Downstream River Training Structures: Submerged Weir as an Example

River bank erosion can upset the balance of the river and its aquatic habitat, as well as the lands and facilities adjacent to the river. As hydraulic structures, spur dikes are constructed perpendicularly to erodible river banks in an attempt to divert the flow toward the middle of the channel as a measure for reducing and controlling river bank erosion. The expansion of the scour hole at the tip of the spur dike can lead to structure failure. Inclining the horizontal crest of the rectangular spur dikes (called triangular-shaped spur dike) can reduce the scouring at their tips. There are limitations on the design criteria for this newly-introduced structure. In the present study, the scour patterns developed around triangular spur dikes were examined under different hydraulic conditions and compared these patterns with those obtained for common type of spur dikes (rectangular spur dikes). The three-dimensional velocity components around triangular and rectangular spur dikes were also measured. According to the results, maximum scour hole depth and volume were smaller in the triangular spur dikes than in their rectangular counterparts. Quantitatively, on average, maximum scour hole depth and volume in the former were, respectively, 44% and 70% less than those in the latter. Based on the analyzed the flow patterns, it is recommended that the spacing between the triangular spur dikes should not exceed 5.5 times the effective length of the structure.

Study of Scour and Flow Patterns around Triangular-Shaped Spur Dikes

Experimental Investigation of Three-Dimensional Flow Structure and Turbulent Flow Mechanisms Around a Nonsubmerged Spur Dike With a Low Length-to-Depth Ratio

In this study we carried out laboratory experiments to investigate the three-dimensional turbulent flows around a spur dike installed in a straight open channel flume. The experiments are conducted under the two different Froude numbers, 0.100 and 0.185. The three-dimensional instantaneous velocities are measured using the Acoustic Doppler Velocimetry (ADV) to obtain the time-averaged velocities and the turbulence stresses. The measured flow field reveal the existence of the recirculation zones downstream of the dike, which is characterized by high turbulence stresses near its boundaries. The results show that although the overall mean flow patterns between the low and high Froude number cases are very similar to each other, there exist moderate changes in the maximum dimensionless turbulence stresses and the maximum dimensionless bed shear stress with the increase of the Froude number.

Flume experiments for turbulent flow around a spur dike

A transcritical flow occurs when the width and slope of a channel are varying abruptly. In this study, the transcritical flow in a two-dimensional open channel is analyzed by using the shallow-water equations. A weighted average flux scheme that has flux limiter with a total variation diminishing condition is introduced for a second-order accuracy in time and space, and non- spurious oscillations at discontinuous points. A HLLC method with three wane speeds is employed to calculate the Riemann problem. To overcome difficulties resulting from variation of channel sections in a two-dimensional analysis of transcritical flow, the numerical model is developed based on a generalized grid system.∏舀㘲ㄮ㌶〵㬏A灰汩敳⁰桹獩捳

Jeongsook Jeon

Papers

Experimental Investigation of Three-Dimensional Flow Structure and Turbulent Flow Mechanisms Around a Nonsubmerged Spur Dike With a Low Length-to-Depth Ratio

Flume experiments for turbulent flow around a spur dike

A Numerical Analysis of a Discontinuous Flow with TVD Scheme