Tidal stream turbines are becoming an affordable option for harvesting sustainable energy in coastal areas. They can be retrofitted in barrages, providing an integral solution for flood protection and emission-free power generation, within environmental constraints. To optimize the turbine-barrage configuration with respect to these objectives, simulation tools are needed to predict the efficiency of the turbines as well as their impact on the adjacent tidal system. These tools should be based on an accurate representation of the underlying flow processes, which cover a wide range of spatial scales — from meters at the barrage and turbines to tenths of kilometers in the tidal basin. This article presents the development of such a tool by linking an analytical model for turbine fences in barrage gates to a regional flow model. The turbine model is validated with experimental data, and data from a thoroughly monitored tidal energy pilot project. Simulations reveal how clustering the turbines in small arrays can increase their efficiency, owing to array blockage effects, with only little effect on the tidal exchange. We also demonstrate the potential of using turbine fences to manipulate the tidal jet, issued from the barrage, with benefits for coastal — and wildlife protection in the basin. The presented research helps understanding how turbine fences in barrages can be configured with high energy yield and calculated impact to the environment.

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Plastic pollution is a threat for all ecosystems due to its effects on people, animals, and environment (Mai et al., 2020). Rivers are estimated to transport around 0.5 millions tons of plastic per year (Strokal et al., 2023). When plastic enters a river system, it is transported downstream towards the sea but it is also likely to accumulate at specific cross sections and locations, including hydraulic structures (Al-Zawaidah et al., 2021), eventually increasing the risk of floods. Gates, locks, weirs, and bridges are commonly present in rivers and canals and have several functions, including water level reg-ulation, flood safety, and inland water shipping. These can also be found in water treatment plants, hydropower stations as well as debris/plastic collection systems (Honingh et al., 2020). Riverine plastic accumulation is also known to cause geomorphic changes (Al-Zawaidah et al., 2021). In-depth knowledge on how plastic particles accumulate upstream of hydraulic structures is therefore crucial to understand the processes that affect plastic transport, its influence on the safety and functionality of hydraulic structures and their effects on the hydro- and morphody-namic conditions of the flow (Yan Toe et al., 2022). In this research experiments were performed using simplified plastic particles to analyse the processes that lead to the instability of accumulated particles upstream of a simple gate.@en

Knowledge of plastic debris transport mechanism in open waters and its interaction with hydraulic structures (i.e. accumulation and clogging) is of paramount importance for effective waste-removal strategies and sustainable management of plastic debris. To the author’s best knowledge, current models for prediction of plastic debris transport assume a highly simplified geometry, while making use of parameterization of the physical processes, therefore pointing out the need for further research. Herein, the effect of shape and buoyancy on the motion of a single particle was studied employing point-particle approach while the background flow is solved using RANS approach. It is observed that the particles with the same amount of plastic mass but different shape and density showed substantially different behaviors, resulting in different trajectories. Since parametrization and point-particle approach were used, even if the particle size is larger than the mesh size, these preliminary results showed that further validation is required for prediction of accurate trajectory by means of resolved-particle approach.

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Marine biofouling is a major concern in the operational performance of submerged floating tunnels (SFTs). The objective of this research is to extend hydrodynamic conditions in experiments and numerically investigate the effects of marine fouling on the hydrodynamic behavior of SFTs, including flow characteristics and forces on the SFT subject to waves. A sensitivity analysis of roughness parameters including different roughness heights and roughness coverage ratios is carried out. Additionally, the hydrodynamic forces of a roughened SFT with a circular shape and a newly designed parametric shape are compared.

