An advanced micro-bio-loop to produce biogas

The authors report an advanced micro-bio-loop that involves recycling through four steps; namely: microalgae culture; de-oxygenation; anaerobic digestion; and aerobic decomposition. The advanced micro-bio-loop operates under sunlight to produce a continuous stream of biogas without requiring any additional external input or internal output to its surrounds. In comparison to conventional biogas production process, it achieves a net positive energy balance at remarkably different level of 0.0224 kWh MJ−1, with less than 33% of environmental impacts, less than 0.57% of water demand, only 7.35% arable land-use and 0.041% of labor

Heat transfer in the seabed boundary layer

We present a theoretical model of the temperature distribution in the boundary layer region close to the seabed. Using a perturbation expansion, multiple scales and similarity variables, we show how free-surface waves enhance heat transfer between seawater and a seabed with a solid, horizontal, smooth surface. Maximum heat exchange occurs at a fixed frequency depending on ocean depth, and does not increase monotonically with the length and phase speed of propagating free-surface waves. Close agreement is found between predictions by the analytical model and a finite-difference scheme. It is found that free-surface waves can substantially affect the spatial evolution of temperature in the seabed boundary layer. This suggests a need to extend existing models that neglect the effects of a wave field, especially in view of practical applications in engineering and oceanography.</jats:p

Effects of support structures in an LES actuator line model of a tidal turbine with contra-rotating rotors

Computational fluid dynamics is used to study the impact of the support structure of a tidal turbine on performance and the downstream wake characteristics. A high-fidelity computational model of a dual rotor, contra-rotating tidal turbine in a large channel domain is presented, with turbulence modelled using large eddy simulation. Actuator lines represent the turbine blades, permitting the analysis of transient flow features and turbine diagnostics. The following four cases are considered: the flow in an unexploited, empty channel; flow in a channel containing the rotors; flow in a channel containing the support structure; and flow in a channel with both rotors and support structure. The results indicate that the support structure contributes significantly to the behaviour of the turbine and to turbulence levels downstream, even when the rotors are upstream. This implies that inclusion of the turbine structure, or some parametrisation thereof, is a prerequisite for the realistic prediction of turbine performance and reliability, particularly for array layouts where wake effects become significant

Explicit radial basis function collocation method for computing shallow water flows

A simple Explicit Radial Basis Function (RBF) is used to solve the shallow water equations (SWEs) for flows over irregular, frictional topography involving wetting and drying. At first we construct the MQ-RBF interpolation corresponding to space derivative operators. Next, we obtain numerical schemes to solve the SWEs, by using the gradient of the interpolant to approximate the spatial derivative of the differential equation and a third-order explicit Runge-Kutta scheme to approximate the temporal derivative of the differential equation. Then, we verify our scheme against several idealized one-dimensional numerical experiments including dam-break and open channel flows over non-uniform beds (involving shock wave behavior), and moving wet-dry fronts over irregular bed topography. Promising results are obtained

Quantifying multiple uncertainties in modelling shallow water-sediment flows: A stochastic Galerkin framework with Haar wavelet expansion and an operator-splitting approach

The interactive processes of shallow water flow, sediment transport, and morphological evolution constitute a hierarchy of multi-physical problems of significant interests in a spectrum of engineering and science areas. To date, modelling shallow water hydro-sediment-morphodynamic (SHSM) processes is subject to multiple sources of uncertainty arising from input data and incomplete understanding of the underlying physics. A stochastic SHSM model with multiple uncertainties has yet to be developed as most SHSM models still concern deterministic problems and only one has been recently extended to a stochastic setting, but is restricted to a single source of uncertainty. Here we first present a new probabilistic SHSM model incorporating multiple uncertainties within the stochastic Galerkin framework using a multidimensional tensor product of Haar wavelet expansion to capture local, nonlinear variations in joint probability distributions and an operator-splitting-based method to ensure that the modelling system remains hyperbolic. Then, we verify the proposed model via benchmark probabilistic numerical tests with joint uncertainties introduced in initial and boundary conditions, matching established experiments of flow-sediment-bed evolutions driven by a sudden dam break and by a landslide dam failure and large-scale rapid flow-sediment-bed evolution in response to flash flood. The present work facilitates a promising modelling framework for quantifying multiple uncertainties in practical shallow water hydro-sediment-morphodynamic modelling applications

Bed-slope-related diffusion of an erodible hump

In certain conditions, the bed slope has a substantial influence on sediment transport rates and hence morphological evolution. Approaches to account for such influence usually suffer from a high degree of empiricism and/or mathematical complexity. We propose a bed slope related diffusivity parameter, derived from a morphodynamic model previously validated against empirical data for bedload transport on horizontal and steep sloping channels. The proposed diffusivity parameter is easy to include in a conventional morphodynamic model via the modification of a typical bedload formula originally developed for sediment transport in a nearly horizontal channel. A conventional model modified through this parameter proves to yield enhanced results in the case study of a submerged migrating sandbar, by avoiding the generation of unrealistic high-frequency oscillations in the bed profile, yet permitting the expected steepening of its downstream face with time. Other models derived for sloping channels do not satisfy the latter condition. It is also shownthat unre-alistic oscillations can be avoided through numerical means; however, their use should be interpreted carefully from a hydrodynamic and sedimentological viewpoint

