Self-amplifying density waves in hydraulic transport pipelines is a scarcely researched topic. Density waves are in essence the result of a spatial redistributing effect and clustering of solids in hydraulic transport pipelines. Self-amplifying density waves are very undesirable for practical applications, as these waves increasing the risk of pipeline blockages. The two available experimental studies (Talmon et al., 2007; Matoušek and Krupička, 2013) report conflicting properties of the density waves, such as wave length and wave celerity. This new experimental research aims to shed light on the reported differences, by broadly varying particle size and concentration in a new dedicated experiment. The main highlight of this research is that two separate mechanisms were identified that can cause density waves, and Talmon et al. (2007) and Matoušek and Krupička (2013) in hindsight were studying the two different mechanism respectively. Both wave type mechanisms come into effect at mixture velocities close to the deposit limit velocity, and require a stationary bed layer to initiate. The first mechanism is caused by an imbalance of erosion and sedimentation of the bed layer, which is predominant for fine sand particles (∼242μm and ∼308μm in this research). The second mechanism occurs when the bed layer starts sliding, instead of being eroded, and is specific for larger sand sizes (∼617μm and ∼1.08mm in this research). These two mechanisms are clearly distinguishable, having different wave lengths, celerity, amplitudes and amplification rates. The results also show a clear relationship between the mean concentration of a density wave, the wave amplitude and wave celerity specific for each of the two mechanisms.

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This article presents the current state-of-the-art understanding of underwater dilative slope failure (breaching). Experimental investigations are reviewed, providing critical insights into the underlying physics of breaching and pointing out knowledge gaps, which underscore the need for further research. Besides, field observations at several locations across the globe are outlined, highlighting the hazard of breaching and the need for effective coastal management strategies to mitigate the associated risks. Furthermore, existing methods for analyzing and predicting the slope failure evolution are discussed and reflected upon, including analytical approaches and numerical models, ranging from simplified 1D models to advanced 3D coupled flow-soil approaches. Lastly, open questions are posed and key future directions are identified to enhance our understanding of the breaching failure. Overall, this review paper provides a valuable resource for researchers and decision makers involved in slope stability and flow slide risk assessment.@en

A common way to transport solids in large quantities is by using a carrier fluid to transport the solids as a concentrated solid/liquid mixture or slurry through a pipeline. Typical examples are found in dredging, mining and drilling applications. Dependent on the slurry properties and flow conditions, horizontal slurry pipe flow is either in the fixed-bed, sliding-bed or fully-suspended regime. In terms of non-dimensional numbers, the flow is fully characterized by the bulk liquid Reynolds number (Re), the Galileo number (Ga, a measure for the tendency of particles to settle under gravity), the solid bulk concentration (ϕ_b), the particle/fluid density ratio (ρ_p/ρ_f), the particle/pipe diameter ratio (D_p/D_pipe), and parameters related to direct particle interactions such as the Coulomb coefficient of sliding friction (μ_c). To further our fundamental understanding of the flow dynamics, we performed experiments and interface-resolved Direct Numerical Simulations (DNS) of slurry flow in a horizontal pipe. The experiments were performed in a transparent flow loop with D_pipe=4 cm. We measured the pressure drop along the pipeline, the spatial solid concentration distribution in the cross-flow plane through Electrical Resistance Tomography (ERT), and used a high-speed camera for flow visualization. The slurry consisted of polystyrene beads in water with D_p=2mm, ρ_p/ρ_f=1.02, Ga between 40–45 and ϕ_b between 0.26–0.33. The different flow regimes were studied by varying the flow rate, with Re varying from 3272 till 13830. The simulations were performed for the same flow parameters as in the experiments. Taking the experimental uncertainty into account, the results from the DNS and the experiments are in reasonably good agreement. The results for the pressure drop agree also fairly well with popular empirical models from literature. In addition, we performed a parametric DNS study in which we solely varied Re and Ga. In all flow regimes, a secondary flow of Prandtl's second kind is present, ascribed to the presence of internal flow corners and a ridge of densely packed particles at the pipe bottom during transition towards the fully-suspended regime. In the bulk of the turbulent flow above the bed, secondary flow transport of streamwise momentum dominates over turbulent diffusion in regions where the secondary flow is strong and vice versa where it is weak. The transition between flow regimes appears to be governed by the competition between the net gravity force on the particles and shear-induced particle migration from particle–particle interactions. This competition can be expressed by the Shields number, θ. For θ≲0.75, gravity is dominant and the flow is in the fixed-bed regime. For θ≳0.75, shear-induced migration becomes progressively more important for increasing θ. Low-concentration zones flanking the sliding bed start to form at the top corners of the bed, and gradually expand downwards along the pipe wall till the pipe bottom is reached. For θ≳1.5, shear-induced migration is responsible for lifting the particle bed away from the wall, associated with the onset of the suspended regime. For θ≫1, gravity is of minor importance and the mean flow eventually reaches axi-symmetry with a high-concentration particle core at the pipe center and negligible secondary flow.