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The modelling of complex free surface flows is challenging due to the mobility and deformability of the interface and air entrainment characteristics, which are highly affected by turbulence. With the framework of Reynolds averaged Navier–Stokes (RANS) models and the volume of fluid (VOF) method, turbulence quantities at the air–water interface tend to be over-estimated. In this study, interfacial turbulence treatment methods including the buoyancy modification model based on the simple gradient diffusion hypothesis (SGDH) and Egorov’s turbulence damping model are investigated. Furthermore, due to the unconditionally unstable characteristics of the standard k-ε turbulence model, the stabilized k-ε turbulence model is applied as a comparison. The turbulence attenuation performance using different interfacial turbulence treatment methods in the vicinity of the interface is compared and discussed for stratified flows and free overflow weirs for aerated and non-aerated nappe scenarios. The turbulence quantities and free surface profile under different flow conditions are validated against experimental data and an analytical model. The results show that for free surface waves, both the SGDH model and the turbulence damping model give strong improvements in turbulence production compared with the standard k-ε model. The SGDH model augments the turbulence kinetic energy (TKE) in the unstable stratification, leading to unphysical behaviour for the partially dispersed and separated flow.

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Compound weirs are used as flow diversion structures by adding additional flow resistance to the anticipated regions at the rivers. Furthermore, they are used for measuring and regulating flow rates accurately over a wide range of flow depths. When they are used for this purpose, the cross sections are generally selected as symmetrical having the lowest level of the weir at the middle to constrain large flow contractions. However, when used as a flow diversion structure, the cross sections should be adjusted for different degrees of flow contractions to satisfy the anticipated amount of flow resistance. The literature includes several studies in which compound weirs are modelled as flow measurement structures. Modelling them with the aim of flow diversion received little interest in literature. The adaptation of previously proposed analytical and experimental weir models has problems especially in cases with high flow contractions. In this study, the free flow over the compound weirs is modelled numerically with the application of flow diversion in mind. The results are validated by comparing them with the surface velocity measurements obtained using Surface Particle Image Velocimetry (SPIV). The study aims at understanding how far upstream flow redistribution takes place, how big the transverse mass fluxes are and how this affects the flow at the weir openings, at various degree of flow contractions. Three compound weir configurations were used: One with high flow contraction and the other two with moderate to low flow contractions. The numerical model is constructed by using a three-dimensional mesh using OpenFOAM CFD solver. A multiphase flow analysis was conducted by using Volume-of-Fluid (VOF) approach. We have applied RANS modelling with k-ω SST turbulence closure. SPIV experiments were conducted in a 3-meter-wide, 20-m-long rectangular horizontal flume at the Water Lab of Delft University of Technology. A camera was mounted over the flume to record the floating particle positions during flow. The results gave the possibility to quantify transverse distribution of mass transfer among the openings at various degrees of horizontal contractions. The initiation of streamline curvature locations at the upstream were labelled such that a comparison was achieved among the configurations. The numerical model results gave the possibility to complement experimental data regarding the effective flow sections at the weir openings. In summary, the numerical model validated by the SPIV measurements helped understanding the behaviour of flow under high horizontal contraction. However, to develop a correction methodology for high contraction for the simplified 1D weir discharge prediction models, the numerical runs should be extended for various configurations and covering the submerged weir flow as well.@en

Marine biofouling is a major concern in the operational performance of submerged floating tunnels (SFTs). The objective of this research is to investigate the effects of marine fouling (represented by surface roughness) on the hydrodynamic behavior of SFTs, including the hydrodynamic forces on the SFT subject to current-only, wave-only, and combined current-wave flow conditions. The effects of increased surface roughness induced by marine fouling on the dynamic response of an SFT are characterized by hydrodynamic force coefficients, including drag and inertia coefficients. At the Water Lab of Delft University of Technology (TU Delft), experiments have been performed in a wave-current flume to compare the SFTs’ behaviors as affected by different roughness characteristics. In addition, a parametric cross-section for an SFT is presented, and the hydrodynamic performance associated with surface roughness effects on the parametric shape and circular SFT cross-section shape are compared. The results show that the parametric shape can effectively reduce the drag coefficient (C_d) under current-only conditions and lower the inertia coefficient (C_m) when waves are present. As roughness height and coverage ratio increase, C_d generally increases while C_m decreases. However, small differences in C_d and C_m can be observed with regard to roughness parameters for wave-only conditions. The Morison coefficients adapted for a marine-fouled SFT measured in the experiments are compared to predictions from engineering standards and are recommended for engineering practice.

