Predictive model of bulk drag coefficient for a nature-based structure exposed to currents.

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.

Characterising the two-phase flow and mixing performance in a gas-mixed anaerobic digester: Importance for scaled-up applications.

This study aimed to characterise the gas-liquid flow and mixing behaviour in a gas-mixed anaerobic digester by improving phase interaction modelling using Computational Fluid Dynamics (CFD). A 2D axisymmetric model validated with experimental data was set up using an Eulerian-Eulerian method. Uncertainty factors, including bubble size, phase interaction forces and liquid rheology were found to significantly influence the flow field. A more reliable and complete validation was obtained by critical comparison and assessment of the referred experimental data, compared to the models reported in other studies. Additionally, justifiable corrections and predictions in detail were obtained. Mixing was evaluated by trajectory tracking of a large number of particles based on an Euler-Lagrange method. The mixing performance approximated to a laminar-flow reactor (LFR) that distinctly deviated from expected continuous stirred tank reactor (CSTR) design, indicating limited enhancement from the applied gas-sparging strategy in the studied digester. The study shows the importance of a proper phase-interaction description for a reliable hydrodynamic characterisation and mixing evaluation in gas-mixed digesters. Validations, bend to experimental data without a critical assessment, may lead to an inaccurate model for further scaled-up applications.

Natural recovery of infiltration capacity in simulated bank filtration of highly turbid waters.

As a consequence of the suspended sediments in river water, cake formation on the streambed and clogging of the aquifer may occur, leading to a decline in the production yield of riverbank filtration systems, particularly in highly turbid river waters. However, naturally occurring flow forces may induce sufficient scouring of the streambed, thereby self-regulating the thickness of the formed cake layer. This study assessed the recovery of the infiltration capacity in a simulated physically clogged riverbank filtration system, due to self-cleansing processes. A straight tilting flume, provided with an infiltration column at the bottom, was used for emulating clogging, infiltration and self-cleansing. Based on the presented research it may be concluded that the infiltration of a mixture of different sediments, as found in natural water bodies, can already be recovered at low shear stresses. Clay and silt behaved very differently, due to the difference in cohesiveness. Clay was found to produce a persistent sticky cake layer, whereas silt penetrated deeper into the bed, both resulting in an absence of infiltration velocity recovery. A cake layer of fine sand sediments was easiest to remove, resulting in dune formation on the streambed. However, due to deep bed clogging by fine sand particles in a coarser streambed, the infiltration velocity did not fully recover. The interaction between mixed suspended sediments (5% clay, 80% silt, and 15% fine sand) resulted in uneven erosion patterns during scouring of the streambed and recovery of the infiltration velocity is low. Altogether it may be concluded that natural recovery of infiltration capacity during river bank filtration of highly turbid waters is expected to occur, as long as the river carries a mixture of suspended sediments and the sand of the streambed is not too coarse.