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Hydraulic transport pipelines in the dredging, mining and deep sea mining are designed using steady-state methods. However, these methods cannot predict density wave formation. Density waves form a risk for pipeline blockages, therefore there is a need to understand and preferably be able to model the process. The density waves studied in this research are caused by a stationary sediment deposit in the pipeline. This article explores the development of a new transient design model, based on 1-dimensional-two-layer Driftflux CFD. The two layers model the exchange of sediment between the turbulent suspension, and a stationary bed layer, and can therefore model density wave amplification. An empirical erosion-sedimentation closure relationship is applied to model the sediment exchange between the two layers, and is calibrated using experiments. The final model is also validated against a second experiment, specifically for density wave amplification. The experiments and the model show good agreement on the erosion of a stationary bed layer and the growth rate of a density wave and the amplitude of the density wave.

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We have developed and investigated a hydrodynamic model of Deep-Sea Mining (DSM) collector turbidity flows that captures sediment particle aggregation and breakup. Flocculation is expected to have a significant impact on determining the spread patterns of the turbidity flows and the resulting turbidity currents. The recently validated drift-flux model by Elerian et al. (2022) has been coupled to the Population Balance Equation (PBE) for modelling real-life discharge scenarios. This advanced approach accounts for the dynamics of flocculation and offers a comprehensive simulation of discharge systems. We hypothesize that this will produce a more accurate representation of DSM turbidity flows in the near-field region, where the turbulence mixing is expected to be the highest. Particular emphasis is placed on the settling velocity closure, as the flocs that form are porous and have a complex geometry. The flocculation parameters are calibrated using the experiments of Gillard et al. (2019). Finally, we investigate the effect of flocculation in the near-field region by numerically solving the new model in a computational domain of the near-field region. The results indicate that aggregation is the primary mechanism, however, it does not have a visible impact on the turbidity flow in the immediate vicinity, but it is likely to have a substantial effect on the far-field region.

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When dredging rock using a Cutter Suction Dredger the high amount of spillage is problematic, since it prevents an energy efficient removal process. This papers presents a coupled DEM-FVM method to simulate spillage, that can be used for optimizing the design and working method of the Cutter Suction Dredger. In these simulations, the challenge was to model relatively large particles in a complex and rotating geometry. To ensure stability and reduce computational time we used smoothing kernels to map the forces and the concentration between the discrete elements and the fluid mesh. The method is validated for the fluid flow in the rotating cutter head. This model incorporates all physical processes to predict flow induced spillage in cutter heads within feasible calculation times.@en

As a result of the dilation of soil matrix, dense submarine sand slopes can temporarily be steeper than the natural angle of repose. These slopes gradually fail by the detachment of individual grains and intermittent collapses of small coherent sand wedges. The key question is whether steep disturbances in a submarine slope grow in size (destabilizing breaching) or gradually diminish (stabilizing breaching) and thereby limit the overall slope failure and resulting damage. The ability to predict whether the breaching failure is stabilizing or destabilizing is also crucial for the assessment of safety of submarine infrastructure and hydraulic structures located along rivers, lakes, and coasts. Through a set of large-scale laboratory experiments, we investigate the validity of an existing criterion to determine the failure mode of breaching (i.e., stabilizing or destabilizing). Both modes were observed in these experiments, providing a unique set of data for analysis. It is concluded that the existing method has limited forecasting power. This was quantified using the mean absolute percentage error, which was found to be 92%. The reasons behind this large discrepancy are discussed. Given the complexity of the underlying geotechnical and hydraulic processes, more advanced methodologies are required.@en