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Compound weirs can be used as flexibly adjustable structures to regulate the flow and the flow distribution over the cross-section. Examples are found in rivers to create additional resistance in one side of the river bifurcations. It is done to distribute the discharge among the branches properly. They are placed at the flood plains and become active only during flooding. Therefore, the expected flow type over the weirs is generally submerged, unlike the modular weir flow which has been studied a lot in the literature. In this study, an experimental campaign was conducted to understand how flow over compound weirs consisting of 12 sections, differs from the flow over uniform weirs. The analyses were conducted under modular and submerged weir conditions to gain a comparative understanding. Configurations of the compound weirs are important as they may lead to horizontal and vertical contraction of flow at various degrees. In this study, nine compound weir configurations were used to include flow variety and a wide range of applications. The experiments were conducted in a 3-meter-wide, 20-m-long rectangular horizontal flume at the Water Lab of Delft University of Technology. Flow depths at the upstream and downstream sides of the weirs were recorded along with the flow rates. Six discharge values were used to include the effect of discharge variations in the results. The measurement results were compared with the standard formulas from the literature to estimate flow rates over uniform rectangular sharp-crested weirs. Experimental data showed strong deviations from the model in submerged cases and moderate to slight deviations in modular cases. The observed deviations showed dependence on weir configurations and discharge. This indicates that discharge coefficients and head losses cannot be treated per weir section, but should be considered in interaction with neighboring sections.@en

Gas-mixing is commonly applied in anaerobic digesters, yet the resulting flow and hydraulic mixing are difficult to evaluate because of limited full-scale experimental data and uncertainties in integrating sludge rheological data. This study used computational fluid dynamics (CFD) to assess the impact of treated sludge rheology on flow and mixing characterisation in a full-scale biogas-mixed digester. The CFD model, which was firstly validated using a lab-scale setup, showed that flow and mixing predictions depended on the rheological properties, especially at low shear rates. The predicted dominant shear rate was out of the effective shear-rate range of the Ostwald model, leading to flow and mixing performance overestimation. The results indicated that there are limitations in applying the Ostwald model and the conventional approaches for determining dead-zone. The Herschel-Bulkley model was more appropriate for the prevailing low shear rates and predicted large viscosity gradients in the digester, indicating two distinct compartments with different flow and mixing behaviour based on the gas-sparging height: a plug-flow compartment with dominant vertical convection above, and a dead-zone compartment with considerable segregation below. The results showed that the applied gas-sparging induced insufficient flow and mixing, but contributed to the well-functioning of the digester. To correctly assess flow and mixing, the applied rheological data should be in agreement with the type of sludge that is treated in the digester. Our results indicate that the shear rate in the digester must be increased and various options for achieving this are proposed.

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The effects of surface roughness as induced by marine fouling on the hydrodynamic forces on a submerged floating tunnel (SFT) are experimentally and numerically investigated in detail at Reynolds numbers Re = 8.125 × 10³–5.25 × 10⁴. A sensitivity analysis to different roughness parameters including roughness height, skewness, coverage ratio, and spatial arrangement is performed. In addition, an optimized parametric cross-section for an SFT is proposed, and the hydrodynamic performance of the parametric shape and circular SFT cross-section shape with roughness elements is compared. The pressure distribution along the SFT, flow separation and wake characteristics are analyzed to provide a systematic insight into the fundamental mechanism relating the roughness parameters and flow around an SFT. In order to better understand the nonlinear relationships among structural geometry, roughness parameters, flow states, and structural response, an artificial intelligence method using Random Forest (RF) for feature importance ranking is applied. The results show that with the parametric shape, the hydrodynamic forces on the fouled SFT can be effectively mitigated. The roughness height and coverage ratio affect the equivalent blockage and hence, change flow separation and recirculation length in the wake. Lower skewness of the roughness elements can increase the critical Re by changing the relative roughness parameter. Horizontal arrangement of the roughness elements on an SFT generally results in the largest hydrodynamic forces, compared to staggered and vertical distributions. Throughout the feature importance ranking, the flow regime is found to be the most important feature of the hydrodynamics of the SFT. In addition, the SFT cross-section shape and roughness coverage ratio play a dominant role.

