Aerogels and metal–organic frameworks for environmental remediation and energy production

Environmental pollution and climate change are requiring new methods to clean pollutants and produce sustainable energy. Aerogels and metal organic frameworks are emerging as advanced porous materials with higher functionality, high surface area, high porosity and flexible chemistry. Aerogels are dried gels prepared using the sol–gel procedure, whereas metal organic frameworks are networks of organic ligands and metal ions connected by coordination bonds. Applications of aerogels include the removal of heavy metals, CO₂ capture and reduction, photodegradation of pollutants, air cleanup and water splitting. This article reviews the synthesis and types of aerogels and metal organic frameworks, and the application to pollutant removal, energy production including hydrogen, methane reforming, CO₂ conversion and NOx removal.     

Granulation of anammox microorganisms for autotrophic nitrogen removal from wastewater

Excessive discharge of nutrients in waters induces pollution such as such as eutrophication. Conventional methods to treat waters are expensive. Alternatively, anaerobic ammonium oxidation, termed “anammox”, has been recently developped with benefits such as low sludge production, 50% less aeration demand, no external carbon supply, 60% less power consumption, and 90% reduction of greenhouse gas emissions. However, anammox is limited by long start-up periods due to the low growth rate of anammox bacteria. This issue can be solved by complete retention of biomass by reactor modification or by formation of anammox granules. This article reviews the mechanisms of anammox granulation and biogranulation models. We present factors involved in the granulation processes such as hydrodynamic shear force, extracellular polymeric substances, hydraulic retention time, seed sludge and bioreactors. We also discuss the interaction of proteins and polysaccharides in anammox granules.     

Insights on plant interaction between dominating species from patterns of plant association: expected covariance of pin-point cover measurements of two species

It has been suggested that in order to infer ecological processes from observed patterns of species abundance we need to investigate the covariance in species abundance. Consequently, an expression for the expected covariance of pin-point cover measurements of two species is developed. By comparing the observed covariance with the expected covariance we get a new type of information on the spatial arrangement of two species. Here the discrepancy between the observed and expected covariance may be thought of as a measure of the spatial configuration of the two species that may indicate underling ecological processes. The method is applied in a case study of Calluna vulgaris and Deschampsia flexuosa on dry heathland sites. The observed covariance of Calluna and Deschampsia at the level of the sites was positively and significantly correlated with the expected covariance. Negative covariance was observed on sites where both Calluna and Deschampsia had a high cover, which is in agreement with the notion that both species form distinct patches. Oppositely, at sites where both species have a low cover, we found that both the expected and observed covariance were positive. The proposed measure for the expected covariance of two species does capture information on the combined spatial configuration of the two species if the species are common. We show how this may be relevant for understanding the underlying ecological processes leading to the observed covariance.     

Halton iterative partitioning: spatially balanced sampling via partitioning

A new spatially balanced sampling design for environmental surveys is introduced, called Halton iterative partitioning (HIP). The design draws sample locations that are well spread over the study area. Spatially balanced designs are known to be efficient when surveying natural resources because nearby locations tend to be similar. The HIP design uses structural properties of the Halton sequence to partition a resource into nested boxes. Sample locations are then drawn from specific boxes in the partition to ensure spatial diversity. The method is conceptually simple and computationally efficient, draws spatially balanced samples in two or more dimensions and uses standard design-based estimators. Furthermore, HIP samples have an implicit ordering that can be used to define spatially balanced over-samples. This feature is particularly useful when sampling natural resources because we can dynamically add spatially balanced units from the over-sample to the sample as non-target or inaccessible units are discovered. We use several populations to show that HIP sampling draws spatially balanced samples and gives precise estimates of population totals.     

Small-scale entrainment in inclined gravity currents

We investigate the effect of buoyancy on the small-scale aspects of turbulent entrainment by performing direct numerical simulation of a gravity current and a wall jet. In both flows, we detect the turbulent/nonturbulent interface separating turbulent from irrotational ambient flow regions using a range of enstrophy iso-levels spanning many orders of magnitude. Conform to expectation, the relative enstrophy isosurface velocity \(v_n\) in the viscous superlayer scales with the Kolmogorov velocity for both flow cases. We connect the integral entrainment coefficient E to the small-scale entrainment and observe excellent agreement between the two estimates throughout the viscous superlayer. The contribution of baroclinic torque to \(v_n\) is negligible, and we show that the primary reason for reduced entrainment in the gravity current as compared to the wall-jet are 1) the reduction of \(v_n\) relative to the integral velocity scale \(u_T\); and 2) the reduction in the surface area of the isosurfaces.     

