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1. An Eulerian two-phase model investigation on wave-induced scour around a vertical circular cylinder

   Benjamin Tsai, Antoine Mathieu (September 2023, International Conference on Scour and Erosion)

   A three-dimensional Eulerian two-phase flow solver, SedFoam, has been developed for various sediment transport applications. The solver has demonstrated success in modeling sheet flow and bedforms driven by oscillatory flows using a Reynolds-averaged Navier–Stokes (RANS) formulation. However, the accuracy of the RANS formulation for more complex flows, such as scour around structures, requires further evaluation. SedFoam has recently been enhanced to incorporate two-phase large-eddy simulation (LES) capability. In this study, RANS and LES approaches are tested via a three-dimensional case of wave-induced local scour around a single vertical circular pile. Two laboratory experiments, one with an erodible bed and the other with a rigid bed, were chosen for simulation, with both experiments having a Keulegan-Carpenter (KC) number of 10. The k-ω turbulence closure was selected for the RANS simulation, and the dynamic Lagrangian subgrid closure was chosen for the LES simulation. Numerical results reveal that both RANS and LES simulations can resolve lee-wake vortices, although the vortices are significantly weaker in the RANS simulation. In comparison with the LES results, the RANS approach fails to predict horseshoe vortex with sufficient intensity, leading to an underestimation of scour hole depth development. Although the scour depths develop at a very similar rate in the early stage, the scour depth predicted by the RANS simulation quickly reaches equilibrium, while the LES simulation follows the measured trend. These findings indicate that a turbulence-resolving methodology, i.e. LES, is necessary for accurate scour simulations. 

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2. Three-phase flow simulation of local scour around a submerged horizontal cylinder

   Huang, Shijie; Huang, Zhenhua (October 2020, Proceedings of the Thirtieth (2020) International Ocean and Polar Engineering Conference)

   null (Ed.)

   Wave-induced scour plays a key role in the stability analysis of coastal structures, submarine pipelines or cables. There is a rich literature in current-induced scour, but more research is needed to understand the characteristics of wave-induced scour and the mechanisms that are important to the scour process. Sediment transport and flow-induced scour are three-phase (air-water-sediment) flow problems in nature and multi-phase flow simulation is a useful tools that can provide information difficult to obtain from physical tests. Most existing numerical models developed for simulating local scours are based on one-way coupling, which neglects effects of sediment phase on hydrodynamics of the flow. The present study uses a three-phase (air, water and sediment) flow model, which allows for a two-way coupling, to simulate wave-induced local scour problems. The three-phase flow model captures the air-water interface using a modified VOF method, and uses an improved rheology for the sediment phase for better results. The model is validated and verified using one set of existing experiment results for local scour around a submerged horizontal pipe. The detailed flow fields of both the sediment phase and the water phase around the scour are analyzed to understand the scour process. All three-phase flow simulations flow simulations on XSEDE’s Stampede2 supercomputers. The applicability of the model to other local scour problems is also discussed. 

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3. Can environmental flows moderate riparian invasions? The influence of seedling morphology and density on scour losses in experimental floods

   https://doi.org/10.1111/fwb.13235  

   Kui, Li; Stella, John C.; Diehl, Rebecca M.; Wilcox, Andrew C.; Lightbody, Anne; Sklar, Leonard S. (December 2018, Freshwater Biology)