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In order to assess the dynamic performance of a submerged floating tunnel (SFT) subject to flow-induced vibration (FIV) conditions in a practical engineering application, a one-way fluid–structure interaction (FSI) model consisting of multi-scale hydrodynamic solvers combined with the finite element method (FEM) is established. A typical long, large aspect ratio SFT is modeled by coupling tube, joint, and mooring components. The SFT is simulated in the time domain under currents, waves, and extreme events. FIV of SFTs with different cross-section shapes is investigated by analyzing each structure's natural frequencies, hydraulic loading frequency, and dominant modes. The results show that FIV of the SFT tube is dominated by wave conditions. The excitation of the SFT's first dominant mode by a large wave height and period should be avoided. Standing and traveling wave patterns and multi-mode response are observed during extreme events. The hydrodynamic forcing and structural dynamic response of the SFT can be effectively reduced by adopting a parametric cross-section.

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Wave height attenuation in vegetation canopies is often all attributed to the drag force exerted by vegetation, whereas other potential dissipation process is often neglected. Previous studies without vegetation have found that opposing currents can induce wave breaking and greatly increase dissipation. It is not clear if similar process may also occur in vegetation canopies. We conducted systematic flume experiments to show that wave breaking in opposing currents can occur in vegetated flows, but only in submerged canopies with shear currents above vegetation top. Subsequently, we developed a new analytical model to understand and assess the contribution of both drag-induced dissipation in the lower vegetation layer and current-induced breaking in the upper free layer. A new generic drag coefficient relation was applied in the model to quantify drag-induced dissipation with various current-wave combinations. It shows that breaking induced by opposing currents constitutes an essential part (up to 87%) of the total dissipation, which leads to considerably higher dissipation than the cases with following currents. Breaking can occur with various submergence ratios and with small opposing currents in the submerged vegetation field. It indicates that similar breaking process is likely to occur in real vegetation fields. The present study reveals and quantifies the current-induced wave breaking process that has not been reported before, which can improve our understanding of vegetation wave dissipation capacity in field conditions.@en

The ability to estimate the erosion rate along an underwater breach face is crucial to understand the evolution of breaching failure. To this end, breaching erosion models were developed and applied in numerical models. However, these erosion models have never been validated, owing to the scarcity of direct measurements of turbidity currents that accompany breaching. The aim of this study is to evaluate the existing breaching erosion models using direct measurements of recently performed laboratory experiments on breaching flow slides. We found out that the erosion model put forward by Mastbergen & Van Den Berg (2003) provides good agreement with the data and performs better than the one proposed by Van Rhee (2015). The latter tends to overestimate the erosion rate, particularly at steeper slopes.@en

Concepts of a submerged floating tunnel (SFT) for novel sea-crossings have been researched in recent years. An SFT tube should be moored afloat by tensioned mooring systems to maintain the tube position under complex hydrodynamic loads. In-line force is amongst the dominant hydrodynamic parameters in the SFT cross-section design and the mooring system reliability evaluation. Selecting a suitable in-line force computation method is crucial to successful and accurate SFT cross-section optimization. The transition SST model is an effective turbulence transition prediction tool in the boundary layer computation subjected to tidal flow at both low and high Reynolds numbers. Two types of parametric Bézier curves applied in airfoil optimization are used to describe the SFT cross-section. We show that an SFT cross-section described by a leading-edge Bézier-PARSEC (BP) curve has better hydrodynamic performance than a trailing-edge BP curve of equal aspect ratio (AR). To avoid large flow separation, an AR not exceeding 0.47 is recommended. An SFT cross-section design should balance hydrodynamic performance and construction cost. The SFT cross-section with AR = 0.47 using the leading-edge BP curve with fixed clearance has a comparatively small in-line force and a minimum material cost.