The hydraulic transport of sediments in sediment–water multiphase mixtures is an important process in nature and many industrial applications. The flows are characterized by complex transient phenomena, in which the overall system scale and the particle scale are equally important. Experimental research into dense mixture flows is focused on measurement of flowrates, differential pressures and concentrations of the suspended sediments. Concentration measurements are especially challenging in the case of coarse particles (beyond millimeter size scale) flowing in dense mixtures, limiting the range of available sensors for accurately measuring the in-situ solids concentrations. For the investigation of transient processes, a quick sensor response is required, which makes concentration measurement based on mixture conductivity an interesting option. This study is focused on combined concentration and pressure measurements in dense sediment–water mixtures with coarse particles in a vertically oriented closed conduit, using differential pressure sensors over the vertical segments and conductivity probes for measuring the mixture concentration. We experimentally investigated the dispersion process of an initially densely packed batch of sand and gravel by measuring the concentration on different segments of the conduit, resulting in data on mixture wall shear stresses for different sand and gravel mixtures and data of attenuation of concentration gradients in vertical upward and downward flow, in the conduit horizontal top section and in the centrifugal pump. We describe in the detail the sensor calibration and data processing method, giving a best practice for the use of conductivity concentration sensors in dense coarse particle mixtures, and we suggest a novel method for analysis of density wave amplification and attenuation based on concentration measurements in general, which allows for the detailed analysis of transient multiphase flow phenomena at pipe system component level.

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Renewable energy installations and energy storage solutions require significant quantities of critical raw materials such as nickel, cobalt and rare earth metals. The supply chains of these raw materials face many difficulties, such as the continuous decrease of mineral ore grades on land. In view of these complications, the motivation to search for new resources has grown, with the deep sea being seen as a potential source of these minerals. Polymetallic nodule mining generates turbidity currents, which could negatively impact the deep-sea environment. For that reason, we investigate this type of current experimentally and numerically in order to characterize the generated turbidity current. Various non-cohesive sediment types, i.e., different particle sizes, and different concentrations are tested using a lock-exchange set-up. Three sediment types (glass beads, silica sand and a 50/50 blend of glass beads and silica sand) with seven initial sediment concentrations are examined. Additionally, for the numerical work, a drift–flux modelling approach is used to simulate the performed lock-exchange experiments. The results show that the front velocities of the currents resulting from the three sediment types increases with increasing initial concentrations inside the lock regardless. Moreover, using the same initial concentration, the difference in front velocities between the generated currents of the three sediment types decreases as the initial concentration increases. When using an initial volumetric concentration of 2.5% and 3%, the difference in front velocities between the generated current of the three sediment types vanishes. Finally, by comparing the numerical and experimental results, the drift–flux model is proven to be a reliable numerical model for predicting the current.

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Dredging is the relocation of soil. Before the soil can be transported, it has to be loosened. This can be done hydraulically (jetting) or mechanically (cutting). Often, water jets are used to erode the soil layer. Over time, pickup functions have been derived to predict the amount of erosion corresponding to the flow conditions. However, existing pickup functions are inaccurate at high flow velocities. During the current study, erosion experiments have been done at high flow velocities (up to 4.7 m/s) corresponding to a bed shear stress of up to 60 Pa and a Shields parameter (θ) of up to 30. The results of these experiments were compared with a number of well-known data sets and pickup functions.

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Density wave amplification in hydraulic transport pipelines forms a high risk to operational continuity, as density waves can lead to system blockages or centrifugal pump drive failures. Recent experimental research, in pipelines which contain long vertical sections, has shown that density waves can amplify at velocities far exceeding the deposit limit velocity, previously thought to be a limiting condition for amplification. The typical design methodology of hydraulic transport pipelines is based on a steady-state philosophy, which assumes that the mixture velocity and sediment concentration are constant in time and space. However, these variations can lead to the amplification of density waves. This article discusses the cause of a new type of density wave amplification mechanism, which is related to slurry dynamics in a pipeline containing vertical sections. This research also presents a 1D Driftflux CFD model which models the aforementioned slurry dynamics and can predict density wave amplification.