Human affinity for rivers

AbstractHuman civilization prospers along rivers worldwide. Here, we investigated human–river relations by revealing the linkage between water and habitability for human settlements, and socioeconomic and cultural development across China. We found that human–water co‐occurrence relationships are self‐similar over sub‐basins for different scales of stream‐order in the river networks. According to the earlier complete demographic census conducted during the reign of the Qing Dynasty (1776), there has been a general tendency for inhabitants to live close to rivers, characterized by population density associated with habitability cored by water under a near‐natural state, which still remains to date (2019) even after long‐term population growth and human interventions. Throughout history, we have extended the linkage of human settlements to humanistic attributes with river networks, derived four different modes of human aggregation towards rivers, and elucidated the geographical diversity of river density, population density, cultural prosperity, and clusters of ethnicity, particularly the Western and the Northeast culture established in the arid (desert) areas, the Huaxia culture in the alluvial plains, the Loess/Nomadic/Southwestern Ethnic culture in the plateaus, and the Qi‐Lu/Wu‐Yue/Linnan culture in coastlands across the whole country. This work is also of significance to understanding long‐term human–water relationships at a global scale.</jats:p

Offshore monopile in the southern North Sea: Part II, simulated hydrodynamics and loading

This paper considers two methods for determining the local wave particle kinematics and hydrodynamic forces on an idealised wind turbine monopile in the southern North Sea using sea-state data from Teesside Offshore Wind Farm. An assessment of local flow hydrodynamics is important with regard to safe access of personnel from a crew transfer vessel to a monopile. The hydrodynamic behaviour was calculated using an analytical solution from linear diffraction theory and numerical predictions using OpenFoam, with both slip and no-slip cylinder boundary conditions. Provided the underlying sea state is unidirectional, it was found that close agreement is obtained between analytical and numerical spectra derived from the time series of local free surface elevation, water particle velocity components and in-line wave force loading on the monopile. Less satisfactory agreement is achieved with sea states possessing a bimodal spectrum, which suggests that bimodal spectra may not be unidirectional

Vertically layered flow structure at confluence of a reservoir and tributary carrying high sediment loads

Enhanced understanding of flow structure at a river confluence is essential for predictions of sediment transport and morphological evolution. To date, however, the confluent flow structure of a reservoir and tributary carrying high sediment loads has remained poorly understood, and may be vertically layered sharply, featuring subaqueous sediment-laden flow, i.e., turbidity currents underneath subaerial clear water. Here a recently established 2D double layer-averaged model, able to resolve the formation, propagation, and recession of turbidity currents, is used to investigate a series of idealized laboratory-scale cases and a prototype case study of the Guxian Reservoir on the Yellow River, China. Four primary patterns of the stable, vertically layered flow structure at a reservoir-tributary confluence are identified: 1) single layers of sediment-laden inflow in both the main channel and tributary, sustained by sufficient vertical mixing; 2) a double layer in the main channel and a single layer of sediment-laden inflow in the tributary, when the sediment-laden flow in the tributary suffices to block intrusion of flow in the main channel; 3) a single layer of sediment-laden inflow in the main channel and a double layer in the tributary, induced by the intrusion of sediment-laden flow from the main channel into clear-water flow with small discharge in the tributary; and 4) double layers in both the main channel and tributary, which may be further divided into three subpatterns, as turbidity current exists in both the main channel and tributary, or in the main channel (tributary) intruding into the tributary (main channel). In response to unsteady discharge and sediment inputs from upstream, the vertically layered flow structure evolves in time, and may fall into one of the patterns identified above. Although bed deformation in the long term may modify the confluent flow, the vertically layered flow pattern remains so far as the present cases are concerned. The findings have implications for sediment transport and morphological evolution at a reservoir–tributary confluence, for which further studies are suggested to inform the optimization of reservoir operation schemes to mitigate capacity loss caused by sedimentation

Watershed-scale environmental risk assessment of accidental water pollution: The case of laoguan river, China

Although co-existing sources of chemicals pose major cumulative environmental threats to watersheds, few risk asse- ssments have specifically tackled the accidental chemical pollution of rivers at watershed-scale. Herein, a Watershed-scale Accidental Pollution Risk Assessment (WAPRA) method was constructed which applied a risk ranking procedure to the whole watershed, and was based on watershed-scale stressors, exposures to and effects of water accidental pollution risk (e.g., sudden occurrences, waterway spread, and acute consequences). Multi-criteria analysis and instantaneous water quality models were used to refine the risk ranking procedure within the framework of a Relative Risk Model (RRM), a regional-scale ecological risk assessment approach. The study area comprised the Laoguan River, a tributary watershed of the Danjiangkou Reservoir, which will eventually feed into the South-to-North Water Diversion Project in China. The resultant map shows that risk is higher in the upstream and downstream reaches, and lower in the middle reaches. The map also indicates that the greatest threat to water quality arises from the upstream heavy metal mine tailings ponds. Sensitivity and uncertainty analyses were performed to validate the robustness of the WAPRA method