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Mangrove vegetation constitutes a natural coastal defence against waves and erosion. Despite their protective role, mangrove ecosystems have experienced continuous degradation over the last decades due to human causes. At retreating mangrove coastlines, bamboo structures are built to create new habitat for mangrove colonization. Existing structures have experienced mixed rates of success due to the lack of a scientific basis in their design. Optimizing future structure designs requires investigating the effect of the bamboo poles on waves. We consequently conducted laboratory experiments to measure wave transformation, hydrodynamic forces, and flow velocities inside cylinder arrays, mimicking bamboo poles, with varying cylinder configurations and orientations. The experiments provided relationships for wave transmission, wave reflection, and the drag coefficients for configurations with volumetric porosities between n = 0.64 − 0.9. Configurations with a small lateral spacing (causing higher blockage) and a relatively longer streamwise spacing (causing less sheltering) exhibit larger forces and dissipation per element. Such arrangements enable optimizing wave dissipation at locations where the wave direction has low variability over the year. Placing the poles horizontally instead of vertically increases the forces and wave dissipation per element in relatively deeper water. Based on the experiments, we developed a conceptual analytical model that predicts wave reflection and dissipation through cylinder arrays, including blockage and sheltering. The model can reproduce the influence of cylinder arrangement on wave transformation, and it suggests that accurate predictions of sheltering and wave reflection are important to find optimal designs. Overall, these results provide useful insights on how to model and optimize the design of structures for mangrove restoration.

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Accurate rheological characterisation of sewage waste activated sludge (WAS) is of high importance for downstream processing related to optimised sludge pumping and mixing, assessment of energy demands and overall process design. However, to elaborate rheological behaviour is often challenging under dynamic operational conditions in practice. In this study, two practical influencing factors were investigated: long-term shearing and temperature. Compared to anaerobic digestate (DGT), concentrated WAS had more complex and stronger thixotropic behaviour. Under the long-term shearing conditions, the sludge thixotropic behaviour was well characterised by two quantified limitation states. Temperature had a striking impact on the rheological properties, which was strongly correlated to solids content and digestion process. The impact discrepancy between the long-term shearing and temperature, implied different mechanisms to shift the equilibrium of hydrodynamic and non-hydrodynamic interactions for structure deformation and recovery. The distinct rheological properties between the two determined states were clearly reflected in pipe flow behaviour, revealing a concrete link between lab-measured sludge rheology and its practical flow performance. The pipe flows were well assessed using the developed Computational Fluid Dynamics model with effective rheological data integration, which is promising for practical design and optimisation of sewage sludge systems.

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Mangrove vegetation provides natural protection against coastal hazards like flooding and erosion. In spite of their economic and societal value, mangrove forests have experienced a worldwide decline due to human activities. Bamboo structures, formed by poles driven into the soil, are being used to create a sheltered environment for mangrove restoration. The lack of design rules for the structures has led to mixed success rates in their implementation. Improving future designs requires a better understanding of how the bamboo poles affect waves and currents. Currents cause drag forces on the poles, which depend on flow acceleration through the elements (blockage), and the distance from wakes of upstream cylinders (sheltering). We developed a model that predicts the bulk drag coefficient of dense arrays of emergent cylinders in a current, including blockage, sheltering and a balance between turbulence production and dissipation. The model could reproduce measured bulk drag coefficients from the literature within a deviation of 20%. The model also showed that anisotropic structures with small spanwise spacing and large streamwise separation maximize the bulk drag coefficient, and the energy dissipation per pole. The application of the model can guide the design of future mangrove restoration efforts.

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