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Renewable energy installations and energy storage solutions require a significant amount of critical raw materials such as nickel, cobalt and rare earth elements. The supply chains of these raw materials face many challenges, e.g., these materials are often found at lower grades on land. These complications motivate the search for new resources. Therefore, the deep sea is looked into as a potential source for such minerals. However, sea bed mining is expected to affect the mined area. One of the concerns is the so-called mining-generated turbidity current, which can cause a negative impact on the deep-sea environment. For that reason, in order to characterize the generated turbidity current, we investigate the generated current experimentally, where cohesive and noncohesive sediment types are tested using a lock-exchange set-up. Three non-cohesive sediment types are tested in order to investigate the effect of the particle size and initial concentration on the propagation velocity of the current. Moreover, one cohesive sediment, i.e illite, is used to compare the propagation velocity in both saline and fresh water. Finally, we used flocculating agents as a proxy to biological matter, to test its influence on the flocculation process. The results show that using or generating larger particle sizes effectively results in a reduced propagation velocity of the current. In addition, the propagation velocity increases in case of higher initial concentrations. In case of cohesive sediment, natural flocculation (i.e flocculation without using flocculants ) occurs faster in saline water than the fresh water. Moreover, using organic flocculants would increase the process of the flocs formation, which results in a lower front velocity and an effectively reduced plume dispersion. @en

The traditional design approach of grabs and other bulk handling equipment consists of manufacturing and testing physical prototypes. A novel design approach is to use a co-simulation of MultiBody Dynamics (MBD) and Discrete Element Method (DEM), in which the virtual prototype of a new concept interacts with bulk solids. Therefore, this study develops and validates a full-scale co-simulation that models the grabbing process of cohesive and stress-history dependent iron ore. First, by executing in-situ measurements during the unloading of a vessel, grab-relevant bulk properties of the cargo, such as penetration resistance, are determined. Second, full-scale grabbing experiments are conducted in the cargo hold, which allows the process to be recorded in realistic operational conditions. Third, full-scale co-simulation is set up using the material model that has been calibrated based on an elasto-plastic adhesive contact model. Fourth, the co-simulation is validated by comparing its predictions to experimental data from various aspects, such as the force in cables and the torque in winches. The validated co-simulation proves that the stress-dependent behaviour of cohesive cargo as it interacts with the grab could be captured successfully. Valuable information such as a grab's kinematics and dynamics, as well as the porosity distribution of collected bulk solids, can be extracted from the simulation, supporting engineers to enhance the design and operation of equipment.

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Hydraulic two-phase transport applied in the dredging, mining, and deep-sea mining industries involves the transportation of sand, gravel, polymetallic nodules, or other particulate tailings as a solids phase and water as a liquid phase. Regardless of the type or size of the granular material, the slurry flow is always subject to transient behavior. Most transient behavior can be attributed to the centrifugal pump as variations in pump pressure and mixture velocity over time, but transients can also be caused by microscopic slurry mechanisms, specifically the amplification of density waves in a pipeline. Density wave amplification in horizontal pipelines at mixture velocities just above the deposition limit velocity was reported and researched in the 1990s. New experiments showing a density wave amplification in a system with combined vertical and horizontal pipelines and at mixture velocities far above the deposition limit suggest that another type of density wave amplification mechanism exists. The newly proposed density wave amplification mechanism is hypothesized to be caused by a change in average particle velocity as the slurry flows from a vertical pipe into a horizontal pipe. Density waves that grow too large cause system blockages or possibly a failure of the pump drive. This article considers centrifugal pump-induced transients and density wave amplification effects separately and how these effects influence each other. Three case studies showing density wave amplification are analyzed, one from the literature and two from new data sets. Furthermore, the causes of these transients are discussed, and where possible, solutions are proposed to avoid these undesirable instabilities.

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In dredging applications, deep sea mining and land reclamation projects typically large amounts of sediments are transported through pipes in the form of hyper concentrated (40% sediment or more) sediment-water mixtures or slurries. In this paper it is investigated how well a generic Euler-Euler CFD-model is capable to model velocity, concentration profiles and the pressure gradient of sediment above deposition limit velocity in a pipeline. This Euler-Euler solver treats both phases as a continuum with its own momentum and continuity equations. The full kinetic theory for granular flows is accounted for (no algebraic form is used) and is combined with a buoyant k-ε turbulence model for the fluid phase. The influence of the mesh size has been checked and grid convergence is achieved. All numerical schemes used are of second-order accuracy in space. The pressure gradient was calibrated by adjusting the specularity coefficient in one calibration case and kept constant afterwards. Simulations were carried out in a wide range of slurry flow parameters, in situ volume concentration (9–42%), pipe diameter (0.05–0.90 m), particle diameter (90–440 μm) and flow velocity of (3–7 m/s). The model shows satisfactory agreement to experimental data from existing literature.