A two-dimensional inviscid model of the gravity-current head by Lagrangian block simulation

A two-dimensional inviscid model of the gravity-current head produced by the release of a relatively small volume of dense fluid from behind a tall lock gate is constructed by Lagrangian block simulation. Three numerical experiments are conducted for the lock’s height-to-length aspect ratios H/L _o  = 8, 4 and 2. The front speeds obtained by the simulations agree with the laboratory observation for a similar range of aspect ratios. The floor velocity in the wake behind these heads is found to be greater than their front speed. The high floor velocity is caused by the impingement of the coherent wake vortex on the floor. It is a condition that permits these gravity-current heads to maintain their structural integrity so that the fine sediments can travel with the head over long distances on the ocean floor. The structural coherence of the current head depends on the lock aspect ratio. The gravity-current head produced by the release from the lock with the highest aspect ratio of H/L _o  = 8 is most coherent and relatively has the greatest floor velocity and the least trailing current behind the head.     

Modelling hydraulic jump using the bubbly two-phase flow method

Hydraulic jumps have complex flow structures, characterised by strong turbulence and large air contents. It is difficult to numerically predict the flows. It is necessary to bolster the existing computer models to emphasise the gas phase in hydraulic jumps, and avoid the pitfall of treating the phenomenon as a single-phase water flow. This paper aims to improve predictions of hydraulic jumps as bubbly two-phase flow. We allow for airflow above the free surface and air mass entrained across it. We use the Reynolds-averaged Navier–Stokes equations to describe fluid motion, the volume of fluid method to track the interface, and the k–ε model for turbulence closure. A shear layer is shown to form between the bottom jet flow and the upper recirculation flow. The key to success in predicting the jet flow lies in formulating appropriate bottom boundary conditions. The majority of entrained air bubbles are advected downstream through the shear layer. Predictions of the recirculation region’s length and air volume fraction within the layer are validated by available measurements. The predictions show a linear growth of the shear layer. There is strong turbulence at the impingement, and the bulk of the turbulence kinetic energy is advected to the recirculation region via the shear layer. The predicted bottom-shear-stress distribution, with a peak value upstream of the toe of the jump and a decaying trend downstream, is realistic. This paper reveals a significant transient bottom shear stress associated with temporal fluctuations of mainly flow velocity in the jump. The prediction method discussed is useful for modelling hydraulic jumps and advancing the understanding of the complex flow phenomenon.     

Part I : <Emphasis Type="Italic">a priori</Emphasis> study of erosion and deposition with large eddy simulation of turbulent flow over multiple 2D sandy Gaussian hills

The micro-scale prediction of sand trapping or take-off over hilly terrains is a crucial issue in semi-arid regions for soil depletion. In this context, large eddy simulations around one or several hills are performed in order to provide statistical parameters to characterize the flow at micro-scales and provide data for mesoscale modelling. We focus on the determination of recirculation zones since they play an important role in solid particle erosion or entrapment. A new wall modeling adapted from Huang et al. (J Turbul 17:1–24, 2016) for rough boundary layers is found to improve the prediction of the recirculation zone length downstream of an isolated hill and is used for all the numerical cases presented here. A geometrical parameterization of the recirculation zones is proposed. When the recirculation region is assumed to have an ellipsoidal shape, the total surface of the recirculation can be obtained from this new parameterization and easily extrapolated to more general dune configurations. Numerical results are compared with experiments performed in our laboratory (Simoëns et al. in Procedia IUTAM 17:110–118, 2015) and good agreement is achieved. We explore general aerodynamic cases deduced from the urban canopy scheme of Oke (Energy Build 11:103–113, 1988). In this scheme the momentum and mass exchange between the upper layer and the space between hills is sorted according to the streamwise hill spacing within three basic cases of skimming, wake or isolated flow. The study of the recirculation zones, the mean velocity and Reynolds stress profiles around an isolated or two consecutive hills with different distances shows that the double hill configuration with 3H separation behaves as much as a whole to the upcoming flow. The vortex formed between the crests does not strongly affect the overall evolution of the outer flow. By an a priori prediction of the preferential zones of erosion and accumulation of fictive particles, it is shown that isolated dunes present more deposition and less erosion than two-hill configurations. The results presented in this study will be discussed in the presence of Lagrangian transport of sand particles above 2D Gaussian hills in future work.