   Abstract Environmental flow releases are an effective tool to meet multiple management objectives, including maintaining river conveyance, restoring naturally functioning riparian plant communities, and controlling invasive species. In this context, predicting plant mortality during floods remains a key area of uncertainty for both river managers and ecologists, particularly with respect to how flood hydraulics and sediment dynamics interact with the plants’ own traits to influence their vulnerability to scour and burial.To understand these processes better, we conducted flume experiments to quantify different plant species’ vulnerability to flooding across a range of plant sizes, patch densities, and sediment condition (equilibrium transport versus sediment deficit), using sand‐bed rivers in the U.S. southwest as our reference system. We ran 10 experimental floods in a 0.6 m wide flume using live seedlings of cottonwood and tamarisk, which have contrasting morphologies.Sediment supply, plant morphology, and patch composition all had significant impacts on plant vulnerability during floods. Floods under sediment deficit conditions, which typically occur downstream of dams, resulted in bed degradation and a 35% greater risk of plant loss compared to equilibrium sediment conditions. Plants in sparse patches dislodged five times more frequently than in dense patches. Tamarisk plants and patches had greater frontal area, larger basal diameter, longer roots, and lower crown position compared to cottonwood across all seedling heights. These traits were associated with a 75% reduction in tamarisk seedlings’ vulnerability to scour compared to cottonwood.Synthesis and applications. Tamarisk's greater survivability helps to explain its vigorous establishment and persistence on regulated rivers where flood magnitudes have been reduced. Furthermore, its documented influence on hydraulics, sediment deposition, and scour patterns in flumes is amplified at larger scales in strongly altered river channels where it has broadly invaded. Efforts to remove riparian vegetation using flow releases to maintain open floodways and/or control the spread of non‐native species will need to consider the target plants’ size, density, and species‐specific traits, in addition to the balance of sediment transport capacity and supply in the river system. 

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4. Experimental study of bed evolution around a non-slender square structure under combined solitary wave and steady current actions

   https://doi.org/10.1016/j.oceaneng.2022.112792  

   Sogut, E. (December 2022, Ocean engineering)

   This paper presents the findings from the laboratory wave flume experiments designed to investigate the formation and evolution of scour around a non-slender, square vertical structure, under three flow conditions, solitary wave, combined solitary wave and steady following current, and combined solitary wave and steady opposing current. The structure was placed on a sandy berm, either fastened to the flume wall or positioned at the centerline of the flume. For the wave only case, the scour on the seaside edge turned out to be deeper than the one on the leeside regardless of the structure’s position. The analyses showed that the depth, width, volume, and location of the scour were all significantly influenced by the introduction of steady currents. The following current, for example, deepened the seaside scour, while leading to shallower leeside scour holes as a result of the backfilling process. Contrary to the opposing current, which shifted the scour area in the upwave direction, the scour was transported downwave under the effect of the following current. The scour depth was determined to be a function of the structure position and the Keulegan-Carpenter number, whereas the scour width mostly depended on the structure’s position. In this regard, the structure fastened to the wall experienced the widest scour area and the largest volume regardless of the flow condition. 

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5. On the Morphodynamics of a Wide Class of Large‐Scale Meandering Rivers: Insights Gained by Coupling LES With Sediment‐Dynamics

   https://doi.org/10.1029/2022MS003257  

   Khosronejad, Ali; Limaye, Ajay B.; Zhang, Zexia; Kang, Seokkoo; Yang, Xiaolei; Sotiropoulos, Fotis (March 2023, Journal of Advances in Modeling Earth Systems)

   Abstract In meandering rivers, interactions between flow, sediment transport, and bed topography affect diverse processes, including bedform development and channel migration. Predicting how these interactions affect the spatial patterns and magnitudes of bed deformation in meandering rivers is essential for various river engineering and geoscience problems. Computational fluid dynamics simulations can predict river morphodynamics at fine temporal and spatial scales but have traditionally been challenged by the large scale of natural rivers. We conducted coupled large‐eddy simulation and bed morphodynamics simulations to create a unique database of hydro‐morphodynamic data sets for 42 meandering rivers with a variety of planform shapes and large‐scale geometrical features that mimic natural meanders. For each simulated river, the database includes (a) bed morphology, (b) three‐dimensional mean velocity field, and (c) bed shear stress distribution under bankfull flow conditions. The calculated morphodynamics results at dynamic equilibrium revealed the formation of scour and deposition patterns near the outer and inner banks, respectively, while the location of point bars and scour regions around the apexes of the meander bends is found to vary as a function of the radius of curvature of the bends to the width ratio. A new mechanism is proposed that explains this seemingly paradoxical finding. The high‐fidelity simulation results generated in this work provide researchers and scientists with a rich numerical database for morphodynamics and bed shear stress distributions in large‐scale meandering rivers to enable systematic investigation of the underlying phenomena and support a range of river engineering applications. 

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