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In dredging, high pressure water jets are commonly applied to assist the mobilization of soil. This work considers the excavation of cohesive soil. The key objective is to predict the development of the cavity in the soil as a function of the undrained shear strength, translation velocity and hydrodynamic pressure of a single nozzle. A generic computational fluid dynamics (CFD) model has been developed that captures both the jet flow and the soil failure in a single framework. The results are compared with data from a previous experimental study. The CFD model predicts the cavity dimensions with reasonable accuracy. In addition the model provides detailed data to study the cyclic nature of the soil failure process. The CFD model is promising and can be applied for more complex nozzle configurations to assist the design process of dragheads and improve production estimates.

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The interest in polymetallic nodule mining has considerably increased in the last few decades. This has been largely driven by population growth and the need to move towards a green future, which requires strategic raw materials. Deep-Sea Mining (DSM) is a potential source of such key materials. While harvesting the ore from the deep sea by a Polymetallic Nodule Mining Tool (PNMT), some bed sediment is unavoidably collected. Within the PNMT, the ore is separated from the sediment, and the remaining sediment–water mixture is discharged behind the PNMT, forming an environmental concern. This paper begins with surveying the state-of-the-art knowledge of the evolution of the discharge from a PNMT, in which the discharge characteristics and generation of turbidity currents are discussed. Moreover, the existing water entrainment theories and coefficients are analyzed. It is shown how plumes and jets can be classified using the flux balance approach. Following that, the models of Lee et al. (2013) and Parker et al. (1986) are combined and utilized to study the evolution of both the generated sediment plume and the subsequent turbidity current. The results showed that a smaller sediment flux at the impingement point, where the plume is transformed into a turbidity current, results in a shorter run-out distance of the turbidity current, consequently being more favorable from an environmental point of view. @en

This paper presents a multi-step DEM calibration procedure for cohesive solid materials, incorporating feasibility in finding a non-empty solution space and definiteness in capturing bulk responses independently of calibration targets. Our procedure follows four steps: (I) feasibility; (II) screening of DEM variables; (III) surrogate modeling-based optimization; and (IV) verification. Both types of input parameter, continuous (e.g. coefficient of static friction) and categorical (e.g. contact module), can be used in our calibration procedure. The cohesive and stress-history-dependent behavior of a moist iron ore sample is replicated using experimental data from four different laboratory tests, such as a ring shear test. This results in a high number of bulk responses (i.e. ≥ 4) as calibration targets in combination with a high number of significant DEM input variables (i.e. > 2) in the calibration procedure. Coefficient of static friction, surface energy, and particle shear modulus are found to be the most significant continuous variables for the simulated processes. The optimal DEM parameter set and its definiteness are verified using 20 different bulk response values. The multi-step optimization framework thus can be used to calibrate material models when both a high number of input variables (i.e. > 2) and a high number of calibration targets (i.e. ≥ 4) are involved.

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The computation time of Discrete Element Method (DEM) simulations increases exponentially when particle size is reduced or the number of particles increased. This critical challenge limits the use of DEM simulation for industrial applications, such as powder flow in silos. Scaling techniques can offer a solution to reduce computation time. In this paper, we have developed a hybrid particle-geometric scaling approach with a focus on Elasto-Plastic Adhesive contact models. It established relationships between particle scaling factors and DEM contact input parameters. The isolated effects of varying particle size and geometric dimensions on bulk properties were also evaluated using uniaxial consolidation, static angle of repose, and ring shear tests. This paper shows how the particle scaling can be applied together with geometric scaling to incorporate two important aspects of bulk materials, their Elasto-Plastic behaviour and their cohesive forces.

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An ideal unit of bulk transport or storage equipment is able to handle cohesive iron ore with consistent productivity. In practice, however, uncontrollable bulk property variations affect the productivity. Therefore, it is important to understand the effect of uncontrollable variations on the process. This study quantifies variability and interdependency of bulk property of a range of cohesive iron ore products. Three different laboratory tests relevant to storage and excavation processes are used. Using a multi-variate experimental plan, three influencing characteristics of iron ore – type, moisture content and consolidation state – are included. A stress-history dependent behavior is captured in both the shear and penetration tests, with the results being highly dependent on the pre-consolidation stress. The outcome of this study enables future research on minimizing the effect of uncontrollable bulk properties variability of iron ore and other cohesive materials in the design procedure of transport and storage processes.

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